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1、1,Plant Reproduction,Zichao Mao,2,Life cycle of plant,3,4,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,Convert SAM （Shoot Apical Meristem) to infloresce
2、nce 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 biolo
3、gical clock Temperature Hormones,7,affecting the transition of vegetative growth to reproductive growth,WT,emf2,Flowering-time genes,emf1,embryonic flower,8,affecting formation of inflorescence and floral meristems,Floral identity genes,Inflorescence (from Inflorescence meristem),Flower (from Floral
4、 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 i
5、nflorescence. 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
6、 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
7、 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
8、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 en
9、vironmental 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,1
10、6,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
11、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 certai
12、n 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
13、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 strum
14、arium(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
15、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
16、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 integrat
17、or 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 i
18、ncluding wheat and poplar,The role of phytochrome in de-etiolation response,Phytochrome: a molecular switching mechanism,The effect of light on the biological clock,Phytoperiodism and control of flowering Short-day plant: required a light period shorter than a critical length to flower long-day plan
19、t: required a light period longer than a critical length to flower Day-neutral plant: unaffected by photoperiod and flower when reaching a certain stage of maturity Vernalization: use of pretreatment with cold to induce flowering,32,The Arabidopsis biological clock,The central oscillator: CCA1, LHY,
20、 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,Bi
21、ological 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 nyctinas
22、tic 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
23、 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 chi
24、lling 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
25、 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 su
26、ppression. 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 flow
27、ering 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 t
28、ails 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 p
29、resent 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
30、 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
31、; 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 r
32、ecruit 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 t
33、o 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
34、 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 ide
35、ntity 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
36、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),st
37、amen-carpel-stamen-carpel,sepal-petal-stamen-carpel,56,wt,pistillata (pi),sepal-sepal-carpel-carpel,sepal-petal-stamen-carpel,apetala3 (ap3),57,sepal-petal-stamen-carpel,sepal-petal-petal-sepal,58,The “ABC” model for flower development,The ABC genes function in the distinct fields. The A and C genes
38、 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 (an
39、d 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,
40、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 v
41、egetative cell grows to produce the pollen tube the generative cell produce 2 sperm cells (mitosis II),69,an ovule primordium emerges as a bump on the inner wall (placenta) of the ovary the megasporocyte undergoes meiosis to produce 4 haploid cells, only one of which (the megaspore) survives.,Female
42、 gametogenesis,70,placental wall,Female gametogenesis,the megaspore undergoes 3 mitotic divisions to produce 8 cells: 3 antipodal cells 2 synergid cells 2 central cell nuclei 1 egg cell (EC),71,Female gametogenesis,placental wall,72,Double fertilization,Pollens land on the stigma, hydrate, and begin
43、 to germinate the pollen tube,73,Pollen tubes grow, by tip growth, down through the stigma and style and into the ovary, toward the ovules. The pollen tube navigates to the micropyle and discharges the two sperm cells.,74,One sperm fertilizes the egg cell to develop into the embryo. the other sperm
44、fertilizes the diploid central cell nucleus to develop into the endosperm.,Micropyle,Egg,Synergids,Antipodal cells,Central cell nuclei,Sperms,Pollen tube,Ovule,Double fertilization,75,Plant reproduction,Ovule (1 to many) Ovary,Silique（长角果）,76,Fruit development,The ovary and other tissue together produce a fruit. Fruit is important for seed dispersal in many species Many foods are also called “vegetables”: tomatoes, pea , squash Fruit size, texture, and sugar content are determined by genes. Ethylene stimulates fruit ripening.,77,Life cycle of a flowering plant,