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Life Cycle of an Angiosperm

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Hello everyone my name is Shri Ram Yadav from Department of Biotechnology IIT Roorkee.I welcome you all for this course of Plant Developmental Biology, for this course I wasassisted by Mister Tushar Garg a PhD scholar in my laboratory.Coming to the introduction of plant developmental biology, so in plants or any higher organismgrowth and developments are very very important and essential processes.And plant developmental biology is a study of how various tissues and organs are developedstarting from a single cell zygote in a complex multicellular plants.How different organs are developed during various phases of their life cycle?So, this course will provide an overview of mechanisms underlying the primary meristemestablishment and maintenance, cell specification and differentiation during various organ developmentin flowering plants.In general, if you look the life cycle of a land plant, it has two major phases: sporophyticphase and gametophytic phase.So sporophytic phase, basically it is diploid phase of life cycle and it is started froma single cell which is called zygote.This zygote which is generated by the fusion or fertilization of male and female gametesand after this fertilization this diploid zygote cells undergo the process of cell divisiontypically mitosis cell division to generate multicellular sporophytic phase.And then at some point of time in their life cycle this sporophytic phase undergo the processof meiosis and this meiosis generates many haploid spores and this haploid spores whenthey undergo the process of cell further cell division mitotic cell division and they generatemulticellular gametophytic stage.And this entire process is called alteration of generation in land plants.So, if you look the overall evolution of land plants.So, the land plants they belongs to streptophytes clade of plant kingdom and this streptophyteclades it includes charyophytes which is green algae and embryophytes which is typicallycalled land plant.And one important thing is that in charyophytes the sporophytic stage is unicellular whereas,in all other land plants this is multicellular.In embryophytes this can be further divided into vascular plants and nonvascular plants.So, the bryophytes which belongs to the nonvascular plants and then lycophytes onwards they belongto the vascular plants.And then at some time point during the evolution particularly from here there is a true leafformation and then later on there is evolution of seeds like structure.So, if you look the event of development of the plants.So, very important events one is the multicellularity in the sporophytic and then the vascular tissue.This vascular tissues are very important because they forms the major transport system in landplants or in higher plants and it is very important for land plant since they are terrestrialthey become sessile.It is very important for them to have a very robust transport system, required to takeminerals and water from the soil.We will be mostly focusing our developments on the angiospermic plant.Further if you look the alteration of generation across the land plants.So, there are few important things which you can see.So, in bryophyte the major part of the bryophyte life cycle is the gametophyte, and sporophytewhich is dependent on the gametophyte.But in fern onwards the major component, the large form is the sporophyte and gametophyteis basically a small independent form.But in seed plants which which includes basically gymnosperm and angiosperm, it has a reducedgametophyte which is basically dependent on the major large sporophyte.So, during this evolution there is, if you look the duration of their life cycle betweensporophytic versus gametophytic stages.So, mosses which is having more gametophytic or more duration of the gametophytic, lessduration of the sporophytic.Whereas, if you go across ferns, gymnosperm, angiosperm, the gametophytic stage are gettingrestricted and sporophytic stage are getting expanded.Now, we will discuss about the developmental events during angiosperms life cycle.Typically an angiospermic plant has two major part; one is the root system, another is theshoot system.And as we discussed earlier a sporophyte is a dominant phase of the angiosperm life andthis entire sporophyte can be divided into three major phases, the one is the embryodevelopment, then vegetative development and reproductive development.The embryo development starts after the process of fertilization and then it terminates bythe seed dormancy.Then vegetative phase starts when seeds are germinating and then during the vegetativephase all the vegetative organs are formed.And then at the certain time point there is a transition, there is a phase transitionwhich is called floral transition where vegetative phase get converted into the reproductivephase and during reproductive phase the reproductive organs like flowers are developed.And in this flower there is a sex organs which is basically anthers and carpel and therethe process of gametophyte is restricted in case of angiospermic plant.So, if you look the embryogenesis and seed development process.So, actually during this embryogenesis process the basic body pattern is established beforethe seed dormancy.So, in overall throughout the embryogenesis process four major things are happening.Number 1, the apical basal axis polarity is established, number 2 radial tissue patterningoccurs which occurs through the spatial organization of the tissues particularly epidermis groundtissue and vascular tissues.Third important things what happens is the positioning of shoot and root apical meristemalong the apical basal axis and finally, there is a formation of cotyledons.So, if you look embryogenesis process.So, embryogenesis typically begins with an asymmetric cell division of zygote which producesa small terminal cell and a large basal cells.The terminal cells makes proper embryo and basal cells produces suspensor.Eventually the suspensors generates a stock or a filament which attach proper embryo tothe maternal tissues.At this stage if you look the shape of embryo, it looks like globular and this stage is calledglobular embryo.During globular embryo stage, the three major body tissues are established, the outer mostlayer which is called epidermis or protoderm, then middle layer is ground meristem or groundtissues and the inner most layer is procambium.Then next things what happens, cotyledon primordia initiates and this results in the change ofthe shape of the embryo, now it becomes heart shape and further this heart shape get convertedinto torpedo stage embryo.And this is the stage around which the shoot apical meristem and root apical meristemsare actually positioned at apex.Shoot apical meristems are positioned at the apical side and root apical meristems arepositioned at the basal side.This is a mature seeds where you have a seed coat and proper embryo which is attached withthe suspensor and in this embryo we have the basic body plants.For example, root apical meristem, shoot apical meristem, pro vascular tissues and at thisstage this embryo undergo the process of dormancy and it can stay for long period of time.The embryo breaks their dormancy during seed germination and which requires a proper externalcues such as temperature, moisture and lights.Seed germination require extensive cell elongation, depending on the region of elongation thepattern of germination can be categorized into two types; one is the epigeal germinationand hypogeal germination.In epigeal germination elongation occurs in the region between cotyledon and the radicalwhich is also called hypocotyl and cotyledons comes above the ground.In Hypogeal type of germination, when elongation occurs in the epicotyl region which meansthe region between the cotyledons and shoot apical meristem, the cotyledons remains belowthe ground in this case.This is the typical example of epigeal germination and hypogeal germination as you can see here,the cot this the region, hypocotyl region when it elongates the cotyledons are comingoutside.In case of hypogeal when the region between shoot apical meristem and cotyledons is elongates.So, there is the cotyledons remains underground.Next developmental stage is vegetative developmental stage.This starts with the seed germination and once you put seeds for the germination plantsundergo mostly post embryonic vegetative development.During primary vegetative phase following developments occurs.First very important thing is the primary growth which occurs at the apical meristemboth shoot apical meristem and root apical meristem.For having continues growth it is very important that plant always maintains the both the meristemsthroughout their life.Next during vegetative development is the vegetative organ formation, the process iscalled organogenesis.Slightly lateral stages or at the same stage you will also have a process of organ patterningwere special tissues are patterned in a proper configuration.And vascular tissues development which is also part of tissue patterning occur simultaneously.So, if you look a typical example of root development.So, this is a typically growing primary root.The primary root can be divided into three zones, this is the meristematic zone, thisis elongation zone and this is the maturation or differentiation zone.And root apical meristem is positioned at very tip of the growing root.And the important process which is root apical meristem maintenance, and this basically generatesthis has a stem cells which generate the cell lineage for tissue patterning later on.In higher region tissue patterning occurs where you can have a clear different layersof the tissues arranged in a proper pattern.In differentiation zone one of the very important process occurs, which is called root branching.In root branching lateral roots are formed.You can see lateral root primordias are initiated first specified then initiated and then finally,lateral root primordia emerged out.Similarly, if you look the shoot development, during shoot development, the stem cell maintenanceoccur in the region which is the shoot apical meristem.This is the region where stem cells are maintained and when this stem cell, when the cells leavesthe meristem in the peripheral zone, it undergo the process of differentiation.During vegetative phase the major organ which forms is the leaf you can see P 1 is the firstleaf primordia, second leaf primordia, third leaf primordia.So, during shoot development the important process which takes place is the root shootapical meristem maintenance, organogenesis, tissue and organ patterning and then shootbranching.Shoot branching normally occurs through the axillary buds and of course, vasculature isequally important.So, once this vegetative process is completed, vegetative development is completed a majortransition occurs in the life cycle of an angiosperm plant and then transition is fromvegetative phase to reproductive phase.Lot of internal and external factor regulates this process, but once the decision is takenplace the vegetative phase transit and it start making the reproductive organs whichare basically flowers and flowers are typically arranged in form of inflorescence.So, if you look at the meristem level.So, during vegetative phase the structure which is responsible for the growth is shootapical meristem, but at the time of transition the identity of the shoot apical meristemis changed, now it becomes inflorescence meristem.The shoot apical meristem, it was making leaf at the flanks, but once its identity is convertedto the inflorescence meristem the flanking primordia is now floral primordia.So, if you look at typically growing inflorescence you will have lot of flowers and then theflowers will have floral organs which is sepal, petal, stamen and carpels.And if you look the stamens and carpels, a very special tissues or very special cellsof stamen and carpels they have a restricted gametophytic phase.If you look this plants, so this is a typical flower, this is typical angiospermic flower.In flower there are four organs mainly the sepals, petals, stamen and carpels.The stamen is male reproductive organ, carpel is female reproductive organ and then theprocess of gametophytic stages initiate from here when anthers which is a male part ofstamen produces pollen grains and carpels which has ovule.When they undergo the process of fertilization they makes seed which contains embryo.And the seeds has all the information, all the basic body plan in embryo and when wetake the seeds, germinate it makes the complete plant.So, the process of gametophytic therefore, can be divided into two classes; one is themegasporogenesis and microsporogenesis.This is flower which is a reproductive organ.So, as I said earlier flower has sepals, petals, stamens and carpels.Stamens and carpels are the reproductive organ whereas, sepals and petals they are accessoryorgan.So, though sepals and petals they do not directly involved in the process of reproduction, butthey play a very very important role in successful completion of the reproduction.And in a flower all these organs are properly organized and arranged, so they arrange inform of whorl.So, during the development it is not only the organ it is not only the identity, buttheir patterning is also equally important.So, if you look sepals they makes the first organ, then petal, stamens and carpel andwe will also see in this course that how this floral organing, organ patterning is happening.Now, during the gametophytic phase, so we will look how female gametophyte developmentsoccurs.So, if you look the female reproductive organ which is carpel in angiosperm, the carpelhas three regions the stigma, style and ovary.In ovary there is a structure called ovule and in ovule we have megaspore mother cell.So, this megaspore mother cell which is diploid in nature, it first undergo the process ofmeiosis to generate four megaspores, four haploid megaspores and then during later stagesthree of this megaspores they degenerate only one remains as a functional which is calledfunctional megaspores.And this functional megaspores undergo the process of three round of mitosis withoutcytokinesis to generate a cells with 8 haploid nuclei.And then later on this haploid nucleis they get rearranged and they make this structurecalled embryo sac.Embryo sac development, so this entire process of embryo sac development can be clearly dividedinto two groups or two stages.The first stage is megasporogenesis, where megaspore mother cells divides through themeiosis to generate haploid functional megaspores.Then another stage is megagametogenesis where this megaspores undergo the process of mitosiscell division and rearrangement to generate a female gametophyte.And if you look female gametophyte this female gametophyte which is also called embryo sacit contains three antipodal cells on the apical region of embryo sacs, then it has two synergidsand one egg cells at the basal end of the embryo sac and then a central cell containingtwo haploid polar nuclei.Similarly, if you look the male gametophyte development.So, as I said that the male organs are stamens; stamen has filaments and anthers this is across section of anther here we have microsporocyte’s which is diploid in the chromosome contentand this microsporocyte’s when it undergo the processes of meiosis I it generates 2haploid nuclei, then meiosis II and it makes a tetrads.This tetrads release and then later on this microspores they undergo the process of anotherround of mitosis and they generate two asymmetric cell; one is called vegetative cell anotheris called generative cells.Later on this generative cells migrate inside and they makes a kind of cell within cellappearance.Here there are two category, in first category the generative nuclei they undergo the processof mitosis II and generate 2 nuclei before the germination or before the pollination.But in second case this they remains in this form and when they get attached with the stigmaof carpel, then this division takes, but in overall because of this divisions there aretwo sperm cells are generated.Next stages of the life cycle is pollination and fertilization, so once the pollens areready.So, this is a typical germinating mature pollens which is which has to a sperm cells and onevegetative cell nucleus and this is mature ovule cells.So, in ovule cells you can clearly see there are antipodal cells; there are synergids,egg cells and polar nuclei.But the pollination is a process through which the pollen grains they get attached with thestigma of carpel and then they germinate, but it is not always the case.So, all the pollens cannot fertilize or cannot pollinate with any kind of carpel.So, there is some self incompatibility issue.So, if pollens are not compatible with the carpel they may not result in the successfulpollination, where either they their germination itself is inhibited or even though if it startgermination, but they cannot reach to the ovule and they cannot fertilize the embryosac.But eventually what happens at in the case where the fertilization is successful, thepollen germinates and it makes a structure called pollen tube, when pollen tube reachthe micropyle of the ovule, there is if you look here it is more clear here the one ofthe synergid cell it secreted some kind of signals and this signal is very importantfor pollen tube to reached to the embryo sac.And once it reach to the embryo sac it releases two of the sperm cells.One sperm cells it goes and fertilize the egg cells and makes diploid zygote.Another sperm cell it goes and fuses with the polar nuclei two polar nuclei and it makesa triploid cells which eventually generates the endosperm.And that is why this process is called double fertilization because there are two roundof fertilization occurs.So, at the end if I summarize this life cycle of angiosperm plant.So, as I said that flowers are the organs where we have the reproductive organs, malereproductive organs as a stamen, female reproductive as organs as carpel.The male reproductive organs they generates microspores or pollens haploid pollens throughthe process of microsporogenesis.Female reproductive organs they generates embryo sac which contains egg cell and polarnucleoid through the process of megasporogenesis.And then the process of fertilization or process of pollination completes the double fertilizationand then this entire structures are developed as a seeds.So, life cycle of angiosperm plants, so it is evident that angiosperm plants alternatestheir life cycle between diploid sporophyte and haploid gametophyte.Sporophyte is an extensive phase whereas, gametophyte is more restricted.Male gametophyte develops in anther of the stamens and contains haploid sperms.Female gametophyte is produced within ovules of the carpel and contains haploid egg cell,two haploid polar nuclei, three antipodal and two synergid cell.Double fertilization is characteristic feature of an angiosperm which occurs when pollentube discharges two sperm nuclei into the female gametophyte.One sperm fertilizes the egg cell to develop a diploid zygote and other combines with twopolar nuclei to produce a triploid endosperm.So, this is all for this class in next class we will discuss about characteristic of plantdevelopment.Thank you.