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Developmental Plasticity and Cell Fate Determination

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So, now we will continue this lecture of characteristic of plant growth and development and anothervery important characteristic of plant development is plasticity.It is extremely important for the plants; considering the fact that plants are sessilein nature.They cannot move from one place to another place which means that they have to face allthe environmental disturbance or all the environmental condition just by standing at one place.And, to cope with this change in the environment they have adopted a very nice mechanism andwhich is plastic in nature.So, the plasticity is basically an adaptability to prevailing environmental condition.I will take couple of example for it and you will appreciate how developmental plasticityis adopted or it is acquired by the plants.So, if you take this example this is couch grass.So, the crouch grass basically it’s shoot grows below the ground as well as above theground.And, what happens when the shoots are growing underground, underground means it has a verydifferent environmental condition than the above ground.Underground the light intensity is less, oxygen availability environmental other factors whichare very different than the above ground.But, what you can see when this grass is growing or when the shoot is growing underground ithas a developmental program which is giving rise to long internode, and leaf like a structurewhich is called scale leaf and more important that at every node so, here is the thingsat node you can see lot of adventitious root development.So, this is important thing.So, there is a developmental program, there is a developmental program which is goingon here already, but once this growing shoots reaches the ground; once it is coming outsidethe ground it switches its developmental program.There is a change the first thing what happens that now it start making short internode so,short internode.So, the internode length is reduced to do that it has to change; it’s the basic programof developmental biology.And, what happens here another very important thing that now there is a formation of trueleaf blade like a structure.And, the most important thing what is happening that this shoot at the node it is no longermaking the roots.So, what it has to do developmentally, first thing what it has done?It has changed its program for making long internode to the short internode.It has major changes, changes in their program of the leaf development.Now, the leaf is different, different types of leaf and more important thing when it wasunderground it was having active developmental program for adventitious root development.But, once it is coming above ground it has to shut down the program of adventitious rootdevelopment.This is another case of plasticity, developmental plasticity.This if you look water crow foot which is a kind of aquatic plants.And, it grows in a way that a portion of these plants are submerged in the water, then aportion which is at the surface or at the interface of water and air and then a certaina part of this plant is aerial which is outside.And, that is why this plant makes three types of leaf.The leaves which are submerged they have a very different morphology, they have a verydifferent structure, they have a different function.If you see these which are basically at the surface of air and water or at the interfaceof air and water its morphology is very different than this one and the aerial leaves they aretotally different.So, this is one example where the program, developmental programs are being changed dependingon the environment; where this developmental program is happening, what is the surroundingenvironment for this development to happen.Next characteristic feature of plant development is how cell fate is determined.So, if you remember a previous class where we have discussed that the first step of organogenesisor differentiation is cell fate specification or identity of a cell or specifying identityhow this is possible.In animals this is widely known that the cell lineage is playing a very important role whichmeans that the ancestors of the cell, the cell which is giving rise to the daughtercells they are also passing the information for its identity.But, in case of plant you will realize that there are some mechanism of lineage dependentdevelopment, but which is very very restricted.And, the major developmental fate is defined by position dependent mechanism which meansthat the cell fate is determined by mostly by the neighboring cells, who is next to thecells, not entirely by the ancestors.This is important because in case of plants the cell migration is not occurring; duringanimal development cell migration plays a very very important role in the development.Whereas, in case of plant due to presence of a very rigid cell wall, very tightly gluedcell wall the plant lack the cell migration which means that the neighboring cell in caseof plant development is going to play a very very crucial and very important role.So, if you look that the information which is helping for a cell to take it’s a properfate it can be either cell instrinsic or cell extrinsic.Cell instrinsic means, is it lineage dependent or position dependent.If you look this schematic diagram if it is a lineage based mechanism then what happensif consider this is cell A, this is cell B; they are may be next to each other.But, cell A will already have its own information for a specification, B will have another informationfor the specification.And, the cells which are being generated from the A, they will carry this information fromthe ancestor.So, this descendants cells or the daughter cells they are going to take the identityor the fate as instructed by their parent cell.But, if this is position based mechanism then the parent cells they do not have any informationfor specification.They just divide and give rise the daughter cells, now the daughter cells which are nextto each other they pass the information.So, this cell might get information from this cell and this cell might get information fromthis cell.And, based on that they define or they decide what is my relative position in the body andthat helps them to take the identity and one way of doing it is the clonal analysis.So, in clonal analysis what we do we use a heritable visible marker such as pigment mutants.So, for example, if you take this plant or in any part of the plant and if you mutagenizethem or you expose them for some kind of mutagen and try to find out a mutant where, some cellsthey lack some kind of pigments, let’s assume chlorophyll.So, let’s so, if you take this leaf and if you irradiate them at early stage whatyou see that you might find a cell here which is losing the pigment.And, then later on at the mature leaf if you see there is a patch of cells which are losingthese things.But, if you take this leaf and irradiate at the later stage when the development is quitebit completed then you see that these patches are relatively smaller in size.So, what this tells?This tells that through the clonal analysis you can actually identify the lineage andthe cells which are coming from the lineage.So, there is a strong correlation between lineage as well as the position of it ok.Another way of detecting or or checking this is chimera analysis.So, chimeras are a kind of plant which might have two or more distinct cell population.So, if you take this example so, let us assume that this is a kind of top view of a of aplant and it has a kind of sectorial chimeras.So, one small portion or sector of this growing meristem has certain marked cells and othersthey are unmarked cells.And, if plants which are coming from this meristem, if you look them you can clearlysee that some regions of these plants which are coming from these cells they are the markedone.And, these two experiments using these two experiment you can actually trace out thelineage of a particular cell.Another kind of chimera could be the periclinal chimera, here what happens for example, ifyou the if you look this one.So, it is known that the meristem the shoot apical meristem it has basically three layers:layer L 1, L 2, L 3.And, it is known that epidermis which is the outermost layer of any organ it derives fromthe L 1, and L 2 is usually giving a cell layer which is beneath the epidermis.So, if you mark this cell layers the L 2 cell layers and then if you make a anatomy of themature leaf which is coming from here; what you could see that the cell layers which isbeneath the epidermis they have the marked cell.But, apart from that there are other cells as well which are away from these cells, theyhave also got the marked cell property.But, in case of epidermis it was observed that both the mechanisms functions.So, for example, the differentiation of epidermis is kind of position dependent as well as thelineage dependent for example, if you look the mother cell.So, the specification of mother cell; mother cell for the stomata or mother cell for thetrichome.So, if you look the epidermis leaf epidermis there are three types of cells, pavement cellsand the cells which are going to make stomata or the cells which are going to make trichome.And, what has been seen that from epidermis to mother cell formation it is mostly contactdependent, position dependent.But, the mother cell which is responsible for the stomata it is lineage dependent.Once it is the cell fate is determined it will follow the lineage and it will make thestomata whereas, the trichome development is basically adopting position dependent mechanism.Another very important technique which can be used to actually distinguish whether cellcell fate is determined by lineage or by position is laser ablation.So, laser ablation is a technique through which you can very specifically, targetedway you can kill some cell and then you look what happens to that cells.So, first example if you take I am going to kill this or if you kill the cell which iscalled quiescent cell center.So, this is a typical epical root apical meristem; in root apical meristem these cells are veryvery important cell.Why?Because, they pass the information they are very important to maintain the stem cell nichein the root apical meristem.So, all the cells which are directly in contact with QC they have meristemetic capabilityand what happens if you kill this QC or if you laser ablate this QC what was observedthat.So, if you look here this is procambium, procambium cells; what happens if you kill this QC throughthe laser ablation the some cells of the procambium they comes at the place of QC and then theymake new QC.What it means?It means that the fate or identity of procambium is now changed to the QC identity.And, if this happens here the cells which are in this region they originally they werealso procambium cells.They change their identity and then they make new columella cells which is typically presentbelow the QC.So, if you look here so, there is a change of the identity this suggests or this tellsthat the information is not lineage dependent.The fate the new fate determination or new QC or new columella they are taking a newfate depending on their position, not what was their initial programming or what wastheir information inside it.Similarly, another very important tissue if you look here in the root tip here this isQC; in QC these are the cells which are called initial cells.These are the cells which are meristematic cells and in case of root there are differentcell layers.So, cell layers are like epidermis, cortex, endodermis pericycle and vascular cells.And, what is important here that each cell; each initial cells they give rise to one layerof the cells, particularly here.If you look this initial cell it will give like this cell layer, but there is one cellwhich is here.If you look this cell this is initial cell because it is in direct contact with the QCand this cell actually eventually give like two layers.So, the layer of cortex and layer of endodermis they are originated from a single initial.And, this initial is called cortex endodermis initial.So, what happens this initial first it divides, divides and then it makes a kind of periclinaldivision.And, this periclinal division give rise to two cells parallel cells and one cells thenit continues making endodermis another cells it start making cortex.But, what is important here if you laser ablate this CEI what you see?What we see that the cells which are in the pericycle, this is the inner pericycle layerthe cells they basically are getting pushed in and they are taking the place of CEI.And, what happens when they take this place they undergo the process of a periclinal celldivision.And, the cells which are outside they now switch their identity and they become cortexendodermis initial; whereas, the cell which is inside they retain its capability or itsidentity as pericycle cells.So, both of these experiment suggest that the new cell fate is not inherited or it isnot coming from the mother cell.But, it is getting acquired depending on the place of the cell where this cell new cellis positioned.If position is very important in case of plant growth and development what are this positionalsignal,?how a particular cell realize or how a particular cell acquire a particular identity?how it gets the information from the neighboring cells?And, there are certain mechanisms which has been identified the first mechanism is morphogen.Morphogen are the signaling molecules which basically is responsible for making a kindof for providing a cell fate or providing identity.And, one of such molecule very important molecule is auxin, auxin is a plant hormone.The auxin distribution is very important, it is being transported in the polar mannerand there is a formation of gradient.So, auxin distribution, biosynthesis, storage, distribution everything is important.And, the distribution of auxins some places it makes gradient which means that the somecells they always get high amount of auxin, some cells they get low amount of auxin.Not only presence or absence of auxin, but critical amount of auxin is very very importantfor switching on a certain developmental program.And, another thing which is important with respect to auxin is its concentration.So, some developmental programs are switched by auxin, once auxin reaches a particularamount of concentration which we called auxin maxima.So, if you see here if you look this picture so, this picture has luciferase marker, butthis luciferase marker is driven under DR 5 promoter.DR 5 promoter is a promoter which has auxin response element which means that it is responsiveto auxin hormone.So, wherever auxin response is there this promoter will light up.And, what you see you can clearly see a correlation that different regions of the developing primaryroots they have a very high amount of signals as compared to other regions and this is calledauxin maxima.And, another important thing if you make a correlation you see that this lateral rootsare coming from this region wherever there is high level of auxin or wherever there isauxin maxima.So, this tells that somehow the auxin maxima is correlated with the lateral root developmentwhich means that it might be responsible to initiate lateral root development or the programwhich is specific for the lateral root development.How it is doing?It might be activating some set of genetic network.Where it is activating?It might be activating in some of the cells which is competent of giving lateral rootprimordia; so, one mechanism which is through morphogen.Another mechanism; why a particular cell acquires a particular fate is typical receptor mediatedsignaling.This is example we will discuss in detail when we will see shoot apical meristem maintenanceor root apical meristem maintenance.But, here to say that there are some receptors in one cell which is CLAVATA 1 and 2 here.And, there are some signaling molecules which is secreted by the neighboring cells.And, this neighboring cell they secrete a some kind of signaling molecule which is receivedby the receptors and then these receptors they initiate a kind of genetic developmentalprogram, a developmental program which is important.So, the second mechanism of acquiring a special fate, position dependent special fate is throughthis mechanism.Third and very important mechanism in case of plant is molecular trafficking.So, as I said that the plant cell wall are very rigid, it is very tightly glued.Plant cannot move from one place to another place, their position is fixed during thedevelopment.Then how they communicate, and, to break the barrier of communication plant has developedof wonderful way of cell to cell communication which is called symplastic cell to cell communication.And, this can help in the short as well as long distance trafficking; trafficking means,there are lot of molecules, signaling molecules in forms of protein, in forms of messengerRNA, in forms of micro RNA they have been identified.And, they have been shown that they are being generated at one place and they are movingto the another place.The movement can be short distance, just next to the neighboring cell or even they can movethrough the transport system to a very very long distance.So, how this happens?If you take this example this is a protein which is called SHORTROOT protein, it is atranscription factor very important transcription factor and if you look its expression pattern.So, here is the transcriptional fusion construct for SHORTROOT, here is the translational fusionconstruct.So, what do you mean by transcriptional fusion construct?So, when you take promoter of SHORTROOT and put GFP downstream it.So, this is GFP and this is SHORTROOT promoter.So, what happens wherever this promoter is active GFP will be expressed and you can seethe signal.But, in translational fusion what you are doing you are making, you are taking SHORTROOTpromoter, you are taking SHORTROOT gene and you are just removing the stop codon and translationallyfusing with GFP.So, this is you are basically fused SHORTROOT protein with the GFP.So, here what you can do?The first thing is that the promoter will ensure the domain of expression and this proteinthe GFP will ensure the localization of the SHORTROOT protein.And, if you compare these two things what happens in the transcriptional fusion wherejust the transcriptional domain where the gene is getting transcribed you can see thisis the root tip.So, this is endodermis this layer is endodermis, and these layer is this entire layer is thevascular tissues.And, what you can see signal is only in the vascular tissues which means that SHORTROOTpromoter is actively transcribing only in the vascular tissues, proteins are being madeonly in the vascular tissue.But, when you track the protein SHORTROOT protein what happens that the proteins arepresent in the vascular tissue, but they are also present in the endodermis; this is veryimportant.So, from where this protein is coming and another interesting thing you can see herethat in endodermis protein is localized to the nucleus.So, from where this protein is coming which means that one thing is clear from here thatthere is a protein mobility.SHORTROOT protein which is getting synthesized in the vascular tissue it is moving to theendodermis because in endodermis transcription is not happening.So, gene is not getting expressed in the endodermis, the endogenous gene is not getting expressed,but protein is present.And, then we know that this is happening through a very very special structure in case of plantwhich is plasmodesmata.Plasmodesmata are the nanopores which basically is between two cells and they are providinga path, a symplastic path through which molecules can move from one cell to another cell.But, there are a lot of question, is the movement free?No, the movement through the plasmodesmata is highly regulated.So, only those molecules can move through the plasmodesmata which are supposed to move.Those molecule which are not supposed to move they cannot move because, if everything movesfrom this cell to this cell and everything moves from this cell to this cell then boththe cell will lose their own specific identity.So, both the cells they are ensuring their specific identity, but at the same time theyare allowing certain molecules to move from each other from one cell to another cell.And, this is a very very unique and very important feature which plant cells has acquired duringthe process of development and playing very important role in plant and growth and development.We will cover this in detail in the later classes, but if you look this picture is SHORTROOTprotein moving through the Plasmodesmata? and how it has been tested?So, there is a mechanism or there is a program through which what you can do, you can actuallyblock the plasmodesmata between endodermis and the vascular tissues.So, as in the previous slide I showed you that these proteins are the SHORTROOT proteinsare being made or being transcribed and translated in the vascular tissues and then protein ismoving to the endodermis.If somehow we block the plasmodesmata which is present in the cell wall between endodermisand the vascular tissue we should block the movement.And, this is what you can see here when we block this plasmodesmata you can clearly seethat the signal SHORTROOT protein signals are disappearing from the endodermis.Look here this is completely disappeared after 24 hour of the blocking plasmodesmata.So, this clearly tell that the critical or very important transcription factor, SHORTROOTprotein is basically getting move or getting trafficked or getting transfer from one cellto another cell specifically through plasmodesmata.Another important characteristic feature of plant growth and development is totipotencynature of the plants.What it means?Every cell in the plants in principle they have a capability that they can undergo theprocess of cell division cell differentiation de-differentiation anything and they can makea kind of plant.And imagine why this is so, important again I would say that plants are sessile they cannotmove.So, they have to have a mechanism or robust mechanism that under adverse condition, ifeverything they lose even they retain some kind of living cells they should revert backand they should propagate back to that one.So, this slide if you look here there are some natural mechanism of regeneration andthis regeneration mechanism there is a genetic program, there is a regulatory program forthis process; some of them we will discuss here, first thing if you look here and here.So, if somehow the root tips or shoot tips are damaged in a plant or if you cut themwhat happens?You know that the apical meristems are positioned in the shoot tips and the root tips.If they are lost so, there is a mechanism plant has acquired a mechanism that they canactually reconstitute, reconstruct the meristem, this is because of their regeneration capability.Another important thing if there is a damage if there is a tissue damage again plant hasa mechanism through which they can repair the damage.This is again getting activated through the process of regeneration, one thing which Idiscussed in previous class was the apical dominance.So, what I said that when plant is primarily growing then there are axillary meristem,axillary meristems are specified, but their growth is inhibited.Because, plant first want to ensure a certain amount of growth particularly they want totransit to the reproductive phase, they want to ensure first successful reproduction.And, then the signal is being transmitted and lot of axillary branches they startedgrowing.But, what happens if you cut the plant from here, they immediately sense it and sincethis is cut in the differentiated region; if you cut in the meristematic region theycan reconstitute the meristem.But, if you cut the region which is fully differentiated, it is difficult for them tofully reconstitute the meristem.But, what they does they immediately activates its axillary meristem and they started makingthe branches.So, axillary shoot out growth is now getting activated.Another very important thing is de-novo organogenesis.So, this is a property of plant cell.If you take any part of the plant which typically we are using in the process of tissue culturewhich we called explant.And, if you take this plant there is a genetic program through which you can activate a propergenetic program and you can generate a full plants or full plantlets.For examples you can take shoot and you can induce the root; you can take shoot and youcan induce the shoot.Similarly, you can take shoot and you can induce both shoot and root, you can take theroot and you can induce the shoot.So, what it tells that it tells that there are the genetic program, if you give a propersignal, if you give a proper information or proper hormonal combination you can eventuallyactivate a full and proper developmental program for the shoot development as well as rootdevelopment.Another important developmental program which occurs is the lateral root formation.So, as I said when plant is growing in the apical and basal axis it is growing by usingits apex or shoot apical meristem and root apical meristem.But, once these tissues they are entering in the region of differentiation there isa new meristem which is getting generated somewhere here.And, this meristem they acquire a capability and they make lateral root primordia and thenthey eventually they make lateral roots coming from here.So, what happens there is a difference between apex.So, apex; at the apex the meristems are specified as a meristem and they remain as a meristem.So, they are undergoing only the process of cell division.But, here what is happening that at fully differentiated cells they are undergoing theprocess of de-differentiation, through which they are losing their existing identity andthey are acquiring new identity.First the meristematic identity and then later they undergo a new round of differentiationprogram or totally new differentiation program and now they are making a new organ or a newtissue.So, the plants they also have this capability that even though if they are differentiatedthey can initiate if proper signal is given if required, they can undergo the processof de-differentiation and they can undergo the process of re-differentiation.So, if we summarize all the characteristic feature of plant growth and development.So, what we have seen?We have seen that plants are sessile and they are actually very prone to the physical damage.And, that is why they have evolved a developmental pattern that are continuous, responsive tothe environmental cues and regenerative in nature.These features are attained in large mostly by exchange of the information between thecells which can be local as well as long range.And, the cell fate in the developing region is more dependent on the positional cues ascompared to the lineage information.So, we will stop here and we will discuss in next class.Thank you very much.