表观遗传.docx
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表观遗传
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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JournalofExperimentalBotanyAdvanceAccesspublishedJune1,2011
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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
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