高级植物生理学 课件 第七章 植物的生殖生理

1 Plant Reproduction Zichao Mao 2 Life cycle of plant 3 Transition to reproduction Flower organ development Gametogenesis and fertilization Plant Reproduction 4 Transition to reproduction Vegetative phase Reproductive phase Inflorescence Flower 5 Production of flowers involves two transitions in Arabidopsis SC: stem cell P: organ primordia Se: sepal 1.Convert SAM （ Shoot Apical Meristem) to inflorescence meristem (infinite, making lateral organs) 2. Convert inflorescence meristem to floral meristem (terminal, flowers) 6 Factors regulating the transitions Vegetative meristem Inflorescence meristem Floral meristem Genes (flowering-time genes and floral identity genes) Light (photoperiod) The biological clock Temperature Hormones 7 affecting the transition of vegetative growth to reproductive growth WTemf2 Flowering-time genes emf1 embryonic flower 8 affecting formation of inflorescence and floral meristems Floral identity genes Inflorescence (from Inflorescence meristem) Flower (from Floral meristem) 9 terminal flower 1 (tfl1): Convert the inflorescence meristem to the flower meristem. leafy (lfy): produce more inflorescences, delayed flowering Mutations in floral identity genes tfl1 FUNCTION Controls inflorescence meristem identity and is required for maintenance of a indeterminate inflorescence. Prevents the expression of APETALA1 and LEAFY. Also plays a role in the regulation of the time of flowering in the long-day flowering pathway. May form complexes with phosphorylated ligands by interfering with kinases and their effectors Expressed below the apical dome of inflorescence and coflorescence meristems, and in inflorescence stem. Weakly expressed in vegetative phase from day 2 or day 3. Increased expression after commitment to flowering from day 7 on. LEAFY; LFY, FUNCTIONS IN: chromatin DNA binding, transcription factor activity, sequence-specific DNA binding; INVOLVED IN: flower development, maintenance of inflorescence meristem identity, response to gibberellin stimulus, gibberellic acid mediated signaling; LOCATED IN: nucleus; EXPRESSED IN: shoot apex, leaf whorl, embryo, flower, seed; EXPRESSED DURING: 7 growth stages; 12 Factors regulating the transition to reproduction Vegetative meristem Inflorescence meristem Floral meristem EMF TF1 LFY 13 Garner and Allard (1920s) The discovery of photoperiodism Soybeans (Glycine max) planted over a three-month period all flowered about the same time 14 Many more experiments were followed: Eliminate a variety of environmental conditions: Nutrition, temperature, and light intensity Relative length of day and night decides the flowering time Photoperiodism: ability of an organism to measure the proportion of daylight during a 24-hour period 15 Varies according to the latitude and seasonal changes. Photoperiod 16 Critical daylength Critical Daylength (CD) Xanthium(苍耳）: a short day plant, flowers when CD is LESS than 15.5 hours. Hyoscyamus(茛菪 ): a long day plant, flowers when CD is MORE than 11 hours. 17 Plants are induced to flower by different photoperiods short day (SD) : plants are stimulated to flower when the length of day falls below a threshold long day (LD): plants are stimulated to flower when the length of day exceeds a threshold Day neutral (DN): plants flower indifference to the changes of day length. Long-short-day: flowering requires certain number of short days are preceded by a certain number of long days. Short-long-day: flowering requires certain number of long days are preceded by a certain number of short days. Intermediate-daylength: not flowering if the daylength is too short or too long. Plants that respond to lengthening days and flower in the spring or early summer are known as long-day (LD) plants. Short-day (SD) plants flower in the late summer or autumn in response to shortening days and lengthening nights 19 Do plants really measure the length of the daylength? 20 Xanthium flowers when the dark period exceeds 8.5 hours. Hamner and Bonner (1938): Xanthium strumarium(苍耳）, a SD plant with CD = 15.5 hours Short interruption of dark period, even by a pulse of light as short as 1 minute delays flowering. The relative length of dark is not the determining factor. 21 Similar results were obtained with other SD plants. For LD plants A longer dark period inhibits flowering. Light break induces flowering. 22 What tissues/organs perceive photoperiod? 23 Exp. 1: The leaf or apex of Perilla（紫苏 ） (a short day plant) was exposed to different daylength. 24 Exp. 2: Grafting experiment with Perilla 25 the flowering signal is generated in the leaf the signal goes one way: from the leaf to the apex Grafting transmittable The flowering signal: florigen ? vegetative or reproductive growth? SAM Florigen Florigen Florigen Recent studies in Arabidopsis and rice have made a strong case that florigen, or at least acomponent of the floral stimulus, is the floral integrator FT. The FT gene is expressed in leaves, and the protein travels to the meristem where it interacts with another integrator, FD, to initiate the floral transition FT-like genes ( FD, FVE, FCA, FY, and FPA) are ubiquitous in plants and have been found to regulate flowering in a variety of species including wheat and poplar The role of phytochrome in de-etiolation response Phytochrome: a molecular switching mechanism The effect of light on the biological clock wPhytoperiodism and control of flowering wShort-day plant: required a light period shorter than a critical length to flower wlong-day plant: required a light period longer than a critical length to flower wDay-neutral plant: unaffected by photoperiod and flower when reaching a certain stage of maturity wVernalization: use of pretreatment with cold to induce flowering 32 The Arabidopsis biological clock The central oscillator: CCA1, LHY, and TOC1 (these are transcription factors) and other proteins 33 Present in plants, animals, fungi, and some photosynthetic bacteria An internal time measuring system (“clock”) that runs on its own with a periodicity of nearly 24 hours. It can be “reset” by external signals. The biological clock Temperature Biological clocks and circadian rhythms 36 The Arabidopsis biological clock CCA1 and LHY are expressed during the day and together repress expression of TOC1 during the day TOC1 is expressed at night and is required for activation of CCA1 and LHY1, beginning just before morning 37 Lack of the nyctinastic movement: diurnal rise and fall of leaves Altered flowering time in some mutants cca1: early flowering lhy: early flowering toc1: early flowering Some other clock mutants can be late flowering Mutations in the clock genes 38 Temperature: Vernalization Vernalization: low temperature treatment can promote flowering in some plants. The vernalization-effective temperature and duration of low temperature treatment may vary. Vernalization is perceived by the shoot apex. The vernalization state is grafting transmissible. Definition the acquisition or acceleration of the ability to flower by a chilling treatment Plants have evolved many systems to sense their environment and to modify their growth and development accordingly. One example is vernalization, the process by which flowering is promoted as plants sense exposure to the cold temperatures of winter. A requirement for vernalization is an adaptive trait that helps prevent flowering before winter and permits flowering in the favorable conditions of spring. In Arabidopsis and cereals, vernalization results in the suppression of genes that repress flowering. We describe recent progress in understanding the molecular basis of this suppression. In Arabidopsis, vernalization involves the recruitment of chromatin-modifying complexes to a clade of flowering repressors that are silenced epigenetically via histone modifications. vernalization 41 Can be induced quickly Increases plant resistance to freezing stress Does not affect flowering time. Cold acclimation Venalization in cereals Machanism of Vernalization Vernalization-mediated changes in FLC chromatin. (a) Prior to cold exposure, FLC is actively expressed. The complexes that maintain this active chromatin conformation include the PAF complex, which methylates histone 3 tails at lysine 4 and 36 (H3K4triMe and H3K36triMe), a SWR1-like complex, which deposits a histone 2A variant in the nucleosomes of FLC chromatin, and H2B ubiquitinases like HUB1 and HUB2 that ubiquitinate histone 2B tails (H2Bub1). Although FLC is in an active state, there are repressive complexes present such as Polycomb Repression Complex 2 and some degree of lysine 27 methylation of histone 3 (H3K27triMea repressive modification) (b) During cold exposure, FLC repression is initiated. VIN3 is induced, VIN3 and VIL1/VRN5 associate with the Polycomb complex, the density of repressive chromatin modifications such as lysine 27 methylation of histone 3 increases, and repressors such as LIKE HETEROCHROMATIN PROTEIN 1 (LHP1) assemble on FLC chromatin. (c) As vernalization proceeds, the density of repressive modifications, particularly H3K27triMe and lysine 9 methylation of histone 3 H3K9triMe; mediated by an unknown H3K9 methyltransferase (HMTase) increases. (d) Eventually, a mitotically stable state of repression that no longer requires VIN3 is achieved. This mitotically stable state is likely to involve positive feedback loops in which the repressive chromatin modifications serve to recruit the chromatin-modifying complexes including VRN1 to maintain a repressive state. As the FLC locus passes to the next generation, the active chromatin state represented in (a) is re-established 47 Hormone GA regulates flowering time GA1: an enzyme involved in GA biosynthesis ga1: In addition to the dwarf phenotype, the mutant flowers late under LD conditions and does not flower under SD conditions. GA treatment promotes flowering time. 48 Flower development in Arabidopsis Vegetative meristem Inflorescence meristem Floral meristem Flower: sepals, petals, stamens, and carpels Transition to reproduction: Genes & other factors Flower organ development: Organ identity genes One of the early successes of the application of molecular genetics to study plant development was the discovery of a series of genes that act together, in an apparently simple combinatorial model, to specify the identity of the different organs of a flower. Widely known as the ABC model, The cast list of genes has been defined and, great progress has been made in understanding how they are regulated, how they act together, what they do and how they have contributed to the evolution of the flower in its varied forms. 51 Flower organs petal stamen carpel sepal 52 The flower is generated from the floral meristem the floral meristem 53 Produced in 4 concentric whorls with the same order sepal (whorl 1)stamen (whorl 3) petal (whorl 2)carpel (whorl 4) Flower organs ABC model 55 (the ap1 mutant is similar) stamen-carpel-stamen-carpelsepal-petal-stamen-carpel 56 wtpistillata (pi) sepal-sepal-carpel-carpel sepal-petal-stamen-carpel apetala3 (ap3) 57 sepal-petal-stamen-carpelsepal-petal-petal-sepal 58 The “ABC” model for flower development The ABC genes function in the distinct fields. The A and C genes are mutually exclusive in their expression. A B C AP1, AP2 AP3, PI AG 59 The A genes: ap1 or ap2 mutants should (and do) make carpel-stamen-stamen-carpel ap1 or ap2 WT 60 The B genes: ap3 or pi mutants should (and do) make sepal -sepal-carpel-carpel WT ap3 or pi 61 The C genes: ag mutants should (and do) make sepal-petal-petal-sepal WT ag MADS-box proteins (MCM1, AG, DEF, SRF) Plant MADS-box proteins belong to two large families: the type I class, which group with the human SRF protein, and the type II class that groups with yeast MEF2 ABCE model (A)BC model 66 Gametogenesis and Fertilization Flower organ function: 67 generative cell Diploid pollen mother cells undergo meiosis to produce a tetrad of haploid microspores. Each microspore develops into a pollen grain containing two haploid cells (mitosis I): the generative cell (small) The vegetative cell (large) Male gametogenesis 68 the vegetative cell grows to produce the pollen tube the generative cell produce 2 sperm cells (mitosis II) 69 an ovule primordiu