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JournalofExperimentalBotany,Page1of9

doi:

10.1093/jxb/err177

FLOWERINGNEWSLETTERREVIEW

Theroleofepigeneticprocessesincontrollingfloweringtime

inplantsexposedtostress

MahmoudW.Yaish1,*,JosephColasanti2andStevenJ.Rothstein2

1DepartmentofBiology,CollegeofScience,SultanQaboosUniversity,Muscat,Oman

2DepartmentofMolecularandCellularBiology,UniversityofGuelph,Guelph,ON,CanadaN1G2W1

*Towhomcorrespondenceshouldbeaddressed:

E-mail:

myaish@squ.edu.om

Received5April2011;Revised5May2011;Accepted9May2011

Abstract

Plantsinteractwiththeirenvironmentbymodifyinggeneexpressionpatterns.Onemechanismforthisinteraction

involvesepigeneticmodificationsthataffectanumberofaspectsofplantgrowthanddevelopment.Thus,the

epigenomeishighlydynamicinresponsetoenvironmentalcuesanddevelopmentalchanges.Floweringis

controlledbyasetofgenesthatareaffectedbyenvironmentalconditionsthroughanalterationintheirexpression

pattern.Thisensurestheproductionofflowersevenwhenplantsaregrowingunderadverseconditions,andthereby

enhancestransgenerationalseedproduction.Inthisreviewrecentfindingsontheepigeneticchangesassociated

withfloweringinArabidopsisthalianagrownunderabioticstressconditionssuchascold,drought,andhighsalinity

arediscussed.TheseepigeneticmodificationsincludeDNAmethylation,histonemodifications,andtheproduction

ofmicroRNAs（miRNAs）thatmediateepigeneticmodifications.Therolesplayedbythephytohormonesabscisic

acid（ABA）andauxininchromatinremodellingarealsodiscussed.Itisshownthatthereisacrucialrelationship

betweentheepigeneticmodificationsassociatedwithfloralinitiationanddevelopmentandmodifications

associatedwithstresstolerance.Thisrelationshipisdemonstratedbythecommonepigeneticpathwaysthrough

whichplantscontrolbothfloweringandstresstolerance,andcanbeusedtoidentifynewepigenomicplayers.

Keywords:

Epigenetics,flowering,stress,vernalization.

Introduction

Plantscanadapttheirgrowthanddevelopmentalprocessesin

responsetoenvironmentalconditions.Understresscondi-

tionssuchasdrought,highsalt,hightemperature,andhigh

lightintensity,physiologicalprocessesareinducedtoreduce

thecellulardamagecausedbystressand,atthesametime,

alterdevelopmentaltimingtocompletetheirlifecyclein

atimelymanner.Plantsthatexperiencestresstransitionto

reproductivedevelopmentearlierthannon-stress-treated

plants,typicallyattheexpenseofdecreasedseednumberthat

allowsforsomeseedproductiontooccurduringperiodsof

environmentalstress.Stressenvironmentalfactorsthatinduce

floweringhavebeendiscussedthoroughlyinapreviousreview

（WadaandTakeno,2010）.Mostnotably,salicylicacid,which

usuallyinducesdefencegenes,alsoinducesearlyflowering

underUV-Clightstress,presumablybyinteractingwith

keyfloraltranscriptionrepressorssuchasFLOWERING

LOCUST（FT）andothercomponentsoftheautonomous

floweringpathway（Martinezetal.,2004;Wadaetal.,

2010）.However,evidenceofphysicalinteractionisnotyet

available.Similarly,hightemperaturestressinducesflower-

ing,aprocessmediatedbyFLOWERINGLOCUSM

（FLM）andFT,whichintegratesinputfromCONSTANS

andotherfloralinductivepathways（Blazquezetal.,2003;

Balasubramanianetal.,2006）,whilestressassociatedwith

lownitrateavailabilityinducesearlyfloweringthrough

anovelpathway（CastroMarı´netal.,2010）.

Theeffectofstressonfloweringtimecanbeascribed,in

part,toinducedchangesintheepigenome.Epigenetics

referstoheritable,self-perpetuatingchangesingeneactivi-

tiesthatarenotcausedbychangesinnucleotidesequence

andareassociatedwithchemicalmodificationsofchroma-

tin（Bonasioetal.,2010）.Thesemodificationstakeplacein

ªTheAuthor[2011].PublishedbyOxfordUniversityPress[onbehalfoftheSocietyforExperimentalBiology].Allrightsreserved.

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thenucleosomeatdifferentlevelsthroughreversiblebio-

chemicalreactionsthatincludeDNAmethylationand

histonetailmodifications.DNAmethylationoccursby

covalentlyaddingamethylgrouptocytosineoftheDNA

backbone,whilehistonemodificationsoccurwhenspecific

lysineorarginineresidueswithintheaminoacidterminal

tailofhistonesarepost-translationallymodifiedeitherby

acetylation（Grunstein,1997）,ADP-ribosylation（Tanigawa

etal.,1984）,glycosylation（Cervantes-Laureanetal.,1996）,

methylation（ZhangandReinberg,2001）,phosphorylation

（Loetal.,2001）,ubiquitination（Sridharetal.,2007）,or

SUMOylation（Davidetal.,2002;Milleretal.,2010）.

Typically,DNAmethylationleadstoreducedgeneexpres-

sion,whereaashistonemodificationsaremorecomplicated

andcanleadtovariousgeneexpressionalterationsdepend-

ingonthemodification（RichardsandElgin,2002）.More

recently,itwasdiscoveredthatsmallnon-codingRNA

playsanimportantroleinregulatinggeneexpressionby

specifyingDNAmethylationpatterns（MatzkeandBirchler,

2005）.Eachofthesetypesofmodificationshavebeen

involvedinmodulatingflowering,andmanyoftheseare

associatedwithresponsetoabioticandbioticstress.

Plantsthatareadaptedtoextremeenvironmentshavethe

capacitytocopewithadverseenvironmentalcueswith

minimumcellulardamage.Eukaryoticcellsrespondtothe

environmentbymodifyingtheirgeneexpressionprofiles,

aprocesswhichusuallyinvolvesspecificchromatinmod-

ifications.Thecorrelationbetweenepigeneticchangesin

plantsandstresstolerancehasbeenpreviouslydiscussed

（BoykoandKovalchuk,2008;ChinnusamyandZhu,2009;

Alvarezetal.,2010;MChenetal.,2010）.Chromatin

modificationsbothinfluenceandareinfluencedbyother

responsestoabioticandbioticstresses.

DNAmethylation,environmentalstress,and

thefloweringprocess

DNAmethylation（5-methylcytosine）canaccountfor>30%

ofthecytosine（CpG）residuesinplants（Gruenbaumetal.,

1981a,1981b）and>60%inmammals（Gruenbaumetal.,

1981b;Razinetal.,1984）.InArabidopsis,DNAmethylationis

oftenassociatedwithgenerepression（Zilbermanetal.,2007）,

althoughsomereportsshowaweakrelationshipbetweenthe

hypermethylationstatusofthegenesandtheirexpression

level.Forexample,Vaillantetal.（2006）showedthat

expressionlevelsoftheArabidopsis5SrRNAgenerepeats

canbeincreasedintheMORPHEUS’MOLECULE1

（MOM1）mutantlinesdespitethepresenceofthesame

amountofDNAmethylation（Vaillantetal.,2006）.In

addition,recentevidenceshowedthatshort-termheat

stressinducedtheexpressionofgenesinheterochromatin

thatcontaintranscriptionallyinertnon-codingrepeated

DNA.Thisoccursdespitethepresenceofepigenetic

modificationsthataretypicallyassociatedwithrepression

ofgeneexpressionsuchasahighlevelofDNAmethylation

andhistonedeacetylation,suggestingthattheeffectcan

varydependingoncircumstance（Pecinkaetal.,2010;

Tittel-Elmeretal.,2010）.DNAmethylationisoftenapre-

requisiteforgenesilencingviamethylationdirectedbysmall

RNAs.Inparticular,floweringtimeinArabidopsiscanbe

controlledbysilencingofthehomeodomainfloraltranscrip-

tionfactorFWAbydenovomethylationofaspecificregion

withinthe5’endofthetranscribedregion.OnceFWAis

methylated,smallinterferingRNA（siRNA）ismoreefficient

atdirectingfurthermethylationtothelocus,whichsub-

sequentlyenhancesstablegenesilencing（Chanetal.,2006）.

TheArabidopsisgenomeencodes12methyl-CpG-binding

domain（MBD）proteins,whichfunctionwithchromatin

remodellingproteinstoinactivegeneexpression（Bergetal.,

2003;SpringerandKaeppler,2005）.Theseproteinsinteract

withotherproteinstoformchromatinmodifiercomplexes.

Forexample,AtMBD7interactswithargininemethyltrans-

ferase（PRMT11）（Scebbaetal.,2007）,andtheAtMBD5–

AtMBD7proteinsco-localizeinvivoandbindinvitrotothe

DECREASEINDNAMETHYLATION1（DDM1）pro-

tein（Zemachetal.,2008）.Loss-of-functionstudiesshowed

thatplantswithmutationsinAtMBD9displayapleiotropic

phenotypethatleadstoadecreaseinhistoneacetylation

andanincreaseinDNAmethylationattheFLClocus

（Pengetal.,2006;Yaishetal.,2009）.Asaresult,transcript

levelsforFLCdecline,resultinginanearlyflowering

phenotype.Prolongedexposureofsomeplantspeciesto

coldconditionsorvernalizationinducesflowering,aprocess

whichiswidelyconsideredasanon-stresscondition.

AtMBD8hasbeenshowntocontrolfloweringinthe

Arabidopsisvernalization-responsiveC24ecotype.Mutation

ofAtMBD8leadstoadelayinfloweringunderbothlong-

andshort-dayphotoperiods.WhileFLCexpressionisnot

affectedinatmbd8-1,theexpressionofFTandSOC1,

whicharemajorfloweringpromoters,isdown-regulatedin

themutant（Stangelandetal.,2009）.Themechanismby

whichtheexpressionofthesegenesisdecreasedinthe

atmbd8-1mutanthasnotyetbeendetermined.Globalgene

expressionanalysisrevealedthattheC24ecotypedifferen-

tiallyexpressesasetofbioticandabioticstress-related

genesduringthevegetativestagecomparedwiththe

Columbiaecotype.Thisfindingmayhighlightarelationship

betweenthefloweringprocessandstressresponse,although

thedirectroleofMBDproteinsinthestresstolerance

phenotypehasnotbeendetermined.

ThelevelofCpGmethylationinArabidopsisisc