基础分子生物第十章

植物开花时间控制的分子机理MolecularmechanismofplantfloweringtimecontrolByHongweiGuo,PekingUniversity,2014.12.8peachblossomwaterlilychrysanthemumplumblossom春游桃花坞，夏赏绿荷池。秋饮黄花酒，冬吟白雪诗。FactorsregulatingthetransitionsVegetativemeristemInflorescencemeristemFloralmeristemGenes(flowering-timegenesandfloralidentitygenes)Hormones(GA,etc)Daylength(photoperiod)Temperature(vernalization)VegetativegrowthReproductivegrowthTwomajorpathwaysregulatingfloraltransitionPhotoperiodicflowering(光周期开花):day-lengthdependentfloweringtimecontrolVernalization(春化):cold-promotedflowering(“MarylandMammoth”cultivaroftobacco)短日植物（short-dayplant，SDP），一般夏末开花指在昼夜周期中日照长度短于临界值日长才能开花的植物。适当地缩短光照或延长黑暗可提早开花。如：美洲烟草、大豆、晚稻、菊花、苍耳、高粱、日本牵牛、紫苏、黄麻、大麻等。长日植物（long-dayplant，LDP），一般春季开花指在昼夜周期中日照长度大于临界日长才能开花的植物。延长日照长度可促进开花。如：拟南芥、油菜、小麦、黑麦、大麦、菠菜、天仙子、胡萝卜、芹菜、洋葱等。日中性植物（day-neutralplant，DNP）指在任何日照条件下都能开花的植物。如：番茄、黄瓜、茄子、四季豆、辣椒、四季花卉等。（开花）Nightlengthistheauthenticdeterminantfactor!!暗期间断实验-----临界暗期长度植物开花决定于暗期的长度而不是光期的长度。临界暗期指昼夜周期中LDP能够开花的最长暗期长度或SDP开花所需的最短暗期长度。SDP实际上是长夜植物，LDP是短夜植物。SDPLDP光周期营养生长开花光暗开花营养生长营养生长间断白昼开花开花营养生长营养生长开花24hours闪光营养生长开花FloralinductionofleavesofShortDayplantPerrilacrispa“Something”mustbeproducedinleavesand“move”tothemeristemLongjourneytoidentify“Florigen”began(LangandZeevart)MultiplegraftingexperimentwithPerillaLongdistancetransport----thruvascularsystemthefloweringsignalisgeneratedintheleafthesignalgoesfromtheleaftotheapexGraftingtransmittableThefloweringhormone:florigen（开花素）

vegetativeorreproductivegrowth?SAMFlorigenFlorigenFlorigenPhotoperiodicfloweringTwoessentialquestions:1.Howdoestheleafmeasureday-length?2.Whatistheflorigen?

光周期与光敏色素(phytochrome)暗期间断的效果取决于最后一次照射的是红光还是远红光对SDP而言，红光阻止开花，远红光促进开花；对LDP而言，红光促进开花，远红光阻止开花。光敏色素（phytochrome）：感应红光-远红光可逆反应的蛋白红光-远红光可逆反应的存在，表明光敏色素系统参与了成花诱导过程。蓝光受体隐花色素（cryptochrome）也参与植物光周期开花的调节（昼夜节律，生物钟）SignalTrans-ductionPhytochromes(光敏色素）：Red/Far-red

lightreceptorsCryptochromes(隐花色素）：BluelightreceptorsLighthasadualroleinthismodel:-entrainsthecircadianoscillationoflight-anddark-sensitivephases-directlyrequiredfortheproductionofthesignal.(originallyproposedbyBoenning,1936)Gatingmodel*Somerhythmspersistevenwithoutenvironmentalchangesendogenouscontrol:circadianclockcircadianrhythms–approximately24hperiod*Rhythmicityinbehavior,physiologyandbiochemistryoforganismsCircadianClock（生物钟、昼夜节律）*Anticipationofrhythmicchangesintheenvironmentchangesinthephysiologicalstatethatprovidethemwithanadaptiveadvantagecyanobacteria:80%ofgenesareCCGs(circadianclock-regulatedgenes)Inplants:leafmovements,cellelongationrates,stomatalaperture,CO2assimilation,Calvincycle,hypocotylelongation…Arabidopsis:6%areCCGs,peakingatallphasesofdayandnight

Entraining(resettingtheclock):twoclassesofphotoreceptors(phyandcry)canestablishtheperiodlength

andphase.LightREDPRR:photoperiodresponseregulatorCoincidencemodel:PRR

andlightsignalcoexistPRR:photoperiodresponseregulatorLightREDAgeneticapproachGeneticsprovidestheanswersArabidopsis:LongDayPlantFloweringisinducedbyLongDay(LD:i.e.16L/8D)Certainlatefloweringmutantsareblindtophotoperiodcryptochrome2(cry2)phytochromeA(phyA)contstans(co)floweringlocust(ft)Cry2:bluelightreceptorPhyA:Red/Far-redlightreceptorCO:transcriptionalactivatorFT:transcriptionalco-activator(?)COisessentialforphotoperiodicflowering,ascomutantislatefloweringandalmostday-neutral.ItencodesaZinc-fingertranscriptionalregulator.FT

isthedirecttargetgeneofCOCO

is

required

for

FT

expressionFTisalsoaCCGFTmRNAlevelsdeterminefloweringtimeSD:latefloweringLD:earlyfloweringCol-4(WT):earlyfloweringcry2:latefloweringinLD,butnotinSDSDLDLDFTflowerinitiationCOcry2COP1,anE3ligase,isresponsibleforCOubiquitinationanddegradationMolecularmechanismofphotoperiodicfloweringCOP1Amolecularmodelofcircadianclock3)LHY,CCA1repressexpressionofTOC1,theirpositiveregulator1)PHYandCRYasphotoreceptors2)LHY,CCA1andTOC1negativefeedbackloop4)Generationofcircadianrhythms,includingthatofCOforfloweringtime5)ELF3gatesthelightsignalssinceitselfaCCG,thisallowscyclingevenatconstantlight6)ZLPandGIalsoactonlightinputMorningfactors(peakinthemorning):CCA1/LHYCDF1/2/3/5Afternoonfactors:(peakintheafternoon):PRR5/7/9GI/FKF1ClockgenescontrolcomRNAlevelsCCA1/LHY,CDFs,PRRsaretranscriptionfactorsGI/FKF1functionsasanE3ligasetargetingCDFsPhotoperiod-dependentactivationofCO

protein

andFTmRNACOmRNAisregulatedbycircadianclock.COproteinisstabilizedbylightCDF1:circadiandependentfactor,repressingCOexpressionTherefore,FTexpressionisactivatedonlyinlongdayCOproteinmeasuresday-lengthFTmRNAleveldeterminesfloweringtimeCOisthePRRinthecoincidencemodelEveninanartificialshortday(e.g.10L/20D),aslongasthecircadianclockpermitshighlevelsofcomRNAinthedaytime,thisphotoperiodwouldpromotefloweringIntheexternalcoincidencemodel,lightentrainsaperiodicfunction(ochre).Lightalsocausestheproductionofarepressor(red)directlyproportionatetothisfunction.Thus,evenshortnightbreaks,ifgivenatthecorrecttime,canleadtotherepressorsurpassingacriticalthreshold(dashedline).Amodeltoexplainnight

breakresponsesinshortday.FT(orHd3a)isafloral

activatorbothinLDandSDplants(evolutionallyconserved)LongDayPlantShortDayPlantCOinLongDayplantsandsimilarproteins(Hd1)inShortDayplantsareregulatedinoppositeways.InSDP,SDmakesHd1anactivatorofHd3a,whileLDmakesHd1arepressorofHd3a(phy-dependent).phyBInbothspecies,theorthologsFT/Hd3aactdownstreamfromCO/Hd1topromoteflowering.InA.thaliana,COseemstobelargelydispensableinnoninductiveshortdays(becausethereislittleornoprotein),whileinrice,Hd1isactivelyrepressingHd3ainnoninductivelongdays(asdeducedfromgeneticanalyses).MutationalinactivationofthephytochromeB(PhyB)photoreceptorcausesearlyfloweringinnoninductiveconditionsinbothspecies,butfordifferentreasons.InA.thalianaphyBmutants,COproteinisstabilizedandcanactivateFT;inricephyBmutants,Hd1activityisnolongermodifiedbyphytochromeB,whichnormallyturnsHd1intoarepressor(indicatedbyadifferentcolor);floweringisnowaccelerated,becauserepressionisrelievedand,perhapsinaddition,Hd1activatesHd3a.Forsimplicity,itisshownherethatCOandHd1arerecruitedbyotherproteins(possiblyHAPfactors)totheFTandHd3apromoters,althoughthishasnotbeenformallydemonstrated.Similarly,itisnotknownyetwhetherHd1proteinitselfismodifiedbyphytochromeB.ComparisonofthefunctionoftheorthologsCO(Arabidopsis)andHd1(rice)(Kobayashi&Weigel,Genes

and

Development,

2007)Hd1=CO;Hd3a=FTse1=Hd1mutant;se5=phytochromemutant.DaylengthmeasurementinSDP(rice)Hd1expressionpatterninSDPissimilartoCOexpressionpatterninLDP;ActivephymakesHd1arepressorforHd3a.retr2005被《科学》杂志评为当年十大科学发现之一HeatingtheleafofpHSP::FT/fttransgenicplantscanpromoteflowering,andFTmRNAcanbedetectedinSAMhsp:heatshockproteinHowever,themRNAhypothesiswaschallenged

in2006byaPNAS

paper,theoriginalScience

paperwasretractedin2007.5morepapershavebeenpublishedin2007–all

arguedthatFTproteinistheflorigen,inatleast5

differentplant