(B)(A)Sieve plateP-proteinSieve tubeSieveplateelement8poregogModifiedLateral sieveg80plastidarea80880Sieve tubeelementCompanionSmoothcellendoplasmicreticulumBranchedplasmodesmatCytoplasmVacuolePlasmamembraneChloroplast酒ThickenedFigure 6-1primary waliNucleusSieveplateMitochondrioporeSieveplate
Figure 6-1
Figure 6-2MiddlelamellaProtein particles onPlasmamembraneouter leaflet of ERCellwallTonoplastEndoplasmicCytoplasmreticulumVacuolePlasmodesmaDesmotubuleProtein particles onProteinparticles oninnerleafletofERinnerleafletofwithappressedERplasmamembrane
Figure 6-2
(B)ERPlasmamembrane(A)CytoplasmEndoplasmicreticulumDesmotubuleNeckCentralrodCell wallDesmotubulePlasmamembraneMiddleCell walllamellaCentral cavityCytoplasmicsleeveERCrosssectionsFIGURE1.27Plasmodesmatabetweencells,(A)Electronmicrograph of a wall separating two adjacent cells,showingOOCentral cavitytheplasmodesmata. (B)Schematicview ofa cell wall withtwoplasmodesmatawith different shapes.The desmotubuleCentral rodis continuous withthe ER ofthe adjoining cells.Proteins lineSpokelikeCytoplasmicthe outer surface ofthe desmotubuleand the inner surface offilamentoussleevetheplasma membranethe two surfaces arethought tobeproteinsconnected by filamentousproteins.Thegapbetweentheproteins lining the two membranesapparently controls themol-ecularsievingpropertiesofplasmodesmata.(AfromTilneyFigure 6-2-1etal.1991;BafterBuchananetal.2000.)
Figure 6-2-1
AcquiringphloemsapAphidsare small insectsthatremovenutrientsfromphloembymeans ofa needlelike mouthpart called a styleta.Aphid withstylet inplace.b.When theexperimenterphloenremovestheaphid'sbody,phloemsapisavailablesapcollection andanalysis.styletstyletFigure 6-4phloemb
Figure 6-4
MouthpartSieve tubememberMouthpart(b)25μmaAphidsusedtostudytranslocationinphloem.(a)Matureaphid,a tiny insectabout3to6mmin length,feedingona stem.(b)LMofphloemcells,showinga sieve tube memberthat has been penetrated by theaphid mouthpartFigure 6-4-1(a,DwightKuhn;b,M.H.Zimmerman,Science,Vol.133,pp.73-79[Fig.4],13Jan1961.Copyright2002bytheAmericanAssociationfortheAdvancementofScience
Figure 6-4-1
COSmallveinCellwall-apoplastSugarpathwaySievePlasmodesmaelementsCompanioncellsSugarCytoplasm-symplastActive loadingpathwaySugarPhloemparenchymacellBundlesheathcellFigure 6-5Mesophyll cellPlasmamembranecomplex,but they could also enter the apoplast earlier inFIGURE10.14 Schematicdiagramofpathwaysofphloemloadingin sourceleaves.Inthetotallysymplasticpathwaythepathand then moveto the small veins.In any case,thesugars areactivelyloaded intothe companion cellsandsugars move from one cell to anotherin the plasmodessieveelements fromtheapoplast.Sugars loaded intothemata,all the way from themesophyll to the sieve elementsInthepartlyapoplasticpathwaysugarsentertheapoplastcompanioncellsarethoughtto movethroughplasmodes-at somepointFor simplicity,sugarsare shown here enter-mataintothesieveelementsingtheapoplast nearthesieve element-companion cell
Figure 6-5
TABLE10.4Figure 6-5-1Patterns inapoplasticand symplasticloadingApoplasticloadingSymplasticloadingSucroseTransport sugarOligosaccharides in addition to sucroseTypeofcompanionOrdinary companion cellsIntermediary cellscell intheminor veinsortransfercellsFewAbundantNumberofplasmodesmataconnectingthesieveelementsandcompanioncellstosurrounding cellsXylemvesselPlasmodesmataCompanionIntermediarycellcellPhloemparenchymaSieve elementSource:DrawingsaftervanBeletal.1992Note:Somespeciesmayloadbothapoplasticallyandsymplastically,sincedifferenttypesofcompanioncellscanbefoundwithin.theveinsofasinglespecies
Figure 6-5-1
sieve element-companioncellcomplexFigure 6-6APaseATPADPSucrose-H+synporterHSucrsucroseHigh HtLOW H+concentrationconcentrationFIGURE 1O.16ATp-dependentsucrosetransport in sieveelement loading.In the cotransport model of sucrose loadingintothe symplast of the sieve element-companionl cellcomplex,the plasmamembrane ATPase puimps protons ouof the cellinto theapoplast,establishinga highprotonconcentration there. The energy in this proton gradient is thenusedto drive the transport of sucrose into the symplast ofthesieve elementcompanioncell complexthroughasucrose-H+symporter
Figure 6-6
Bundle sheathcellIntermediarycellSieve elementGlucoseGalactoseFructoseSucroseSucroseRaffinosePlasmodesmaSucrose,synthesizedintheIn the intermediary cellsRaffinose and stachyosemesophyll,diffusesfromraffinose(and stachyose)are able to diffuseintothe bundle sheath cellsare synthesized fromthe sieve elements. As ainto the intermediary cellssucroseand galactose,result,the concentrationthrough the abundantthus maintaining theof transportsugarrisesinplasmodesmatadiffusion gradient forthe intermediary cells andsucrose.Because of theirthe sieve elements.larger sizes,theyare notFigure 6-7able todiffuseback intothemesophyll
Figure 6-7
Xylemvessel elementsPhloem sieveelementsCompanioncellH.OSource cellH.OSugar atthe source,Activephloemillustratedherebysucrosew=-1.1MPalloading intoYw=-0.8MPa(redspheres)isactively2sieve elements0.6MPaP=yp=-0.7MPaloadedinto thesievedecreases the-1.7MPaelement-companioncellYs=-0.1MPasolute potential,complex.water enters,and high turgorpressureresults.SucroseHOPressure-drivenbulkflowofwaterandsolutefromsourcetosinkH2OSinkcellTranspirationstream园H20At the sink,sugars areunloaded.ActivephloemunloadingHoYw=-0.4MPalincreasesthePw=-0.6MPap=0.3MPasolute potential,"p=-0.5MPawaterflowsout,P=-0.7MPaYs=-0.1MPaanda lowerSucroseturgorpressureresults.FiGURE10.10Pressure-flowmodeloftranslocationinthephloem.Possible values forywyo,and y,in theFigure 6-9xylemand phloemareillustrated.(AfterNobel 1991.)
Figure 6-9