Plant cell wall polymers as precursors for biofuels

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Plantcellwallpolymersasprecursorsforbiofuels

MarkusPauly1andKennethKeegstra

Theconversionofplantbiomassintoliquidtransportationfuelsisacomplexprocessthatcouldbesimpli?edbyalteringtheratiosofthecellwallpolymersthatconstitutethemainbiomasscomponents.Thecompositionofbiomassvariesnaturallydependinguponplantspeciesandcelltype,includingsomehighlyspecializedwallsthatconsistmainlyofasinglecomponent.Progressisbeingmadeinunderstandingthemolecularbasisofthesenaturalvariationsinwallcomposition.ThisnewknowledgewillbeavaluableresourcethatcanbeusedduringeffortstogeneratedesignerbiofuelcropsusingeitherselectedbreedingmethodsorrecombinantDNAtechniques.

Address

DOEGreatLakesBioenergyResearchCenter,DOEPlantResearchLab,DepartmentofBiochemistryandMolecularBiology,MichiganStateUniversity,EastLansing,MI48824,USA

Correspondingauthor:Keegstra,Kenneth(keegstra@msu.edu)1

Currentaddress:EnergyBiosciencesInstitute,DepartmentofPlantandMicrobialBiology,CalvinLab,UniversityofCalifornia-Berkeley,Berkeley,CA,USA.

CurrentOpinioninPlantBiology2010,13:305–312ThisreviewcomesfromathemedissueonPhysiologyandmetabolism

EditedbyUweSonnewaldandWolfB.FrommerAvailableonline22ndJanuary20101369-5266/$–seefrontmatter

#2010ElsevierLtd.Allrightsreserved.DOI10.1016/j.pbi.2009.12.009

alsoqualitativedifferencesintheircomponents.Sensitiveanalyticaltechniquescanbeusedtoidentifyandcharac-terizespeci?cwallcomponentsatthetissueorcellularlevel.Sensitivemassspectrometricmethodshavebeendevelopedtoanalyzewallfragmentsreleasedbyenzymesfromlaser-dissectedplanttissues[3??].Notsurprisingly,thevasculartissuescontainedverydifferentcomponentsfromthoseinotherleaftissues.Evendifferencesinthesubstitutionpatternsofthehemicellulosexyloglucanbetweentheouterwallofleafepidermalcellsandthewallsderivedfromtheentirelayerwereobserved,indi-catingthattherearelocalstructuraldifferenceswithinxyloglucanfromdifferentpartsofasinglecell.Moresigni?cantly,theuseofmolecularprobes,suchasspeci?cstains[4]andantibodies,revealsanamazingdiversityofpatterns([5];Figure1).Anumberofmonoclonalanti-bodiesthatrecognizedifferentepitopesonxylan[6],arabinan[7??,8],orxyloglucan[9,10??,11]havebeenusedtodemonstrateveryspeci?cpatternsofdistributionamongcelltypesandwithinthewallofsinglecells.Weneedtoconsiderthiscompositionalcomplexityofplantfeedstockswhenevaluatingandassessingtherobustnessoftechnicalprocessesconvertingbiomasstobiofuels.

Wallcompositionandfuelproductionprocesses

Lignocellulosicbiomasscanbeutilizedasafeedstockforbiofuelproductioninanumberofways.Dependingontheprocess,differentattributesinwallcompositionarerequiredordesired.Oneapproach,similartothatusedinthecornethanolindustry,entailsdegradationofthepolysaccharidesinthelignocellulosicfeedstocktomono-saccharides,andtheirsubsequentfermentationtoethanolorotheradvancedbiofuels.Theremainingligninresiduecanbeusedtogenerateheat,forexample,forethanoldistillation.Themajorchallengeinthisprocessisthatplantshaveevolvedwallstructuresthatarerecalcitranttobiologicaldegradation[1,12].Hence,thebiomassmustbesubjectedtosuchenergy-intensiveandcost-intensivetreatmentsassteam,weakacid,ornon-aqueousammonia[13].Inaddition,ahighenzymeloadingisrequiredtoreleasefermentableglucosemonomersfromcellulosebecauseofitstightlinkagewithhemicellulosesandlignininnativewalls.Therefore,onemajorresearchobjectiveofplantscientistsistomakewallsmoreopenandaccessibletoenzymaticdegradation.Onewaytoenhanceenzymeaccessibilitywouldbetoincreasethewatersolubilityofpolysaccharides.Thisshiftintheratiooflesssolubletomoresolublepolysaccharidescouldbeachievedbyanincreaseintheabundanceofamorphousglucanchainsratherthancrystallinemicro?brilsofcellulose;additionofsidechainsubstitutionstothebackboneofhemicelluloses,

CurrentOpinioninPlantBiology2010,13:305–312

Introduction

Plantcellwallpolymershavereceivedsigni?cantatten-tioninrecentyearsbecausetheyarethemajorcom-ponentsintheplantbiomassthatisunderconsiderationasasourceofreducedcarbontopartiallyreplacefossilfuels.Althoughplantbiomassisoftenconsideredashavingauniformcomposition,thereisinfactsubstantialdiversityincompositionthatarisesfromtwoimportantareas.First,differentspeciesofplantshavesigni?cantdifferencesintheproportionsofcellulose,hemicellulose,andligninfoundintheirbiomassand,further,importantdifferencesinthetypesofhemicellulosesand/ortheratiosofmonomersinlignin[1,2].Inadditiontothesespecies-speci?cdifferences,theaveragecompositionofasinglespecies,whichisoftenquiteuniform,hidesagreatdealofdiversity.Everyplantconsistsofmanydifferentcelltypes,eachwithauniquecellwallthatcontainsnotonlydifferentratiosofwallcomponents,butsometimes

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Figure

1

Anotherissueisthefermentabilityofthereleasedmono-saccharides.Currently,yeastsusedintheethanolfer-mentationprocessutilizeonlyhexosessuchasglucoseandmannose.However,themostcommonhemicellulosicpolysaccharidesconsistmainlyofxyloseandarabinose,pentosesthatdonotfermentsoreadily.Progresshasbeenmadeindevelopingspecializedyeastandbacterialstrainsthatcanfermentthesepentoses,buttheyarenotyetveryef?cient[16].Therefore,onedesirablechangeinbiomassisanincreasedabundanceofhexose-containingpolymerssuchascelluloseormannansratherthanxylans.Anotherprocessusedtoconvertlignocellulosicbiomasstofuelisacatalyst-basedchemicalprocess[17,18].How-ever,thisprocessishamperedbyinhibitingcomponents,suchasphenoliccompoundsoraliphaticacids,presenttovaryingdegreesinthedegradedbiomass[19].Oneplantbreedinggoalshouldthusbetoreducetheabundanceofsuchcompoundstoaminimum.

Analternativewaytoproducefuelsisbycombustionandgasi?cationoflignocellulosicstosyngas(carbon-monox-ide,carbon-dioxide,andhydrogengas)thatcanbetrans-formedtoethanolviamicrobes[20]orhydrocarbonsbytheFischer–Tropschprocess.Theadvantageofthegasi-?cationprocessisthatnotonlythepolysaccharidesbutalsothecarbonpresentinligninisavailableforfuelproduction.Fortheseprocesses,thecompositionandaggregationstatusofthevariouspolymerswithinthelignocellulosicmaterialdoesnotplayarole,butlowwaterandashcontentaredesirable[21].

Nativestructuraldiversityofplantcellwalls

Thewallcompositionandstructureofdifferentiatedcellsinplantsarediverse.Sectionsofplanttissuesstainedforlignin[69]andviewedwithaninvertedmicroscope(panela)orwithfluorescent-taggedantibodiesdirectedagainstspecificpolysaccharideepitopes[7??,8]andviewedbyfluorescencemicroscopy(panelsb–f)revealsthatthewallstructuraldiversitycanbeobservednotonlyindifferentcelltypes(a,b,f),butalsoindifferentlayersorareasofasinglecell(c,d,e).

a:Sectionofthefirstinternodeofamaizestemstainedwith

phloroglucinol(courtesyofDebraGoffner,CNRSCastanetTolosan).b:Sectionfromthefirstinternodeofamaizestemstainedwith

Mirande’sreagent(courtesyofDebraGoffner,CNRSCastanetTolosan).c/d:ImmunofluorescencedetectionofpecticarabinanepitopespresentintheparenchymacellsofanArabidopsisinflorescencestem.Twoantibodiesrecognisingdifferentarabinanepitopes(c-LM13;d-LM16)labeldistinctdifferentregionsofthosecells(courtesyofPaulKnox,UniversityofLeeds).

e:Intobaccostemsectionsacertainpecticarabinanepitope(LM16)isobservedonlyinxylemfibercellsandphloemcells.

f:Maturefibersofhemp:localizationofanarabinogalactan-proteinepitope(visulaizedbyJIM14)attheinnersideofsecondarywalls(courtesyofPaulKnox,UniversityofLeeds).

Whenthinkingaboutstrategiesforchangingthecompo-sitionofplantcellwalls,whetherintermsofabundanceofcertainpolymersorsubstitutionpatternsofspeci?cpoly-mers,onecangainsigni?cantinsightsbylookingintonature’s‘laboratory’.Certainplantspecieshaveevolvedspecializedtissuesthathaveunusualwallcompositions,thatis,theycontainelevatedlevelsofcellulose,hemi-cellulose,orlignin(foroverviewseeFigure2).

Specializedcellsthatmakecellulose

therebydecreasinghydrogenbondingwithcellulosemicro?brils;thecreationofamoreeasilydegradableligninthroughtheintroductionofspeci?ceasilycleavablemono-lignols[2,14??];and/orareductionoflignin–hemicelluloselinkages[15].

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Oneofthebest-studiedexamplesistheseedtrichomeofcotton,inwhichthe?bercellshavesecondarywallscontainingalmostpurecellulose[22].Studiesofthissystemhaverevealedmanyimportantfeaturesofcellu-losebiosynthesis,includingthe?rstidenti?cationofthecellulosesynthasegenesinplants[23].Inrecentyears,progresshasbeenmadeinidentifyingtheregulatoryeventsthatallowthedepositionofalmostpurecelluloseincotton?bers[24].Cotton?berdevelopmentisacom-plexprocessthatinvolvesmanyevents,includingtheactionofvarioushormones[25],butsomeofthekeytranscriptionfactorshavebeenidenti?ed[26,27].Adetailedunderstandingofthemetabolicandregulatoryeventsneededforasinglecelltoconvertalmostallofits

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CellWallsandBiofuelsPaulyandKeegstra307

Figure

2

Examplesofspecializedtissuesinplantsthatproducepredominantlyasinglecellwallpolymer.

Cellulose—Maturecottonplants(upperleft)producecottonfibers(lowerleft),whosesecondarywallsthatarenearlypurecellulose(courtesyofCandiceHaigler,E.Roberts,andE.Hequet,NCStateUniversity).Poplartrees(upperright)producetensionwood(lowerright)withwallsthatarealsonearlypurecellulose(courtesyofFrankTelewskiandJameelAl-Haddad,MichiganStateUniversity).

Hemicelluloses—Fenugreekplants(upperleft)produceseedscontainingendospermwalls(lowerleft)thatarelargelygalactomannan.Psylliumplants(uppercenter)produceseedssurroundedbyamucilaginouslayer(lowercenter)withwallsrichinarabinoxylan.Nasturtiumplants(upperright)

produceseedswherethecotyledoncells(lowerright)haveawallthatislargelyxyloglucan(courtesyofMarleneCameron,CurtisWilkerson,MichiganStateUniversity).

Lignin—Pinetrees(upperpanel)producecompressionwood(lowerpanel)thatisenrichedinligninwithadifferentcompositionfromnormalwood(courtesyofFrankTelewskiandJameelAl-Haddad,MichiganStateUniversity).

carbonresourcesintoasinglepolysaccharide,cellulose,shouldbeusefulineffortstomanipulateandenhancecellulosedeposition,therebyincreasingC6sugarabun-danceinbiofuelcrops.

Specializedcellsthatmakeasinglehemicellulose

theworkofCocuronetal.[35],whousedstudiesofdevelopingnasturtiumseeds,whichstorexyloglucan,toprovideevidencethatthexyloglucanglucansynthaseisencodedbyaCslCgene.

Thefactthattheseseedsarecapableofmakingasinglehemicellulosicpolysaccharide,whilenotmakingotherpartsofthewall,indicatesthatthesebiosyntheticpath-wayscanberegulatedindependentlyfromotherwallcomponents.Furthermore,theuseofcellwallpolymersasreservecarbohydratesoccursinseveralunrelatedplants[28],indicatingthatthistraithasarisenindepen-dentlymanytimesduringtheevolutionoflandplants.Ifcorrect,thislogicwouldleadtotheconclusionthatonlyafewchangesareneededtoaccomplishthischangeinregulationandthatitcouldbedonebydesigntomodifythewallsofbiofuelcrops.

Changesinwallcompositionduetoenvironmentalresponses

Manyplantspeciescreateaspecialcellwallintheendospermorcotyledoncellsofdevelopingseeds[28].Thesecellwallpolysaccharidesserveasreservecarbo-hydrates,beingsynthesizedduringseeddevelopmentandlatermobilizedduringseedlinggermination.Seedsthatstoregalactomannanorxyloglucanareabundantinnature[28],butthereareseedsorothertissuesthatproduceotherpolymersinspecializedcells,suchaspsylliumseeds,whichmakearabinoxylaninaspeciallayerofepidermalcells[29].

Studiesofdevelopingseedsystemshaveledtotheidenti?cationofgenesandenzymesinvolvedinhemi-cellulosebiosynthesis.Forexample,studiesofdevelop-ingguarseedsledtotheconclusionthatmannansynthaseisencodedbyaCslAgene[30],andmorerecentstudieshavecon?rmedthatthisconclusionisvalidformostplantspecies[31,32],includingcoffee[33].Recently,Goubetetal.[34]usedreversegeneticstoprovideconvincinginvivoevidencetofurthersupporttheearlierconclusions.Anotherexamplecomesfrom

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Plantshavethecapacitytoreacttoenvironmentalstimulibychangingtheirmetabolism.Reprogrammingcellwallpolymerbiosynthesisisnoexception.Forexample,asaresponsetomechanicalstresses,suchaswindorchangesingravitationalstimuli,treescandevelopspecializedtissuesknownasreactionwood[36].Thistypeofwoodenablesthereturnofstemsbacktoaverticalorientation.

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Reactionwoodofgymnosperms,knownascompressionwood,developsonthelowersideofbranches.Regulargymnospermwoodcontainsligninthatlackssyringylunitsandconsistsalmostentirelyofguaiacylunits,whereascompressionwoodcontainshigheramountsofligninthatisparticularlyenrichedinp-hydroxyphenylunits[37].Thehigherabundanceisprobablyduetoelevatedexpressionofenzymesintheligninmetabolicpathway[38].Consequently,silencingoneofthosegenes(4-coumarate-ligase)resultedinanupto50%reductionoflignininthetrachearyelements[39],severelydwar?http://wendang.chazidian.com-pressionwoodalsohasalterationsinpolysaccharideswithreducedlevelsofcelluloseandglucomannanscomparedtoregularwood.

Bycontrast,thereactionwoodofangiosperms,suchaspoplar,developsontheuppersideofbranchesandistermedtensionwood.Tensionwood?bersformadistinctadditionalinnergelatinouslayer,theG-layer,withhighlyelevatedcellulosecontent(95%vs.45%inregularvesselelements;[40]).AdetailedanalysisofthetranscriptomeandmetabolomeoftheG-layerinpoplardemonstratedahighertranscriptlevelofsucrosesynthase,suggestinganincreasedcarbon?uxintocelluloseproduction[41].Con-comitantly,pathwaysfortheproductionofthehemicel-lulosesandligninweredownregulated.Also,speci?ccellulosesynthasegenesareupregulatedintheG-layer,probablybecausetheycontainmechanicalstress-respon-siveelementsintheirpromoters[42].Despiteitsreducedabundance,xyloglucanispresentintheG-layeranditsmetabolismbyxyloglucanendotransglycosylaseshasbeenproposedtoplayamajorroleinconferringthemechanicalpropertiesoftheG-layeranditsconnectiontotheadjacentS2layer[40,43].

Anotherexampleofadistinctwallstructuremadebyaplantcellisthepapillaformedatthesitewhereapathogenicfungusattemptstopenetrateaplantcell.Apapilla,alocalappositionofamulti-layeredwallstructure,containscallose,structuralproteins,andlig-nin[44].Allofthesecomponentsareproducedasaresultoftheplantcell’ssensingmechanismtriggeredbyinvadingpathogensandarenecessarytoforma?rstlineofdefenseagainstthispathogen[45].Geneticevidencehasbeenpresentedthatcomponentsofthesecretorysystemareresponsibleforthedistinctlocationofthepapilla[46].

Thelessonthatonecanlearnfromthesevariouswallsystemsisthatplantcellsareabletoundergorepro-grammingoftheirwallbiosyntheticmachinery,in-cludingbutnotlimitedtodivertingthe?owofassimilatedcarbon,alteredregulationofglycansynthases,glycosyltransferases,monolignolsyntheticenzymes,andthesecretorysystem.Thisreprogram-mingresultsinfundamentallydifferentandunique

CurrentOpinioninPlantBiology2010,13:305–312

wallmaterialsthathaveanalteredratioofcomponentsorevendifferentcomponents.Understandinghowthecellaccomplishesthistaskwillprovidevaluableknowl-edgethatwillenablerationalchangesofcellwallcompositionthatwillallowenhancedbiofuelpro-duction.

Strategiesformanipulatingwallcomposition

Knowledgeregardingtheregulationofwallpolymerbiosynthesisremainslargelyelusive.Themechanismsbywhichplantcellsregulatethe?owofcarbonintowallpolymersandbalancethequantitiesofthevariouswallpolymersarehottopicsforresearch.Photosyntheticassimilatesareusedasthebuildingblocksforwallpoly-saccharidebiosynthesis.Oneofthe?rstenzymesthatutilizesthenewly?xedcarbonissucrosesynthase(SuSy),whichconvertssucroseintoUDP-glucoseandfructose.Recently,ithasbeenshownthatoverexpressingSuSyinpoplarleadstoahigherproportionandabsoluteamount(2–6%)ofcellulosein?bercells[47????],demon-stratingthatthisenzymemightbeakeydeterminantregulatingcellulosebiosynthesis.Interestingly,theincreaseinwallcellulosecontenthadverylittledetri-mentaleffectontheplant,asgrowthandbiomassremainedsimilartonon-transformedplantswhengrowninthegreenhouse.

UDP-glucoseorotherhexose-phosphatesareusedasasubstrateforthenucleotidesugarconversionpathwaythatresultsinthesynthesisofthe14differentnucleotidesugarsnecessaryforthevariouswallpolymers[48].Hence,manipulatingtheseenzymescouldleadtoaltera-tionincellwallcomposition.Althoughmostofthegenesencodingthenucleotidesugarconversionenzymesareknownandtheiractivitieshavebeendemonstratedinvitro[48],veryfewstudieshavedemonstratedthatmanipulationoftheirexpressionleadstosigni?cantcellwallchanges.Overexpressionoftheseenzymeshassofarnotyieldedplantswithalteredwallcompositions,possiblybecauseoftheredundancyofgenesencodingmanyofthesugarnucleotidemetabolizingenzymes.Forexample,aconcomitantreductionofseveralUDP-glu-coseepimerasetranscriptsledtonotonlyadecreaseinwallgalactoselevels[49],butalsoseverechangesinrootgrowth.Hence,suchchangeswouldpresumablyleadtolessbiomassintheplant.

Anotherstrategyformodifyingthecompositionofcellwallsistoupregulateordownregulatethelevelsofglycosyltransferasesandglycansynthasesthatpolymer-izethesugarsfromnucleotidesugarsintopolysacchar-ides.Cavalieretal.[10??]createdanArabidopsisplantlackingtwogenesencodingxyloglucanxylosyltrans-ferasesandshowedthatxyloglucancouldnotbedetectedinmutantplants.Interestingly,themutantplantsgrewanddevelopedrelativelynormally,althoughtheywereslightlysmallerthanwild-typeplantsandhad

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abnormalroothairs.Itremainstobedeterminedwhatpolymersaresubstitutingforxyloglucansintheprimarywallofthesemutantplants,butoneimportantcon-clusionfromthesestudiesisthatitispossibletomakedramaticalterationsinthehemicellulosecompositionofaplantwithoutseriousconsequencesforitsabilitytogrowandreproduce.

Reidetal.[50]werethe?rsttosuccessfullymodifythecompositionofacellwallpolysaccharidebyoverexpres-singageneencodingaglycosyltransferase.Theyusedaconstitutivepromotertoexpressthegalactomannangalactosyltransferasefromfenugreekintobaccoplants.Thehigherlevelsofgalactosyltransferaseactivityledtoanincreaseinthegalactosylsubstitutionofthegalacto-mannanthatisnormallyfoundinthewallsofseedendospermcells.Effortstomodifythemannancontentofseedsbyoverexpressionofamannansynthasegeneledtoamorecomplexresult.Naoumkinaetal.[51]usedaseed-speci?cpromotertooverexpressthemannansynthasegenefromguarinMedicagotruncatulaplants.ThetransgenicMedicagoseedshadalowerratherthanahigherlevelofgalactomannan,butthemolecularweightandviscosityofthepolymersweresigni?cantlyincreased.Inaddition,theauthorsfoundthatoverex-pressionofthemannansynthasegenecausedlargechangesinthelevelsofvarioussugarsandsugaralcoholsaswellassigni?cantchangesintheexpressionofmorethan900genes,withmorethan300genesupregulatedandalmost600genesexpressedatlowerlevels.Themajorconclusionfromthisstudyisthatoverproductionofacellwallpolysaccharidecannotbeaccomplishedsimplybyupregulationofthegeneresponsibleforpolymerbackbonesynthesis.

Onesystemwhereconsiderableprogresshasbeenmadeinunderstandingtheregulationofwallpolymerbiosyn-thesisonamolecularlevelisdifferentiatingvascularelements,especiallythedepositionofsecondarywallsinxylemcells(seethereviewbyDemuraandYe[52]inthisissue).Theseuniquecellwallsarethemostabun-dantcomponentofharvestedbiomasscropsandthusdeservespecialmention.Manyofthegenesinvolvedincelluloseandligninbiosynthesishavebeenidenti?edandprogressisbeingmadeonidentifyingthegenesrequiredforthebiosynthesisofxylan[53–55],thehemi-cellulosefoundinthevasculartissuesofmostpotentialbioenergycrops.Moreimportantly,keyregulatorygenesthatcontroltheformationofvascularelements[56,57]andmanyofthecomponentsinthetranscriptionalnet-workthatcontrolssecondarywallbiosynthesishavebeenidenti?edinrecentyears[58–61],includingtwotranscriptionalactivatorsoftheligninbiosyntheticpath-way[62????].Itisinterestingtoconsiderthepossibilitythatdiscretetranscriptionfactorsindependentlyregulatethecellulose,hemicellulose,andligninpath-ways.Thedepositionofthesethreecomponentsis

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normallycoordinatedduringwallbiosynthesis[63].Ifitispossibletocontrolthedepositionofeachseparately,thenitshouldbepossibletomodifysecondarywallsinwaysthatwillallowtheproductionofdesignerwallsinbiomasscrops.

Itshouldnotbesurprisingthatthe?http://wendang.chazidian.combiningtheinformationofkeyregulat-orygeneswiththepromotersspeci?cforvariouscelltypesofthevascularsystem[56,57]shouldmakeitpossibletomodifythecompositionofthesecondarycellwallsofvariouscelltypesinstemsorothervegetativetissues.

Investigatingthebiologicalconsequences

Anystrategytoimprovethecompositionoflignocellu-losicsasfeedstocksfortheproductionofbiofuelsneedstoassesstheperformanceofthemodi?edplants.Owingtothemultiplefunctionsofthewallduringthelifecycleofaplant,therecouldbenumerousproblems,includingareductioninplantgrowthandaconcomitantreductioninlignocellulosicbiomass.Biomassyieldconcessionssuchasdwar?smhavebeenshowninanumberofmutants[64],whereasinotherstheyieldundergreenhousecon-ditionsdoesnotseemtobeaffected[65].Alteringthewallscanleadtochangesincellmorphology.Onedetri-mentalexampleofchangesinsecondarywallstructureareirregularxylemcells[66]leadingtoalossinwater-transportingcapacityresultinginareductioninbiomass.Anotherfunctionofthewallistowardoffplantpathogenssuchasbacteriaandfungi[45];structuralchangesinthewallmightleadtoincreasedsusceptibility.However,inafewcaseswherepathogeninfectionhasbeentestedonwallmutants,resistancewasnotreduced[67],andinsomecaseswasevenincreased[68].

Concludingremarks

Plantbiomassisbeingconsideredasafeedstockfortheproductionofbiofuelsandotherchemicals.Becausethetraitsthataredesirablefortheseusesvarydependinguponthemethodutilizedforbiomassprocessing,andbecausethedesirabletraitsdifferfromthoseneededformoretraditionalusesofplantsforfoodand?berpro-duction,muchdiscussionhasfocusedonmodifyingthecompositionandcellwallpropertiesinwaysthatwillimprovethebiomassforthesenewapplications.Assuchmodi?cationsareconsidered,itisimportanttobeawarethatnaturehasalreadycreatedplantcellswithdiversewalls,notonlythosesurroundingthedifferentiatedcellsfoundineveryplant,butalsothoseinvariousspecializedcellsthatoccurinmanyplants.Understandinghowthewallsofthesedifferentiatedandspecializedcellsaresynthesizedwillprovidevaluablecluesonhowtomodifythecellwallsofplantsthataregrownasdesignerbiofuelcrops.

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