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Characteristics of Plant Growth and Development

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Welcome to the second lecture of Plant Developmental Biology.So, in this lecture what we are going to talk, about the Characteristics feature of PlantGrowth and Development.So, if you see the entire process of growth and development is basically it covers a formationof a complete structure or complete body of a plant, three dimensional structure of aplant from a single cell which is called zygote.So, the entire process from a single cell to a multicellular structure is called growthand development and it is very important that in case of plants both growth and developmentsare highly highly coordinated.It is because the development is mostly post embryonic, it starts after the germinationas you will see in the later part.So, if you define plant growth, it is defined as irreversible change in the size and biomassof a plant.And development is basically irreversible change in the state to achieve a particularspecified structure or specified function.So, one thing is very clear, so you are going to make a very complex multicellular organismand that is starting from a single cell, which means that there has to be a process, a mechanismto generate a large number of cells from the single cell and this process is called celldivision.Cell division means here I am going to talk about the mitosis cell division because duringgrowth and development only mitosis cell divisions are involved except a very small part of thegametes development.So, during the growth what happens there is a process of cell division which generatesa large number of cells, and this is important.This is important because if you want to grow you need a large number of cells, but at thesame time you cannot just produce the cells, this cells has to go and take a proper identitythis is very very important.For example, if you look this typical plant.The plant has a root system and shoot system and if you look an organ which is placed hereit is very different than the organ which is present here.So, the leaf is very different in terms of structure, in terms of anatomy, in terms ofcellular organization and in terms of the function.So, how this all processes or how all this different types of organs are specified iscovered in process of development.So, during for the growth of the plant the two major fundamental process which is veryimportant is cell division and cell elongation.So, cell division is the process which generate the cell and cell elongation to certain extentit helps in growth of certain organs, but the major contribution for the growth anddevelopment is cell division.Similarly, for development the cells which are coming off after the division they haveto take a proper identity, they have to undergo the process of specification, this is veryimportant.And how this happens?This happens through a very fundamental process which we call cell differentiation.So, cell differentiation is the process together with the cell division ensures a proper organformation or development of proper organs.Whereas, cell division and cell elongation these two processes together they ensure propergrowth in the plant.Another important thing which is worth to note down here is that where this processof cell division occurs.So, in a growing plants there are certain specified position where the process of celldivision occurs.So, one is the shoot apex, root apex then we will come in the later slides and you willsee, that there are some meristematic activity in the radial pattern, we have meristematicactivity at the axial side, we will see in detail.As I said the process of plant growth and development is highly coordinated, which meansthat how this is being achieved this is very very important.This is being achieved by a coordinated cell division and cell differentiation.So, which means that two very important process which has to be running simultaneously inparallel; the one thing is that organization and maintenance of the meristem.Meristem is a population of cells which has a stem cell property.So, stem cell, why you need stem cells?Stem cells are the cells which can undergo the process of cell division without enteringin the process of cell differentiation.So, which is very important in case of plant, so and the second process which takes placeand which is a result of cell differentiation is organogenesis.So, both the process has to be maintained simultaneously.So, there has to be a region, there has to be a mechanism to produce the cell, then therehas to be a mechanism that some of the cells they retain their property as a stem cellsand the some of the cells they should enter in the process of differentiation.What is the meaning of differentiation?What occurs during the differentiation?The first thing what happens, once this cells when it leave the region where it is meristematic,it enters in the process of cell fate determination, this is the first thing.So, the cell has to take a proper fate for the development.So, this is a stage when the identity is established.So, let us assume if cell is going to make leaf then it has to take a leaf fate, if ithas to make flower it has to make flower fate, this is in general, but how this happens?Then second process what occurs during the organogenesis is the tissue specification.So, one organ is not made up of a single tissue there are multiple tissue and different tissuehas different structure, different function, different organization.So, the second stage after fate determination is tissue specification, here the specialization,tissues specialization occur.Then the next thing what is being ensured is the patterning, so the tissues are alwayspatterned in a proper manner, there is a fixed pattern of the tissues.For example, epidermis is the outer layer, so it will always remain as a outer layer.The layer which is just below the epidermis has to be cortex or hypodermis whatever dependingon the tissues.Then another interesting thing what happen during the process of development is the polarity.So, if you look the proper or complete structure or shape of an organ it has mostly polarity.If I draw here, so if you look leaf here.So, leaf at the base is very different than the tip.So, this is the polarity and this is also being established during the process of development.And the final thing what happens in the development is organ determinacy.Whether this growth or the development should continue or it should stop.So, this is called determinacy.So, continuity is finally, ensured in the development, if organ is it depends on theorgan, if organ is determinate in nature then up to after a certain time point it will stop.If organ is indeterminate in the nature it will continue to grow.So, if I take few example two example here, so this is shoot primary development and thisis root primary development and here is the region where meristem is maintained.In case of root here is the region where meristem is maintained.So, if you look here the central zone; the central zone is the region where shoot apicalmeristem or population of shoot stem cells are present.So, when these stem cells are present in this region, it will only divide it will undergothe process of cell division and the cell differentiation is inhibited.But once this cells leave this region and enter in the peripheral region then it undergothe process of cell differentiation and depending on the position, signal and information ittakes a proper cell fate it undergo the proper process or cell specification and differentiationto make a specific organ.Similar is the case in case of root.So, if you look the region of root apical meristem here meristem is maintained, butwhen the this cells enters the initial cells they specifies some tissue those tissues arepatterned, if you go to the maturation zone or if you go to the differentiation zone,then new organs are being formed.This all process are essential as well as highly coordinated.As I said previously that the one of the major function of growth and development is to establisharchitecture.Plant architecture is very important parameter, it is extremely important parameter.It not only provides a proper shape or proper morphological structure, but it also helpsin plant to adapt in a different environmental condition this is very important.And all therefore, the process of development has to be properly organized and this entireprocess of plant development occurs across the 4 axis.So, if I draw here, so you can see if let’s consider this is a typical growing plant.This is root, so we have a root system, we have a shoot system.So, the first axis where growth and development occurs is this axis which we called apical-basalaxis and this occurs this is mostly responsible for the primary growth.So, and this is because of the activity of root apical meristem and shoot apical meristem.Second axis through which growth and development occurs is central peripheral axis or radialaxis.So, as we know that during primary growth the major growth occurs across the apicalbasal axis, but at a certain time point there is another growth which is called secondarygrowth which starts in this direction.So, the second axis is established as a central peripheral axisThen certain point at the later stage if you look this structure let us assume this considerthat this is a leaf, this is a structure.So, if you look the leaf has a tip and it has a base and this axis is called proximaland distal axis.This is established with respect to the primary meristem, the growing primary meristem.So, if you look this is the primary axis, so the axis, the point which is close to theprimary axis is called proximal region and the point which is away from the primary axisis called distal region.And another and final axis across which growth and development occurs is adaxial-abaxialaxis.This is particularly important for the organs, so if you look this leaf the upper surfaceof the leaf and lower surface of the leaves they are very different.They have a different characteristic feature, they have a total different developmentalprogram.And the region if you if you look here the face which is towards the meristem is calledadaxial and the face which is away from the meristem is called abaxial.So, all this growth and development together they occurs in all the three dimensional directionand this gives a proper plant architecture.So, now, we will come to the typical characteristic feature of plant growth and development.So, if you remember the first class where we have talked about that, in case of plantsthe growth the development is continuous first very important thing.So, plant has capability they can grow throughout their life this is very important.And another very interesting thing is that the most of the development is post embryonicin nature, this is in contrast with the animals.If you look the development of animals most of the development most of the primary developmentis completed during the process of embryogenesis.Whereas, if you look the mature seed which contains our embryo there is not much developmentoccurs.The only thing what has occur is the basic body plant is established during the processof embryogenesis.Shoot apical meristem and root apical meristem.So, basically meristem which is across the apical basal axis is established.Radial meristem is established, the procambium is established and then this embryogenesishas stopped.Now this plant this seed is under dormancy stage.When we take the seed and when we put for the germination, then all the organogenesisor most of the organogenesis start which is as I said it is post embryonic and post germination.But the question here again I will say the same thing what happens that since seed isa very small structure and embryo is inside the seed once this embryo start elongating.So, this embryo is undergoing the process of both simultaneously growth and development.So, it has to ensure that it achieve the growth at the same time it also has to ensure thatall organs are being made properly.Another very important characteristic feature of growth and development in case of plantis indeterminate versus determinate growth.So, in case of indeterminate growth what happens that, when growth and development is happeningand it is not getting terminated.It is happening throughout the life and this is this is a case of typically in case ofshoot apical meristem.So, if you look the shoot apical meristems are normally in higher plants or in perennialplants they continuously grow, they grow throughout their life.This kind of growth is called indeterminate growth.In case of determinate growth this mostly occurs in the lateral organs like leaf, flowerwhere what happens that at a certain time point or when this structure or when the organhas developed certain kind of organs or taken a particular shape or achieved a particularsize, then growth is totally stopped or it is terminated this process is called determinategrowth.And sometime this is also reversible sometime indeterminate growth may converted into thedeterminate growth.For example, if you if you take the case of flower development.So, typically inflorescence is the meristem which is indeterminate in nature, but whenthe inflorescence meristem makes floral primordia or floral meristem at the flank, the floralmeristem after making all this organs which is sepal, petals, stamen and carpel’s onceit has developed the carpel, the growth basically terminates here and the all the meristemswhich were present here is totally consumed.So, this is a process of gaining determinacy.So, one indeterminate meristem gave rise a determinate meristem this is one very importantprocess in the development of plant organ.Since all this process are important then they must be tightly regulated.The process of determinacy, process of growth, process of development has to be regulatedby certain regulator and needless to say if you disrupt these genetic regulators or ifyou mutate them you are going to see the developmental defect.For example, if you take the case of this mutant which is TFL1 which is terminal flower1.So, if you look the wild type Arabidopsis plant you see the development how it happensthe wild type plants during the vegetative development it makes several leaves whichis called rosette leaves and then this plants undergo the process of reproductive developmentswhen this inflorescence are made and then a very special kind of leaves which is arecauline leaves are made.And during this process when there is a inflorescence meristem identity is established, then inflorescencemeristem will start making flowers on the flanks and these flowers once they maturethey undergo the process of fertilization and they give rise to the structure whichcontains the seeds.So, if you look this is the siliques and in the siliques the seeds are there containingthe embryo.So, if you look a single single inflorescence you can clearly find that on the basal sideof the inflorescence the organogenesis is almost completed, all the organs are madeand the meristems are terminated, but at the same time if you look the tip of the inflorescencestill there are the process of organogenesis going on and you can still see some new flowersgrowing.So, this tip or the inflorescence meristem it has the property, but in the mutant backgroundwhat is happening you look there is development of only few leaves and then this inflorescenceit make some flowers and then get terminated by making a terminal flower that is why itis called terminal flower mutant.So, essentially if you look this process in this mutant there is a gain of determinacy.So, normally it is achieved pre mature before the proper development is completed.On the other hand if you look this mutant which is hen1.In hen1 mutant, what is happening?There is loss of determinacy.So, if you look this is a, the top view of the Arabidopsis inflorescence.In this inflorescence you have a certain number of flowers and this flowers are determinatein nature, but in the mutant background you can see that the number of flowers are increased,organs are increased which means that they are not being terminated when they are supposedto be this is a case of loss of determinacy.So, the process of growth and development has to be genetically regulated and this regulationis very important to give rise a proper and defined plant architecture ok.So, as you see a typical mature plant, what you will find?You will find that plant has lot of organs, lot of different types of organs and theyare not achieved simultaneously some organs are developed at a particular time othersare developed slightly later.So, the total architecture or total body plan of a plant is achieved in three major development.The first one is primary growth and development.So, during primary growth and development the the growth actually occurs across theapical basal axis.As we have seen previously, so the growth is mostly in apical and basal manner and thisis mostly responsible for the growth and what is?This is because of the activity of shoot apical meristem here and root apical meristem here.And as I said there has to be a proper balance between the cell division and cell differentiation.So, this and the same thing is here in the root meristem in shoot meristem it is forthe vegetative organs for after the transition it is further reproductive organ.In root meristem the stem cells are maintained here and these stem cells are responsiblefor providing cells for different tissues which are growing in the tip.And of course, this is this also has to be regulated if you see this case this is a mutantwhich is called STM; STM is for Shoot Meristemless.So, in this mutant the meristem is terminated.So, if you look here this is a top view of growing Arabidopsis.So, this is the region where shoot apical meristems it are positioned and you can seethis growing shoot apical meristem is allowing all this lateral organs which is leaf hereto develop and form.But in this mutant somehow this meristem is not being maintained for very long periodof time which means that since meristem cannot be maintained only very few lateral organsare made and eventually this meristem is terminated.Similarly, if you look the plethora mutants.PLETHORA are very important transcription factor and they are responsible for maintainingroot apical meristem activity.So, in this mutant background the root apical meristem activity is not maintained and theprimary root or the meristem cells are getting consumed early and that’s why you cannotsee a proper root development.So, both the primary growth at the shoot apex as well as the root apex are regulated byvery important developmental regulators.Now, coming to the secondary growth and development.So, normally there is a pattern, so as as if you remember or if you recall the previouspart of this lecture what we have seen during the process of embryogenesis that apart fromthe root apical meristem and shoot apical meristem there is another meristem which isbeing specified which is called procambium which is basically lateral meristem or youcan say the radial meristem.But during primary growth this meristems are not getting activated.So, meristem is there the region has been already marked that this is the region whichis undergoing the process of secondary growth.Secondary growth is nothing, it is a growth which occurs to increase the diameter of agrowing plant.So, and this as I said earlier this happens in the central and peripheral radius.So, if you look this, typical root and if you make a cross section at early stage thisis the structure.So, if you look the structure you can see here or you can take shoot no problem makecross section here and you will find here.So, these meristems the procambium meristems they are basically positioned here and theyare part of the vascular bundles.So, vascular bundles are very important tissues which are going to make the transport system.And in transport system if you have, they have a primary phloem, primary xylem and inbetween there is a tissue called procambium.So, usually during primary development the procambium is not activated this only primaryphloems and primary xylems are made.But what happens when there is a decision taken place that now plant should enter theprocess of secondary growth typically this happens when primary growth is achieved significantlyso that they can sustain secondary growth.And then what happens here?This procambium cells these are actually lateral meristem they get activated and once theyactivated they start making a kind of ring like this and then they produce they startedproducing secondary phloem secondary xylems.And this secondary phloem secondary xylems basically they are in radial in nature andthey are responsible for thickening of the stem.So, this process this is absolutely important needless to say that they also need to beregulated as if you look these mutants it is clearly evident that this is a typicalwild type Arabidopsis root which is under process of secondary growth.But in this mutant background this here secondary growth is already activated, but if you lookthese different mutants the secondary growth is not so evident as you can see in the wildtype.So, this tells that the secondary growth is very important and it is genetically regulated.Now, the third growth and development which is lateral growth and lateral development.This occurs most of the time at later stage of the development.And this is the one which completes the architecture.This gives total proper architecture and most important thing what happens during this processis branching.Branching is very important process in plants.It is important because if you have more branches there is opportunity to produce more seeds,there is opportunity to have more kind of reproductive capability.And another important thing these process occurs through two axis; across the two axis.Proximal distal axis, abaxial-adaxial axis.And we have this branching process both in the shoot as well as in the root.So, if you look here in the shoot this is the apex of the shoot, but if you come downthere is a place where is some kind of meristematic activity and this meristematic activity isbasically called axillary meristem.And this axillary meristem again I would say that during primary growth they are usuallysuppressed.So, meristem are already defined, but they are not getting activated this is becauseof a process called apical dominancy in case of plants, but once the decision is beingtaken place this meristematic activity or this axillary meristem they get activatedand then they will make axillary buds and these axillary buds they will start makingaxillary branches and this axillary branches they will have a shoot apical meristem andthey will do the process of primary growth they will continue the process very similarto the primary growth.So, this is in the shoot in root what happens?In root this is your primary root; primary root basically because of the activity ofroot apical meristem, but at certain point of time in the development a new meristemis being generated here post embryonically and this meristem is called lateral root meristem.And these meristem when they undergo the process organogenesis they makes lot of root branchingin this roots are called lateral root they are very important for giving a proper rootarchitecture for increasing the absorption capability of a root by increasing the overallsurface area of the root.Here are some some mutants for this.So, if you look these two mutants they are, if you see this is the wild type plants thereare certain amount of branches, but if you look both the mutants this both the mutantsthey have high amount of branches the branching is increased, the shoot branching is increased.So, this is a kind of loss of apical dominant kind of phenotype.On the other hand if you look in the root, so these are two transcription factor thisis double mutant of auxin response factor 7 and auxin response factor 19.And in this double mutant what is happening?You can clearly see that the apex meristems are very much working fine.So, you can see the shoot apical meristem is here; root apical meristem is here it isevident by the fact that this mutants are making shoot as well as the root tissues,primary root tissues.But what is more important if you look the wild type at this stage this wild types theyhave made lot of lateral roots; lot of lateral roots, but these mutants they do not haveany lateral roots which tells that there is a specific genetic regulatory modules or thereis a regulatory program which is very specific for the lateral root development or for thebranching and which is independent of the mechanism or the program which is regulatingprimary meristematic activity.So, these all together suggest that the growth and development in plants are highly coordinatedand highly regulated.So, to summarize this lecture, so what we have discussed today, we have discuss theprocess of growth and development and what we learn that the process of development incase of plants are mostly post embryonic and we have discussed the primary growth, secondarygrowth and the lateral growths which are characteristic feature of plants growth and development andthis all processes together ensures a proper plant architectures.So, now, we will continue this lecture in next class.Thank you.