基礎(chǔ)分子生物第十章

植物開花時間控制的分子機理MolecularmechanismofplantfloweringtimecontrolByHongweiGuo,PekingUniversity,2014.12.8peachblossomwaterlilychrysanthemumplumblossom春游桃花塢，夏賞綠荷池。秋飲黃花酒，冬吟白雪詩。FactorsregulatingthetransitionsVegetativemeristemInflorescencemeristemFloralmeristemGenes(flowering-timegenesandfloralidentitygenes)Hormones(GA,etc)Daylength(photoperiod)Temperature(vernalization)VegetativegrowthReproductivegrowthTwomajorpathwaysregulatingfloraltransitionPhotoperiodicflowering(光周期開花):day-lengthdependentfloweringtimecontrolVernalization(春化):cold-promotedflowering(“MarylandMammoth”cultivaroftobacco)短日植物（short-dayplant，SDP），一般夏末開花指在晝夜周期中日照長度短于臨界值日長才能開花的植物。適當?shù)乜s短光照或延長黑暗可提早開花。如：美洲煙草、大豆、晚稻、菊花、蒼耳、高粱、日本牽牛、紫蘇、黃麻、大麻等。長日植物（long-dayplant，LDP），一般春季開花指在晝夜周期中日照長度大于臨界日長才能開花的植物。延長日照長度可促進開花。如：擬南芥、油菜、小麥、黑麥、大麥、菠菜、天仙子、胡蘿卜、芹菜、洋蔥等。日中性植物（day-neutralplant，DNP）指在任何日照條件下都能開花的植物。如：番茄、黃瓜、茄子、四季豆、辣椒、四季花卉等。（開花）Nightlengthistheauthenticdeterminantfactor!!暗期間斷實驗-----臨界暗期長度植物開花決定于暗期的長度而不是光期的長度。臨界暗期指晝夜周期中LDP能夠開花的最長暗期長度或SDP開花所需的最短暗期長度。SDP實際上是長夜植物，LDP是短夜植物。SDPLDP光周期營養(yǎng)生長開花光暗開花營養(yǎng)生長營養(yǎng)生長間斷白晝開花開花營養(yǎng)生長營養(yǎng)生長開花24hours閃光營養(yǎng)生長開花FloralinductionofleavesofShortDayplantPerrilacrispa“Something”mustbeproducedinleavesand“move”tothemeristemLongjourneytoidentify“Florigen”began(LangandZeevart)MultiplegraftingexperimentwithPerillaLongdistancetransport----thruvascularsystemthefloweringsignalisgeneratedintheleafthesignalgoesfromtheleaftotheapexGraftingtransmittableThefloweringhormone:florigen（開花素）

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

光周期與光敏色素(phytochrome)暗期間斷的效果取決于最后一次照射的是紅光還是遠紅光對SDP而言，紅光阻止開花，遠紅光促進開花；對LDP而言，紅光促進開花，遠紅光阻止開花。光敏色素（phytochrome）：感應紅光-遠紅光可逆反應的蛋白紅光-遠紅光可逆反應的存在，表明光敏色素系統(tǒng)參與了成花誘導過程。藍光受體隱花色素（cryptochrome）也參與植物光周期開花的調(diào)節(jié)（晝夜節(jié)律，生物鐘）SignalTrans-ductionPhytochromes(光敏色素）：Red/Far-red

lightreceptorsCryptochromes(隱花色素）：BluelightreceptorsLighthasadualroleinthismodel:-entrainsthecircadianoscillationoflight-anddark-sensitivephases-directlyrequiredfortheproductionofthesignal.(originallyproposedbyBoenning,1936)Gatingmodel*Somerhythmspersistevenwithoutenvironmentalchangesendogenouscontrol:circadianclockcircadianrhythms–approximately24hperiod*Rhythmicityinbehavior,physiologyandbiochemistryoforganismsCircadianClock（生物鐘、晝夜節(jié)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《科學》雜志評為當年十大科學發(fā)現(xiàn)之一HeatingtheleafofpHSP::FT/fttransgenicplantscanpromoteflowering,andFTmRNAcanbedetectedinSAMhsp:heatshockproteinHowever,themRNAhypothesiswaschallenged

in2006byaPNAS

paper,theoriginalScience

paperwasretractedin2007.5morepapershavebeenpublishedin2007–all

arguedthatFTproteinistheflorigen,inatleast5

differentplant