3 Transition to reproduction Flower organ developmentGametogenesis and fertilizationPlant Reproduction
4 Transition to reproduction Flower?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 readyInflorescenceVegetative phaseReproductive phase
5 Production of flowers involves two transitions in Arabidopsis Convert SAM to inflorescence meristem (infinite, making lateral organs)2. Convert inflorescence meristem to floral meristem (terminal, flowers)SAM convert to meristems that can make flowersInfluorescence mersitems can produce more lateral branches – can further branch into tertiary branchesInfinite branching as long as plant’s age allows – produces branches and leaf organsConvert influorescence to floral mersitem – make final flower (cluster of flowers at each end of inflorescence shoot)Once floral meristem finishes function to make flowers, some cell activity lost at that region – terminal organSC: stem cellP: organ primordiaSe: sepal
6 Factors regulating the transitions Vegetative meristemGenes (flowering-time genes and floral identity genes)Light (photoperiod)The biological clockTemperatureHormonesMany factors regulate transition-Vegetative mersitem can convert into inflorescence meristemInflorescence meristemFloral meristem
7 Flowering-time genesaffecting the transition of vegetative growth to reproductive growthFlowering-time genes regulate general transition from vegetative to reproductive growthEmf1 and emf2 mutants flower during seedling stage – don’t have to go thru long process of making more shoot, root, etc.Very early on, seedlings can make flowers – genes negative regulator of reproduction(early flowering mutants – negative regulator of reproduction; promote vegetative growth – so under normal conditions, go to vegetative growth)WTemf1emf2embryonic flower
8 Floral identity genesaffecting formation of inflorescence and floral meristemsFlower (from Floral meristem)Floral identity genes – inflorescence and floral meristem affectedFlower from floral meristem and inflorescence from inflorescence meristem – how regulated?Other types of mutants can tell functionInflorescence (from Inflorescence meristem)
9 Mutations in floral identity genes Terminal flower 1 gene normal function is positive regulator of inflorescence (when mutant, lose inflorescence shoot activity = flower early; SAM (vegetatuve) converted to floral mersitem (for making flowers))Leafy – delay flowering; thus leafy gene acts as negative regulator of inflorescence meristemterminal flower 1 (tfl1):Convert the inflorescence meristem to the flower meristem.leafy (lfy): produce more inflorescences,delayed flowering
10 Factors regulating the transition to reproduction Vegetative meristemEMFInflorescence meristemLFYTFLFloral meristem
11 The discovery of photoperiodism Garner and Allard (1920’s)Soybeans (Glycine max) planted over a three-month period all flowered about the same time
12 Many more experiments were followed: Eliminate a variety of environmental conditions: Nutrition, 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
13 PhotoperiodVaries according to the latitude and seasonal changes.
14 Critical daylength Critical Daylength (CD) Xanthium: a short day plant, flowers when CD is LESS than 15.5 hours.Hyoscyamus: a long day plant, flowers when CD is MORE than 11 hours.
15 Plants are induced to flower by different photoperiods short day (SD) : plants are stimulated to flower when the length of day falls below a thresholdlong day (LD): 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 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.
16 Do plants really measure the length of the daylength?
17 Hamner and Bonner (1938): Xanthium strumarium, a SD plant with CD = 15 Hamner and Bonner (1938): Xanthium strumarium, a SD plant with CD = 15.5 hoursXanthium flowers when the dark period exceeds 8.5 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.
18 Similar results were obtained with other SD plants. For LD plantsA longer dark period inhibits flowering.Light break induces flowering.SD: A longer dark period promotes flowering.Light break inhibits flowering.
20 Exp. 1: The leaf or apex of Perilla (a short day plant) was exposed to different daylength. When leaf part treated w/ short day (regardless w/ what apex treated) – plants made flowerWhen only treated single leaf w/ short day – plant couldn’t make flowerThe leaf, not the apex perceives photoperiod
21 Exp. 2: 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 transmittable
22 The 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 transmittableSAMFlorigen 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 revolutionizedFlorigen?FlorigenFlorigen
23 The biological clockPresent in plants, animals, fungi, and some photosynthetic bacteriaAn internal time measuring system (“clock”) that runs on its own with a periodicity of nearly 24 hours. It can be “reset” by external signals.TemperaturePresent in many different organismsVery important to control daily activities of organismsMovement of leaves, when to make flowers, etc.Central component of clock is central oscillator – internal time measuring machine in many organisms – can run independently of external factors (intrinsic) – can also be regulated by external factors such as light and temperatureOutput of clock represented in rhythmic activity – circadian
24 The Arabidopsis biological clock The central oscillator: CCA1, LHY, and TOC1 (these are transcription factors) and other proteinsThree TF regulate expression of each other-TOC1 is activator of CCA1 and LHY-CCA1 and LHY are repressors that inhibit TOC1In morning, TOC1 accumulate to certain abundant level – w/ more TAC1, other two genes expressed in morningMore expression of these two genes, more of those two proteins made – those two proteins bind to each other to form heteodimer – bind to promoter of TOC1 gene to inhibit its expressionWhen have more LHY and CCA1 = less TOC1 proteinAt end of day, TOC1 protein eventually disappear thru protein degradation and lack of making new TOC1 protein, so at end, lack of TOC1 will make expression of these two genes turned off – so no more LHY and CCA1 protein madeIn evening, no more genes to expression this point, eventually LHY and CC1 protein disappear – as a result, TOC1 no longer being de-repressed – so expressed and accumulate in eveningIn morning, CCA1 and LHY more thus TOC1 once again repressed
25 The Arabidopsis biological clock CCA1 and LHY are expressed during the day and together repress expression of TOC1 during the dayThese three make up central component – expressed in circadian mannerExpression of CCA1 and LHY and TOC1 is circadian regulatedArabidopsis seedling expresses CCA1::luciferaseTOC1 is expressed at night and is required for activation of CCA1 and LHY1, beginning just before morning
26 Mutations in the clock genes Lack of the nyctinastic movement: diurnal rise and fall of leavesAltered flowering time in some mutantscca1: early floweringlhy: early floweringtoc1: early floweringSome other clock mutants can be late floweringDifferent type of mutations – some make clock go fast and others make clock go slower
27 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.Vernalization—low temperature treatment can promote flowering time in some plantsLow temperature treatment at earlier time but can still affect later plant development for plants to make flowers or notAffective temperature can vary and in different plantsSome plants don’t need vernalization - ex. plants grown in cold regions – have to go thru winter time so have to be vernalized – in high temp, don’t need vernalizationPerceived by shoot apex – if treat dry seeds w cold temp – don’t matter – only when seed germinates, shoot apex active and can perceive cold temperature and affect floweringNot known which molecule induced – induced stage can stay and later on affect floweringAlso grafting transmissible
28 Cold acclimationCold acclimation is different – couple of days of cold weather before deep winter – very important for plant b/c repeated cold treatment to trees can dramatic increase plant tolerance to freezing temps later on in deep winter timeCold acclimation can be induced quickly (not long term like vernalization) and it doesn’t affect flowering timeCan be induced quicklyIncreases plant resistance to freezing stressDoes not affect flowering time.
29 Hormone GA regulates flowering time GA1: 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.GA positively regulates flowering time
30 Inflorescence meristem Flower development in ArabidopsisVegetative meristemInflorescence meristemTransition to reproduction:Genes & other factorsFlower organ development stage less regulated by environmental factors (light, temp) , but more by intrinsic - genes (organ identity genes)Floral meristemFlower:sepals, petals, stamens,and carpelsFlower organ development:Organ identity genes
31 Flower organs petal stamen carpel sepal Petals - attract polinators Stamen – filament and anther (pollen sac)Carpel – stigma (flower perceives pollen; pollens land) and style – long or short neck – connects stigma to ovary; Ovary houses ovules – contain egg cells and central nuclei for double fertilizationSepal – green leaf like structure; degenerative leaves – can perform limited photosynthesis – protect flower bud before it opensReceptacle – holds the flower
32 The flower is generated from the floral meristem Floral meristem can give rise to cluster of flowers, but in some one flower meristem can only give rise to one flowers (ex. Roses)the floral meristem
33 Produced in 4 concentric whorls with the same order Flower organsProduced in 4 concentric whorls with the same ordersepal (whorl 1) stamen (whorl 3)petal (whorl 2) carpel (whorl 4)w/ Arabiposis flowers – many mutants identified to understand processes of flower organ formationFour whorls –3rd whorl – 6 stamensMiddle – two carpels
34 sepal-petal-stamen-carpel stamen-carpel-stamen-carpel Mutants have weird flower structure – puzzle for many yearsApetala2-2 mutant = only see stamen, carpel, stamen, etc.Ap1 mutant is similarsepal-petal-stamen-carpelstamen-carpel-stamen-carpel(the ap1 mutant is similar)
35 Aptala3 and Pistillata = see sepal, sepal, carpel, carpel (lose petal and stamen; 2nd and 3rd whorl are lost)wtapetala3 (ap3)pistillata (pi)sepal-petal-stamen-carpelsepal-sepal-carpel-carpel
36 sepal-petal-stamen-carpel sepal-petal-petal-sepal Agamous 1 – see sepal and petal only; so lost two inner whorlssepal-petal-stamen-carpelsepal-petal-petal-sepal
37 The “ABC” model for flower development AP1, AP2BAP3, PICAGABC model used to explain weird flower structureA, B, and C genes function to specify different structures in diff. whorlsA+B = 2nd, B+C = stamen, and C alone = carpel = only when functioning in all three categories, have normal flowerA and C genes are antagonizing each other in their function – only in certain physical regionThe ABC genes function in the distinct fields.The A and C genes are mutually exclusive in their expression.
38 WTAp1 and ap2 mutants are A genes – if knock them out, C will expand to A territory – don’t have sepal and petal, but will have carpel in sepal position b/c C expanded into A region and C only specifies carpelSo, get carpel-stamen-stamen-carpelap1 or ap2The A genes: ap1 or ap2 mutants should (and do) make carpel-stamen-stamen-carpel
39 WTAp3 and pi are B genes, if lost them, lost combination of AB and BC, so lose petal and stamen and have sepal-sepal-carpel-carpelap3 or piThe B genes: ap3 or pi mutants should (and do) make sepal-sepal-carpel-carpel
40 The C genes: ag mutants should (and do) make sepal-petal-petal-sepal WTIf don’t have C genes, loss of C, make A expand to C’s territory and lose stamen and carpelSo have sepal-petal-petal-sepalABC genes are all transcription factors that act as master regulators to control downstream genes – all together to regulate floral organ formationAll ABC genes function within physical restriction spaceThe ABC genes are TF that set up the boundary of gene expression to determine which organ will be formed, and whereAdditional downstream genes are required to ensure the property development of each organagThe C genes: ag mutants should (and do) make sepal-petal-petal-sepal
41 Flower organ function: Gametogenesis and FertilizationGametogenesis involves 2 meiosis – takes place in two different locations
42 Male gametogenesisDiploid 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):Pollen mother cells undergo meiosis to produce tetrad of haploid microspores – attach to each otherLater separate into individual pollen cells – undergo mitosis to give rise to two cells for each pollen cellOne cell engulfes the other cell (small = generative cell – lives inside the large, vegatative cell)the generative cell (small)The vegetative cell (large)generativecell
43 Vegetative cell grows to produce pollen tube Goal of pollen tube is to deliver male sex cells to femaleGenerative cell produce 2 sperm cells via mitosis II – 2 sex cells for double fertilizationthe vegetative cell grows to produce the pollen tubethe generative cell produce 2 sperm cells (mitosis II)
44 Female gametogenesisan ovule primordium emergesas a bump on the inner wall(placenta) of the ovarythe megasporocyteundergoes meiosis to produce4 haploid cells, only one ofwhich (the megaspore)survives.Ovule inner wall can bud to form ovule primordium – one of the cells undergo meiosis to produce 4 cells – only one survivesFurther divides three rounds to make 8 cells
45 Female gametogenesis placental wall the megaspore undergoes 3 mitotic divisions to produce 8 cells:3 antipodal cells2 synergid cells2 central cell nuclei1 egg cell (EC)3 antipodal cells (die eventually)2 synergid cells located right at entrance of ovule – micropylePollen tube navigate all the way to micropyle and deliver sperm cellIn end, synergid cells die – only 3 cells survive
47 Double fertilizationPollens land on the stigma, hydrate, and begin to germinate the pollen tube
48 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.Pollen tube doesn’t penetrate but navigates in b/w of cells
49 Double fertilizationOne sperm fertilizes the egg cell to develop into the embryo.the other sperm fertilizes the diploid central cell nucleus to develop into the endosperm.OvuleAntipodal cellsCentral cell nucleiOne sperm cell will fuse w/ egg – into embryo and other will fuse w/ central nuclei – into endospermEggSynergidsPollen tubeMicropyleSperms
50 Plant reproductionOvule (1 to many)OvarySilique
51 Fruit development The ovary and other tissue together produce a fruit. Fruit is important for seed dispersal in many speciesMany foods are also called “vegetables”: tomatoes, pea pods, squashFruit size, texture, and sugar content are determined by genes.Ethylene stimulates fruit ripening.