Review Cel Plants to power:bioenergy to fuel the future Joshua S.Yuan1.2.3,Kelly H.Tiller4,Hani Al-Ahmad1.5,Nathan R.Stewart1.2 and C.Neal Stewart Jr1 1 Department of Plant Sciences,University of Tennessee,Knoxville,TN 37996,USA 2 University of Tennessee Institute of Agriculture Genomics Hub,University of Tennessee,Knoxville,TN 37996,USA 3 Current address:Institute for Plant Genomics and Biotechnology,Texas A&M University,College Station TX 77843,USA 4 Department of Agricultural Economics and the Office of Bioenergy Programs,University of Tennessee,Knoxville,TN 37996,USA 5 Department of Biology Biotechnology,An-Najah National University,Nablus,West Bank,Palestine Bioenergy should play an essential part in reaching tar- is also an important raw product in the chemical industry. gets to replace petroleum-based transportation fuels with Therefore,ethanol production has a particularly important a viable alternative,and in reducing long-term carbon role in transforming petroleum-based economies to bio- dioxide emissions,if environmental and economic sus- mass-based sustainable and environment-friendly econ- tainability are considered carefully.Here,we review omies. different platforms,crops,and biotechnology-based improvements for sustainable bioenergy.Among the Ethanol processing platforms different platforms,there are two obvious advantages Ethanol can be produced using agricultural products such as to using lignocellulosic biomass for ethanol production: starch and sugar,or lignocellulosic biomass(see Glossary; higher net energy gain and lower production costs.How- Figure la and b).Currently >10 billion gallons ofethanol is ever,the use of lignocellulosic ethanol as a viable alterna- produced globally per year from starch(maize)and sugar tive to petroleum-based transportation fuels largely (sugarcane and sugar beet)through mature industrialized depends on plant biotechnology breakthroughs.We procedures,including hydrolysis of starch and fermentation examine how biotechnology,such as lignin modification, ofsugar(Figure la)[3,6].Starch and sugar-based ethanol is abiotic stress resistance,nutrition usage,in planta expres- often referred to as a first-generation biofuel. sion of cell wall digestion enzymes,biomass production, feedstock establishment,biocontainment of transgenes, metabolic engineering,and basic research,can be used to Glossary address the challenges faced by bioenergy crop pro- Agriculture residuals (or residues):straw or 'stover'that are left in the field duction after harvest,or forest product 'waste'such as woodchips. Bioenergy feedstock:either the biomass crops themselves or the raw material Multiple choices for bioenergy that is input into the biorefinery. Biofuel cells:various electrochemical systems that can generate a current with Bioenergy refers to renewable energy from biological the electron or proton donated from microorganisms,often through oxidation sources that can be used for heat,electricity and fuel, reaction. and their co-products.There has been a resurgence of Carbon balance:also known as carbon dioxide balance;calculated as carbon dioxide emitted by biomass production and usage subtracted from the carbon interest in bioenergy recently,and several articles have dioxide fixed in the plant material,both above ground and underground.Thus, already addressed the potential impact of biotechnology on a negative carbon balance is desirable. renewable energy [1-5].However,in this review we will Fermentation:the conversion of sugars into ethanol by microorganisms under anaerobic conditions. integrate several of the key components of bioenergy, Greenhouse effect (also known as global warming):worldwide rise in including feedstock,processing platforms,enabling bio- temperature caused by particula ch as methane and carbon dioxide, technologies,ecological effects and economics,to gauge trapping heat from the sun on the Earth's surface.These gases are therefore called 'greenhouse gases' how plant biotechnology might impact bioenergy efficiency Lignocellulosic biomass:plants grown for ethanol production using the entire and sustainability.We will discuss the crucial choices of aboveground biomass.'Lignocellulosic'refers to the plant biomass that is composed of cellulose,hemicellulose and lignin polymers.Biomass can be feedstock (e.g.starch,sugar,fatty acid or cellulose)and hydrolyzed and resulting sugars can be for ethanol production and energy product (e.g.ethanol,biodiesel and others),the potentially other biofuels. economic feasibility and the pros and cons of different Net energy balance (NEB):the difference between the energy output and the energy input for biomass production and processing. choices,and the major technical breakthroughs needed Net energy ratio(NER):an altemative measure of energy gain consisting of the to develop a sustainable bioenergy industry. ratio of the energy output the energy nput for biomass production and processing. Pretreatment:an initial physical or chemical treatment to disassemble cell wall Choices of platforms components,typically involving factors such as high temperature and extreme In terms of modern bioenergy,ethanol,biodiesel and bio- pH. Recalcitrance:resista nce gas are the three major bioenergy products.Ethanol and of pla ell drolysis for the release of fermentable sugars. biodiesel can be used as transportation fuels,and ethanol Saccharification:the release of products such as cellulobiose and glucose from cellulose via chemical hydrolysis or enzymatic reactions. Corresponding author:Stewart Jr,C.N.(nealstewart@utk.edu). 1360-1385/$-see front matter 2008 Elsevier Ltd.All rights reserved.doi:10.1016/j.tplants.2008.06.001 Available online 16 July 2008 421
Plants to power: bioenergy to fuel the future Joshua S. Yuan1,2,3 , Kelly H. Tiller4 , Hani Al-Ahmad1,5 , Nathan R. Stewart1,2 and C. Neal Stewart Jr1 1 Department of Plant Sciences, University of Tennessee, Knoxville, TN 37996, USA 2 University of Tennessee Institute of Agriculture Genomics Hub, University of Tennessee, Knoxville, TN 37996, USA 3Current address: Institute for Plant Genomics and Biotechnology, Texas A&M University, College Station TX 77843, USA 4 Department of Agricultural Economics and the Office of Bioenergy Programs, University of Tennessee, Knoxville, TN 37996, USA 5 Department of Biology & Biotechnology, An-Najah National University, Nablus, West Bank, Palestine Bioenergy should play an essential part in reaching targets to replace petroleum-based transportation fuelswith a viable alternative, and in reducing long-term carbon dioxide emissions, if environmental and economic sustainability are considered carefully. Here, we review different platforms, crops, and biotechnology-based improvements for sustainable bioenergy. Among the different platforms, there are two obvious advantages to using lignocellulosic biomass for ethanol production: higher net energy gain and lower production costs. However, the use of lignocellulosic ethanol as a viable alternative to petroleum-based transportation fuels largely depends on plant biotechnology breakthroughs. We examine how biotechnology, such as lignin modification, abiotic stress resistance, nutrition usage,in plantaexpression of cell wall digestion enzymes, biomass production, feedstock establishment, biocontainment of transgenes, metabolic engineering, and basic research, can be used to address the challenges faced by bioenergy crop production. Multiple choices for bioenergy Bioenergy refers to renewable energy from biological sources that can be used for heat, electricity and fuel, and their co-products. There has been a resurgence of interest in bioenergy recently, and several articles have already addressed the potential impact of biotechnology on renewable energy [1–5]. However, in this review we will integrate several of the key components of bioenergy, including feedstock, processing platforms, enabling biotechnologies, ecological effects and economics, to gauge how plant biotechnology might impact bioenergy efficiency and sustainability. We will discuss the crucial choices of feedstock (e.g. starch, sugar, fatty acid or cellulose) and energy product (e.g. ethanol, biodiesel and others), the economic feasibility and the pros and cons of different choices, and the major technical breakthroughs needed to develop a sustainable bioenergy industry. Choices of platforms In terms of modern bioenergy, ethanol, biodiesel and biogas are the three major bioenergy products. Ethanol and biodiesel can be used as transportation fuels, and ethanol is also an important raw product in the chemical industry. Therefore, ethanol production has a particularly important role in transforming petroleum-based economies to biomass-based sustainable and environment-friendly economies. Ethanol processing platforms Ethanol can be produced using agricultural products such as starch and sugar, or lignocellulosic biomass (see Glossary; Figure 1a and b). Currently >10 billion gallons of ethanol is produced globally per year from starch (maize) and sugar (sugarcane and sugar beet) through mature industrialized procedures, including hydrolysis of starch and fermentation of sugar (Figure 1a) [3,6]. Starch and sugar-based ethanol is often referred to as a first-generation biofuel. Review Glossary Agriculture residuals (or residues): straw or ‘stover’ that are left in the field after harvest, or forest product ‘waste’ such as woodchips. Bioenergy feedstock: either the biomass crops themselves or the raw material that is input into the biorefinery. Biofuel cells: various electrochemical systems that can generate a current with the electron or proton donated from microorganisms, often through oxidation reaction. Carbon balance: also known as carbon dioxide balance; calculated as carbon dioxide emitted by biomass production and usage subtracted from the carbon dioxide fixed in the plant material, both above ground and underground. Thus, a negative carbon balance is desirable. Fermentation: the conversion of sugars into ethanol by microorganisms under anaerobic conditions. Greenhouse effect (also known as global warming): worldwide rise in temperature caused by particular gases, such as methane and carbon dioxide, trapping heat from the sun on the Earth’s surface. These gases are therefore called ‘greenhouse gases’. Lignocellulosic biomass: plants grown for ethanol production using the entire aboveground biomass. ‘Lignocellulosic’ refers to the plant biomass that is composed of cellulose, hemicellulose and lignin polymers. Biomass can be hydrolyzed and resulting sugars can be used for ethanol production and potentially other biofuels. Net energy balance (NEB): the difference between the energy output and the energy input for biomass production and processing. Net energy ratio (NER): an alternative measure of energy gain consisting of the ratio of the energy output and the energy input for biomass production and processing. Pretreatment: an initial physical or chemical treatment to disassemble cell wall components, typically involving factors such as high temperature and extreme pH. Recalcitrance: resistance of plant cell walls to hydrolysis for the release of fermentable sugars. Saccharification: the release of products such as cellulobiose and glucose from cellulose via chemical hydrolysis or enzymatic reactions. Corresponding author: Stewart Jr, C.N. (nealstewart@utk.edu). 1360-1385/$ – see front matter 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.tplants.2008.06.001 Available online 16 July 2008 421
Review Trends in Plant Science Vol.13 No.8 Starch Lignocellulosic biomass Methanol Plant lipid Hydrolysis Pretreatment Transesterification Glucose or fructose Disrupted biomass Biodiesel Sucrose Fermentation Hydrolysis Liquidification and pyrolysis Bioethanol Glucose,xylose and others Biomass (a) (c) Fermentation Biomass and other organic waste Bioethanol Anaerobic gasification (b) Biogases (d) TRENDS in Plant Science Figure 1.Processing platforms for producing different biofuels with various feedstocks.(a)Starch and sugar-based first generation ethanol.In this platform,starch can be hydrolyzed into monosaccharides,where sugar can be further fermented into ethanol.(b)Lignocellulosic ethanol.Lignocellulosic ethanol production involves pretreatment of biomass material,hydrolysis for monosaccharide production,and fermentation to produce ethanol.Lignocellulosic ethanol is believed to be the major component of the second generation of bioenergy.(c)Biodiesel.Biodiesel production often involves the transesterification of fatty acids.(d)Biomass gasification.Biomass can be used to produce methanol,carbon monoxide,hydrogen or other gases through a process of gasification. Even though the production of ethanol from starch nical hurdles can be scaled.Lignocellulosic feedstock can represents the most convenient and technically advanced be acquired from either dedicated biomass crops or forestry option for bioenergy in the USA,it would result in severe and agricultural residuals [5.9-131. competition between energy and food supplies,which is The key obstacle for transitioning from starch-based to probably not sustainable in the long term given that the lignocellulosic biofuels is the complicated structure of the net energy and carbon dioxide balance of the platform is cell wall,which is,by nature,resistant to breakdown-the not favorable (Table 1)[7,8].Therefore,in temperate recalcitrance problem.Current processes for lignocellulosic regions,biofuel (ethanol for now)production from ligno- biomass include pretreatment,saccharification (hydroly- cellulosic biomass represents the best choice if key tech- sis),and fermentation (Figure 1b)[14].Improvement or Table 1.Comparison of different platforms and bioenergy crops Platforms" Feedstock" NEB NERb C02 Annual Estab- Germ- Agricul. Ecological Refs GJ/ha/yr balance feedstock lishment plasm practiced benefits Ethanol from Maize 10-80 1.5-3.0 Positive Yes +++0 +++ +++ [4,28,30,31] starch or Sugarcane 55-80 3.0-4.0 Positive No +++ +++ +++ [4,28,30,311 sucrose Sugar beet 40-100 2.5-3.5 Positive Yes +++ +++ [28] Sweet 85-300 5-10 Positive Yes +++ ++ ++ [28,92] sorghum Ethanol from Miscanthus 250-550 15-70 Possibly Yes/No +++ [28 lignocellulosic negative feedstocks Switchgrass 150-500 10-50 Possibly No +++ [4,28,30,31.93 negative Poplar 150-250 10-20 Possibly No ++ ++ +++ [24,301 negative Biodiesel Soybean -20-10 0.2-0.6 Positive Yes ++ +++ +++ [28 Canola -5-2 0.7-1.0 Positive Yes +++ +++ +++ [28 Sunflower -10-0 0.3-0.9 Positive Yes +++ ++ +++ [28 Multiple platforms and crops are compared in a synthesis integrating information from multiple studies PAbbreviations:NEB,energy balance;NER,net energy ratio,which is the ratio of output to input energy needed to produce a fuel from a feedstock Favorable features are indicated by symbols,with +++being the most favorable. dAgricul.Practice,agricultural practice:how advanced is the current status of farming,harvesting,and processing. 422
Even though the production of ethanol from starch represents the most convenient and technically advanced option for bioenergy in the USA, it would result in severe competition between energy and food supplies, which is probably not sustainable in the long term given that the net energy and carbon dioxide balance of the platform is not favorable (Table 1) [7,8]. Therefore, in temperate regions, biofuel (ethanol for now) production from lignocellulosic biomass represents the best choice if key technical hurdles can be scaled. Lignocellulosic feedstock can be acquired from either dedicated biomass crops or forestry and agricultural residuals [5,9–13]. The key obstacle for transitioning from starch-based to lignocellulosic biofuels is the complicated structure of the cell wall, which is, by nature, resistant to breakdown – the recalcitrance problem. Current processes for lignocellulosic biomass include pretreatment, saccharification (hydrolysis), and fermentation (Figure 1b) [14]. Improvement or Figure 1. Processing platforms for producing different biofuels with various feedstocks. (a) Starch and sugar-based first generation ethanol. In this platform, starch can be hydrolyzed into monosaccharides, where sugar can be further fermented into ethanol. (b) Lignocellulosic ethanol. Lignocellulosic ethanol production involves pretreatment of biomass material, hydrolysis for monosaccharide production, and fermentation to produce ethanol. Lignocellulosic ethanol is believed to be the major component of the second generation of bioenergy. (c) Biodiesel. Biodiesel production often involves the transesterification of fatty acids. (d) Biomass gasification. Biomass can be used to produce methanol, carbon monoxide, hydrogen or other gases through a process of gasification. Table 1. Comparison of different platforms and bioenergy crops Platformsa Feedstocka NEBb GJ/ha/yr NERb CO2 balance Annual feedstock Establishment Germplasm Agricul. practiced Ecological benefits Refs Ethanol from starch or sucrose Maize 10–80 1.5–3.0 Positive Yes +++c +++ +++ + [4,28,30,31] Sugarcane 55–80 3.0–4.0 Positive No +++ +++ +++ + [4,28,30,31] Sugar beet 40–100 2.5–3.5 Positive Yes +++ ++ +++ + [28] Sweet sorghum 85–300 5–10 Positive Yes +++ ++ ++ ++ [28,92] Ethanol from lignocellulosic feedstocks Miscanthus 250–550 15–70 Possibly negative Yes/No + + + +++ [28] Switchgrass 150–500 10–50 Possibly negative No + + + +++ [4,28,30,31,93] Poplar 150–250 10–20 Possibly negative No + ++ ++ +++ [24,30] Biodiesel Soybean 20–10 0.2–0.6 Positive Yes ++ +++ +++ + [28] Canola 5–2 0.7–1.0 Positive Yes +++ +++ +++ + [28] Sunflower 10–0 0.3–0.9 Positive Yes +++ ++ +++ + [28] a Multiple platforms and crops are compared in a synthesis integrating information from multiple studies. b Abbreviations: NEB, energy balance; NER, net energy ratio, which is the ratio of output to input energy needed to produce a fuel from a feedstock. c Favorable features are indicated by + symbols, with +++ being the most favorable. d Agricul. Practice, agricultural practice: how advanced is the current status of farming, harvesting, and processing. Review Trends in Plant Science Vol.13 No.8 422
Review Trends in Plant Science Vol.13 No.8 replacement of these processes is crucial for increasing byproducts.Maize can be used as a bioenergy crop in two efficiency and for decreasing biofuel production costs ways:the starch in seeds can be used to produce ethanol, Obviating pretreatment along with simultaneous sacchar- and the crop residuals(termed stover)could potentially be ification and fermentation are two important factors that used to produce lignocellulosic ethanol.Sorghum (Sor- would decrease the cost oflignocellulosic ethanol production ghum bicolor)is the fifth most cultivated cereal crop in [14].Technology breakthroughs are badly needed. the world and is grown for grain,forage,sugar and fiber. Sorghum could also be used for bioenergy in several ways Biodiesel production processes Both the starch in the grain and the sugar could be feed- Biodiesel is a biofuel requiring far simpler processing tech- stock for ethanol fermentation using current technology nology compared with that required for ethanol.Biodiesel is platforms,and crop residuals could be useful for lignocel- a mixture of diesel fuel with oils from plant seeds,algae or lulosic ethanol production.Two features make sorghum a other biological sources such as animal renderings that have particularly attractive bioenergy crop.First,there would been transesterified for removal ofglycerol [15].A variety of not be strong competition between the use of land for food plant species are currently used for biodiesel production or for energy because the seeds can be used for food and including soybean,rapeseed and canola,sunflower and feed and the stems could be optimized for different plat- palm [15].After oil is extracted from plant organs it is forms of ethanol production,which is particularly import- transesterified,leading to biodiesel methyl or ethyl esters ant for heavily populated developing countries such as as products(Figure lc).Other potential choices for biodiesel China.Second,sorghum is drought and heat tolerant, include using terpenoid products from Copaifera species which would enable the usage of marginal land that is ('diesel tree')as biodiesel directly,or engineering the plant not suitable for the cultivation of many other crops [19]. terpenoid pathway to produce large amounts of sesquiter- Besides sorghum and maize,the residuals of other crops penes and diterpenes.As an alternative to diesel fuel, such as wheat and rice are also expected to be useful for biodiesel already has a niche in the current transportation lignocellulosic ethanol production. fuel system and is widely used,but production is relatively low.The sources and forms of biodiesel are diverse,and it is Sugar-producing plants important to consider the different environmental and Sugar can be used for direct fermentation of ethanol. economic factors that apply in the production of different Sugarcane(Saccharum officinarum)and sugar beet (Beta types of biodiesel [15].For example,feedstocks are as vulgaris)are the major sugar-producing plants.Sugar- diverse as soybean and cooking grease waste.Processing cane is adapted to warm temperate to tropical areas, feedstock to fuel is relatively simple,leading to widespread whereas sugar beet is grown in temperate areas.There- production but of variable quality. fore the two sugar crops occupy different geographical niches.Brazil is a successful example of a country that Biogas production process has reduced its gasoline usage by producing bioenergy. A third modern choice for bioenergy is biogas,which is The Brazilian national ethanol program,which is based produced from a variety of organic wastes,including plant solely on sugarcane,produces 4.2 billion gallons ofethanol straw,through gasification(Figure 1d).Biogases include a year [8),although the resultant ecological and environ- methane,hydrogen and carbon monoxide.Gasification mental effects are still debatable [20,211.Most ethanol using current technologies has a low net energy balance production using sugar beet takes place in Europe;how- and its usefulness therefore might be limited [16,17]. ever,using sugar beet to produce ethanol could potentially Besides the traditional biogases,hydrogen production by increase soil erosion and lower the net energy balance. green algae and microbes has been proposed as a potential Other sugar-producing crops include energy cane, source for a third generation biofuel [51.Unlike hydrogen improved cultivars of sugarcane and varieties of sweet production from other biomass sources,algae-based hydro- sorghum.All the above crops are annuals,with the excep- gen production uses a biological water-splitting reaction,in tion of sugarcane.Perennials are more desirable than which hydrogenase uses the photosynthetic electron trans- annuals as bioenergy feedstocks because they do not need port chain to reduce protons for hydrogen production [18] to be reseeded each growing season and therefore cultiva- Hydrogenase engineering for increased oxygen tolerance tion costs are lower. and systems biology research of genes and pathways involved in hydrogen production are needed to realize Dedicated bioenergy feedstocks the potential of this platform[18] Perennial bioenergy feedstocks are important sources of lignocellulosic biomass production.Switchgrass (Panicum Choices of crops and feedstocks virgatum)has been proposed as the major perennial feed- Bioenergy crops can be classified into the following four stock in the USA because it is widely adapted,has high groups:traditional cereal crops,traditional sugar-produ- biomass production,high C-4 photosynthetic efficiency, cing crops,dedicated lignocellulosic biomass feedstocks, and efficient use of water and nitrogen.Switchgrass and oilseed crops for biodiesel. yield is around 10 to 25 Mg/ha/yr depending on latitude, nutrition and other factors.Hybrid Miscanthus,including Traditional cereal crops Miscanthus x giganteus,is another highly favored Cereal crops are a major source for starch-based ethanol biomass feedstock,mainly in Europe.However,Mis- production.Maize(Zea mays)is an important food and feed canthus x giganteus is a sterile triploid clone that requires crop,used as processed food,oil,fodder,a vegetable and vegetative propagation.Similar to switchgrass,Mis- 423
replacement of these processes is crucial for increasing efficiency and for decreasing biofuel production costs. Obviating pretreatment along with simultaneous saccharification and fermentation are two important factors that would decrease the cost of lignocellulosic ethanol production [14]. Technology breakthroughs are badly needed. Biodiesel production processes Biodiesel is a biofuel requiring far simpler processing technology compared with that required for ethanol. Biodiesel is a mixture of diesel fuel with oils from plant seeds, algae or other biological sources such as animal renderings that have been transesterified for removal of glycerol [15]. A variety of plant species are currently used for biodiesel production including soybean, rapeseed and canola, sunflower and palm [15]. After oil is extracted from plant organs it is transesterified, leading to biodiesel methyl or ethyl esters as products (Figure 1c). Other potential choices for biodiesel include using terpenoid products from Copaifera species (‘diesel tree’) as biodiesel directly, or engineering the plant terpenoid pathway to produce large amounts of sesquiterpenes and diterpenes. As an alternative to diesel fuel, biodiesel already has a niche in the current transportation fuel system and is widely used, but production is relatively low. The sources and forms of biodiesel are diverse, and it is important to consider the different environmental and economic factors that apply in the production of different types of biodiesel [15]. For example, feedstocks are as diverse as soybean and cooking grease waste. Processing feedstock to fuel is relatively simple, leading to widespread production but of variable quality. Biogas production process A third modern choice for bioenergy is biogas, which is produced from a variety of organic wastes, including plant straw, through gasification (Figure 1d). Biogases include methane, hydrogen and carbon monoxide. Gasification using current technologies has a low net energy balance and its usefulness therefore might be limited [16,17]. Besides the traditional biogases, hydrogen production by green algae and microbes has been proposed as a potential source for a third generation biofuel [5]. Unlike hydrogen production from other biomass sources, algae-based hydrogen production uses a biological water-splitting reaction, in which hydrogenase uses the photosynthetic electron transport chain to reduce protons for hydrogen production [18]. Hydrogenase engineering for increased oxygen tolerance and systems biology research of genes and pathways involved in hydrogen production are needed to realize the potential of this platform [18]. Choices of crops and feedstocks Bioenergy crops can be classified into the following four groups: traditional cereal crops, traditional sugar-producing crops, dedicated lignocellulosic biomass feedstocks, and oilseed crops for biodiesel. Traditional cereal crops Cereal crops are a major source for starch-based ethanol production. Maize (Zea mays) is an important food and feed crop, used as processed food, oil, fodder, a vegetable and byproducts. Maize can be used as a bioenergy crop in two ways: the starch in seeds can be used to produce ethanol, and the crop residuals (termed stover) could potentially be used to produce lignocellulosic ethanol. Sorghum (Sorghum bicolor) is the fifth most cultivated cereal crop in the world and is grown for grain, forage, sugar and fiber. Sorghum could also be used for bioenergy in several ways. Both the starch in the grain and the sugar could be feedstock for ethanol fermentation using current technology platforms, and crop residuals could be useful for lignocellulosic ethanol production. Two features make sorghum a particularly attractive bioenergy crop. First, there would not be strong competition between the use of land for food or for energy because the seeds can be used for food and feed and the stems could be optimized for different platforms of ethanol production, which is particularly important for heavily populated developing countries such as China. Second, sorghum is drought and heat tolerant, which would enable the usage of marginal land that is not suitable for the cultivation of many other crops [19]. Besides sorghum and maize, the residuals of other crops such as wheat and rice are also expected to be useful for lignocellulosic ethanol production. Sugar-producing plants Sugar can be used for direct fermentation of ethanol. Sugarcane (Saccharum officinarum) and sugar beet (Beta vulgaris) are the major sugar-producing plants. Sugarcane is adapted to warm temperate to tropical areas, whereas sugar beet is grown in temperate areas. Therefore the two sugar crops occupy different geographical niches. Brazil is a successful example of a country that has reduced its gasoline usage by producing bioenergy. The Brazilian national ethanol program, which is based solely on sugarcane, produces 4.2 billion gallons of ethanol a year [8], although the resultant ecological and environmental effects are still debatable [20,21]. Most ethanol production using sugar beet takes place in Europe; however, using sugar beet to produce ethanol could potentially increase soil erosion and lower the net energy balance. Other sugar-producing crops include energy cane, improved cultivars of sugarcane and varieties of sweet sorghum. All the above crops are annuals, with the exception of sugarcane. Perennials are more desirable than annuals as bioenergy feedstocks because they do not need to be reseeded each growing season and therefore cultivation costs are lower. Dedicated bioenergy feedstocks Perennial bioenergy feedstocks are important sources of lignocellulosic biomass production. Switchgrass (Panicum virgatum) has been proposed as the major perennial feedstock in the USA because it is widely adapted, has high biomass production, high C-4 photosynthetic efficiency, and efficient use of water and nitrogen. Switchgrass yield is around 10 to 25 Mg/ha/yr depending on latitude, nutrition and other factors. Hybrid Miscanthus, including Miscanthus giganteus, is another highly favored biomass feedstock, mainly in Europe. However, Miscanthus giganteus is a sterile triploid clone that requires vegetative propagation. Similar to switchgrass, MisReview Trends in Plant Science Vol.13 No.8 423
Review Trends in Plant Science Vol.13 No.8 canthus x giganteus is also a C-4 perennial plant positive NEB can be considered as economically and conferring most of the advantages of switchgrass.Mis- environmentally sustainable.This is particularly import- canthus shows greater cold tolerance and hence might ant when considering which crops and conversion pro- perform better at higher latitudes.The yield of Mis- cesses might be worthy of substantial biotechnology canthus x giganteus has been reported to be between 7 investment.Even though the economics of corn starch- and 38 Mg/ha/yr and potentially has better nitrogen usage based ethanol and biodiesel production is currently com- than switchgrass [22,23]. petitive with gasoline,their NEB is fairly low or even Another group ofdedicated bioenergy feedstocks is woody negative,in contrast to the favorable NEB of lignocellu- plants,including hybrid poplar,willow and pines.Hybrid losics,as shown in Table 1 [10,28,29].If lignocellulosic poplar is considered a model woody biomass feedstock biomass can be efficiently converted into ethanol,a NEB because of its broad adaptation,available genome sequence of up to 600 GJ/ha/yr is a reasonable expectation,which and transformation techniques,and fast growth.The bio- would provide the highest NEB of all first or second mass accumulation of hybrid poplar is reported to bebe- generation platforms.Recent efforts to build biorefineries tween ~7 to 20 Mg/ha/yr depending on the nutrition and for lignocellulosic biomass processing are the first step to environmental conditions [24-26].From the perspective of fulfilling such potential;however,both low recalcitrance biomass production,switchgrass and hybrid Miscanthus feedstocks and new biocatalysts to improve the proces- seem to have the potential to produce more biomass com- sing efficiency are needed to realize this potential.Among pared with that produced by poplar.Given that a short the different bioenergy crops,switchgrass,Miscanthus, rotation for trees is five years,there is a time lag before and sorghum could potentially produce the highest NEB poplars can be harvested,and then,only the wood is har- 28.30.311 vested.Woody biomass does have a storage advantage over herbaceous feedstocks.However,geography,land-use pat- Environmental and ecological benefits of different terns,agronomy,economics and biology are likely to result platforms in multiple feedstock use.Because of the advantages of Different bioenergy platforms have different pros and cons perennial feedstock,efforts have been put into developing from an ecological and agricultural perspective(Table 1) perennial bioenergy feedstocks via breeding [27]. [41.The near-term economic advantages of ethanol pro- duction from maize and biodiesel production from soybean Plants for biodiesel are often counter-balanced by the detrimental effects of In temperate areas,annual oilseeds such as soybean agricultural practices on the environment.By contrast, (Glycine max),canola (Brassica napus),and sunfower perennial feedstocks such as switchgrass can help to (Helianthus annuus)have all been used as biodiesel feed- decrease soil erosion,improve water quality,and protect stocks.Palm oil (Arecaceae)trees have been successfully natural diversity [4,29,32-34]Perennial biomass crops used as biodiesel plants in the tropics.If we consider also complement food-based and feed-based agriculture potential biodiesel feedstocks for temperate use,the trans- instead of competing with it. portation costs for palm oil would be prohibitively expens- ive for export and would have a positive net energy Global climate change and bioenergy choices balance.In the case of soybean,canola and sunflower, Biomass crops and bioenergy production as an offset to the energy output from grain was estimated to be ~10 fossil fuel have the potential to ameliorate global warming. to 40 GJJ/ha,which is considerably lower than the ~200 Not only does the offset mean that less 'old'carbon is 500 GJ/ha energy gain from lignocellulosic biomass [28]. released into the atmosphere,but the underground bio- Hence,we might conclude that lignocellulosic biomass will mass of perennial biomass crops also acts as a carbon sink. have a greater demand than biodiesel feedstocks. For example,the capacity of Miscanthus x giganteus to fix There are other candidates for bioenergy feedstocks that carbon dioxide is estimated to be 5.2 to 7.2 t C/ha/yr,which are too numerous to detail in this review.Alternative results in a negative carbon balance where more carbon bioenergy plants include additional crops(e.g.sweet sor- dioxide is fixed than emitted [27].In a recent study of ghum),Camelina,grasses (e.g.big bluestem),trees (e.g. maize,switchgrass,soybean,alfalfa,hybrid poplar and willow),and even algae.Potentially,green algae could be reed canarygrass (Phalaris arundinacea),only poplar used for hydrogen production,oil production for biodiesel and switchgrass had a negative carbon balance(carbon platforms,and even biomass production for a bioethanol fixation of ~2t C/ha/yr)[35].However,two recent publi- platform,depending on the biotechnology breakthroughs cations indicate that first generation ethanol platforms Hydrogen is believed to be an important component of the actually have far higher carbon dioxide emissions com- third generation of bioenergy and can be adapted as differ pared with that released from fossil fuels [20,21].Accord- ent energy sources.Many factors determine the choice of ing to one of the studies,even the best lignocellulosic bioenergy crops;these are summarized in Table 1 and ethanol is predicted to have a positive carbon balance discussed further in the next section. 20].These contradictory estimates are a product of the different methods and models used to assess carbon Bioenergy:environmental,ecological and economic release and fixation.Despite the differences,there is a considerations consensus that second generation ethanol production Net energy balance of different platforms using lignocellulosic platforms should lead to a lower Net energy balance (NEB)is an important concept in carbon balance as compared with first generation plat- choosing a bioenergy platform because only a high forms.Enhancing the ability of perennial feedstocks to 424
canthus giganteus is also a C-4 perennial plant conferring most of the advantages of switchgrass. Miscanthus shows greater cold tolerance and hence might perform better at higher latitudes. The yield of Miscanthus giganteus has been reported to be between 7 and 38 Mg/ha/yr and potentially has better nitrogen usage than switchgrass [22,23]. Another group of dedicated bioenergy feedstocks is woody plants, including hybrid poplar, willow and pines. Hybrid poplar is considered a model woody biomass feedstock because of its broad adaptation, available genome sequence and transformation techniques, and fast growth. The biomass accumulation of hybrid poplar is reported to be between 7 to 20 Mg/ha/yr depending on the nutrition and environmental conditions [24–26]. From the perspective of biomass production, switchgrass and hybrid Miscanthus seem to have the potential to produce more biomass compared with that produced by poplar. Given that a short rotation for trees is five years, there is a time lag before poplars can be harvested, and then, only the wood is harvested. Woody biomass does have a storage advantage over herbaceous feedstocks. However, geography, land-use patterns, agronomy, economics and biology are likely to result in multiple feedstock use. Because of the advantages of perennial feedstock, efforts have been put into developing perennial bioenergy feedstocks via breeding [27]. Plants for biodiesel In temperate areas, annual oilseeds such as soybean (Glycine max), canola (Brassica napus), and sunflower (Helianthus annuus) have all been used as biodiesel feedstocks. Palm oil (Arecaceae) trees have been successfully used as biodiesel plants in the tropics. If we consider potential biodiesel feedstocks for temperate use, the transportation costs for palm oil would be prohibitively expensive for export and would have a positive net energy balance. In the case of soybean, canola and sunflower, the energy output from grain was estimated to be 10 to 40 GJ/ha, which is considerably lower than the 200– 500 GJ/ha energy gain from lignocellulosic biomass [28]. Hence, we might conclude that lignocellulosic biomass will have a greater demand than biodiesel feedstocks. There are other candidates for bioenergy feedstocks that are too numerous to detail in this review. Alternative bioenergy plants include additional crops (e.g. sweet sorghum), Camelina, grasses (e.g. big bluestem), trees (e.g. willow), and even algae. Potentially, green algae could be used for hydrogen production, oil production for biodiesel platforms, and even biomass production for a bioethanol platform, depending on the biotechnology breakthroughs. Hydrogen is believed to be an important component of the third generation of bioenergy and can be adapted as different energy sources. Many factors determine the choice of bioenergy crops; these are summarized in Table 1 and discussed further in the next section. Bioenergy: environmental, ecological and economic considerations Net energy balance of different platforms Net energy balance (NEB) is an important concept in choosing a bioenergy platform because only a high positive NEB can be considered as economically and environmentally sustainable. This is particularly important when considering which crops and conversion processes might be worthy of substantial biotechnology investment. Even though the economics of corn starchbased ethanol and biodiesel production is currently competitive with gasoline, their NEB is fairly low or even negative, in contrast to the favorable NEB of lignocellulosics, as shown in Table 1 [10,28,29]. If lignocellulosic biomass can be efficiently converted into ethanol, a NEB of up to 600 GJ/ha/yr is a reasonable expectation, which would provide the highest NEB of all first or second generation platforms. Recent efforts to build biorefineries for lignocellulosic biomass processing are the first step to fulfilling such potential; however, both low recalcitrance feedstocks and new biocatalysts to improve the processing efficiency are needed to realize this potential. Among the different bioenergy crops, switchgrass, Miscanthus, and sorghum could potentially produce the highest NEB [28,30,31]. Environmental and ecological benefits of different platforms Different bioenergy platforms have different pros and cons from an ecological and agricultural perspective (Table 1) [4]. The near-term economic advantages of ethanol production from maize and biodiesel production from soybean are often counter-balanced by the detrimental effects of agricultural practices on the environment. By contrast, perennial feedstocks such as switchgrass can help to decrease soil erosion, improve water quality, and protect natural diversity [4,29,32–34] Perennial biomass crops also complement food-based and feed-based agriculture instead of competing with it. Global climate change and bioenergy choices Biomass crops and bioenergy production as an offset to fossil fuel have the potential to ameliorate global warming. Not only does the offset mean that less ‘old’ carbon is released into the atmosphere, but the underground biomass of perennial biomass crops also acts as a carbon sink. For example, the capacity of Miscanthus giganteus to fix carbon dioxide is estimated to be 5.2 to 7.2 t C/ha/yr, which results in a negative carbon balance where more carbon dioxide is fixed than emitted [27]. In a recent study of maize, switchgrass, soybean, alfalfa, hybrid poplar and reed canarygrass (Phalaris arundinacea), only poplar and switchgrass had a negative carbon balance (carbon fixation of 2 t C/ha/yr) [35]. However, two recent publications indicate that first generation ethanol platforms actually have far higher carbon dioxide emissions compared with that released from fossil fuels [20,21]. According to one of the studies, even the best lignocellulosic ethanol is predicted to have a positive carbon balance [20]. These contradictory estimates are a product of the different methods and models used to assess carbon release and fixation. Despite the differences, there is a consensus that second generation ethanol production using lignocellulosic platforms should lead to a lower carbon balance as compared with first generation platforms. Enhancing the ability of perennial feedstocks to Review Trends in Plant Science Vol.13 No.8 424
Review Trends in Plant Science Vol.13 No.8 increase their below-ground biomass accumulation will be table sugars [55].Lignin biosynthesis in monocot species important for increased carbon storage [81. should be studied further so as to be able to modify lignin biosynthesis intelligently in perennial grass feedstocks Economic considerations Current biofuels were marginally profitable before recent Preprocessing in planta:expression of cellulases and petroleum prices spiked.Ethanol production from maize cellulosomes was US$0.48 per gasoline energy equivalent liter(EEL)in Plant cell walls can be degraded by individual cellulases or 2005.when the price of gasoline was US$0.46 per liter [291 in concert by cellulosomes.which consist of a suite of However,the recent spike in the price of global crude oil enzymes.In planta expression of cellulases and cellulo- has made maize-based ethanol production very profitable. somes could potentially reduce the cost of enzymatic sac- The situation is similar for biodiesel production:soybean- charification of lignocellulosic biomass at the biorefinery by based biodiesel production was US$0.55 per diesel EEL in providing the enzymes needed for cell wall degradation. 2005 [29].Despite these potential profit margins, The effectiveness of in planta expression of free cellulases traditional crop-based biofuel production has led to direct or cellulosomes is still controversial because digestion is competition between food and energy and,as a result,the complicated by the subcellular localization of enzyme(s), recent crude oil price increase has already led to the global glycosylation of extracellular proteins,effective enzyme infation of the cost of food,feed and associated products. combinations,and the requirement for inducible expres- There is an imminent need to move to lignocellulosic sion to avoid premature cell wall digestion. biomass-based platforms.However,lignocellulosic ethanol Regardless of the challenges,preliminary research has production using the current platforms is not profitable shown the successful apoplastic expression of active Acid- [361.The lignocellulosic ethanol price is still as high as othermus cellulolyticus cellulase El in maize and tobacco US$0.70 to US$1.0 per EEL.However,as processing tech- [56,57].Moreover,recent research has indicated that nologies mature and biomass crops are modified for higher maize plants showing in planta expression of cellulase yield and lower recalcitrance,the cost of lignocellulosic- had higher biomass conversion efficiency (58].Researchers based ethanol production is expected to decrease to rival have indicated that no detrimental effects were found by maize grain platforms.Indeed,based on break-even price apoplast targeting of El and that cellulase activity aided yield,recent analysis has already indicated that switch- biomass conversion when plant material was milled grass is a more profitable crop than traditional crops such [56,59].These differences result from the different sub- as sorghum and maize [10].The competitiveness of ligno- cellular localization and activation mechanisms of the cellulosic ethanol as a sustainable energy supply in the enzymes.For example,heat-activated cellulase enzymes USA will therefore heavily depend on biotechnology break- such as El have no detrimental effects to plants growing in throughs to reduce cost and improve processing efficiency. typical ambient temperatures [55.561. Further study is necessary to determine different strat- Plant biotechnology solutions for bioenergy egies for in planta cell wall digestion enzyme expression, Novel enabling biotechnologies are crucial for reducing the with high-throughput approaches for optimized sub-cellu- costs of bioenergy production,particularly of lignocellulo- lar localization and different combinations of enzymes.In sic ethanol.The key issues include rapid domestication, addition,fundamental research still needs to be performed overcoming recalcitrance,efficient breakdown of cellulose, to modify cellulases for improved catalytic efficiency,ther- and increasing biomass and lipid production for ethanol mal stability,performance under extreme conditions, and biodiesel,respectively [37].Although aspects of these along with protein modifications designed to lead to important areas will be discussed individually below,it is reduced apoplastic glycosylation.Many cell-wall degrading important to solve these problems in concert. bacteria use cellulosomes,a suite of enzymes for cell wall hydrolysis [60].Cellulosomes have not yet been expressed Modification of lignin biosynthesis in transgenic plants,but the correct assemblage of cellulo- Lignin might be the most crucial molecule in need of some components in the plant apoplast potentially have modification for lignocellulosic feedstocks.It has been the promise of decreasing recalcitrance and facilitating the established that reducing lignin biosynthesis can lead to post-harvest hydrolysis of cellulose,which in turn might lower recalcitrance and higher saccharification efficiency aid simultaneous saccharification and fermentation [13.38-54].Recent studies have indicated two important aspects for lignin modification.First,both lignin content Abiotic stress resistance and composition are important.Although it is codependent Suboptimal water and other abiotic stresses are limiting on efficient processes to fractionate lignin,a more uniform factors for biomass production;stress tolerance traits are lignin structure might facilitate more efficient cell-wall therefore important to enable feedstock to be produced on degradation for fuel production (John Ralph,personal marginal or sub-marginal lands not favorable for food communication).Second,the pretreatment of biomass crops.Drought-,metal-,salt-,cold-and heat-stress all might even be rendered unnecessary if lignin content falls induce some similar responses in plants,yet each of these below a critical threshold,which would enhance down- stresses will induce a different set of genes [61].The stream enzymatic saccharification and fermentation steps upstream pathways for salt and drought stresses have for improved efficiency [55].Therefore,switchgrass,Mis- been well-characterized in Arabidopsis thaliana [62-64], canthus or poplar feedstocks with modified lignin can but until recently have led to only limited success in improve the efficiency of biomass conversion into fermen- translational research to produce field crop abiotic stress 425
increase their below-ground biomass accumulation will be important for increased carbon storage [8]. Economic considerations Current biofuels were marginally profitable before recent petroleum prices spiked. Ethanol production from maize was US$0.48 per gasoline energy equivalent liter (EEL) in 2005, when the price of gasoline was US$0.46 per liter [29]. However, the recent spike in the price of global crude oil has made maize-based ethanol production very profitable. The situation is similar for biodiesel production: soybeanbased biodiesel production was US$0.55 per diesel EEL in 2005 [29]. Despite these potential profit margins, traditional crop-based biofuel production has led to direct competition between food and energy and, as a result, the recent crude oil price increase has already led to the global inflation of the cost of food, feed and associated products. There is an imminent need to move to lignocellulosic biomass-based platforms. However, lignocellulosic ethanol production using the current platforms is not profitable [36]. The lignocellulosic ethanol price is still as high as US$0.70 to US$1.0 per EEL. However, as processing technologies mature and biomass crops are modified for higher yield and lower recalcitrance, the cost of lignocellulosicbased ethanol production is expected to decrease to rival maize grain platforms. Indeed, based on break-even price yield, recent analysis has already indicated that switchgrass is a more profitable crop than traditional crops such as sorghum and maize [10]. The competitiveness of lignocellulosic ethanol as a sustainable energy supply in the USA will therefore heavily depend on biotechnology breakthroughs to reduce cost and improve processing efficiency. Plant biotechnology solutions for bioenergy Novel enabling biotechnologies are crucial for reducing the costs of bioenergy production, particularly of lignocellulosic ethanol. The key issues include rapid domestication, overcoming recalcitrance, efficient breakdown of cellulose, and increasing biomass and lipid production for ethanol and biodiesel, respectively [37]. Although aspects of these important areas will be discussed individually below, it is important to solve these problems in concert. Modification of lignin biosynthesis Lignin might be the most crucial molecule in need of modification for lignocellulosic feedstocks. It has been established that reducing lignin biosynthesis can lead to lower recalcitrance and higher saccharification efficiency [13,38–54]. Recent studies have indicated two important aspects for lignin modification. First, both lignin content and composition are important. Although it is codependent on efficient processes to fractionate lignin, a more uniform lignin structure might facilitate more efficient cell-wall degradation for fuel production (John Ralph, personal communication). Second, the pretreatment of biomass might even be rendered unnecessary if lignin content falls below a critical threshold, which would enhance downstream enzymatic saccharification and fermentation steps for improved efficiency [55]. Therefore, switchgrass, Miscanthus or poplar feedstocks with modified lignin can improve the efficiency of biomass conversion into fermentable sugars [55]. Lignin biosynthesis in monocot species should be studied further so as to be able to modify lignin biosynthesis intelligently in perennial grass feedstocks. Preprocessing in planta: expression of cellulases and cellulosomes Plant cell walls can be degraded by individual cellulases or in concert by cellulosomes, which consist of a suite of enzymes. In planta expression of cellulases and cellulosomes could potentially reduce the cost of enzymatic saccharification of lignocellulosic biomass at the biorefinery by providing the enzymes needed for cell wall degradation. The effectiveness of in planta expression of free cellulases or cellulosomes is still controversial because digestion is complicated by the subcellular localization of enzyme(s), glycosylation of extracellular proteins, effective enzyme combinations, and the requirement for inducible expression to avoid premature cell wall digestion. Regardless of the challenges, preliminary research has shown the successful apoplastic expression of active Acidothermus cellulolyticus cellulase E1 in maize and tobacco [56,57]. Moreover, recent research has indicated that maize plants showing in planta expression of cellulase had higher biomass conversion efficiency [58]. Researchers have indicated that no detrimental effects were found by apoplast targeting of E1 and that cellulase activity aided biomass conversion when plant material was milled [56,59]. These differences result from the different subcellular localization and activation mechanisms of the enzymes. For example, heat-activated cellulase enzymes such as E1 have no detrimental effects to plants growing in typical ambient temperatures [55,56]. Further study is necessary to determine different strategies for in planta cell wall digestion enzyme expression, with high-throughput approaches for optimized sub-cellular localization and different combinations of enzymes. In addition, fundamental research still needs to be performed to modify cellulases for improved catalytic efficiency, thermal stability, performance under extreme conditions, along with protein modifications designed to lead to reduced apoplastic glycosylation. Many cell-wall degrading bacteria use cellulosomes, a suite of enzymes for cell wall hydrolysis [60]. Cellulosomes have not yet been expressed in transgenic plants, but the correct assemblage of cellulosome components in the plant apoplast potentially have the promise of decreasing recalcitrance and facilitating the post-harvest hydrolysis of cellulose, which in turn might aid simultaneous saccharification and fermentation. Abiotic stress resistance Suboptimal water and other abiotic stresses are limiting factors for biomass production; stress tolerance traits are therefore important to enable feedstock to be produced on marginal or sub-marginal lands not favorable for food crops. Drought-, metal-, salt-, cold- and heat-stress all induce some similar responses in plants, yet each of these stresses will induce a different set of genes [61]. The upstream pathways for salt and drought stresses have been well-characterized in Arabidopsis thaliana [62–64], but until recently have led to only limited success in translational research to produce field crop abiotic stress Review Trends in Plant Science Vol.13 No.8 425
Review Trends in Plant Science Vol.13 No.8 tolerance.Improved cold and drought tolerance has been biosynthesis and modification enzymes:the overexpres- reported in tobacco and potato by transformation with the sion of cellulose synthase in poplar has led to higher gene encoding DREB1A(dehydration response element lignocellulosic biomass biosynthesis [78].Overall,the pro- B1A),which is driven by a promoter of a stress-responsive duction of biomass can be further increased with the water channel,RD29A [65,66].Rice plants with induced engineering of plant hormone response genes or genes expression of a NAC (for NAM,ATAF and CUC)-type involved in developmental processes [70-74,77,79,80]. transcriptional factor,OsNAC6,have been shown to enhance tolerance to both high salinity and plant patho- Male sterility and biocontainment gens 67]. Male sterility is another desirable feature for feedstock More research is required to understand the effects of development to prevent transgene escape from genetically abiotic stresses on bioenergy crops from two different modified feedstock [1].Induced male sterility is one perspectives.First,genetic variation among different cul- approach to limit transgene flow.Male sterility can be tivars should be explored both in the laboratory and in field induced in plants by either knocking out the expression studies,which will guide breeding and genetic engineering of genes important in pollen development or pollen-specific for feedstock improvements.Indeed,switchgrass shows silencing of major metabolic genes [81-83].Another large phenotypic variation for water and cold-stress toler- approach to prevent transgene flow is the excision of the ance even within cultivars [11,68,69]:the different culti- transgene in pollen through pollen-specific recombinase vars might be incorporated in breeding programs for activity [84,85].Most of the proposed bioenergy crops such better-adapted feedstocks.Second,basic science from as switchgrass have wild relatives,and transgene fow is model species needs to be translated into field crop im- expected to be a major issue limiting the application of provement,and many key stress-response genes identified genetic engineering in these species.Preventing transgene in model species should be explored for the genetic modi- flow is therefore an important issue for feedstock improve- fication of bioenergy feedstocks such as switchgrass and ment by genetic modification. poplar. Metabolic engineering Increasing biomass production and yield Metabolic engineering will play an important role in The importance of altering plant growth and development improving biodiesel,biomass and sugar production.The to increase the biomass production for bioenergy cannot be future of biodiesel will largely depend on metabolic engin- over-emphasized.Given that most lignocellulosic biomass eering to improve oil content and composition in seeds crop candidates are relatively undomesticated,rapid pro- [15,86-89].Previous oilseed research has focused mainly gress should be attainable.First of all,the molecular on changing fatty acid profiles,particularly for nutritional mechanisms controlling plant architecture need to be bet- purposes [87,89].Recent efforts have also led to an increase ter understood.Current knowledge in the field can be in lipid production via induced expression of key exogenous translated into developing bioenergy feedstocks with desir- lipid biosynthesis genes [88].Metabolic engineering can able architectural features such as dwarf stature and erect also help to increase the production of sugar and starch for leaves.It has been shown that these features can be ethanol production using current platforms 90].For achieved by modifying biosynthesis or signal transduction example,recent research has indicated that the overex- for key plant growth hormones including GA(gibberellic pression of a bacterial sucrose isomerase in vacuoles could acid),IAA(indole-3-acetic acid)and brassinosteroids [70- double the sucrose yield for sugarcane [90].Metabolic 74].Biotechnology could make rapid improvements in engineering will also become an important approach for bioenergy feedstocks using genomics-guided improve- increasing non-fuel bioproducts,and advanced bioproducts ments.For example,GA pathway genes such as Agai might be the greatest long-term benefit of the current (gibberellic acid-insensitive)could be introduced into biofuels research spike.Although it is possible that some switchgrass to dwarf the plants,which should produce a alternative,non-biobased,fuel could ultimately replace crop with an increased annual biomass that is easier to petroleum,plastics and other bioproducts will require harvest [75].In addition,dwarfing might also help to new feedstocks in the absence of petroleum feedstocks. change the lignin content of the overall biomass.Following Overall,plant biotechnology will play a central role in dwarfing,biomass allocation should shift to the leaves.The the next generation of bioenergy options to produce ligno- leaves of switchgrass have been shown to contain a lower cellulosic feedstocks with higher yield,better water-use proportion of lignin than that found in stems [76];dwarfing efficiency,greater net energy gain,lower recalcitrance, would increase the cellulosic content needed as feed,or for enhanced abiotic stress tolerance,and improved ecological saccharification and fermentation needed for ethanol pro- benefits,such as better carbon fixation and water and soil duction.One of the major goals of poplar domestication is conservation. to produce dwarf Trees-pathways that are controlled by auxin,GA and brassinosteroids could potentially be Fueling the future altered to achieve this goal [70-74]. The future of bioenergy will depend on breakthrough Second,developmental programming of feedstock needs technologies.However,the importance of basic research to be altered to increase biomass production.For example, on pathways and genes involved in cell wall biosynthesis, delaying the onset of flowering has been reported to result plant development,and metabolite production should not in increased biomass [77].Third,biomass production can be ignored.Translational systems biology is needed for also be increased by the genetic modification of cell wall biofuel applications (Figure 2)[91].The use of 'omics 426
tolerance. Improved cold and drought tolerance has been reported in tobacco and potato by transformation with the gene encoding DREB1A (dehydration response element B1A), which is driven by a promoter of a stress-responsive water channel, RD29A [65,66]. Rice plants with induced expression of a NAC (for NAM, ATAF and CUC)-type transcriptional factor, OsNAC6, have been shown to enhance tolerance to both high salinity and plant pathogens [67]. More research is required to understand the effects of abiotic stresses on bioenergy crops from two different perspectives. First, genetic variation among different cultivars should be explored both in the laboratory and in field studies, which will guide breeding and genetic engineering for feedstock improvements. Indeed, switchgrass shows large phenotypic variation for water and cold-stress tolerance even within cultivars [11,68,69]; the different cultivars might be incorporated in breeding programs for better-adapted feedstocks. Second, basic science from model species needs to be translated into field crop improvement, and many key stress-response genes identified in model species should be explored for the genetic modi- fication of bioenergy feedstocks such as switchgrass and poplar. Increasing biomass production and yield The importance of altering plant growth and development to increase the biomass production for bioenergy cannot be over-emphasized. Given that most lignocellulosic biomass crop candidates are relatively undomesticated, rapid progress should be attainable. First of all, the molecular mechanisms controlling plant architecture need to be better understood. Current knowledge in the field can be translated into developing bioenergy feedstocks with desirable architectural features such as dwarf stature and erect leaves. It has been shown that these features can be achieved by modifying biosynthesis or signal transduction for key plant growth hormones including GA (gibberellic acid), IAA (indole-3-acetic acid) and brassinosteroids [70– 74]. Biotechnology could make rapid improvements in bioenergy feedstocks using genomics-guided improvements. For example, GA pathway genes such as Dgai (gibberellic acid-insensitive) could be introduced into switchgrass to dwarf the plants, which should produce a crop with an increased annual biomass that is easier to harvest [75]. In addition, dwarfing might also help to change the lignin content of the overall biomass. Following dwarfing, biomass allocation should shift to the leaves. The leaves of switchgrass have been shown to contain a lower proportion of lignin than that found in stems [76]; dwarfing would increase the cellulosic content needed as feed, or for saccharification and fermentation needed for ethanol production. One of the major goals of poplar domestication is to produce dwarf Trees – pathways that are controlled by auxin, GA and brassinosteroids could potentially be altered to achieve this goal [70–74]. Second, developmental programming of feedstock needs to be altered to increase biomass production. For example, delaying the onset of flowering has been reported to result in increased biomass [77]. Third, biomass production can also be increased by the genetic modification of cell wall biosynthesis and modification enzymes: the overexpression of cellulose synthase in poplar has led to higher lignocellulosic biomass biosynthesis [78]. Overall, the production of biomass can be further increased with the engineering of plant hormone response genes or genes involved in developmental processes [70–74,77,79,80]. Male sterility and biocontainment Male sterility is another desirable feature for feedstock development to prevent transgene escape from genetically modified feedstock [1]. Induced male sterility is one approach to limit transgene flow. Male sterility can be induced in plants by either knocking out the expression of genes important in pollen development or pollen-specific silencing of major metabolic genes [81–83]. Another approach to prevent transgene flow is the excision of the transgene in pollen through pollen-specific recombinase activity [84,85]. Most of the proposed bioenergy crops such as switchgrass have wild relatives, and transgene flow is expected to be a major issue limiting the application of genetic engineering in these species. Preventing transgene flow is therefore an important issue for feedstock improvement by genetic modification. Metabolic engineering Metabolic engineering will play an important role in improving biodiesel, biomass and sugar production. The future of biodiesel will largely depend on metabolic engineering to improve oil content and composition in seeds [15,86–89]. Previous oilseed research has focused mainly on changing fatty acid profiles, particularly for nutritional purposes [87,89]. Recent efforts have also led to an increase in lipid production via induced expression of key exogenous lipid biosynthesis genes [88]. Metabolic engineering can also help to increase the production of sugar and starch for ethanol production using current platforms [90]. For example, recent research has indicated that the overexpression of a bacterial sucrose isomerase in vacuoles could double the sucrose yield for sugarcane [90]. Metabolic engineering will also become an important approach for increasing non-fuel bioproducts, and advanced bioproducts might be the greatest long-term benefit of the current biofuels research spike. Although it is possible that some alternative, non-biobased, fuel could ultimately replace petroleum, plastics and other bioproducts will require new feedstocks in the absence of petroleum feedstocks. Overall, plant biotechnology will play a central role in the next generation of bioenergy options to produce lignocellulosic feedstocks with higher yield, better water-use efficiency, greater net energy gain, lower recalcitrance, enhanced abiotic stress tolerance, and improved ecological benefits, such as better carbon fixation and water and soil conservation. Fueling the future The future of bioenergy will depend on breakthrough technologies. However, the importance of basic research on pathways and genes involved in cell wall biosynthesis, plant development, and metabolite production should not be ignored. Translational systems biology is needed for biofuel applications (Figure 2) [91]. The use of ‘omics’ Review Trends in Plant Science Vol.13 No.8 426
Review Trends in Plant Science Vol.13 No.8 Genome sequence of Genome sequence of bioenergy feedstock model species Genomics Functional Comparative Genomics and genomics genomics. genetics studies Gene discovery Microbe Gene function Gene function cellulases and of bioenergy of model cellulosomes feedstocks species Biotechnology In planta Increase Down- exoression regulation biomass by Engineering of cell wall Abiotic stress of cell wall Male sterility Increase lipid Develop new regulating and other and sugar perennial of lignin enhancement degradation plant biogenesis and biocontainment production feedstock biosynthesis development degradation enzymes TRENDS in Plant Science Figure 2.Genomics-based biotechnology research for bioenergy.Plant biotechnology research greatly depends on the availability of genome information.The process from genomics to gene discovery to biotechnology is shown.Basically,the genomics level information(green boxes)will lead to gene discovery(yellow boxes),which can in turn be translated into biotechnology for feedstock improvement(light-blue boxes).In addition to plant biology research,studies using microorganisms(deep-blue box) are also important. techniques should be helpful to study the genes,proteins 5 Sims,R.E.H.et al.(2006)Energy crops:current status and future and metabolites from different tissues and at different prospects.Glob.Change Biol.12,2054-2076 developmental stages to correlate the features and struc- 6 Goldemberg,J.(2007)Ethanol for a sustainable energy future.Science 315.808810 tures of cell walls with genes of interest for guiding 7 Farrell.A.E.et al.(2006)Ethanol can contribute to energy and further gene discovery and biotechnology-based feedstock environmental goals.Science 311,506-508 improvements. 8 De Oliveira,M.E.D.et al.(2005)Ethanol as fuels:energy,carbon Furthermore,bioenergy is not,and should not be,lim- dioxide balances,and ecological footprint.Bioscience 55,593-602 9 Smeets,E.M.W.and Faaij,A.P.C.(2007)Bioenergy potentials from ited to higher plants,although higher plants are likely to forestry in 2050-an assessment of the drivers that determine the provide the most important feedstock for first and second potentials.Clim.Change 81,353-390 generations of biofuels.Studies of microbes that have the 10 Monti,A.et al.(2007)A full economic analysis of switchgrass under capacity to digest plant cell walls will also be important different scenarios in Italy estimated by BEE model.Biomass.Bioen. components of bioenergy research.In addition,green algae 31,177-185 should be considered as a potential feedstock choice 11 Somleva,M.N.(2006)Switchgrass (Panicum virgatum L.).Methods Mol.Biol.344,65-73 because of their fast growth.Biofuel cells might also be 12 Sanderson,M.A.et al.(2006)Switchgrass as a biofuels feedstock in the an option if more mature technologies become available USA.Can.J.Plant Sei.86,1315-1325 through engineering breakthroughs.Overall,bioenergy 13 Boerjan,W.(2005)Biotechnology and the domestication offorest trees. research is emerging as a field full of opportunities to Curr.Opin.Biotechnol.16,159-166 14 Ragauskas,A.J.et al.(2006)The path forward for biofuels and re-shape the energy supply of human society. biomaterials.Science 311,484-489 15 Ma,F.R.and Hanna,M.A.(1999)Biodiesel production:a review. Acknowledgements Biores.Tech.70,1-15 We appreciate funding from the DOE Bioenergy Science Center,the 16 Borjesson,P.and Berglund,M.(2006)Environmental systems analysis Southeastern Sun Grant Center,and the Tennessee Agricultural of biogas systems-part 1:fuel-cycle emissions.Biomass.Bioen.30, Experiment Station.We are also grateful for the critical reviews of this 469-485 paper and for contributions from Jonathan R.Mielenz. 17 Borjesson,P.and Berglund,M.(2007)Environmental systems analysis of biogas systems.-partⅡ正：the environmental impact of replacing various reference systems.Biomass.Bioen.31,326-344 References 18 Ghirardi,M.L.et al.(2007)Hydrogenases and hydrogen 1 Stewart,C.N.(2007)Biofuels and biocontainment.Nat.Biotechnol.25, photoproduction in oxygenic photosynthetic organisms.Annu.Rev. )83)R4 Plant Biol.58,71-91 2 Somerville,C.(2007)Biofuels.Curr.Biol.17,R115-R119 19 Tuck,G.et al.(2006)The potential distribution of bioenergy crops in 3 Rass-Hansen,J.et al.(2007)Bioethanol:fuel or feedstock?.Chem. Europe under present and future climate.Biomass.Bioen.30,183-197 Tech.Biotechnol.82.329-333 20 Searchinger,T.et al.(2008)Use of U.S.croplands for biofuels increases 4 Tilman,D.et al.(2006)Carbon-negative biofuels from low-input high- greenhouse gases through emissions from land-use change.Science diversity grassland biomass.Science 314,1598-1600 319.1238-1240 427
techniques should be helpful to study the genes, proteins and metabolites from different tissues and at different developmental stages to correlate the features and structures of cell walls with genes of interest for guiding further gene discovery and biotechnology-based feedstock improvements. Furthermore, bioenergy is not, and should not be, limited to higher plants, although higher plants are likely to provide the most important feedstock for first and second generations of biofuels. Studies of microbes that have the capacity to digest plant cell walls will also be important components of bioenergy research. In addition, green algae should be considered as a potential feedstock choice because of their fast growth. Biofuel cells might also be an option if more mature technologies become available through engineering breakthroughs. Overall, bioenergy research is emerging as a field full of opportunities to re-shape the energy supply of human society. Acknowledgements We appreciate funding from the DOE Bioenergy Science Center, the Southeastern Sun Grant Center, and the Tennessee Agricultural Experiment Station. We are also grateful for the critical reviews of this paper and for contributions from Jonathan R. Mielenz. References 1 Stewart, C.N. (2007) Biofuels and biocontainment. Nat. Biotechnol. 25, 283–284 2 Somerville, C. (2007) Biofuels. Curr. Biol. 17, R115–R119 3 Rass-Hansen, J. et al. (2007) Bioethanol: fuel or feedstock? J. Chem. Tech. Biotechnol. 82, 329–333 4 Tilman, D. et al. (2006) Carbon-negative biofuels from low-input highdiversity grassland biomass. Science 314, 1598–1600 5 Sims, R.E.H. et al. (2006) Energy crops: current status and future prospects. Glob. Change Biol. 12, 2054–2076 6 Goldemberg, J. (2007) Ethanol for a sustainable energy future. Science 315, 808–810 7 Farrell, A.E. et al. (2006) Ethanol can contribute to energy and environmental goals. Science 311, 506–508 8 De Oliveira, M.E.D. et al. (2005) Ethanol as fuels: energy, carbon dioxide balances, and ecological footprint. Bioscience 55, 593–602 9 Smeets, E.M.W. and Faaij, A.P.C. (2007) Bioenergy potentials from forestry in 2050 – an assessment of the drivers that determine the potentials. Clim. Change 81, 353–390 10 Monti, A. et al. (2007) A full economic analysis of switchgrass under different scenarios in Italy estimated by BEE model. Biomass. Bioen. 31, 177–185 11 Somleva, M.N. (2006) Switchgrass (Panicum virgatum L.). Methods Mol. Biol. 344, 65–73 12 Sanderson, M.A. et al. (2006) Switchgrass as a biofuels feedstock in the USA. Can. J. Plant Sci. 86, 1315–1325 13 Boerjan, W. (2005) Biotechnology and the domestication of forest trees. Curr. Opin. Biotechnol. 16, 159–166 14 Ragauskas, A.J. et al. (2006) The path forward for biofuels and biomaterials. Science 311, 484–489 15 Ma, F.R. and Hanna, M.A. (1999) Biodiesel production: a review. Biores. Tech. 70, 1–15 16 Bo¨rjesson, P. and Berglund, M. (2006) Environmental systems analysis of biogas systems–part 1: fuel-cycle emissions. Biomass. Bioen. 30, 469–485 17 Bo¨rjesson, P. and Berglund, M. (2007) Environmental systems analysis of biogas systems–part II: the environmental impact of replacing various reference systems. Biomass. Bioen. 31, 326–344 18 Ghirardi, M.L. et al. (2007) Hydrogenases and hydrogen photoproduction in oxygenic photosynthetic organisms. Annu. Rev. Plant Biol. 58, 71–91 19 Tuck, G. et al. (2006) The potential distribution of bioenergy crops in Europe under present and future climate. Biomass. Bioen. 30, 183–197 20 Searchinger, T. et al. (2008) Use of U.S. croplands for biofuels increases greenhouse gases through emissions from land-use change. Science 319, 1238–1240 Figure 2. Genomics-based biotechnology research for bioenergy. Plant biotechnology research greatly depends on the availability of genome information. The process from genomics to gene discovery to biotechnology is shown. Basically, the genomics level information (green boxes) will lead to gene discovery (yellow boxes), which can in turn be translated into biotechnology for feedstock improvement (light-blue boxes). In addition to plant biology research, studies using microorganisms (deep-blue box) are also important. Review Trends in Plant Science Vol.13 No.8 427
Review Trends in Plant Science Vol.13 No.8 21 Fargione,J.et al.(2008)Land clearing and the biofuel carbon debt. mutations on lignification and vascular integrity.Phytochemistry 61 Science319,1235-1238 221-294 22 Danalatos,N.G.et al.(2007)Potential growth and biomass 51 Chabannes,M.et al.(2001)Strong decrease in lignin content without productivity of Miscanthus x giganteus as affected by plant density significant alteration of plant development is induced by simultaneous and N-fertilization in central Greece.Biomass.Bioen.31,145-152 down-regulation of cinnamoyl CoA reductase (CCR)and cinnamyl 23 Price,L.et al.(2003)Identifying the yield potential of alcohol dehydrogenase(CAD)in tobacco plants.Plant J.28,257-270 Miscanthus x giganteus:an assessment of the spatial and temporal 52 Meyermans,H.et al.(2000)Modifications in lignin and accumulation of variability of M.x giganteus biomass productivity across England and phenolic glucosides in poplar xylem upon down-regulation of caffeoyl- Wales.Biomass.Bioen.26,3-13 coenzyme A O-methyltransferase,an enzyme involved in lignin 24 Zabek,L.M.and Prescott,C.E.(2006)Biomass equations and carbon biosynthesis.J.Biol.Chem.275,36899-36909 content of aboveground leafless biomass of hybrid poplar in coastal 53 Li,L.et al.(2000)5-Hydroxyconiferyl aldehyde modulates enzymatic British Columbia.For.Ecol.Manage.223,291-302 methylation for syringyl monolignol formation,a new view of 25 Christersson,L.(2006)Biomass production of intensively grown monolignol biosynthesis in angiosperms.J.Biol.Chem.275,6537- poplars in the southernmost part of Sweden:observations of 6545 characters,traits and growth potential.Biomass.Bioen.30,497-508 54 Hu,W.J.et al.(1999)Repression of lignin biosynthesis promotes 26 Bunn,S.M.et al.(2004)Leaf-level productivity traits in Populus grown cellulose accumulation and growth in transgenic trees.Nat. in short rotation coppice for biomass energy.Forestry 77,307-323 Biotechnol.17,808-812 27 Clifton-Brown,J.C.et al.(2007)Carbon mitigation by the energy crop, 55 Chen,F.and Dixon,R.A.(2007)Lignin modification improves Miscanthus.Global Change Biol.13,2296-2307 fermentable sugar yields for biofuel production.Nat.Biotechnol.25, 28 Venturi,P.and Venturi,G.(2003)Analysis of energy comparison for 759-761 crops in European agricultural systems.Biomass.Bioen.25,235-255 56 Dai,Z.et al.(2005)Optimization of Acidothermus cellulolyticus 29 Hill,J.et al.(2006)Environmental,economic,and energetic costs and endoglucanase (E1)production in transgenic tobacco plants by benefits of biodiesel and ethanol biofuels.Proc.Natl.Acad.Sci.U.S.A transcriptional, post-transcription and post-translational 103.11206-11210 modification.Transgenic Res.14,627-643 30 Hill,J.(2007)Environmental costs and benefits of transportation 57 Biswas,G.C.G.et al.(2006)Expression of biologically active biofuel production from food-and lignocellulose-based energy crops. Acidothermus cellulolyticus endoglucanase in transgenic maize A review.Agron.Sustain.Dev.27,1-12 plants.Plant Sci.171,617-623 31 Agrawal,R.et al.(2007)Sustainable fuel for the transportation sector 58 Ransom,C.et al.(2007)Heterologous Acidothermus cellulolyticus 1,4- Proc.Natl.Acad.Sci.U.S.A.104,4828-4833 B-endoglucanase E1 produced within the corn biomass converts corn 32 Samson,R.et al.(2005)The potential of C perennial grasses for stover into glucose.Appl.Biochem.Biotechnol.137,207-219 developing global BIOHEAT industry.Crit.Rev.Plant Sci.24,461-495 59 Ziegelhoffer,T.et al.(2001)Dramatic effects of truncation and sub- 33 Semere,T.and Slater,F.M.(2007)Ground flora,small mammal and cellular targeting on the accumulation of recombinant microbial bird species diversity in miscanthus (Miscanthus x giganteus)and reed cellulase in tobacco.Mol.Breed.8.147-158 canary-grass (Phalaris arundinacea)fields.Biomass.Bioen.31,20-29 60 Bayer,E.A.et al.(2004)The cellulosomes:multienzyme machines for 34 Semere,T.and Slater,F.M.(2007)Invertebrate populations in degradation of plant cell wall polysaccharides.Annu.Rev.Microbiol. miscanthus (Miscanthus x giganteus)and reed canary-grass 58.521-554 (Phalaris arundinacea)fields.Biomass.Bioen.31,30-39 61 Maggio,A.et al.(2006)Osmogenetics:Aristotle to Arabidopsis.Plant 35 Adler,P.R.et al.(2007)Life-cycle assessment of net greenhouse-gas Cel18.1542-1557 flux for bioenergy cropping systems.Ecol.Appl.17,675-691 62 Chinnusamy,V.et al.(2005)Understanding and improving salt 36 Soccol,C.R.et al.(2005)Brazilian biofuel program:an overview.J.Sci. tolerance in plants.Crop Sci.45,437-448 Ind.Res.64,897-904 63 Hirayama,T.and Shinozaki,K.(2007)Perception and transduction of 37 Himmel,M.E.(2007)Biomass recalcitrance:engineering plants and abscisic acid signals:keys to the function ofthe versatile plant hormone enzymes for biofuels production.Science 315,804-807 ABA.Trends Plant Sci.12,343-351 38 van der Rest,B.et al.(2006)Down-regulation of cinnamoyl-CoA 64 Song,C.P.et al.(2005)Role of an Arabidopsis AP2/EREBP-type reductase in tomato (Solanum lycopersicum L.)induces dramatic transcriptional repressor in abscisic acid and drought stress changes in soluble phenolic pools.J.Exp.Bot.57,1399-1411 responses.Plant Cell 17,2384-2396 39 Talukder,K.(2006)Low-lignin wood-a case study.Nat.Biotechnol. 65 Kasuga,M.et al.(2004)A combination of the Arabidopsis DREB1A 24,395-396 gene and stress-inducible rd29A promoter improved drought-and low- 40 Sticklen,M.(2006)Plant genetic engineering to improve biomass temperature stress tolerance in tobacco by gene transfer.Plant Cell characteristics for biofuels.Curr.Opin.Biotechnol.17,315-319 Physiol.45,346-350 41 Rastogi,S.and Dwivedi,U.N.(2006)Down-regulation of lignin 66 Behnam,B.et al.(2007)Arabidopsis rd29A::DREBIA enhances biosynthesis in transgenic Leucaena leucocephala harboring O. freezing tolerance in transgenic potato.Plant Cell Rep.26,1275-1282 methyltransferase gene.Biotechnol.Prog.22,609-616 67 Nakashima,K.et al.(2007)Functional analysis of a NAC-type 42 Ralph,J.et al.(2006)Effects of coumarate 3-hydroxylase down- transcription factor OsNAC6 involved in abiotic and biotic stress- regulation on lignin structure.J.Biol.Chem.281,8843-8853 responsive gene expression in rice.Plant J.51,617-630 43 Higuchi,T.(2006)Look back over the studies of lignin biochemistry. 68 Casler,M.D.et al.(2004)Latitudinal adaptation of switchgrass J.Wood Sci.52,2-8 populations.Crop Sci.44,293-303 44 Davison,B.H.et al.(2006)Variation of S/G ratio and lignin content in a 69 Lee,D.K.and Boe,A.(2005)Biomass production of switchgrass in Populus family influences the release of xylose by dilute acid central South Dakota.Crop Sci.45,2583-2590 hydrolysis.Appl.Biochem.Biotechnol.129-132,427-435 70 Peng.J.et al.(1999)Green revolution'genes encode mutant 45 Pedersen,J.F.et al.(2005)Impact of reduced lignin on plant fitness. gibberellin response modulators.Nature 400,256-261 Crop Sei.45,812-819 71 Sasaki,A.et al.(2002)Green revolution:a mutant gibberellin- 46 Grabber,J.H.(2005)How do lignin composition,structure,and cross- synthesis gene in rice -new insight into the rice variant that linking affect degradability?A review of cell wall model studies.Crop helped to avert famine over thirty years ago.Nature 416,701-702 Sci.45.820831 72 Sakamoto,T.et al.(2006)Erect leaves caused by brassinosteroid 47 Alizadeh,H.et al.(2005)Pretreatment of switchgrass by ammonia fiber deficiency increase biomass production and grain yield in rice.Nat. explosion (AFEX).Appl.Biochem.Biotechnol.121-124,1133-1141 Biotechnol.24.105-109 48 Lu,J.et al.(2004)Lignin reduction in transgenic poplars by expressing 73 Morinaka,Y.et al.(2006)Morphological alteration caused by antisense CCoAOMT gene.Prog.Nat.Sci.14,1060-1063 brassinosteroid insensitivity increases the biomass and grain 49 Boerjan,W.et al.(2003)Lignin biosynthesis.Annu.Rev.Plant Biol.54 production of rice.Plant Physiol.141,924-931 519-546 74 Choe,S.et al.(2001)Overexpression of DWARF4 in the brassinosteroid 50 Anterola,A.M.and Lewis,N.G.(2002)Trends in lignin modification:a biosynthetic pathway results in increased vegetative growth and seed comprehensive analysis of the effects of genetic manipulations/ yield in Arabidopsis.Plant J.26,573-582 428
21 Fargione, J. et al. (2008) Land clearing and the biofuel carbon debt. Science 319, 1235–1238 22 Danalatos, N.G. et al. (2007) Potential growth and biomass productivity of Miscanthus x giganteus as affected by plant density and N-fertilization in central Greece. Biomass. Bioen. 31, 145–152 23 Price, L. et al. (2003) Identifying the yield potential of Miscanthus giganteus: an assessment of the spatial and temporal variability of M. giganteus biomass productivity across England and Wales. Biomass. Bioen. 26, 3–13 24 Zabek, L.M. and Prescott, C.E. (2006) Biomass equations and carbon content of aboveground leafless biomass of hybrid poplar in coastal British Columbia. For. Ecol. Manage. 223, 291–302 25 Christersson, L. (2006) Biomass production of intensively grown poplars in the southernmost part of Sweden: observations of characters, traits and growth potential. Biomass. Bioen. 30, 497–508 26 Bunn, S.M. et al. (2004) Leaf-level productivity traits in Populus grown in short rotation coppice for biomass energy. Forestry 77, 307–323 27 Clifton-Brown, J.C. et al. (2007) Carbon mitigation by the energy crop, Miscanthus. Global Change Biol. 13, 2296–2307 28 Venturi, P. and Venturi, G. (2003) Analysis of energy comparison for crops in European agricultural systems. Biomass. Bioen. 25, 235–255 29 Hill, J. et al. (2006) Environmental, economic, and energetic costs and benefits of biodiesel and ethanol biofuels. Proc. Natl. Acad. Sci. U. S. A. 103, 11206–11210 30 Hill, J. (2007) Environmental costs and benefits of transportation biofuel production from food- and lignocellulose-based energy crops. A review. Agron. Sustain. Dev. 27, 1–12 31 Agrawal, R. et al. (2007) Sustainable fuel for the transportation sector. Proc. Natl. Acad. Sci. U. S. A. 104, 4828–4833 32 Samson, R. et al. (2005) The potential of C4 perennial grasses for developing global BIOHEAT industry. Crit. Rev. Plant Sci. 24, 461–495 33 Semere, T. and Slater, F.M. (2007) Ground flora, small mammal and bird species diversity in miscanthus (Miscanthus x giganteus) and reed canary-grass (Phalaris arundinacea) fields. Biomass. Bioen. 31, 20–29 34 Semere, T. and Slater, F.M. (2007) Invertebrate populations in miscanthus (Miscanthus x giganteus) and reed canary-grass (Phalaris arundinacea) fields. Biomass. Bioen. 31, 30–39 35 Adler, P.R. et al. (2007) Life-cycle assessment of net greenhouse-gas flux for bioenergy cropping systems. Ecol. Appl. 17, 675–691 36 Soccol, C.R. et al. (2005) Brazilian biofuel program: an overview. J. Sci. Ind. Res. 64, 897–904 37 Himmel, M.E. (2007) Biomass recalcitrance: engineering plants and enzymes for biofuels production. Science 315, 804–807 38 van der Rest, B. et al. (2006) Down-regulation of cinnamoyl-CoA reductase in tomato (Solanum lycopersicum L.) induces dramatic changes in soluble phenolic pools. J. Exp. Bot. 57, 1399–1411 39 Talukder, K. (2006) Low-lignin wood – a case study. Nat. Biotechnol. 24, 395–396 40 Sticklen, M. (2006) Plant genetic engineering to improve biomass characteristics for biofuels. Curr. Opin. Biotechnol. 17, 315–319 41 Rastogi, S. and Dwivedi, U.N. (2006) Down-regulation of lignin biosynthesis in transgenic Leucaena leucocephala harboring Omethyltransferase gene. Biotechnol. Prog. 22, 609–616 42 Ralph, J. et al. (2006) Effects of coumarate 3-hydroxylase downregulation on lignin structure. J. Biol. Chem. 281, 8843–8853 43 Higuchi, T. (2006) Look back over the studies of lignin biochemistry. J. Wood Sci. 52, 2–8 44 Davison, B.H. et al. (2006) Variation of S/G ratio and lignin content in a Populus family influences the release of xylose by dilute acid hydrolysis. Appl. Biochem. Biotechnol. 129–132, 427–435 45 Pedersen, J.F. et al. (2005) Impact of reduced lignin on plant fitness. Crop Sci. 45, 812–819 46 Grabber, J.H. (2005) How do lignin composition, structure, and crosslinking affect degradability? A review of cell wall model studies. Crop Sci. 45, 820–831 47 Alizadeh, H. et al. (2005) Pretreatment of switchgrass by ammonia fiber explosion (AFEX). Appl. Biochem. Biotechnol. 121–124, 1133–1141 48 Lu, J. et al. (2004) Lignin reduction in transgenic poplars by expressing antisense CCoAOMT gene. Prog. Nat. Sci. 14, 1060–1063 49 Boerjan, W. et al. (2003) Lignin biosynthesis. Annu. Rev. Plant Biol. 54, 519–546 50 Anterola, A.M. and Lewis, N.G. (2002) Trends in lignin modification: a comprehensive analysis of the effects of genetic manipulations/ mutations on lignification and vascular integrity. Phytochemistry 61, 221–294 51 Chabannes, M. et al. (2001) Strong decrease in lignin content without significant alteration of plant development is induced by simultaneous down-regulation of cinnamoyl CoA reductase (CCR) and cinnamyl alcohol dehydrogenase (CAD) in tobacco plants. Plant J. 28, 257–270 52 Meyermans, H. et al. (2000) Modifications in lignin and accumulation of phenolic glucosides in poplar xylem upon down-regulation of caffeoylcoenzyme A O-methyltransferase, an enzyme involved in lignin biosynthesis. J. Biol. Chem. 275, 36899–36909 53 Li, L. et al. (2000) 5-Hydroxyconiferyl aldehyde modulates enzymatic methylation for syringyl monolignol formation, a new view of monolignol biosynthesis in angiosperms. J. Biol. Chem. 275, 6537– 6545 54 Hu, W.J. et al. (1999) Repression of lignin biosynthesis promotes cellulose accumulation and growth in transgenic trees. Nat. Biotechnol. 17, 808–812 55 Chen, F. and Dixon, R.A. (2007) Lignin modification improves fermentable sugar yields for biofuel production. Nat. Biotechnol. 25, 759–761 56 Dai, Z. et al. (2005) Optimization of Acidothermus cellulolyticus endoglucanase (E1) production in transgenic tobacco plants by transcriptional, post-transcription and post-translational modification. Transgenic Res. 14, 627–643 57 Biswas, G.C.G. et al. (2006) Expression of biologically active Acidothermus cellulolyticus endoglucanase in transgenic maize plants. Plant Sci. 171, 617–623 58 Ransom, C. et al. (2007) Heterologous Acidothermus cellulolyticus 1,4- b-endoglucanase E1 produced within the corn biomass converts corn stover into glucose. Appl. Biochem. Biotechnol. 137, 207–219 59 Ziegelhoffer, T. et al. (2001) Dramatic effects of truncation and subcellular targeting on the accumulation of recombinant microbial cellulase in tobacco. Mol. Breed. 8, 147–158 60 Bayer, E.A. et al. (2004) The cellulosomes: multienzyme machines for degradation of plant cell wall polysaccharides. Annu. Rev. Microbiol. 58, 521–554 61 Maggio, A. et al. (2006) Osmogenetics: Aristotle to Arabidopsis. Plant Cell 18, 1542–1557 62 Chinnusamy, V. et al. (2005) Understanding and improving salt tolerance in plants. Crop Sci. 45, 437–448 63 Hirayama, T. and Shinozaki, K. (2007) Perception and transduction of abscisic acid signals: keys to the function of the versatile plant hormone ABA. Trends Plant Sci. 12, 343–351 64 Song, C.P. et al. (2005) Role of an Arabidopsis AP2/EREBP-type transcriptional repressor in abscisic acid and drought stress responses. Plant Cell 17, 2384–2396 65 Kasuga, M. et al. (2004) A combination of the Arabidopsis DREB1A gene and stress-inducible rd29A promoter improved drought- and lowtemperature stress tolerance in tobacco by gene transfer. Plant Cell Physiol. 45, 346–350 66 Behnam, B. et al. (2007) Arabidopsis rd29A::DREB1A enhances freezing tolerance in transgenic potato. Plant Cell Rep. 26, 1275–1282 67 Nakashima, K. et al. (2007) Functional analysis of a NAC-type transcription factor OsNAC6 involved in abiotic and biotic stressresponsive gene expression in rice. Plant J. 51, 617–630 68 Casler, M.D. et al. (2004) Latitudinal adaptation of switchgrass populations. Crop Sci. 44, 293–303 69 Lee, D.K. and Boe, A. (2005) Biomass production of switchgrass in central South Dakota. Crop Sci. 45, 2583–2590 70 Peng, J. et al. (1999) ‘Green revolution’ genes encode mutant gibberellin response modulators. Nature 400, 256–261 71 Sasaki, A. et al. (2002) Green revolution: a mutant gibberellinsynthesis gene in rice – new insight into the rice variant that helped to avert famine over thirty years ago. Nature 416, 701–702 72 Sakamoto, T. et al. (2006) Erect leaves caused by brassinosteroid deficiency increase biomass production and grain yield in rice. Nat. Biotechnol. 24, 105–109 73 Morinaka, Y. et al. (2006) Morphological alteration caused by brassinosteroid insensitivity increases the biomass and grain production of rice. Plant Physiol. 141, 924–931 74 Choe, S. et al. (2001) Overexpression of DWARF4 in the brassinosteroid biosynthetic pathway results in increased vegetative growth and seed yield in Arabidopsis. Plant J. 26, 573–582 Review Trends in Plant Science Vol.13 No.8 428
Review Trends in Plant Science Vol.13 No.8 75 Peng,J.et al.(1997)The Arabidopsis GAI gene defines a signaling 84 Mlynarova,L.et al.(2006)Directed microspore-specific recombination pathway that negatively regulates gibberellin responses.Genes Dev. of transgenic alleles to prevent pollen-mediated transmission of 11,3194-3205 transgenes.Plant Biotechnol.J.4.445-452 76 Jung,H.J.G.and Vogel,K.P.(1992)Lignification of switchgrass 85 Luo,K.et al.(2007)GM-gene-deletor':fused loxP-FRT recognition (Panicum virgatum)and big bluestem (Andropogon gerardii)plant sequences dramatically improve the efficiency of FLP or CRE parts during maturation and its effect on fiber degradability.J.Sci. recombinase on transgene excision from pollen and seed of tobacco Fo0 d Agric.59,169-176 plants.Plant Biotechnol.J.5,263-274 77 Salehi,H.et al.(2005)Delay in flowering and increase in biomass of 86 Chapman,K.D.et al.(2001)Transgenic cotton plants with increased transgenic tobacco expressing the Arabidopsis floral repressor gene seed oleic acid content.J.Am.Oil Chem.Soc.78,941-947 FLOWERING LOCUS C.J.Plant Physiol.162,711-717 87 Thelen,J.J.and Ohlrogge,J.B.(2002)Metabolic engineering of fatty 78 Shoseyov,O.et al.(2003)Modulation of wood fibers and paper by acid biosynthesis in plants.Metab.Engin.4,12-21 cellulose-binding domains.In Application of Enzymes to 88 Vigeolas,H.et al.(2007)Increasing seed oil content in oil-seed rape Lignocellulosics.ACS Symposium Series 855 (Mansfield,S.D.and (Brassica napus L.)by over-expression of a yeast glycerol-3-phosphate Saddler,J.N.,eds),pp.116-131,American Chemical Society dehydrogenase under the control of a seed-specific promoter.Plant 79 Sakamoto,T.et al.(2003)Genetic manipulation of gibberellin Biotechnol.J.5,431-441 metabolism in transgenic rice.Nat.Biotechnol.21,909-913 89 Wu,G.et al.(2005)Stepwise engineering to produce high yields of very 80 Dodd,A.N.et al.(2005)Plant circadian clocks increase long-chain polyunsaturated fatty acids in plants.Nat.Biotechnol.23. photosynthesis,growth,survival,and competitive advantage. 1013-1017 Science309,630-633 90 Wu,G.and Birch,R.G.(2007)Doubled sugar contentin sugarcane plants 81 Ariizumi,T.et al.(2004)Disruption of the novel plant protein NEFI modified to produce a sucrose isomer.Plant Biotechnol.J.5,109-117 affects lipid accumulation in the plastids of the tapetum and exine 91 Yuan,J.S.et al.(2008)Plant systems biology comes of age.Trends formation of pollen,resulting in male sterility in Arabidopsis thaliana. Plant Sci.13,165-171 PlantJ.39,170-181 92 Wu,X.et al.(2007)Factors impacting ethanol production from grain 82 Li,S.F.et al.(2007)Suppression and restoration of male fertility using sorghum in the dry-grind process.Cereal Chem.84,130-136 a transcription factor.Plant Biotechnol.J.5,297-312 93 Wu,M.et al.(2006)Energy and emission benefits of alternative 83 Khan,M.S.(2005)Plant biology-engineered male sterility.Nature 436, transportation liquid fuels derived from switchgrass:a fuel life cycle 783-785 assessment.Biotechnol.Prog.22,1012-1024 429
75 Peng, J. et al. (1997) The Arabidopsis GAI gene defines a signaling pathway that negatively regulates gibberellin responses. Genes Dev. 11, 3194–3205 76 Jung, H.J.G. and Vogel, K.P. (1992) Lignification of switchgrass (Panicum virgatum) and big bluestem (Andropogon gerardii) plant parts during maturation and its effect on fiber degradability. J. Sci. Food Agric. 59, 169–176 77 Salehi, H. et al. (2005) Delay in flowering and increase in biomass of transgenic tobacco expressing the Arabidopsis floral repressor gene FLOWERING LOCUS C. J. Plant Physiol. 162, 711–717 78 Shoseyov, O. et al. (2003) Modulation of wood fibers and paper by cellulose-binding domains. In Application of Enzymes to Lignocellulosics. ACS Symposium Series 855 (Mansfield, S.D. and Saddler, J.N., eds), pp. 116–131, American Chemical Society 79 Sakamoto, T. et al. (2003) Genetic manipulation of gibberellin metabolism in transgenic rice. Nat. Biotechnol. 21, 909–913 80 Dodd, A.N. et al. (2005) Plant circadian clocks increase photosynthesis, growth, survival, and competitive advantage. Science 309, 630–633 81 Ariizumi, T. et al. (2004) Disruption of the novel plant protein NEF1 affects lipid accumulation in the plastids of the tapetum and exine formation of pollen, resulting in male sterility in Arabidopsis thaliana. Plant J. 39, 170–181 82 Li, S.F. et al. (2007) Suppression and restoration of male fertility using a transcription factor. Plant Biotechnol. J. 5, 297–312 83 Khan, M.S. (2005) Plant biology–engineered male sterility. Nature 436, 783–785 84 Mlynarova, L. et al. (2006) Directed microspore-specific recombination of transgenic alleles to prevent pollen-mediated transmission of transgenes. Plant Biotechnol. J. 4, 445–452 85 Luo, K. et al. (2007) ‘GM-gene-deletor’: fused loxP-FRT recognition sequences dramatically improve the efficiency of FLP or CRE recombinase on transgene excision from pollen and seed of tobacco plants. Plant Biotechnol. J. 5, 263–274 86 Chapman, K.D. et al. (2001) Transgenic cotton plants with increased seed oleic acid content. J. Am. Oil Chem. Soc. 78, 941–947 87 Thelen, J.J. and Ohlrogge, J.B. (2002) Metabolic engineering of fatty acid biosynthesis in plants. Metab. Engin. 4, 12–21 88 Vigeolas, H. et al. (2007) Increasing seed oil content in oil-seed rape (Brassica napus L.) by over-expression of a yeast glycerol-3-phosphate dehydrogenase under the control of a seed-specific promoter. Plant Biotechnol. J. 5, 431–441 89 Wu, G. et al. (2005) Stepwise engineering to produce high yields of very long-chain polyunsaturated fatty acids in plants. Nat. Biotechnol. 23, 1013–1017 90 Wu, G. and Birch, R.G. (2007) Doubled sugar content in sugarcane plants modified to produce a sucrose isomer. Plant Biotechnol. J. 5, 109–117 91 Yuan, J.S. et al. (2008) Plant systems biology comes of age. Trends Plant Sci. 13, 165–171 92 Wu, X. et al. (2007) Factors impacting ethanol production from grain sorghum in the dry-grind process. Cereal Chem. 84, 130–136 93 Wu, M. et al. (2006) Energy and emission benefits of alternative transportation liquid fuels derived from switchgrass: a fuel life cycle assessment. Biotechnol. Prog. 22, 1012–1024 Review Trends in Plant Science Vol.13 No.8 429
