1、Plant ReproductionZichao Mao Life cycle of plant Transition to reproduction Flower organ development Gametogenesis and fertilizationPlant ReproductionTransition to reproductionVegetative phaseReproductive phaseInflorescenceFlowerProduction of flowers involves two transitions in ArabidopsisSC:stem ce
2、llP:organ primordiaSe:sepalConvert SAM（Shoot Apical Meristem)to inflorescence meristem(infinite,making lateral organs)2.Convert inflorescence meristem to floral meristem(terminal,flowers)Factors regulating the transitionsVegetative meristemInflorescence meristemFloral meristemGenes(flowering-time ge
3、nes and floral identity genes)Light(photoperiod)The biological clockTemperature Hormones affecting the transition of vegetative growth to reproductive growthWTemf2Flowering-time genesemf1embryonic floweraffecting formation of inflorescence and floral meristemsFloral identity genesInflorescence(from
4、Inflorescence meristem)Flower(from Floral meristem)terminal flower 1(tfl1):Convert the inflorescence meristem to the flower meristem.leafy(lfy):produce more inflorescences,delayed floweringMutations in floral identity genestfl1 FUNCTION Controls inflorescence meristem identity and is required for ma
5、intenance 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 plexes with phosphorylated ligands by interfering with kinases and their effectors Expressed below the apic
6、al dome of inflorescence and coflorescence meristems,and in inflorescence stem.Weakly expressed in vegetative phase from day 2 or day 3.Increased expression after mitment to flowering from day 7 on.LEAFY;LFY,FUNCTIONS IN:chromatin DNA binding,transcription factor activity,sequence-specific DNA bindi
7、ng;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;Factors regulating the transition to reprodu
8、ctionVegetative meristemInflorescence meristemFloral meristemEMFTF1LFYGarner and Allard(1920s)The discovery of photoperiodismSoybeans(Glycine max)planted over a three-month period all flowered about the same timeMany more experiments were followed:Eliminate a variety of environmental conditions:Nutr
9、ition,temperature,and light intensityRelative length of day and night decides the flowering timePhotoperiodism:ability of an organism to measure the proportion of daylight during a 24-hour period Varies according to the latitude and seasonal changes.PhotoperiodCritical daylengthCriticalDaylength(CD)
10、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.Plants are induced to flower by different photoperiodsshort day(SD):plants are stimulated to flower when the length of day falls below a thresholdlong day(LD):
11、plants are stimulated to flower when the length of day exceeds a thresholdDay 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 num
12、ber 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 summe
13、r or autumn in response to shortening days and lengthening nights Do plants really measure the length of the daylength?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 pul
14、se of light as short as 1 minute delays flowering.The relative length of dark is not the determining factor.Similar results were obtained with other SD plants.For LD plantsA longer dark period inhibits flowering.Light break induces flowering.What tissues/organs perceive photoperiod?Exp.1:The leaf or
15、 apex of Perilla（紫苏）(a short day plant)was exposed to different daylength.Exp.2:Grafting experiment with Perilla the flowering signal is generated in the leaf the signal goes one way:from the leaf to the apex Grafting transmittableThe flowering signal:florigen?vegetative or reproductive growth?SAMFl
16、origenFlorigenFlorigen Recent studies in Arabidopsis and rice have made a strong case that florigen,or at least aponent 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 initi
17、ate 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 poplarThe role of phytochrome in de-etiolation responsePhytochrome:a molecular switching mechanismThe effect of light on the bi
18、ological clockwPhytoperiodism and control of floweringPhytoperiodism and control of floweringwShort-day plant:required a light period shorter than Short-day plant:required a light period shorter than a critical length to flowera critical length to flowerwlong-day plant:required a light period longer
19、 than a long-day plant:required a light period longer than a critical length to flowercritical length to flowerwDay-neutral plant:unaffected by photoperiod and Day-neutral plant:unaffected by photoperiod and flower when reaching a certain stage of maturityflower when reaching a certain stage of matu
20、ritywVernalization:use of pretreatment with cold to Vernalization:use of pretreatment with cold to induce floweringinduce floweringThe Arabidopsis biological clockThe central oscillator:CCA1,LHY,and TOC1(these are transcription factors)and other proteins33 Present in plants,animals,fungi,and some ph
21、otosynthetic 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 clockTemperatureBiological clocks andcircadian rhythmsThe Arabidopsis biological clock CCA1 and LHY are expressed during the d
22、ay 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 morningLack of the nyctinastic movement:diurnal rise and fall of leavesAltered flowering time in some mutantscca1:early floweringlhy:early flower
23、ingtoc1:early floweringSome other clock mutants can be late flowering Mutations in the clock genesTemperature:VernalizationVernalization:low temperature treatment can promote flowering in some plants.The vernalization-effective temperature and duration of low temperature treatment may vary.Vernaliza
24、tion 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 exa
25、mple 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
26、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 plexes to a clade of flowering repressors that are silenc
27、ed epigenetically via histone modifications.vernalization Can be induced quickly Increases plant resistance to freezing stress Does not affect flowering time.Cold acclimationVenalization in cerealsMachanism of Vernalization Vernalization-mediated changes in FLC chromatin.(a)Prior to cold exposure,FL
28、C is actively expressed.The plexes that maintain this active chromatin conformation include the PAF plex,which methylates histone 3 tails at lysine 4 and 36(H3K4triMe and H3K36triMe),a SWR1-like plex,which deposits a histone 2A variant in the nucleosomes of FLC chromatin,and H2B ubiquitinases like H
29、UB1 and HUB2 that ubiquitinate histone 2B tails(H2Bub1).Although FLC is in an active state,there are repressive plexes present such as Polyb Repression Complex 2 and some degree of lysine 27 methylation of histone 3(H3K27triMea repressive modification)(b)During cold exposure,FLC repression is initia
30、ted.VIN3 is induced,VIN3 and VIL1/VRN5 associate with the Polyb plex,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
31、 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
32、involve positive feedback loops in which the repressive chromatin modifications serve to recruit the chromatin-modifying plexes 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 Hormone GA regula
33、tes flowering timeGA1:an enzyme involved in GA biosynthesisga1: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.Flower development in ArabidopsisVegetative meristemInflorescence meristemFloral
34、meristemFlower:sepals,petals,stamens,and carpelsTransition to reproduction:Genes&other factorsFlower 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 ap
35、parently simple binatorial 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
36、 to the evolution of the flower in its varied forms.Flower organs petal stamen carpel sepalThe flower is generated from the floral meristem the floral meristemProduced in 4 concentric whorls with the same ordersepal(whorl 1)stamen(whorl 3)petal(whorl 2)carpel(whorl 4)Flower organs ABC model (the ap1
37、 mutant is similar)stamen-carpel-stamen-carpelsepal-petal-stamen-carpelwtpistillata(pi)sepal-sepal-carpel-carpelsepal-petal-stamen-carpelapetala3(ap3)sepal-petal-stamen-carpelsepal-petal-petal-sepalQuickTime and aTIFF(LZW)decompressorare needed to see this picture.The“ABC”model for flower developmen
38、t The ABC genes function in the distinct fields.The A and C genes are mutually exclusive in their expression.ABCAP1,AP2AP3,PIAGThe A genes:ap1 or ap2 mutants should(and do)make carpel-stamen-stamen-carpelap1 or ap2WTThe B genes:ap3 or pi mutants should(and do)make sepal-sepal-carpel-carpelWTap3 or p
39、iThe C genes:ag mutants should(and do)make sepal-petal-petal-sepalWTagMADS-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 MEF2ABCE model(A)BC model Gametogenesis an
40、d FertilizationFlower organ function:generative cellDiploid 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 the v
41、egetative cell grows to produce the pollen tube the generative cell produce 2 sperm cells(mitosis II)an ovule primordium emerges as a bump on the inner wall (placenta)of the ovarythe megasporocyte undergoes meiosis to produce 4 haploid cells,only one of which(the megaspore)survives.Female gametogene
42、sisplacental wallFemale gametogenesisthe megaspore undergoes 3 mitotic divisions to produce 8 cells:3 antipodal cells 2 synergid cells 2 central cell nuclei 1 egg cell(EC)Female gametogenesisplacental wallDouble fertilizationPollens land on the stigma,hydrate,and begin to germinate the pollen tubePo
43、llen 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.One sperm fertilizes the egg cell to develop into the embryo.the other sperm fertilizes the diploid central cell nucleus
44、to develop into the endosperm.MicropyleEggSynergidsAntipodal cellsCentral cell nucleiSpermsPollen tubeOvuleDouble fertilizationPlant reproductionOvule(1 to many)OvarySilique（长角果）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.Life cycle of a flowering plant2n2n2n2n1n2n
