Plant cell wall components

Plant systems: Extracellular matrix components of plants-cell wall, cellulose and hemicelluloses, extensins, WAKs, secondary wall structure, pits-primary and secondary pits and their development, plasmodesmota-structure and functions, pectins, cutins, lignins, turnover of cell wall components

1. PLANT CELL COMPONENTS
Plantsystems:Extracellularmatrixcomponentsofplants-cellwall,celluloseand
hemicelluloses,extensins,WAKs,secondarywallstructure,pits-primaryand
secondarypitsandtheirdevelopment,plasmodesmota-structureandfunctions,
pectins,cutins,lignins,turnoverofcellwall
Dr. M. THIPPESWAMY

2. •Cannotmaketheirownfoodsothey
havetoeatfood
•Animalcellsaremoreroundshaped
•Animalcellsdonothaveacellwall
•Animalcellsdonothavealargevacuole
•Theydonothavechloroplast
•Animalcellscannotmakesugar
•Plantcellsgothroughphotosynthesis
•Plantcellsareintheshapeofarectangle
•Plantcellshaveacellwall
•Plantcellshavealargevacuole
•Plantcellshavechloroplast
•Plantcellsusechloroplasttostoreenergy
Animal cell Plant cell

3. •Thecellwallisaselectivefilterwhosepermeabilityis
controlledlargelybypectinsinthewallmatrix,including
proteinslargerthan20kDa,islimited.
•Thislimitationmayaccountforwhymanyplanthormonesare
small,water-solublemolecules,whichcandiffuseacrossthe
cellwallandinteractwithreceptorsintheplasmamembraneof
plantcells.
•Theprimarycellwall,generallyathin,flexibleandextensible
layerformedwhilethecellisgrowing.
•Thesecondarycellwall,athicklayerformedinsidetheprimary
cellwallafterthecellisfullygrown.Itisnotfoundinallcell
types.Somecells,suchastheconductingcellsinxylem,possess
asecondarywallcontaininglignin,whichstrengthensand
waterproofsthewall.
•Themiddlelamella,alayerrichinpectins.Thisoutermostlayer
formstheinterfacebetweenadjacentplantcellsandgluesthem
together.
Plant cell wall

4. Primary wall composition and architecture
•Primarywallscomposedofpolysaccharideswithlesseramountsofstructuralglycoproteins(hydroxyproline-rich
extensins),phenolicesters(ferulicandcoumaricacids),ionicallyandcovalentlyboundminerals(e.g.calcium
andboron),andenzymes.
•Theprimarywallconsistsof50%hemicellulose,25%ofcelluloseandinlesseramountoffats,proteinsetc…..
•Extensin,aglycoproteinthatcontain
hydroxyprolineandserine.Mosthydroxyproline
residuesarelinkedtoshortchainsofarabinoseand
theserineresiduesarelinkedtogalactose.
•Lignin,amacromoleculecomposedofhighly
cross-linkedphenolicmolecules,isamajor
componentofsecondarywalls.
•Cellulose-alarge,linearpolysaccharidecomposed
of1,4-linkedβ-D-glucosepolymerizestheenzyme
calledcellulosesynthase,assemblesspontaneously
intomicrofibrilsstabilizedbyhydrogenbonds.

5. Hemicellulose-branchedpolysaccharidesthatare
structurallyhomolgoustocellulosebecausethey
haveabackbonecomposedof1,4-linkedβ-D-
hexosylresidues.Thepredominanthemicellulosein
manyprimarywallsisxyloglucan.Other
hemicellulosesfoundinprimaryandsecondarywalls
includeglucuronoxylan,arabinoxylan,glucomannan,
andgalactomannan.
Pectin-afamilyofcomplexpolysaccharidesthatall
contain1,4-linkedα-D-galacturonicacid.
Todatethreeclassesofpecticpolysaccharideshave
been characterized: Homogalacturonans,
rhamnogalacturonans,andsubstitutedgalacturonans.
Some of the functions of the primary wall:
•Structural and mechanical support.
•maintain and determine cell shape.
•resist internal turgor pressure of cell.
•control rate and direction of growth.
•ultimately responsible for plant architecture and form.
•carbohydrate storage -walls of seeds may be metabolized.
•protect against pathogens, dehydration, and other
environmental factors.
•source of biologically active signalling molecules.
•cell-cell interactions.

6. •Thethickerandstrongersecondarywall,
whichaccountsformostofthe
carbohydrateinbiomass,isdepositedonce
thecellhasceasedtogrow.
•Thesecondarywallsofxylemfibers,
tracheids,andsclereidsarefurther
strengthenedbytheincorporationoflignin.
•Chemicallysecondarywallsaremadeupof
cellulose,hemicellulosewithlignin,chitin
orsuberin.
•Itsometimesconsistsofthreedistinctlayers
-S1,S2andS3-wherethedirectionofthe
cellulosemicrofibrilsdiffersbetweenthe
layers.
The secondary wall
Unlike cellulose, pectin and hemicellulose are synthesized in the Golgi apparatus and transported to the cell surface where they
form an interlinked network that helps bind the walls of adjacent cells to one another and cushions them.

7. •Thewall-associatedkinases,orWAKs,are
receptor-likekinasesthatarelinkedtothepectin
fractionofthecellwall,andhaveacytoplasmic
proteinkinasedomain.
•WAKsarerequiredforcellexpansion,cell
elongation,morphogenesis,pathogenresponse,
andtheirexpressionisactivatedbynumerous
environmentalstimuli.
•WAKsreceptorsforbothpectininthecellwall,
andforpectinfragments,oligogalacturonicacids
(OGs),generatedduringsomepathogenattacks.
Wall-Associated Kinases
•Thesereceptor-likeproteinscontainacytoplasmicserinethreoninekinase,atransmembranedomain,anda
lessconservedregionthatisboundtothecellwallandcontainsaseriesofepidermalgrowthfactorrepeats.
•Undoubtedly,wall-associatedproteinsservecomplexandbiologicalroleswithregardtowallstructure.

8. Infungi,thecellwallistheouter-mostlayer,externaltotheplasma
membrane.Thefungalcellwallisamatrixofthreemaincomponents:
•Chitin:polymersconsistingmainlyofunbranchedchainsofβ-(1,4)-
linked-N-AcetylglucosamineintheAscomycotaandBasidiomycota,
orpoly-β-(1,4)-linked-N-Acetylglucosamine(chitosan)inthe
Zygomycota.Bothchitinandchitosanaresynthesizedandextrudedat
theplasmamembrane.
•Glucans:glucosepolymersthatfunctiontocross-linkchitinor
chitosanpolymers.β-glucansareglucosemoleculeslinkedviaβ-
(1,3)-orβ-(1,6)-bondsandproviderigiditytothecellwallwhileα-
glucansaredefinedbyα-(1,3)-and/orα-(1,4)bondsandfunctionas
partofthematrix.
•Proteins:enzymesnecessaryforcellwallsynthesisandlysisin
additiontostructuralproteinsareallpresentinthecellwall.Mostof
thestructuralproteinsfoundinthecellwallareglycosylatedand
containmannose,thustheseproteinsarecalledmannoproteinsor
mannans.
Fungal cell wall
Chemical configurations of the different monosaccharides(glucose
and N-acetylglucosamine) and polysaccharides (chitin and cellulose)
A close-up of the wing of a leafhopper; the wing is composed of chitin.

9. Functions of cell wall
•Cellwallsprimaryfunctionismechanicalsupport.Itactslikeaskeletalframeworkoftheplants.
•Cellwallistoughandhashightensilestrength.Stillplantcellisfullypermeabletowaterandsolutes.
•Plantcellwallhasminutewaterfilledchannelsthroughwhichwater,hormonesandgasespassestoandfro.
•Cellwallshowsplasticityandelasticityduringcellgrowth.
•Ithelpstomaintainthebalanceofintracellularosmoticpressurewiththatofitssurroundings
•Lignificationofsecondarywallsgreatlyenhancescompressivestrengthpermittingwoodystructuresto
reachthesky.
•Cellwalluponlignificationbecomesdeadasitbecomesimpermeableandthusprotoplasmhasnoaccessto
takeupsolutesthatiswhylignifiedtissueisalwaysdead.
•Ligninprovidedextramechanicalstrengthandalsoprovidesawaterresistantchannelfortransportof
solutes.

10. •Thecytosolsofadjacentanimalorplant
cellsoftenareconnectedbyfunctionally
similarbutstructurallydifferent“bridges”
calledgapjunctionsinanimalsand
plasmodesmatainplants.
•Thesestructuresallowcellstoexchange
smallmoleculesincludingnutrientsand
signals,facilitatingcoordinated
functioningofthecellsinatissue.
•Thecellwallisperforatedbynarrowpores
orpitsatmanyplaces.
•Plasmodesmataarenarrowchannelsthroughthewallboundbyplasmalemmmacontainingcytoplasmand
oftenadesmotubule.
•Thedesmotubuleisthecenrtalcoreandiscomposedofproteinsubunitsconsistsofmodifiedmembranous
structurecontinuouswiththeendoplasmicreticulumoftheadjoiningcells.
Plasmodesmata

11. •ThesemembraneousstructuresprobablyoriginatefromtheER.
•Desmotubuleactsasvalvetocontrolthedirectionofflowof
materials.
•Plasmodesmatahavebeenshowntotransportproteins(including
transcriptionfactors),messengerRNA,viroids,andviral
genomesfromcelltocell.
•Oneexampleofaviralmovementproteinsisthetobaccomosaic
virusthoughttobindtothevirus'sowngenomeandshuttleit
frominfectedcellstouninfectedcellsthroughplasmodesmata.
•Itisconcernedwiththetransportofmaterialsfromonecellto
anotherandalsoconductionofstimuli.
•InthecaseofcertainparasiteslikeViscum,Loranthus,Cuscuta
etcplasmodesmataconnectionsexistbetweenthehaustoriaand
thecellsoftheirhost.
•Throughthesechannelsfoodandvirusaretransported.
The structure of a primary plasmodesma. CW=Cell wall CA=Callose
PM=Plasma membrane ER=Endoplasmic reticulum DM=Desmotubule
Red circles=Actin Purple circles and spokes=Other unidentified proteins.

12. Thepitsareformedinpairslyingagainsteachotherontheopposite
sidesofthewall,andmorphologicallymorecorrecttheyarecalled
‘pitpairs’.
•Simplepits:Thesimplepitiscircular,oval,polygonal,elongated
andoccurinparenchymacells,inmedullaryrays,inphloem
fibres,companioncells,andintracheidsofseveralflowering
plants.
•Borderedpits:Theyoverarchingsecondarywallfoundinthe
vesselsofmanyangiospermsandinthetracheidsofmany
conifers.Theyaremorecomplexandvariableintheirstructure
thansimplepits.Theoverarchingsecondarywallwhichenclosesa
partofthepitcavityiscalled,thepitborder,whichopensoutside
byasmallroundedmouthknownaspitaperture.
•Halfborderedpitpair:Insomecases,borderedpithasa
complementarysimplepit.Suchapitpairiscalledhalfbordered
pitpair.
Pit Pairs: Structure and Types

13. •Blindpits:Somepitsdonothateallycomplementarypit.Suchpits
arecalledblindpits.
•Compoundpits:Sometimes,thereisonepitonside.Butthereare
twoormorecomplementarypitsonoppositeside.Suchpitsarecalled
compoundpits.
•Torus:Thetorusmayremainincentralpositionoritmayshifttothe
lateralposition.Asthetorusisshiftedtothelateralpositionthepit
aperturecloses,andthepassageoftheprotoplasmmaytakeplace
onlybydiffusionthroughtorus.
Patterns of bordered pits:
The bordered pits in vessel walls of angiosperms
show three main types of arrangement:
(i)Scalariform pitting (ladder like arrangement)
(ii)Opposite pitting and (horizontal pairs)
(iii)Alternate pitting.(rectangular outlines)

14. •Cutinisoneoftwowaxypolymersthatarethemain
componentsoftheplantcuticle,whichcoversallaerial
surfacesofplants.
•Theothermajorcuticlepolymeriscutan,whichismuch
morereadilypreservedinthefossilrecord.
•Cutinconsistsofomegahydroxyfattyacidsandtheir
derivatives,whichareinterlinkedviaesterbonds,forminga
polyesterpolymerofindeterminatesize.
•Therearetwomajormonomerfamiliesofcutin,theC16
andC18families.TheC16familyconsistsmainlyof16-
hydroxypalmiticacidand9,16-or10,16-dihydroxypalmitic
acid.
•TheC18familyconsistsmainlyof18-hydroxyoleicacid,
9,10-epoxy-18-hydroxystearicacid,and9,10,18-
trihydroxystearate.
•Theprimaryfunctionoftheplantcuticleis
asawaterpermeabilitybarrierthat
preventsevaporationofwaterfromthe
epidermalsurface,andalsoprevents
externalwaterandsolutesfromentering
thetissues.
•"Thewaxysheetofcuticlealsofunctions
indefense,formingaphysicalbarrierthat
resistspenetrationbyvirusparticles,
bacterialcells,andthesporesorgrowing
filamentsoffungi”.
Cutin

15. •Pectinispresentinthemiddlelamella,primarycellandsecondary
wallsandisdepositedintheearlystagesofgrowthduringcell
expansion.
•Itpectinplaysanimportantroleintheformationofhigherplantcell
walls,whichlendstrengthandsupporttoaplantandyetarevery
dynamicstructures.
•Ingeneral,thepolymericcompositionofprimarycellwallsin
dicotyledonousplantsconsistsofapproximately35%pectin,30%
cellulose,30%hemicellulose,and5%protein.
•Grassescontain2–10%pectinandwoodtissueca5%.Incellwallsofsomefruitsandvegetables,thepectin
contentcanbesubstantiallyhigherandtheproteincontentlower.
•Second,pectininfluencesvariouscellwallpropertiessuchasporosity,surfacecharge,pH,andionbalanceand
thereforeisofimportancetotheiontransportinthecellwall.
•Pectinoligosaccharidesinducelignificationandaccumulationofproteaseinhibitorsinplanttissues.
Pectin

16. •Ligninisaclassofcomplexorganicpolymersthatform
importantstructuralmaterialsinthesupporttissuesofvascular
plantsandsomealgae.
•Ligninsareparticularlyimportantintheformationofcellwalls,
especiallyinwoodandbark,becausetheylendrigidityanddo
notroteasily.Chemically,ligninsarecross-linkedphenolic
polymers
•Thecompositionofligninvariesfromspeciestospecies.In
gereral63.4%carbon,5.9%hydrogen,0.7%ash(mineral
components),and30%oxygen(bydifference),corresponding
approximatelytotheformula(C
31H
34O
11)
n.
•Ligninfillsthespacesinthecellwallbetweencellulose,
hemicellulose,andpectincomponents,especiallyinvascularand
supporttissues:xylemtracheids,vesselelementsandsclereid
cells.
Lignin

17. •Theplantcell-wallcompartmentcanhavehighmetabolicactivityandmaybeengagedinimportantevents
relatedtoplantcellfunctionanddevelopment,includingcontrolofgrowthandmorphogenesis,cell-cell
recognition,diseaseresistance,andsignaling.
•Changesinthechemicalstructureofthecellwallpolysaccharidesunderlietheprocessofcellwallloosening
leadingtocellelongation.
•Cellwalloftheplantiscomposedofmanycomplexcarbohydratesandproteins,whichundergorapidturnover
duringtheprocessofcellelongation.
•Cellulosemicrofibrilsarelinkedbytwomajorgroupsofpolysaccharidenetwork,i.e.pecticsubstancesand
xyloglucans.
•Cellwallcomponentsturnoveroccurthroughavarietyofchemicalmodificationbycleavage(hydrolysis)of
cellwallpolysaccharides.
•Cellwallpolymerbreakdownhasbeenassociatedwithcelldivision,growth,xylemdifferentiation,the
abscissionofvariousplantorgansandpollengerminationandpollentubegrowth.
Turnover of cell wall components

18. •Theproteinsplayaimportantroleincellwallturnoverbyhydrolyzingthepolysaccharidesofcelluloseand
hemicellulosewithcellulasesandglucanases.Glycosidasesareenzymeswhichhydrolyzeoligosaccharides
tomonomers.
•Plantcellgrowthistheresultofturgor-dependentextensionofthecellwallthattheprimarygrowth
promotingactionsofauxinweakensthewallwhichallowedittostretch.
•TheobservationthatincreasedRNAandproteinsynthesisaccompaniedhormone-promotedtissue
elongationledtotheproposalthatenhancementofdenovosynthesisofhypotheticalwall-loosening
enzymeswastheprimaryactionofauxin.
•Thepulse-chasestudyofcellwallturnoverinepicotylsegmentswerestudiedwithlabelled
14
C-glucose.
Theradioactiveepicotylsegmentswereincubatedfor3hrsinunlabeledglucose.
•IAAwasthenprovidedandchangesinlabeledwallsugarsweredetectedanauxin-dependentlossof
14
C-
xyloseand
14
C-glucosefromtheepicotylwall.
•Thesyntheticanddegradativereactionsarebalancedtoprovidecoordinationofwallturnoverwithallof
theotherprocessesthatareessentialtoplantcelldevelopment.

19. THANK YOU