Systems Analysis of Shoot Apical Meristem Growth and Development

14 Pages·2012·0.59 MB·English

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REVIEW
Systems Analysis of Shoot Apical Meristem Growth and
Development: Integrating Hormonal and Mechanical Signaling
JamesA.H.Murray,aAngharadJones,aChristopheGodin,bandJanTraasc,1
aSchoolofBiosciences,CardiffUniversity,Cardiff,WalesCF103AX,UnitedKingdom
bVirtualPlants,CentredeCoopérationInternationaleenRechercheAgronomiquepourleDéveloppment,InstitutNationaldela
RechercheAgronomique,InstitutNationaldeRechercheenInformatiqueetenAutomatique,UniversitéMontpellier2,34095
Montpelliercedex5,France
cLaboratoiredeReproductionetDéveloppementdesPlantes,UnitéMixtedeRecherche,InstitutNationaldelaRecherche
Agronomique,CentreNationaldelaRechercheScientifique,ÉcoleNormaleSuperieurdeLyon,UniversitéClaudeBernardLyonI,
69364Lyoncedex07,France
The shoot apical meristem (SAM) is a small population of stem cells that continuously generates organs and tissues. This
review covers our current understanding of organ initiation by the SAM in Arabidopsis thaliana. Meristem function and
maintenance involves two major hormones, cytokinins and auxins. Cytokinins appear to play a major role in meristem
maintenanceandincontrollingmeristematicproperties,suchascellproliferation.Self-organizingtransportprocesses,which
arestillonlypartiallyunderstood,leadtothepatternedaccumulationofauxinatparticularpositions,whereorganswillgrow
out.Amajordownstreamtargetofauxin-mediatedgrowthregulationisthecellwall,whichisadeterminantforbothgrowth
ratesandgrowthdistribution,butfeedbackswithmetabolismandthesyntheticcapacityofthecytoplasmarecrucialaswell.
Recentworkhasalsopointedatapotentialroleofmechanicalsignalsingrowthcoordination,buttheprecisemechanismsat
workremaintobeelucidated.
INTRODUCTION:THESHOOTAPICALMERISTEM, initiated. Superimposed on this zonation is an organization in
PHYLLOTAXIS,ANDPATTERNING layers, with a so-called tunica layer at the meristem surface,
containing cells that divide in anticlinal (perpendicular to the
Plantsgrowandgenerateneworgansthroughouttheirlifespan.
surface)directionsonly.AstrikingfeatureoftheSAMinmost
Thisispossiblethroughtheactivityofsmallpopulationsofcells,
species is its capacity to generate organs in very regular ar-
called meristems, which continuously generate organs and tis-
rangements (Figure 1). Although subsequent growth of the
sues. Thisreviewdeals with the aerial partsof the plant, where
stem can affect patterning, these arrangements of the pri-
so-called shoot apical meristems (SAMs) play a central role. mordia, also called phyllotaxis, largely contribute to the final
Positioned atthetipsofshoots,theyproducedifferenttypesof
architecture of the plant. Different types of phyllotaxis exist:
organs (leaves, flowers, and floral organs), depending on the
from simple alternate arrangements, where organs are pro-
physiologicalstateoftheplant.Asecondtypeofaerialmeristem,
duced in alternate, opposite positions, to the more complex
thesecondarymeristem,isresponsibleforthesecondarygrowth
whorled phyllotactic patterns where organs are arranged in
inthegirth(thickening)ofstems.Thesehaveanumberofchar-
multiplespirals.Thetypeofphyllotaxisisspeciesdependent,
acteristics and molecular mechanisms in common with SAMs
but it is not necessarily constant throughout development
Schraderetal.,2004),butwillnotbediscussedfurtherhere.In-
(reviewed in Steeves and Sussex, 1989; for a historical per-
stead, we will focus on the SAM itself and describe our current
spective,seeAdleretal.,1997).
knowledgeonhowthismeristemandadjacenttissuescontribute
Here,wegiveabriefoverviewofourcurrentunderstandingof
tocreatingplantarchitecture.
organ initiation by the SAM in Arabidopsis thaliana, including
The SAM is a highly organized structure, and a simple his-
positioningandoutgrowthattheshootapex.Inthiscontext,an
tologicalanalysisissufficienttodistinguishdifferentzonesand
important concept is that multicellular organisms are complex
layers(Figure1).Atthesummitofthemeristematicdomelies
systems(i.e.,arecomposedofelementsthatinteractaccordingto
theso-calledcentralzone,whereasmallpopulationofstemcells
localrules).Theseinteractionsleadtotheemergenceofcollective
assures the maintenance of the meristem. This central zone is
propertiesatahigherleveloforganization.Thus,macromolecules
surrounded by a peripheral zone, where organ primordia are self-organize into well-defined, cellular structures and molecular
networkstoformcells,whileatahigherleveloforganizationcells
interact to assemble into tissues and organs with particular
[email protected]. shapes. Importantly, these emergent properties cannot be de-
www.plantcell.org/cgi/doi/10.1105/tpc.112.102194 ducedfromaddinguplocalbehavior(i.e.,thewholeisnotsimply
ThePlantCellPreview,www.aspb.orgã2012AmericanSocietyofPlantBiologists.Allrightsreserved. 1of13
2of13 ThePlantCell
Figure1. TheOrganizedStructureoftheArabidopsisSAMIsGeneratedthroughaCombinationofHormonalandMechanicalSignalsandtheCellular
ResponsestoTheseSignals.
(A)TheSAMconsistsofthecentralzone(CZ),containingthestemcellsandthesurroundingperipheralzone(PZ).Primordia(P),theprogenitorsofnew
lateral organs, are initiated in the PZ. TheCZ is maintained by an underlying organizing center (OC). Belowthe OC is the rib zone (RZ), which is
responsiblefortheelongationofthestem.Indicotyledonousspecies,thetwooutermostcelllayers(L1andL2)divideanticlinallyandmakeuptheso-
calledtunicalayer.Fromthethirdcelllayer(L3)inwards,celldivisionstakeplaceinnonuniformorientations,resultinginthelessorganizedtissue
structureknownasthecorpus.OrganinitiationislargelycontrolledbyauxintransportanddistributionwithintheL1ofthetunica(see[E]),whereas
maintenanceoftheCZrequirescommunicationbetweenthetunicaandthecorpus(see[C]).
(B) Primordia are spacedaccordingto aregular patternor phyllotaxis. In Arabidopsis,spacingfollowsa spiralphyllotaxy.P9 indicates the oldest
primordiumandP1theyoungest.Successiveorgansareseparatedbyanangleof;137.5.Thepositionatwhichthenextprimordium(i1)willbe
initiatedcanbepredictedbasedonthisrule.
(C) and (D) Cells in the OC express the transcription factor WUS, which promotes the expression of CLV3, a small peptide that moves into the
surroundingtissue.Inthecentralzone,CLV3interactswiththereceptor-likekinaseCLAVATA1,inhibitingWUSexpressionandpromotingstemcell
fate.Thehormonecytokinin(CK)isalsorequiredtoestablishandmaintainthecentralzone.ThetranscriptionfactorSTMupregulatestheexpressionof
IPTgenesthatareratelimitingforcytokininbiosynthesis.CytokininactivatesA-typeARRtranscriptionalregulatorsviaaphosphorelaysystem.A-type
ARRsstimulatedownstreamcytokininresponsesbutalsodownregulatetheexpressionofWUS.WUSinhibitstheexpressionofA-typeARRs,creating
anegativefeedbackloopthatregulatessizeandpositionoftheorganizingcenterand,thus,ofthecentralzone.Consistentwiththisroleforcytokinin,
thecytokininresponsereporterTwo-Component-Output-Sensor:GreenFluorescentProtein(TCS:GFP)(MüllerandSheen,2008)(D)isdetectedathigh
levelsintheorganizingcenter,aspreviouslyshownbyYoshidaetal.(2011).
(E)to(G)Thepositionsofprimordiaaredeterminedbyauxinmaxima(orange)thatarecreatedbyself-organizingpatternsofauxintransport(arrows)(E).
TheauxinresponsereporterDR5:3xVENUS-N7isdetectedinprimordiabeforeoutgrowthbegins(seecircledi1in[F]).Directionalmovementofauxinis
producedbytheactivityofPIN1proteins,whichtransportauxinoutofcellsandarepolarlylocalized.ImmunolocalizationofPIN1intheL1oftheSAM
showsthatPIN1proteinsareorientedtowardtheauxinmaxima(G).
(H)and(I)MechanicalprincipledirectionsofstressesexperiencedbythetissuevariesacrosstheSAMasaresultofthegeometryofthestructureand
as a result of local differences in growth rates. Stresses at the dome of the meristem are isotropic (redarrows), whereas inthe fold betweenthe
outgrowingprimordiaandtheSAM,thedirectionofstressesishighlyanisotropic(bluearrows).Microtubulesorientaccordingtotheprincipledirection
ofthemechanicalstressthattheyexperience.[I]showsadetailofameristemexpressingMicrotubuleAssociatedProtein4:GreenFluorescentProtein
(aspreviouslyshowninHamantetal.,2008).Notethatmicrotubulesatthemeristemdomedonotshowaclearorientation(asterisk),butinorgan
boundariesamarkedcoordinationoftubulesisobserved(arrowhead).
([D],F.BesnardandT.Vernoux,unpublisheddata,constructfromMüllerandSheen[2008];[G],reprintedfromBarbierdeReuilleetal.[2006],Figure3F;[F]
A.Jones,unpublisheddata,constructfromLaskowskietal.,2008,[I],J.Traas,unpublisheddata,liveimagingasdescribedinFernandezetal.[2010].)
a sum of the parts). This is partially linked to the presence of theseinteractions,whichcanbebothchemicalandmechanicalin
multiple feedbacks between the different levels of organization nature.
(Figure 2). Considering living systems as complex systems has
importantimplications.First,sinceemergentpropertiescannotbe
PATTERNINGREQUIRESSIGNALING:AFOCUSON
understoodbystudyinglocalpropertiesalone,complexsystems
CHEMICALSIGNALS
mustbe analyzed atmultiplescales. Second, since novelprop-
erties at higher scales depend on the interactions between the Wefirstdiscussthemainsignalingmoleculesinvolvedinsettingup
individual elements, it is essential to consider the properties of and maintaining patterning at the SAM. This includes two major
ShootMeristemGrowthandDevelopment 3of13
Figure2. Cytokinin,Auxin,andMechanicalForcesProvideInputsThattheCellRespondstobyRegulatingaRangeofCellularProcessesThatAffect
Growth.
Auxin-induced pathways are indicated by magenta arrows, cytokinin-induced pathways are indicated by green arrows, and pathways induced by
changesinthemechanicalforcesexperiencedbythecellareindicatedbybluearrows.Blackarrowsindicatepathwaysthatmaybeinducedbymore
thanoneinput.Notethatmanyprocessescanberegulatedbymorethanonepathwayandthatsomepathwaysfeedbackinsuchawayastochange
theinput,forexample,auxinconcentrationinthecellwallleadstochangesinauxintransportandthereforeaffectsauxinconcentration.Itisalso
importanttonotethatgrowthofthecell,andthosearoundit,willaffectbothmechanicalforcesandhormonegradients.Therefore,thesystemmustnot
onlybeconsideredovermultiplelevelsoforganizationbutalsoasadynamicsystemovertime.
hormones,cytokininsandauxins.Otherclassesofplanthormones, SHOOT MERISTEMLESS (STM), a homeodomain transcription
including brassinosteroids, gibberellins, and strigolactones, have factor of the Class 1 KNOX gene family, plays a major role in
been associated with SAM function (reviewed in Alabadí et al., SAMfunction(Longetal.,1996).STMisexpressedthroughout
2009),buttheirrolethereislesswelldefined.Therefore,theywill the nonorganogenic regions of the SAM but excluded from
notbediscussedinmoredetail. regionsofprimordiaformation.ItisessentialbothforSAMes-
tablishment during embryogenesis and its subsequent mainte-
CytokininsandPatterningoftheMeristemProper
nance,sinceitmaintainstheenvironmentinwhichthestemcell
Cytokinins have been associated with the SAM proper (i.e., the nichecanbesustained(Scofieldetal.,2008).STMactsinpart
peripheralandcentralzones).First,thereisastronglinkbetween by promoting cytokinin synthesis byincreasing ISOPENTENYL
cytokininsandtheundifferentiatedstateofthemeristematiccellsin TRANSFERASE (IPT) gene expression (Jasinski et al., 2005;
thecentralzone.ThecentralregionsoftheSAMhavehighcyto- Yanaietal.,2005).Conversely,expressingIPTundercontrolof
kininlevelsandactivatedcytokininresponses,whichappeartobe the STM promoter can partially rescue some aspects of stm
importanttosustainthestem cell population(Shanietal.,2006). mutant phenotypes, as can external applications of cytokinin
This has been studied in detail in Arabidopsis. In this species, (Yanaietal.,2005).
4of13 ThePlantCell
Cytokininsignalinghasalsobeenassociatedwithmaintaining the SAM. The most abundant form, indole-3-acetic acid (IAA),
thespatialboundariesofthecentralzoneofthemeristem.Inthis is not able to diffuse freely from cell to cell, and its transport
region,alocalsignalingloopoperatesinthemaintenanceofthe throughouttissuesisfacilitatedbyauxinimportersandexporters
stemcellpopulationatthemeristemsummit.Thismaintenanceis at the plasma membranes (recently reviewed in Grunewald and
directedbyanorganizingcenterintheinternallayersofthemer- Friml, 2010). In particular, the auxin exporters (transmembrane
istem, comprised of cells expressing the transcription factor proteins of the so-called PIN-FORMED or PIN family) show
WUSCHEL(WUS)(Mayeretal.,1998;Figure1C),whichpromotes a polar localization within individual cells. Since the localization
stem cell identity in the overlying cells. It also induces them to betweenneighboringcellsisoftencoherent,ithasbeenassumed
express the peptide ligand CLAVATA3 (CLV3), which via in- thatthesetransporterscreatefluxesofauxinthroughthetissues,
teractionwiththereceptor-likekinaseCLV1,feedsbacktoinhibit causingtheformationofauxinmaximaandminima.
WUSexpressionandmaintainastemcellpoolofaconstantsize The first auxin exporter to be identified was PIN-FORMED1
(Schoofetal.,2000).Thisfeedbackloopinteractswiththedown- (PIN1) (Okada et al., 1991; Gälweiler et al., 1998) in Arabidopsis.
stream signaling network of cytokinin, comprised of membrane- The pin1mutanthas aneedle-likeinflorescence stem,dueto its
localizedreceptors,whichuponcytokininperceptionactivateboth inabilitytoinitiateorgansfromtheSAM.PIN1isstronglyexpressed
positive(B-type)andnegative(A-type)transcriptionalregulators intheSAM,whereitismainlyfoundintheoutermostcelllayerand
(ARRs) via a phosphorelay system (Hwang et al., 2012) and invasculartissues.Thepin1phenotypecanbecomplementedby
positions the WUS expression domain relative to the L1 layer local external auxin applications, which are able to induce the
(Gordonetal.,2009).EctopicexpressionofWUSrepressesthe formationofflowerbuds(Reinhardtetal.,2000).Theseinitialob-
negative A-type ARRs, which stimulate downstream cytokinin servationssuggestedthathighlocalauxinconcentrationsarere-
responses(Leibfriedetal.,2005).Conversely,overexpressionof quiredfortheinitiationofaneworganandthatPINtransportersare
an A-type ARR inhibits WUS expression. Cytokinins, therefore, required for the creation of such auxin maxima. Direct evidence
notonlycontributetomaintainingSAMcellsinthemeristematic demonstratingtheexistenceofauxinmaximaattheperipheryof
state,butarealsoinvolvedinregulatingthesizeofthestemcell themeristemisstilllackingduetotheabsenceofatruemarkerfor
population at the meristem summit. Computational modeling auxinconcentrations.However,thisscenarioissupportedbythree
demonstratesthatwhilethefeedbackbetweenWUSandCLV3 differentlinesofevidence.First,thepatternsofpolarlocalizationof
is sufficient to produce a stem cell nicheof a determined size, PIN1 (Wisniewska et al., 2006) suggest auxin fluxes directed to-
additionalfeedbackcreatedbythecytokininsignalingpathway wardtheyoungprimordia(Benkováetal.,2003;Reinhardtetal.,
must also be included to produce accurate positioning of the 2003;BarbierdeReuilleetal.,2006).Second,thesyntheticDR5
stem cell niche that is maintained when cell divisions are in- promoter, which is activated by auxin-responsive transcription
cluded(Chickarmaneetal.,2012). factors,isstronglyexpressedinveryyoungorgans,indicatingthe
Cytokinin also mediates light responses of the SAM. When activationofauxin-inducedgenesduringtheearlystagesoforgan
shoot apices are grown in the dark, organ primordial are not initiation (Benková et al., 2003; Barbier de Reuille et al., 2006).
formed. Application of cytokinin alone is sufficient to increase Third,arecentlydevelopedperceptionsensorthatisdegradedin
apicalgrowthandleadtoinitiationofprimordia(Yoshidaetal., thepresenceofhighauxinconcentrationsconfirmsthepresence
2011).Thissuggeststhatcytokininstimulatesgrowth,whichcan of auxin maxima at the sites of organ formation (Vernoux et al.,
thenberedirectedtoorganinitiationbyauxin. 2011).However,itshouldbenotedthatinthevegetativeSAMof
Itisnotablethatauxinresponsesareexcludedfromthecentral pin1 mutants, leaf primordia are formed whose positioning is
region,evenwhenhighlevelsofauxinaredirectlyplacedonthe nonrandom, suggesting that further PIN1-independent mecha-
meristem tip (Reinhardt et al., 2003). Since combined modeling nismsexist(Guenotetal.,2012).
and experimental approaches have shown that auxin levels Asecondsetoftransportersassociatedwithauxindistribution
are high in the SAM center (Barbier de Reuille et al., 2006; attheSAMisthefamilyofAUX/LIKEAUX(LAX)influxcarriers(for
Vernoux et al., 2011), there must be active repression of auxin a thorough functional analysis, see Péret et al., 2012). The
responsesalongsidethehighlevelsofcytokininsignaling,butthe foundingmemberofthegenefamily,AUX1,isexpressedatthe
mechanism by which this is achieved is unknown. Conversely, outercelllayer(alsocalledL1)oftheSAM(Bennettetal.,1996;
at least in Arabidopsis, cytokinin responses are apparently re- Reinhardtetal.,2003).Theproteinseemstobeevenlylocalized
pressed when cells leave the meristem and are incorporated overallmembranesoftheL1,indicatingthatitisnotinvolvedin
in the young primordia, since young primordia express ARABI- the creation of hormone fluxes. Instead, it might concentrate
DOPSISPHOSPHOTRANSFERPROTEIN6(Bartrinaetal.,2011). auxinatthemeristemsurface.TheAUX1sequenceshowsahigh
Thisencodesanonfunctionalphosphotransferproteinthatneg- degreeofsimilaritywiththreeothersequencesintheArabidopsis
ativelyinterfereswiththecytokinin-activatedphosphorelaychain genome, named LAX1, LAX2, and LAX3 (Swarup and Bennett,
mentionedabove(Mähönenetal.,2006). 2003), but even the quadruple mutant is still able to produce
aviable,moderatelyfertileplant,albeitwithsignificantchangesin
architecture.Itisthereforeconceivablethatotherproteins,such
PatterningattheMeristemPeriphery:ACentralRolefor
as certain P-Glycoprotein transporters, which have also been
AuxinandAuxinTransport
associatedwithauxinimport,sharearedundantfunctionwiththe
Manylinesofevidence,includingexperimentsinvolvingthepre- membersoftheLAXfamilyinthemeristem(Bouttéetal.,2007).
ciseplacementofauxinontheSAMsurface,showthatauxinis Anotherclue onthe precise role ofAUX1 comes froma double
themajorsignalassociatedwithorganinitiationandpositioningat pin1aux1mutant.Auxinapplicationsdonotinducesingleflowers
ShootMeristemGrowthandDevelopment 5of13
ontheapexofsuchadoublemutant,butratherverylarge,fused PINendocytosisandproductionwerecombinedwithextracellular
organs, suggesting that somehow AUX1 is required for the auxin signaling. This model was able to reproduce several pat-
restrictionoforganboundaries(Reinhardtetal.,2003).Altogether, terning processes. In parallel, Heisler et al. (2010) proposed a
theavailabledataindicatethattheformationoflocalauxinmaxima model involving wall mechanics to explain transport against the
mainly depends on the action of PIN exporters at the meristem gradient(seealsobelow).
surface. AUX and LAX proteins act to facilitate organ positioning,
probablybyguaranteeingasufficientsupplyofauxinintheL1layer. BeyondAuxinMaxima
What happens once a localized auxin maximum is generated?
ModelsforPolarAuxinTransportRegulation:Canalization
Thehormoneappearstoactviatwodistinctpathways.(1)First,it
orUp-the-Gradient?
interacts with AUXIN BINDING PROTEIN1 (ABP1), an auxin
What coordinates auxin fluxes in the meristem? The principles binding protein localized at the plasma membrane and endo-
plasmicreticulum(Robertetal.,2010;Xuetal.,2010).Theprecise
underlyingtheobservedtransportpatternscouldbeverysimple.
functionandregulationofABP1isstillpoorlyunderstood,butit
Jönsson et al. (2006) and Smith et al. (2006) proposed models
appearstomediaterapidcellularresponsesinvolvingmembrane
where cells check the auxin concentrations in their direct envi-
trafficandthecytoskeletonasdiscussedbelow.(2)Thesecond
ronment and subsequently pump auxin toward neighbors with
responsepathwayinvolvesacomplexnetworkoftranscriptional
a higher concentration (i.e., they move auxin against the con-
regulators(reviewedinLeyser,2006;GuilfoyleandHagen,2007;
centrationgradient).Becausethesepatterningprocessesrequire
Chapman and Estelle, 2009). This network is composed of the
the interaction of hundreds of cells, it is impossible to estimate
so-called auxin response factors (ARFs), transcription factors
on a purely intuitive basis if a particular scenario is plausible or
that activate or repress auxin-regulated genes. In the absence
not.Therefore, computational modeling wasused asa powerful
of auxin, ARFs themselves are inactivated by dimerization with
meanstotestthishypothesis.Interestingly,thesemodelsshowed
that transport against the gradient is sufficient to reproduce re- repressors of the Aux/IAA family (Guilfoyle and Hagen, 2007;
Szemenyeietal.,2008).Inthepresenceofauxin,theAux/IAAsare
alisticPIN1patternsandtogeneratedifferenttypesofphyllotactic
targetedtotheproteasomebyanubiquitinligasecomplex(Leyser,
patterns (Jönsson et al., 2006; Smith et al., 2006). It should be
2006;ChapmanandEstelle,2009)thatfunctionsasanauxinre-
noted that these models show that the mechanism is plausible,
ceptor. In this process, auxin promotes the interaction between
butbynomeansdotheyrepresentabsolute,mathematicalproof.
Aux/IAAproteinsandtheTIR1F-boxprotein(orhomologs)ofthis
Indeed,othermodelsareabletoexplainthedistributionofauxin
complex (Dharmasiri et al., 2005a, 2005b; Kepinski and Leyser,
transporters.Anotherhypothesis,forinstance,wasbasedonthe
2005;Tanetal.,2007).Theauxin-induceddegradationofAux/IAAs
pioneeringworkofSachs(1969),whoproposedtheexistenceof
a positive feedback between flux and transport. It was sub- thenreleasesARFstoregulatetranscriptionoftheirtargetgenes.
Vernoux et al. (2011) found that a subset of 13 ARFs and 12
sequently shown that, in principle, this mechanism is able to
amplify small fluxes and can potentially create canals of auxin AUX/IAAs were coexpressed at the SAM. Interestingly, most of
thesegenesweremorestronglyexpressedattheperipheryofthe
betweenhormonesourcesandsinks(Mitchison,1981).Arangeof
meristemwheretheorgansareinitiated.Expressionwasmostly
experimentssupportsthecanalizationhypothesis,atleastinthe
weak at the meristem center. A second important observation
innertissuesoftheplant,whereitcanaccountfortheformationof
concerned a large-scale two-hybrid interaction analysis, which
veinationpatterns(Scarpellaetal.,2004,2006;Saueretal.,2006).
suggested a very simple network structure, where the 25 ARFs
Note that this would imply the coexistence of two radically dif-
andAux/IAAsfunctionasadouble-brakesystem.Intheabsence
ferentmechanismsforPINallocationinthemembrane,onebased
onfluxsensing(intheinnertissues)andtheotheronlocalcon- of auxin, ARF transcriptional activators are predicted to be in-
hibitedbothbytheinteractionwithAux/IAAsandcompetitionwith
centration sensing (at the meristem surface). Therefore, Stoma
a second group of ARFs that function as transcriptional re-
et al. (2008) tested whether canalization could potentially also
pressors.Bycontrast,inthepresenceofauxin,theARFactivators
accountforthebehaviorofauxintransportersattheSAMsurface.
arereleasedfromtheinhibitionbyAux/IAA,butstillmustcompete
Usingadedicatedcomputersimulationtool,theyshowedthatthis
withARFrepressorsforspecificsites.Theverysimplestructureof
was indeed the case, thus providing a unifying concept for the
thisnetworkprovidesrobustnesstothesystem.Inaddition,the
controlofauxindistributionintheplant(Stomaetal.,2008).More
strongexpressionlevelsatthemeristemperipherysuggesthigh
recently, Bayer et al. (2009) tested a third scenario where both
auxinsensitivitythere.
modes of auxin transport (i.e., up-the-gradient and canalization)
coexist.Oneortheothermechanismwouldprevail,dependingon
the absolute auxin concentrations (Bayer et al., 2009). Further
FROMSIGNALSTOGROWTHPATTERNS:FROM
experimentsarenowrequiredtodistinguishbetweenflux-based
BIOCHEMISTRYTOPHYSICS
andotherhypotheses.Testingofthemodelswillfirstofallrequire
identification of the molecular mechanism controlling PIN1 po- Hormonesfirstinteractwithperceptionanddownstreamsignaling
larity.Itwillthenbepossibletodetermineifthesemechanismsare pathways involving a network of receptors and transcriptional
compatiblewithanyoftheproposedhypotheses.Someattempts regulators. Next, these regulators must somehow control local
to construct more mechanistic models (including cellular pro- growthrates,buthowisthisachieved?Howdolocalbiochemical
cesses) have already been made. Wabnik et al. (2010), for ex- activitiestranslateintoparticularchangesingeometry,andwhat
ample,proposedaflux-basedmodelwhereinauxinfeedbackson feedbacksexistbetweengrowthandgeometry?Thesequestions
6of13 ThePlantCell
are further complicated by the fact that a growing biological Shapechangesdependnotonlyongrowthrates,butalsoon
system is not simply a geometrical structure in which chemical growthdirections,forexample,growthanisotropy,whichinturn
gradients and molecular networks operate. It is also a physical depends on the orientation of the cellulose microfibrils in the
structuregovernedandconstrainedbymechanicalcues.There- extracellular matrix (i.e., the cell wall texture). Walls with highly
fore,ifwewanttounderstandhoworgansareinitiated,wealso oriented cellulosefibers tendtogrowperpendicular tothe main
needtotakeintoaccountthephysicalpropertiesofthesystem.In fibrildirection.Celluloseissynthesizedbymembrane-bounden-
plants,thismeansunderstandingthenatureoftheplantcellwall. zymecomplexes,andthereisstrongevidencethatthemicrotu-
bular cytoskeleton guides these enzymes and thuscontrols cell
walltexture.Howpreciselythisoccursisstillnotwellunderstood,
TheCellWallasaCentralElementinGrowthControl
butitseemsthatmicrotubules,throughtheirtightassociationwith
How do the individual cells of a tissue grow? Mechanistically, theplasmamembrane,somehowrestrictthemovementsofcel-
plantcellsareunderhighinternalturgorpressure.Theyarepre- lulosesynthasecomplexes(Bringmannetal.,2012).Whatregu-
vented from bursting by the dense network of cellulose micro- latesthestructureofthemicrotubulecytoskeletonisalsounclear,
fibrilsthatarecross-linkedtoeachotherbyotherpolysaccharides but recent evidence indicates that microtubule dynamics and
withinthecellwall(Keegstra,2010).Cellexpansioncanonlytake stability couldplay a role. Studies wheremicrotubuleswere de-
placeaslongasthecellsareunderpressure,andthereisstrong polymerized showed that organs still tend to grow out but that
evidence that the irreversible yielding of the cell wall to this organboundariesarenolongerdefinedphysically,showingthat
pressureisinextricablylinkedwithcellgrowth.Awidelyaccepted cell anisotropy is required for tissue folding (buckling) between
hypothesis proposes that, for a given turgor pressure, cell ex- floral primordia and the rest of the meristem. A second role for
pansion is driven initially by cell wall loosening and subsequent wallanisotropyisinthestemofthegrowinginflorescence,which
remodelingandsynthesis.Ingrowingleaves,thishasbeenlinked loses its cylindrical shape when microtubules are removed
toacidificationofthecellwall(Dale,1986)andtheactivityofcell (Hamantetal.,2008).Thesedatashowthatwhiletheorientation
wall–loosening enzymes suchas expansins (Cosgrove, 1996)or of cell growth is not required for organ level patterning, it is
pectin-modifyingenzymes.Expansins causecell wall relaxation, necessaryforitscorrectelaborationthroughmorphogenesis.This
which appears to involve the disruption of hydrogen bonds be- exemplifies the principle that instructions (patterns) can be es-
tweencellulosemicrofibrilsand cross-linkingglycansinthewall tablishedatahigherlevelbutcanonlybeimplementedthrough
(reviewedinLietal.,2003).Importantly,classicalexperimentsby thecorrectbehavioroflowerlevelelements.
Fleming et al. (1997) showed that local, external applications of
expansins could also induce outgrowths on tomato (Solanum
HormonalRegulationofCellWallProperties
lycopersicum)meristems,andseveralexpansingenesareindeed
expressedathighlevelsinyoungorganprimordia.Morerecently, Theprimaryhormonelinkedwithwallmodificationsisauxin.Ev-
Peaucelle et al. (2008, 2011) also showed a link with pectin- idencesuggeststhatauxincanaffectcellwallstructurethrough
modifying enzymes. Pectins, which form the amorphous matrix bothtranscriptionalandnontranscriptionalpathways,althoughit
surrounding the rigid cellulose microfibrils, are believed to play isimportanttoappreciatethatmuchofthedataoncellgrowthin
a major role inthe definitionofcellwall stiffness,depending on plants is derived primarily from expanding cells outside of the
thedegreeofmethylationofthepectins.Indeed,Peaucelleetal. meristematic regions, and the general applicability of these
(2008)foundthatpectinmethylesterase(PME)activityinthewall mechanismstomeristemcellsisnotcertain.
iscorrelated with organ initiation. Inhibiting PMEspreventedor- Auxin-mediated growth has classically been described by
ganoutgrowthcausingtheformationofpin-likeapices. the acid growth hypothesis, which proposes a nontranscrip-
In summary, the data suggest that a combination of wall- tional mechanism for the acidification-linked loosening of the
modifying activities causes organs to bulge out as a conse- wall.Inexpandingnonmeristematiccells,wherecellgrowthis
quenceofchangestheyproduceinthephysicalpropertiesofthe largely associated with fluid uptake and vacuole expansion,
wall(Figures1Hto1J).Todemonstrateeffectivelythatthisisthe applicationofexogenousauxinsrapidlyinducesH+extrusion
case, several groups have started directly to determine these from the cytosol into the cell wall due to auxin-induced acti-
physicalparameters,inparticularwallstiffness.Thisworkisstillin vation of plasma membrane H+-ATPases through phosphory-
anearlyphaseandhassofaronlyprovidedapartialview,which lation(Takahashietal.,2012)andreducedendocyticrecycling
neverthelesstendstoconfirmthatpatterningatthemeristemis (Paciorek et al., 2005). Cell wall–modifying expansin proteins
correlatedwithchangesinwallstiffness.Inparticular,theslow- show a measurable increase in activity within the pH range
growingcellsatthetipofthemeristematicdomehavewallsthat created in the cell wall by this movement of H+ (McQueen-
areprobablystifferthanthoseattheperiphery(Milanietal.,2011; Mason et al., 1992), so the electrophysiological response to
Kierzkowskietal.,2012),whereasorganoutgrowthiscorrelated auxin can be linked to a known mechanism of cell wall loos-
with a reduced elasticity, in particular in the inner layers of the ening.ThemovementofH+alsostimulatesuptakeofK+from
meristem (Peaucelle et al., 2011). However, it should be cau- thecellwallintothecytosolthroughtheactivationofvoltage-
tionedthatexperimentsshowingthatcellwalllooseningcanin- gated ion channels (Philippar et al., 1999; Bauer et al., 2000).
duce cell growth (e.g., in transgenic lines) are not proof of This inward movement both rectifies the hyperpolarization of
causation in normal growth. This will require elucidation of the the membrane caused by the movement of H+ and serves to
interaction mechanisms between cellwallgrowth and other as- lower the osmotic potential of the cytosol such that water is
pectsofcellulargrowth. drawnintothecellandturgorpressureincreased.
ShootMeristemGrowthandDevelopment 7of13
Ithaslong beensuggested thatABP1 functionsas an auxin tissueiselastic;(2)theouterepidermalwallisunderauniformturgor
receptor in the acid growth pathway. Electrophysiological pressuregeneratedbytheinternaltissues;and(3)this,inturn,
responsesarealteredincellsthatunder-oroverexpressABP1 limits meristem growth (Hamant et al., 2008, and references
(Rucketal.,1993;Leblancetal.,1999)andanABP1knockout therein).If theseassumptionsarecorrect,themeristemcanbe
mutant appears to have defects in cell growth during embryo- considered as a pressure vessel, a shell inflated by an inner
genesis (Chen et al., 2001). The exact function of ABP1 has pressure.Sincethephysicalpropertiesofthesetypesofobjects
remainedenigmatic,butsignificantprogresshasrecentlybeen are well known, a theoretical distribution of the principalstress
madeindemonstratingthatcellwall–localizedABP1isrequired directions can be calculated that largely depends on the ge-
for auxin to regulate clathrin coat–mediated endocytosis of ometry of the system. Interestingly, these predicted patterns
plasma membrane–localized proteins, including H+ ATPase atthemeristemsurfacematchedthoseofthemicrotubulesnext
(Pacioreketal.,2005;Robertetal.,2010). totheouterepidermalwall:forexample,(1)randomlyorientedat
The acid growth hypothesis offers an explanation for rapid thedomeoftheSAMandyoungprimordia,(2)orientedalongthe
cellular expansion. However, itisacceptedthata transcriptional organboundaries, and (3)perpendicular to the longaxis of the
responseisrequiredtosustaingrowthoverlongerperiodsoftime growing stem. This led to the hypothesis that microtubules
sinceasupplyofcellwallproteinswillbeneeded.Manymutants somehowalignalongthemainforcedirections.Sincetheyorient
associatedwiththeAux/IAApathwayhavedwarfedphenotypes, the cellulose microfibrils, they would reinforce the wall aniso-
indicating reduced auxin-induced growth, including the axr3-1 tropically to resist the forces. This would tend to reinforce or
mutant, a gain-of-function mutation of IAA17 that prevents its maintainthetissuestressesand,asaresult,causetissuefolding
degradationinresponsetoauxin(Leyseretal.,1996;Worleyetal., atorganboundariesandguaranteetheformationofacylindrical
2000).Transcriptomicanalysisoftheaxr3-1mutantidentified108 stem. In this context, by providing directional, positional in-
genesthatwererepressedinthemutantcomparedwiththewild formation to the cells, the physical forces would function as
type, of which 28 were associated with roles in cell wall bio- a signal able to coordinate growth over long distances. This
synthesisandremodeling.Normally,inresponsetoauxin,IAA17 hypothesiswasfurthersupportedbyseveralexperiments.First,
would be degraded and the repression of these genes relieved. applying external mechanical forces to the meristem caused
Theseregulatedgenesincludedbothstructuralcellwallproteins changesintheorientationofthemicrotubulesoveraperiodof4
andproteinswithcellwall–modifyingactivities,suchasexpansins to 6 h. Second, similar changes of the microtubules were ob-
andPMEs(Overvoordeetal.,2005),which,asmentionedearlier, served along predicted stress patterns around ablated cells
havebeendemonstratedtobeassociatedwithorganinitiationin (Hamantetal.,2008;Heisleretal.,2010;Uyttewaaletal.,2012).
theSAM(Pienetal.,2001;Peaucelleetal.,2008,2011). This behavior of the microtubules has some interesting impli-
Understanding the different pathways through which auxin cationsforauxinsignaling.Heisleretal.(2010)observedthatthe
modifiesthecellwallhighlightsanimportantpoint,namely,that mainorientationofthemicrotubulararraysisfrequentlyparallelto
the structure of the cell wall can be regulated by cytoplasmic the membranes that bear PIN proteins. In addition, PIN proteins
processes, such as vesicular trafficking and protein synthesis. movetomembranesthatareparalleltomicrotubulereorientationin
Coordinationofprocessesoccurringinthecytoplasmandinthe responsetocellablationswithsimilardynamics.Therefore,there
cellwallisthereforeimportantforcellgrowth,andwewillreturn seems to be some type of coupling between the organization of
latertotheroleofthecytoplasmingrowth. the cytoskeleton and auxin transport. In this context, it is in-
teresting to note that in leaf epidermal pavement cells, actin-rich
domains and domains where microtubules are attached are well
GrowthCoordination:ARoleforMechanicalSignals?
separated(Xuetal.,2010).Interestingly,theactin-richdomainsare
So far, we have mainly considered the physical properties and also labeled by PIN proteins in these cells. It was proposed that
resulting growth patterns as an output of chemical patterning. thisseparationofmicrotubuledomainsandactin/PIN-richdomains
However, evidence has started to accumulate indicating that was regulated by small Rho GTPases. If such a system indeed
physicalpropertiesthemselvescouldfeedbackonthechemical operates in the SAM, it is tempting to propose that the effect of
gradientsandcellularstructures. mechanics on cytoskeleton organization also feeds back on cell
Different parts of the meristem grow at different rates, and even polarityandauxintransport,althoughthisremainstobeproven.
neighboring cells can have very different expansion rates. This is Althoughthemechanicalfeedbackhypothesisisattractive,itis
typicallythecaseattheorganboundaries,wherecellsgrowmuch alsofacingsomemajorchallenges.Inparticular,itisnotobvious
moreslowlythantheirdirectneighborsintheyoungformingorgan howtheforcesaresensedandtranslatedbythecells.Aglimpseof
(Kwiatkowska and Dumais, 2003). As a result, there needs to be whatasensingmechanismmightlooklikewasobtainedbyana-
some mechanical compromise between these neighbors. In other lyzing the botero mutant in Arabidopsis (Uyttewaal et al., 2012).
words, the overall tensions generated in growing tissues have the BOTERO encodes a katanin (i.e., a microtubule-severing protein
potential to feed back on local expansion rates (for review, see involvedinmicrotubulebundling).Knockingoutthegenereduces
HamantandTraas,2010).Hamantetal.(2008)proposedahypoth- considerablythecapacityofmicrotubulestoorganizeintoordered
esis where patterned fields of forces at the meristem surface in- arrays and simultaneously reduces growth coordination at the
fluencecellwallandmicrotubuleorganization.Tothisend,theyhad meristem.Asaresult,thesurfaceoftheSAMbecomeshighlyir-
to consider the overall mechanical properties of the meristem. Al- regularandthemutantisnolongerabletoformwell-definedorgan
thoughthesearenotwellcharacterized,anumberofpropertiesofthe boundaries.Partoftheforce-sensingmechanismcouldtherefore
meristemhavebeenproposedintheliterature:(1)themeristematic be based on microtubule dynamics. Another element involved in
8of13 ThePlantCell
such a mechanism could be the cell wall itself. Obviously even Evidenceisbeginningtosuggestthatmechanismsanalogous
minutechangesinstresspatternswillimmediatelycausechanges tothosefoundinanimalsarelikely alsotoberelevanttoplant
inwallstructure,includingchangesintherelativepositionsofthe cells. Indeed, plants have both the TOR pathway (Garrocho-
cellulosefibers.Thisinformation,stored,asitwere,inthecellwall, Villegas and Sanchez de Jiménez, 2012; Robaglia et al., 2012;
could potentially be transmitted to the plasma membrane and XiongandSheen,2012)anditsimportanttargetS6K(Henriques
thencetothecytoplasm. etal.,2010),andTORhasbeenshowntohaveagrowth-limiting
function in plant cells as in animals (Henriques et al., 2010).
SIGNALINGANDGROWTH:BEYONDTHEWALL Phosphorylation of S6K has been used as an assay of TOR
activitytoidentifyregulatorsofthepathway.InArabidopsiscell
culture, S6K activity and ribosome biogenesis are upregulated
GrowthandCytoplasmicSynthesis
uponremovalofstarvation(Turcketal.,2004).Thisisconsistent
Thepreviousparagraphsmightgivetheimpressionofarelatively withtheproposedancestralrolefortheTORpathwayinnutrient
straightforward scenario where growth patterns depend on wall sensingandresponse.
loosening,synthesis,andcellulosemicrofibrilorientation,largelyin Increasing evidence is also linking auxin to changes in these
response to auxin signals. The situation is more complex, how- cytoplasmic aspects of cell growth. Auxin increases the overall
ever,asallcellsarefilledwithcytoplasm.Therefore,growthisalso protein synthetic capacity of the cell due to multiple effects, in-
associatedwithanincreaseincytoplasmrequiringtheproduction cluding increasing the rate of translation, the accumulation of
of macromolecules, primarily proteins, itself dependent on ribo- rRNA and of ribosomal proteins, as well as their recruitment to
some number and activity. It is unclear whether this could be ribosomalcomplexes(Garcia-Hernandezetal.,1996).Auxinalso
triggered by cell wall–derived processes or whether alternative regulatesthephosphorylationofribosomalproteinsbyS6K(Turck
pathways coordinate overall metabolic activities, including wall et al., 2004), and this auxin-dependent activation of S6K is de-
synthesis. In other words, does cell wall expansion drive the in- pendentonPI3kinasejustastheIGFpathwayisinanimals(Turck
ternalprocessesorviceversa?Onedirectlineofevidencecomes etal.,2004).Downstream targetsoftheTORpathway,including
fromthehighlylocalizedinductionofexpansingeneexpressionon EBP1andtranslationallycontrolledtumorprotein(TCTP),arealso
tobacco(Nicotianatabacum)SAMs,whichledtotheinductionof conserved in plants and have functions in stimulating protein
normalleafprimordia(Pienetal.,2001).Inductionontheflankof synthesis and growth. TCTP acts as the guanine nucleotide ex-
anexistingprimordiumalsoledtoanoutgrowthofthedeveloping change factor of the Ras GTPase that controls TOR activity in
leaf blade. This might suggest that cell wall loosening is driving Drosophila melanogaster, and its reduced level in Arabidopsis
growthandthatthesynthesisofcytoplasmsimplyfollows.How- causesreducedgrowth(Berkowitzetal.,2008).EBP1expression
ever,therearemultiplelinesofevidencesuggestingthatthisisnot inplantsiscorrelatedwiththeexpressionofgeneswithfunctions
necessarilythecase.InhibitionofDNAsynthesis,forexample,also inribosomebiogenesis,andithasbeenshowntoaffectgrowthin
arrestsgrowth(Grandjeanetal.,2004)and mutantsperturbedin adose-dependentmanner.DegradationofEBP1proteinhasbeen
diverse metabolic activities also show reduced size. Therefore, shown to be regulated by auxin; when auxin is present, degra-
thereisnodoubtthatgrowthrequiresmultiplefeedbacksbetween dationofEBP1isinhibited(Horváthetal.,2006).
thecellwallandcytoplasmicsynthesis.
The coordination between metabolic activity and growth has
GrowthandCellDivision:AnotherLevelofComplexity
beenintensivelystudiedinanimalsbecauseofitscloserelation-
shipwithcelltransformationleadingtotumorigenesis(Jorgensen Themeristemisdistinguishedbyanotherimportantcharacteristic,
andTyers,2004).TheTORkinaseplaysakeyroleinstimulating whichiscelldivision.Thelinkbetweengrowthandthecellcycle
growth in response to nutrient signals (yeast) or growth factors hasbeensubjecttodebate.Ontheonehand,celldivisionisoften
(metazoans) acting by multiple routes. These include increasing used as synonymous to growth; onthe other hand,ithas been
theproductionofribosomesthroughmultiplemechanisms,such suggestedthatcelldivisionisratherunimportanttoplantgrowth,
asstimulatingrRNAandribosomalproteinsynthesis,aswellasby servingsimplytosubdividetissuespaceintoconvenientlysized
activatingribosomalproteinS6kinase(S6K).Thephosphorylation units(KaplanandHagemann,1991;Kaplan,2001).Nevertheless
ofS6enhancesthetranslationofasubsetofRNAs,andS6Kalso thereappearstobeacloseinterplaybetweengrowthanddivision
activates eukaryotic initiation factor 4E to stimulate translation. intheSAM,sincemeristematiccellsizeismaintainedwithinquite
LossoffunctionofTORoritsdownstreameffectors,includingS6K tight boundaries as a consequence of cell division activity. The
and ribosomal protein genes, reduces cell growth. These effects precise coordination of cell division and growth in the SAM
are clearly seen in minute flies with crippled ribosomal protein suggeststhatitmayplaysignificantroles.Thiscouldbelinkedto
productionaswellass6kmutants.Intheseexamples,theeffects coupling between growth and the cell cycle or could be con-
oncellsizearedifferent,withminuteflieshavingnormalsizedcells nectedwiththesignificanceofthenewcellwallsandmembranes
ands6kverysmallcells,suggestingS6Kalsoaffectsthecoupling generatedduringdivision.Thesecreatenewmechanicalelements
ofcellgrowthanddivision(JorgensenandTyers,2004).Signaling thatmaybeimportantinmorphogenesisoroverallstructurebut
toTORinanimalsoccursthroughreceptors,suchastheIGFre- alsoallowauxingradientstobeestablished,sincethesearebuilt
ceptor via PI3 kinase, PDK1, and Akt. An alternative pathway in up through the polar concentration of PIN proteins in specific
animals involves Ras and myc, which promotes ribosome syn- areasoftheplasmamembrane.Inaddition,whilecelldivisionmay
thesis as well as directly regulating genes encoding metabolic notbeimportantinmorphogenesisprocessesinmeristemswith
enzymesandotherproteinsrequiredforcellgrowth. theexceptionofspecificformativedivisionsofinitialcells(Sozzani
ShootMeristemGrowthandDevelopment 9of13
et al., 2010), overall growth is strongly dependent on cell pro- important regulatory mechanism within the cell cycle, and cell
ductionrateasexemplifiedinplantsdwarfed by bonsaipruning responses to auxin also involve proteolysis. In this regard, it is
(Korner et al., 1989) or by a jasmonate-dependent response to notablethatauxinhasbeenlinkedtochangesinthestabilityof
herbivores(ZhangandTurner,2008). cellcycleregulators,promotingturnoverofthenegativeregulator
The inter-relationship between cell growth and the cell cycle KRP2 (Sanz et al., 2011) and, conversely, the stability of cell
is complex and much discussed. In other systems, three basic cycle–promoting E2FB. This provides a mechanism for auxin to
relationships have been described, involving variously (1) cell stimulateprogressionofthecellcycleintoS-phase;indeed,ec-
growthtriggeringdivisionatathresholdcellsize,(2)independent topic E2FB expression can remove the auxin dependence of
controls of growth and cell division, or (3) common upstream cultured tobacco BY-2 cells (Magyar et al., 2005). Auxin also
pathways(JorgensenandTyers,2004).Inanimals,examplescan promotesdegradationofdifferentiation-enhancingE2FC,aswell
befoundofallthree,whereasinyeast,theprimarymechanismof as the F-box protein SKP2A involved in E2FC turnover, in the
triggering division is cell size and nutrient sensing (reviewed in lattercasebydirectbindingofauxininamechanismanalagousto
Jorgensen and Tyers, 2004). Upstream metabolic activity is TIR1 activation. Hence, auxin simultaneously promotes degra-
measured by overall protein translation rates through the syn- dation of E2FC and of its degradation machinery (Jurado et al.,
thesis of unstable regulatory components, such as cyclins. In 2010).Itseemsclearthatauxinplaysimportantregulatoryrolesin
animals,transcriptionalcontrolofD-typecyclins(CYCD)andthe thecellcyclebyregulatingthedegradationofproteinsotherthan
subsequent activation of their partner kinase CDK4 promotes Aux/IAAs and that auxin both interacts with the control of cell
division by initiating phosphorylation of the retinoblastoma (RB) growthanddirectlyimpingesonaspectsofcellcyclecontrol.
protein,whichinturnactivatesE2Ftranscriptionfactors(Coqueret, Classically, cytokinin was identified as a plant hormone re-
2002). The same CYCD control in Drosophila appears to act quiredtosustaincelldivisionandinhibitdifferentiationinplantcell
mainlybypromotingcellgrowth(Meyeretal.,2000). cultures.Theprimarymechanismofinteractionofcytokininswith
Heterodimeric complexes of E2F proteins with their dimer- the cell cycle is through transcriptional regulation of the D-type
ization partners (DP proteins) are important in regulating the ex- cyclins of the CYCD3 class (Soni et al., 1995; Riou-Khamlichi
pression of genes requiredforthe entry into the S-phaseofthe etal.,1999),whoseectopicexpressioniscapableofdrivingcells
cell cycle as well as other cell cycle–related processes and cell into the cell cycle (Dewitte et al., 2003; Menges et al., 2006).
growth.InArabidopsis,E2FAandE2FB(withDPA)promoteentry CYCD3 activity is required to sustain normal response of both
intoS-phase,whileE2FCanditspartnerDPBrepresscellcycle divisionanddifferentiationtocytokinin(Riou-Khamlichietal.,
progression and are associated with differentiation and expan- 1999; Dewitte et al., 2007). CYCD3 activates cyclin-dependent
sion-driven growth outside the meristems (Inzé and De Veylder, kinase activity of CDKA (Healy et al., 2001), whose inhibition
2006).OverexpressionofE2FBpromotesprematureentryintothe causes differentiation of SAM cells (Gaamouche et al., 2010).
S-phase ofthecell cycle and thus reducesboththedurationof Unlike CDKA, CYCD3 is an unstable protein subject to rapid
thecellcycleandcellsize,indicatingthatitmaybeinvolvedinthe turnover(Planchaisetal.,2004),providingarapidresponsesen-
couplingofcellsizetocelldivision(Heetal.,2004;Magyaretal., sor to the presence of both cytokinin and Suc (Riou-Khamlichi
2005; Sozzani et al., 2006). Similarly, overexpression in Arabi- etal.,2000).TherearethreeCYCD3genesinArabidopsis,and
dopsisof the D-type cyclin CYCD3;1, the upstream regulator of all are strongly expressed in different regions of the SAM
theRB-E2Fpathway,reducescellsize,whereasmutationhasthe (Dewitteetal.,2007).
conversephenotype (Dewitteet al.,2003, 2007). Transcript pro- Bothauxinandcytokininthereforeappeartocontrolthemitotic
filingofinducedexpressionofE2Fsshowthatgenesinvolvedin cell cycle through the conserved CYCD-RB-E2F pathway that
cell wall biogenesis are wellrepresented among E2FA/DPA reg- regulates the progression of cells from G1-phase into the mitotic
ulatedgenes(Vliegheetal.,2003;deJageretal.,2009),including cellcycleinS-phase,usingdifferentmechanismsactingatdifferent
a number of expansins, pectinesterase, and pectin lyases, and points:cytokininthroughtranscriptionalregulationoftheupstream
somearemodulatedwithin6h,suggestingthattheymaybedi- componentCYCD3andauxinprimarilythroughthepromotingthe
recttargets(deJageretal.,2009). stabilityofthedownstreamE2FBtranscriptionfactor.
Auxinisabsolutelyrequirednotonlyforcellgrowthbutalsofor
CONCLUSIONSANDPERSPECTIVE
celldivisioninbothcellculturesandinplanta,butthemechanism
bywhichitactsonthecellcyclehasprovedsurprisinglyelusive.
FromSignalingtoShape
Could auxin act on division through an indirect effect of its role
in promoting cell growth? Indeed, although a number of cell A scenario emerges whereby, through a process of self-
cycleregulatorsandcyclin-dependentkinaseactivityhavebeen organization, plant hormones adopt robust and dynamic dis-
reportedtorespondtoauxin(Johnetal.,1993;Oakenfulletal., tribution patterns at the SAM. This distribution then leads to
2002),inmostcasesthisisonarelativelylongtimescale.How- patterns of growth. Major downstream targets of growth regu-
ever,auxindoesseemtohaveadirecteffectonseveralkeycell lationarethestructuralelementsofthecell,andinparticularthe
cycle regulators in the CYCD-RB-E2F pathway. Kip-Related cell wall, which is a determinant for both growth rates and
Proteins (KRP), also known as Inhibitors of CDK (ICKs), are in- growth distribution, but feedbacks with cytoplasmic synthesis
hibitors of CYCD activity, and expression of KRP2 is rapidly and metabolic networks are crucial as well. How this coor-
downregulated by auxin (Himanen et al., 2002). However, tran- dinationworksisnotwellunderstood.
scriptomicanalysishasnotidentifiedageneraltranscriptionallink Auxinappearstobethecommonlinkingfactor,promotingcell
betweenauxinandcellcycleregulators.Proteindegradationisan wallsynthesis,metabolicactivity,andcellgrowth,aswellascell
10of13 ThePlantCell
cycleactivity.Itrepresentsthebestcandidateforlinkingandco- regardingauxindistributionscouldonlybetestedyearslater,after
ordinating biochemical and morphogenetic aspects of growth, novel auxin sensors were developed. In a similar manner, pre-
sinceitisintimatelyinvolvedwithorganinitiationaswellashaving dictions regarding mechanics at the SAM will have to await the
cellulareffectsonbothgrowthanddivisionandcellwallsynthesis. developmentofnovelmethodstoprobetheseproperties.
Itisclearthattheseprocessesarehighlyinterconnected,anditis
probably not meaningful to talk about an upstream activator of
ACKNOWLEDGMENTS
eithergrowthordivision.Rather,atthecelllevelasattheorgan
level,theinterplayoffeedbackcircuitsprovidesarobustsystem ThisarticlewaspartiallyfundedbytheERASysBio+initiativeunderthe
thatlinkstheseprocessestogethertoensureorderlygrowthwithin EuropeanUnionEuropeanResearchAreaNetworkPlusschemeinFP7
the parameters of certain cell sizes. In contrast with auxin,cyto- through the program grant iSAM to A.J., J.A.H.M., J.T., and C.G. and
kininappearstolinkspecificallytocelldivisioncontrolandnotto aEuropeanResearchCounciladvancedgrant(Morphodynamics)toJ.T.
cell growth, but again we should note the interconnections be-
tweenauxinandcytokininresponses(Marhavýetal.,2011).
AUTHORCONTRIBUTIONS
In complement with chemical signaling, mechanics is also
likelytoplayaroleinpatterning.Forcesgeneratedbydifferential Allauthorscontributedtotheconceptionandwritingofthisreview.
growth rates and turgor pressure clearly have the potential to
generate positionalinformationfortheindividualcells,butpre-
ciselyhowthisoccursisfarfromclear.Thisispartiallybecause ReceivedJuly9,2012;revisedSeptember11,2012;acceptedOctober
the tools to probe forces and physical properties of the SAM 15,2012;publishedOctober30,2012.
haveyettobedeveloped.
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