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 421Plants 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 tar￾gets to replace petroleum-based transportation fuelswith a viable alternative, and in reducing long-term carbon dioxide emissions, if environmental and economic sus￾tainability 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. How￾ever, the use of lignocellulosic ethanol as a viable alterna￾tive 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 plantaexpres￾sion 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 pro￾duction. 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 bio￾technologies, 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 bio￾gas 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 bio￾mass-based sustainable and environment-friendly econ￾omies. 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