Presentation on theme: "Molecular mechanism of plant flowering time control"— Presentation transcript:
1Molecular mechanism of plant flowering time control 第十章 基因和发育第二讲植物开花时间控制的分子机理Molecular mechanism of plant flowering time controlBy Hongwei Guo, Peking University,
2When to flower is a matter of the survival of species Transition to reproductionWhen to flower is a matter of the survival of species?Main inflorescence shoot in middle, that can branch into lateral shootAt end of each shoot – place to make flowerEach inflorescence shoot can make leaves and lateral organsWhat regulates transition from vegetative growth (main function to increase biomass of plant) to making plant sexually readyVegetative phaseReproductive phase
3Two major pathways regulating floral transition Photoperiodic flowering (光周期开花): day-length dependent flowering time controlVernalization (春化): cold-promoted flowering
8Floral induction of leaves of Short Day plant Perrila crispa “Something” must be produced in leaves and “move” to the meristemLong journey to identify “Florigen” began(Lang and Zeevart)
9Multiple grafting experiment with Perilla Only single leaf from one of the plant treated – all 5 plants can make flowerSomething made in leaf can be transferredFlowering signal is grafting transmittableLong distance transport---- thru vascular tissue
10The flowering signal: florigen vegetative or reproductive growth?the flowering signal is generated in the leafthe signal goes one way: from the leaf to the apexGrafting transmittableSAMFlorigenFlorigen can be transmitted to other plants thru graftingFlower locus T protein claimed to be florigen – but some disputesIf you find signal or molecule that can greatly change flowering time – agriculture revolutionizedFlorigenFlorigen
11Photoperiodic flowering Two essential questions:1. How does the leaf measure day-length?2. What is the florigen?
12Genetics provides the answers Arabidopsis: Long Day PlantFlowering is induced by Long Day (LD)Certain late flowering mutants are blind to photoperiodcryptochrome 2 (cry2)phytochrome A (phyA)contstans (co)flowering locus t (ft)Cry2: blue light receptorPhyA: Far-red light receptorCO: transcriptional co-activatorFT: transcriptional co-activator (?)
17(originally proposed by Boenning, 1936) Light has a dual role in this model:- entrains the circadian oscillation of light- and dark-sensitive phases- directly required for the production of the signal.
33Cry inhibits CO protein degradation in light When CO mRNA peaks at midnight in SDs, COP1 is predominantly localized in the nucleus. CRY interacts with COP1 but is not able to repress its activity. COP1–CO interaction results in ubiquitination and degradation of CO (A), whereas when CO mRNA peaks in the afternoon in LDs, light activation of CRY during the daytime might mediate translocation of COP1 from nucleus to cytoplasm. Consequently, CO is able to accumulate and activate the transcription of FT to promote flowering (B). U, ubiquitin.
35Photoperiod-dependent activation of CO protein and FT mRNA CDF1: circadian dependent factorCO protein measures day-lengthFT mRNA level determines flowering timeCO mRNA is regulated by circadian clock.CO protein is stabilized by lightTherefore, FT expression is activated only in long day
36FT (or Hd3a) is a floral activator both in LD and SD plants Long Day PlantShort Day Plant(Kobayashi & Weigel, 2007)CO in Long Day plants and similar proteins (Hd1) in Short Day plants are regulated in opposite ways
41However, the mRNA hypothesis was challenged in 2006 by a PNAS paper, the original Science paper was retracted in more papers have been published in 2007 – all argued that FT protein is the florigen, in at least 5 different plant species: Arabidopsis, rice, tomato, tobacco, pumpkin.Yuval Eshed‘s lab in Israel cloned tomato FT gene from a tomato mutant, sft. They overexpressed SFT in tomato, resulting in early flowering, but they found little transgenic SFT mRNA in the apical meristem。
64So…….. FT protein is a “florigen” CO – transcription factor in leaves, respond to day-length (photoperiod sensor)FT – RAF kinase inhibitor protein, travels in phloem from leaf to SAM, target of CO
65Photoperiodic flowering in plants FT proteinGIFT proteinHd3a protein
66Besides the photoperiod-dependent regulation, floral transition is under controls of many other cues.
67Vernalization (春化)– Promoting flowering with cold
68Some plants need winter to flower Vernalization:Acquisition of the competence to flower in the spring by exposure to the prolonged cold of winter.Plants are genetically identicalNo vernalizationVernalizationExposed as a seedling to 4ºC for 40 days.
69A role of temperature in the plant calendar Changes in day length are a reliable indicator of seasonal progression, but day length per se is not completely informative of the time of year.Vernalization – the process whereby flowering is promoted by a cold treatment given to a fully hydrated seed (i.e., a seed that has imbibed water) or to a growing plant.- dry seeds do not respond to cold treatment- without cold treatment, plants that require vernalization show delayed flowering or remain vegetative
70Vernalization may involve epigenetic changes in gene expression Requirements and features of vernalization:- requires cell division and DNA replication- requires stable changes in gene expression in meristem (even after the signal that induced the change, i.e. cold, is removed → epigenetic regulation)Epigenetic: A heritable change in gene expression that is controlled by modifications in DNA methylation and/or chromatin structure.- from yeast to mammalsArabidopsis: gene acting as repressor of flowering: FLOWERING LOCUS C (FLC)FLC - encodes MADS box transcription factor, delaying floral transition- represses transcription of AGAMOUS-LIKE 20 (AGL20)/SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1), encoding MADS boxtranscription factor accelerating flowering- highly expressed in non-vernalized SAM- after vernaliztion, gene is epigenetically switched off for remainder of plant’s life cycle, permitting flowering in response to long days to occur
71Vernalization blocks the expression of FLC in cold-requiring winter annual Arabidopsis ecotypes winter annual ecotypesWinter annualwithout coldWinter annual after 40 days coldWinter annual without cold, but with flc mutationFLC mRNA
72for vernalization insensitive (vin) mutants Genetic screeningfor vernalization insensitive (vin) mutantsVRN2: Homolog of Su(z)12VRN1: Myb DNA binding proteinVIN3: PHD finger proteinLHP1: LIKE-HETEROCHROMATIN PROTEINVIL1: PHD finger proteinVIN5: Histone arginine methyl transferaseVIN7: PAH2 domain proteinGenes are involved in the regulation of chromatin structureFewer;;;
73Other genes involved in control of flowering by vernalization VERNALIZATION (VRN) 1 and VRN2both are required for maintenance of low levels of FLC mRNA that are established by cold treatment once plants are exposed to warmer conditionsWild typeTime vernalizedDays at 20ºC after vernalizationFLC mRNARole of VRN2 is to maintain the repression of FLC expression.vrn2Time vernalizedDays at 20ºC after vernalizationFLC mRNA
74VRN2 encodes a gene related to Drosophila Polycomb-group (PcG) genes ArabidopsisSU(Z)12DrosophilaIn Drosophila, PcG proteins act in large protein complexes. They maintain the repression of transcription of homeotic genes, once the pattern of expression of these genes has been established during early embryo development.
75Polycomb-group complexes in Drosophila repress gene expression by modifying histones Histone 3 is a major target for modifications – those above activate gene expression, those below repress it.Polycomb-group proteins promote the methylation of K9 and K27 (H3K9Me, H3K9Me) – gene repressionVRN2 involved in chromatin remodeling → vernalization down-regulates FLC by epigenetic mechanisms
76Repressed chromatin (OFF) Histone codeActive chromatin (ON)Repressed chromatin (OFF)High in:Acetylation, H3K4Me, H3S10PHigh in:H3K9Me, H3K27MeModified histone could be recognized by activation/repression complexes and establish stable activation/repression chromatin
77Dynamics of FLC chromatin Active FLC ChromatinHigh inAc; H3K4Me; H3S10PH2A.Z Histone variantRepressed FLC ChromatinH3K9Me; H3K27MeH4R3Me2; LHP1VIL1, VIN5WINTER!VIN3, VIL1, VIN5 VRN2, VRN1, etc.ON; FallOFF; Spring(from Sung and Amasino, 2005)
78Memory of winter can be mitotically stable From Lang & Melchers memory in Hyocyamus niger
79FLC repression by vernalization is mitotically stable VIN3VIN3UBQWe recapitulated fall/winter/spring seasons. Prior to vernalization as shown in red here represents fall condition, and blue indicated that plants are in the cold as in winter.
80Molecular basis of vernalization response FLC is a potent repressor of flowering.Competence: in Arabidopsis, is determined largely by FLC expression level. Vernalization leads to competence via repression of FLC.Mitotic stability: Vernalization-mediated repression of FLC via histone modifications that are hallmarks of epigenetic silencing
81No vernalizationFLC expression is subject to positive and negative regulation