plant cell size ref1-plant cell 2013

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AbscisicAcidRegulatesEarlySeedDevelopmentinArabidopsisbyABI5-MediatedTranscriptionofSHORTHYPOCOTYLUNDERBLUE1

ZhiJuanCheng,1XiangYuZhao,1XingXingShao,1FeiWang,ChaoZhou,YingGaoLiu,YanZhang,andXianShengZhang2

StateKeyLaboratoryofCropBiology,CollegeofLifeSciences,ShandongAgriculturalUniversity,Taian,Shandong271018,ChinaORCIDID:0000-0002-3129-5206(X.S.Z.)

SeeddevelopmentincludesanearlystageofendospermproliferationandalatestageofembryogrowthattheexpenseoftheendosperminArabidopsisthaliana.Abscisicacid(ABA)hasknownfunctionsduringlateseeddevelopment,butitsrolesinearlyseeddevelopmentremainelusive.Inthisstudy,wereportthatABA-de cientmutantsproducedseedswithincreasedsize,mass,andembryocellnumberbutdelayedendospermcellularization.ABSCISICACIDDEFICIENT2(ABA2)encodesauniqueshort-chaindehydrogenase/reductasethatfunctionsinABAbiosynthesis,anditsexpressionpatternoverlapsthatofSHORTHYPOCOTYLUNDERBLUE1(SHB1)duringseeddevelopment.SHB1RNAaccumulationwassigni cantlyupregulatedintheaba2-1mutantandwasdownregulatedbytheapplicationofexogenousABA.Furthermore,RNAaccumulationofthebasic/regionleucinezippertranscriptionfactorABSCISICACID-INSENSITIVE5(ABI5),involvedinABAsignaling,wasdecreasedinaba2-1.Consistentwiththis,seedsizewasalsoincreasedinabi5.WefurthershowthatABI5directlybindstotwodiscreteregionsintheSHB1promoter.OurresultssuggestthatABAnegativelyregulatesSHB1expression,atleastinpart,throughtheactionofitsdownstreamsignalingcomponentABI5.Our ndingsprovideinsightsintothemolecularmechanismsbywhichABAregulatesearlyseeddevelopment.

INTRODUCTION

Doublefertilizationinhigherplantsinitiatestheformationofseeds,whichhaveadiploidembryoandatriploidendospermthatdevelopinconcert.ThisdevelopmentalprocessconsistsofseveralstagesinArabidopsisthaliana(Baudetal.,2002).Fol-lowingfertilization,thebasicembryonicpatternbecomesestab-lished,whiletheendospermproliferates,undergoingasyncytialphaseandacellularizationphase(Boisnard-Lorigetal.,2001).Thetimingofendospermcellularizationcorrelateswithnuclearproliferationandthereforemayin uenceseedsize(Sundaresan,2005;Kangetal.,2008).Indeed,precociouscellularizationoftheendospermledtodecreasednuclearproliferationandseedsizereduction,whereasdelayedcellularizationpromotednuclearproliferationandincreasedseedsize(Garciaetal.,2003,2005;Luoetal.,2005).Followingendospermproliferation,intheearlymaturationstage,theembryogrowsrapidlybyabsorbingnu-trientsfromtheendosperm(Baudetal.,2002).Duringthelaststage,orlatematurationstage,theembryoappearsmetabolicallyquiescentandbecomestoleranttodesiccation(Baudetal.,

authorscontributedequallytothiswork.correspondencetozhangxs@sdau.edu.cn.

Theauthorresponsiblefordistributionofmaterialsintegraltothe ndingspresentedinthisarticleinaccordancewiththepolicydescribedintheInstructionsforAuthors(doc.guandang.net)is:XianShengZhang(zhangxs@sdau.edu.cn).Some guresinthisarticlearedisplayedincoloronlinebutinblackandwhiteintheprintedition.OnlineversioncontainsWeb-onlydata.canbeviewedonlinewithoutasubscription.doc.guandang.net

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2002),resultinginamatureseedcontainingtheembryo,wrappedwithanendospermandanoutsidecoat(Scottetal.,1998;HaughnandChaudhury,2005).

Anumberofgenesthatregulateintegumentdevelopment,embryonicpatternformation,andendospermdevelopmenthavebeendescribed(Leetal.,2010;Belmonteetal.,2013).MutantsofAUXINRESPONSEFACTOR2havelargeseedsthroughthepromotionofcelldivisionintheintegumentandotherorgans(Schruffetal.,2006).ThetranscriptionfactorsLEAFYCOTYLEDON2andFUSCA3playcriticalrolesinem-bryogenesis(Keithetal.,1994;Meinkeetal.,1994).AGAMOUS-like62regulatesendospermcellularization,anditsfunctionallossresultsinsmallseedsduetoprematureendospermcellulari-zation(Kangetal.,2008).ThesignalingregulatorSURROUNDINGREGION-RELATED8,togetherwithWUSCHEL-LIKEHOMEOBOX8,coordinatelymodulatesseedgrowthandsize(FiumeandFletcher,2012).FunctionallossofHAIKU2(IKU2)andMINISEED3(MINI3),encodingareceptor-likekinaseandaWRKYtranscriptionfactor,respectively,alsoproducessmallseedswithprecociousendo-spermcellularizationandreducedembryogrowth(Luoetal.,2005).Recently,IKU2andMINI3werefoundtobeinageneticpathwayregulatedbySHORTHYPOCOTYLUNDERBLUE1(SHB1),whosegain-of-functionmutantsproducelargerseedsduetoenhancedendospermproliferationanddelayedendospermcellularization(Zhouetal.,2009).

Thephytohormoneabscisicacid(ABA)mediatesseveralas-pectsofseeddevelopment,suchasbiosynthesisofstoragecompoundsintheembryo,seeddormancy,andtheinhibitionofprecociousgermination(McCarty,1995;Finkelsteinetal.,2002;Kannoetal.,2010).Duringseeddevelopment,ABAaccumulationexhibitstwopeaks.Duringearlyseedmaturation,ABAmainly

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