Loading

Module 1: Desarrollo de rodaje

Apuntes
Study Reminders
Support
Text Version

Set your study reminders

We will email you at these times to remind you to study.
  • Monday

    -

    7am

    +

    Tuesday

    -

    7am

    +

    Wednesday

    -

    7am

    +

    Thursday

    -

    7am

    +

    Friday

    -

    7am

    +

    Saturday

    -

    7am

    +

    Sunday

    -

    7am

    +

Welcome back to Plant Developmental Biology course. In today’s class we are going todiscuss about Flowering. So, flowering is basically a transition from vegetative phaseto the reproductive phase in the life cycle of a plant.So, when your plant start post-embryonic development after seed germination at a certain time periodor up to certain period of time in the life cycle they only continue the vegetative growth.And that is the period when they make branches they make leaves and then at a particulartime point in their life cycle a major decision is being taken place; where what happens thatthis vegetative plants basically enters in the process of called floral transition andit start the reproductive phase; and this is very important in the life cycle of theplant because, reproductive phase of the life cycle ensures the sexual reproduction. Andsexual reproduction is one of the highly evolved mode of propagation in case of higher plants.So, the sexual reproduction is restricted in an organ or in a part of the plant whichis called flowers, which are developed on a structure which is called inflorescence.So, in today’s class we are going to discuss how this transition occurs, what are the geneticand environmental factors which basically regulates the timing of the transition andones this transition has ensured, how a special organ which is flowers with a special structureand with a special organs like sepal, petals, stamens and carpels are developed.So, if you look it is very important to understand that during the vegetative process reproductivephases are repressed; and it is important because, plant has to gain a certain amountof strength before it enters in the process of reproduction. There was a gene identifiedwhich is called TFL1, TFL is basically TERMINAL FLOWER ONE and when a mutation or when a mutantwas identified in this gene there was few phenotypes one phenotype is that the plantflowered early. So, basically normally if a plant take a certainamount of this is Arabidopsis thaliana. So, if normal Arabidopsis thaliana takes let usassume some time period for example, 40 days to flower basically this plants flowers muchbefore this is another mutant which was early flower mutants. But another things what youcan look and see in the terminal flower mutants, that one thing is that it flowered early,but there is something some special structure so basically these branches are getting terminatedwith a single terminal flower. So, this means that in the life cycle of modeldicot plant Arabidopsis you can look during the vegetative phase these leaves are basicallyformed which is called rosette leaves. Then after transition the shoot apex get convertedinto inflorescence meristem and this inflorescence meristem has a mixture of property.So, if you look the branches which are coming from the inflorescence meristem, they haveindeterminate nature. Whereas, on the branches if you look this floral organ or flowers theyare determinate in nature what it means that branches can grow and they can make a manyflowers, but once a flower is made the meristematic activity is terminated. But here if you lookthe tfl mutant what is happening that this shoot like structure get terminated with asingle determinate flower. So, this was the phenotype which was observedin tfl1 mutant this is the top view or upper look if you look this region. So, you cansee here that here the growth basically stops in a way and there is a not many flowers aremade in this case. So, this is important which suggest that TFL1 could be one important genewhich basically repress the flowering which is a negative regulator of flowering.On the other hand if you look another gene or another mutant which is called leafy, LEAFYis activator of flowering. So, if you have leafy mutant what happens. So, basically ifyou look in the Arabidopsis thaliana plant. So, there are basically three types of structure;type one rosette and then there is a transition from here to here this is the transition whichwe call about the floral transition. And here the first few structures which you look likehere, here they are called coflorescence; they bears a bract like structure. And thenif you come to the higher side there is a flowers which is without bract like structure.So, this is the typical architecture of Arabidopsis inflorescence, but in leafy mutant what ishappening that you can see this is schematic diagram here. So, in wild type you have thisco-florescence and then you have this final flowers, but in case of leafy mutant the co-florescenceare fine, but the later flowers which are basically getting converted like shoot likestructure. So, basically there is a loss of floral identity.So, which suggest that LEAFY is important, so if you consider these two processes theentire and complete process of transition from vegetative shoot apical meristem tillthe flower which has two stages and this final stage is regulated by LEAFY. And this youcan see here if look the expression of LEAFY so you can clearly see that expression inthe inflorescence meristem which is on the apex is very low or not detectable. But ifyou look the flowers or floral primordia which is basically originated at the peripheralregion of the inflorescence meristem, it has a very high amount of LEAFY expression.This suggest that the LEAFY gene is very important in giving identity to the floral meristemwhen you do not have LEAFY, this meristem is basically not acquiring floral meristemidentity instead of that it looks more kind of inflorescence meristem identity. And anotherinteresting thing if you look so this is the side if you look here from where, where thestructure or the expression of LEAFY is very low.But in tfl mutant what is happening that this the side region from where your branches arecoming you can look here that expression of LEAFY is very high. And that is important,because when you have high amount of LEAFY expression here it will convert this meristemto floral meristem identity. This also proves that TFL and LEAFY they looksopposite. So, TFL is may be more important for giving shoot identity or shoot like structureinflorescence identity. Whereas, LEAFY is important for giving identity to the floralmeristem and the critical balance between them basically ensures the time of transitionand the identity of a meristem.So, if you look this process so you can place; TFL is here which is regulating even the firststep of transition. And then LEAFY is ensuring a proper floral meristem identity, in theperipheral region and these are some other mutants the typical schematic diagram of themutants. So, this is wild type and if you look the leafy like structure you can seethat the there is a defect in the floral meristem identity.Another gene which is APETALA ONE which is AP1 this is another important gene which isresponsible for giving floral meristem identity and it together with or in parallel with LEAFYboth are working to ensure the floral meristem identity. And important thing that if youhave double mutant of LEAFY and AP1 you can see that phenotype of both LEAFY as well asAP1 mutant phenotype is enhanced which suggest that they are working in two parallel pathways.And this is your terminal flower when you have terminal flower mutant you have a terminalflower. And another thing what you look if you over express LEAFY it looks like thatit is mutant of terminal flower. So, all these interaction basically suggestthat LEAFY and AP1 they are very very important factor in giving floral meristem identityto the peripheral meristem developing at the inflorescence and TFL is a negative regulatorof floral meristem identity.So, essentially if you will summarize this process then what you can see here that, iflook expression pattern of TFL1. So, in during vegetative phase it is expressed here, here,but in the reproductive phase after floral transition it continue expressing in the inflorescencemeristem. But none of the floral meristem are developingfloral primordia or show any expression of TFL1 whereas, if you look LEAFY in LEAFY whathappens that the inflorescences meristem or the center of meristem does not have LEAFYexpression, but this peripheral region where your floral primordia is going to be initiatethey have very high amount of LEAFY expression and this expression continues in the developingfloral primordia. AP1 has a quite overlapping expression patternlike LEAFY. So, you can see that in the very early floral primordia you have AP1 expressionthen at the developing floral primordia you have a high level of expression at the laterstage the expression is restricted to the first whorl organ which in case of flowersis sepals. So, what happens actually, so at the centerif you look this region in the center you have high amount of TFL1 expression and youdo not you have very low or less amount of LEAFY expression. And when you have high amountof TFL, TFL goes and it repress both AP1 as well as LEAFY in the center region. And sinceyou do not have LEAFY you do not have AP1 which means that you cannot assure any floralmeristem identity in the center so that is it this helps in maintaining inflorescencemeristem identity at the apex. But if you look the peripheral region justhere in the peripheral region your LEAFY is getting activated and when LEAFY is gettingactivated, LEAFY also activates the expression of AP1 and then both LEAFY and AP1 togetherbasically specify or provide identity, floral meristem identity to the peripheral meristem.On the other hand they also does they repress TFL1 in the peripheral apex. See the interactionbetween LEAFY, AP1 and TFL, particularly the antagonistic interaction between TFL and LEAFY;TFL and AP1 ensures inflorescence meristem identity in the center as well as floral meristemidentity in the peripheral region.If you summarize this interaction, so during vegetative phase and the inflorescence meristemyou have TFL1 level very high. And then during floral meristem stage you have LEAFY AP1,AP2. AP2 is another A class of genes which you will see. If you look the transition fromvegetative meristem to inflorescence meristem. In the inflorescence meristem you have expressionTFL1, AP1 and LEAFY are in the in the peripheral meristem and this peripheral meristems aregiving floral meristem. What happens if you have tfl1 mutants? So,since TFL is a negative regulator of LEAFY. So, if you do not have TFL here the LEAFYwill start expressing here AP1 start expressing here. So, all three central and peripheralmeristem all are going to take floral meristem identity.On the other hand if you do not have LEAFY then TFL is going to expand its domain inthe in the peripheral meristem and since you have high amount of TFL1 in the in the peripheraldomain basically it will provide inflorescence meristem identity. Similar kind of thingsyou can look when you have double mutant of AP1 and AP2 here you have TFL1 so it willbe inflorescence meristem. But in the peripheral from region you do not have AP1 which meansthat TFL level will also go very high and LEAFY is alone cannot specify full floralmeristem identity. So, you will have essentially inflorescence meristem identity.So, this regulatory network or this interaction is very crucial in giving a clear floral meristemidentity at the peripheral region of the inflorescence meristem. But what are the genes which arebasically deciding that what is the time when this identity should be given when this transitionfrom vegetative to inflorescence meristem when the vegetative shoot apical meristemchanges its fate and identity and becomes inflorescence meristem?And definitely there is a floral repressor there is a strong this is the hypothesizedwhen it was given in 1995 when the plant developmental biology was still at the early stage and peoplehave started looking very carefully plant developmental biology.So, at that time it was hypothesized that there has to be a strong repressing signalduring the vegetative phase and how to acquire the reproductive phase. So, one thing is thatone mechanism could be if you repress the repressor, that will basically activate theflowering or if you start a new activator of the flowering and when you look all thedata, when you look the mutants identified or the genetic networks you will find thatboth pathways exists to ensure flowering at a particular time period.So, for example, if you look there is a constitutive repression based mechanism and basically ithas the TFL in it; and they basically activate or they strongly activate this process ofthe floral repression during the vegetative phase. But you have long day promotion pathwaybasically day length is very important in long day plants to ensure the flowering.Then, apart from that you have some negative regulators you have gibberellic acid whichis playing very important role in deciding the flowers. And more than that, if you lookhere there is a large number of genes has been identified if you have mutation in thateither they show late flowering phenotype, early flowering phenotype and there are conditionif they can be in long day condition short day condition.So, if you look all those genes and try to understand the genetic pathways or what arethe regulators which basically ensures this process, then what you will realize here.So, these are the four major pathways which are important for regulating the transition.So, if you recall previous slides then you will realize that these two genes which arebasically floral meristem identity genes they are very important AP1 and LEAFY. So, onething what you have to do you have to first ensure activation of AP1 and LEAFY in theperipheral meristem to ensure a proper flower development the first step is this one.So, whatever gene can activate AP1 or LEAFY1 they will basically promote the flowering.And the activity of LEAFY1 and AP1 is regulated by two another important gene one is FT andsecond one is SOC1. And these genes are called flowering time genes and both are positiveregulator of AP1 and LEAFY1. So, if you want to ensure flowering. So, FTand SOC1 has to be activated, but under normal condition or under vegetative growth conditionwhat happens that both of this genes are kept repressed by a repressor which is FLC. So,if you look FLC is basically negative regulator of FT and SOC1. And when there is a propersignal then what happens if you have a proper signal then, the repression of the FLC isrepressed. So, you have two pathway one is basicallysensed by the gibberellic acid pathway where plant hormone gibberellic is very importantin the activating flowering through this one. And another important thing if you look gibberellicacid can activate LEAFY directly, but other pathway if you look the photoperiodic pathwaythey are activating they are positive regulator of FT and SOC1 through CO; CO is CONSTANT.So, basically when there is a suitable photoperiod when there is a time when there is a correctphotoperiod then, this pathway will be activated and this pathway will activate expressionof CONSTANTS and then CONSTANTS will go and activate FT and SOC1.But another two pathways one is the autonomous pathway and the vernalization pathway. Thesetwo pathways basically when they are suitable or when they are promoting the flowering whenthey are in the condition to promote the flowering what they does they repress the FLC expression.So, FLC is a negative regulator of FT and SOC1. So, if you negatively if you repressFLC essentially you are activating FT you are activating SOC1.So, when all the condition for example, if you took a Arabidopsis thaliana. Arabidopsisthaliana is a long day plant if day length is long day if all the autonomous signalingmeans plant is at the right age, right strength and then vernalization signal is positiveif everything is ok, then all this pathway will activate eventually AP1 and LEAFY throughFT and SOC1. And once AP1 and LEAFY is activated, just now you have seen that then the meristemidentity the meristem which is basically coming at the flank of inflorescence will take anidentity of floral meristem. So, there is the switch there is the changeof the identity. So, during vegetative phase you have shoot apical meristem then aftertransition this shoot apical meristem changes the identity and it becomes inflorescencemeristem; when it was shoot apical meristem then the peripheral primordia they were basicallyleaf primordia. But when they become inflorescence meristemthen, same peripheral primordia or the peripheral primordia which is coming here; if this isinflorescence meristem it will become floral primordia. And how it will become floral primordiawhich means that it has to activate some floral primordia specific genes and some of the geneswhich are very important in regulating the floral organ patterning. If it is floral primordiait is going to make flower if it is going to make flower it has to develop flower specificorgans and flower specific organs are basically sepal, petals, stamen and carpals. And thesejob is done by a class of genes which is called A B C and E.These class of genes are very very important and these genes are called floral organ identitygenes they basically specify or they provide a proper identity to each organs in the development.And another important thing if you look here, all these genes are basically getting activatedlater on by LEAFY and AP1. So, if you look AP1 is getting activated by LEAFY. AP1 isdual function gene, so it is floral meristem identity gene at the same time it is alsoA class gene which is responsible for sepal development which you can see in the laterslide. So, basically LEAFY activate AP1, A classgene. LEAFY together with UFO activate B class gene which is AP3 and PI in Arabidopsis andLEAFY together with WUSCHEL activate AGAMOUS, AG gene which is basically C class genes.And then you have another set of genes which are called E class gene.So, if you look this transition process, so how it occurs? Another important thing tomention here that the most of the signals are getting received in the leaf and transitionof the flowering or the decision to change the meristem fate occurs at the apex. Thequestion is that how this signal which is being received in the leaves are getting communicatedto the apex? And there is a long distance signaling involved here and one importantmolecule which is FT which you will see basically it is a mobile florigen, if this is the signalFT can move from leaf to the apex. So, FT is getting activated in the leaf andthen it basically moves through the vascular tissue precisely through the phloem and whenit reaches to the apex in apex it basically activates the genes. So, if you look herethis diagram, so this is your leaf and this is your apex. So, in leaf what happens FTis kept repressed by several mechanism during vegetative phase.So, you can in leaves there are multiple mechanism which are repressing FT, but when there isa photoperiod or right signal and photoperiod is basically activating CONSTANT and CONSTANTis activating FT. And when you have FT and other pathway like vernalization pathway,autonomous pathway, gibberellic acid pathway, they are also working to activate the FT,but they are repressing the repressors. So, eventually if you look here FT is gettingactivated in leaf and then it is moving through the phloem to the shoot apex once it reachedto the shoot apex it basically interact with another gene called FD and these two are veryvery important. And they in the apex if you look here in the center of the meristem theyactivate SOC1, they activate AGAMOUS LIKE 24 and the interaction between SOC1 and AGAMOUSLIKE 24 along with SPL NINE genes are very important in maintaining inflorescence meristemin the center. But at the flank region or at the peripheralregion these pathway or these mechanisms they activate LEAFY, they activate AP1 and thenLEAFY and AP1 or interaction between LEAFY1 and AP1; and then repression of TFL in thisregion essentially ensures floral meristem identity to this peripheral region.So, there are three classes of genes A, B and C and how they express? So, they expressin the overlapping manner. So, A class gene they get activated in thesepal and they continue in the sepal and petal primordia. B class genes get activated frompetal and it is in petal and stamen primordia. And C class genes get activated here in thestamens and carpel primordia. Whereas, E class genes are basically expressed everywhere.So, these genes are very specifically getting activated and LEAFY is involved in activatingthese genes in a very specific domain.So, as I said if you look here LEAFY activate AP1 in the region of the meristem which isresponsible for which is basically sepal and petal zone. Then it activates B class genewhich is AP3 and PI in petal and stamen zones and then it activates, finally, C class genewhich is AGAMOUS in the carpel zones. So, function of LEAFY is not only to givea meristem floral meristem identity, but also to ensure that downstream processes of floralorgan patterning is also getting activated in a very specific manner or in a very domainspecific manner. And if you look here, so basically this is the expression pattern ofAP1 you can see in the developing sepal and petal primordia; if you look AGAMOUS it isonly in the center. And if you look AP3 it is basically in the domain where you havepetal and stamen primordia. And you when you over express LEAFY 1 you can see that expressiondomain of all these genes are getting expanded.So, this is just expression pattern of these genes at early stage and when there is a flowering.And then if you look the E class gene E class gene is very interesting in a way that itworks like a cofactor for A, B, C class genes. So, and there are four E class gene SEPALLATA1,2, 3 and 4 and if you look the expression pattern so basically SEPALLATA1 and 2 is quietbroadly expressed, but SEPALLATA3 and 4 basically expression is restricted.And how they regulate the organ which is important so, this is overview of the expression patternA class gene in the sepal B class gene you have in the petals and stamens and then Cclass genes you have in the center this is A plus B plus C class gene. So, you can havethis kind of specific and overlapping expression pattern and this is very important becausetheir interaction their overlap basically defines the identity.So, if you look sepal in sepal you have only A class gene and E class gene. So, A classgene is here B class gene is here C class genes are here E class genes are here. So,if you look this region only this region which is going to make sepal, sepal it has onlyA class gene and E class gene and this interaction basically gives sepal identity. In petalsif you look this is petal in petal you have B class gene A class gene and E class gene.So, you have A B and E and these three genes together ensures petal identity.Similarly, if you look in the stamens; stamens has B C and E so you have B C and E and thisgives stamen identity. And in very center where you have the carpel you have only Cand E and C and E together ensures carpel identity in sepal you have A, E interactingin petal you have A, B, E interacting.In stamens you have B, C, E interacting and in carpel you have C and E interacting.And this you can see here if you have mutant in this what happens.So, in the wild type pattern the pattern is like that and another important thing hereis that A basically restricts the domain of C and C, they are mutually antagonistic. So,they are restricting their domain. So, if you have A mutant, A class the mutant whathappens that the restriction of C disappears and C can continue expressing here. So, ifyou look here. So, this region is now it is having C and E and C and E will give carpelthen you have B, C, E it will give stamen B, C, E it will give stamen and here you havecarpel. So, if you have A class mutant you can clearlysee that your flower will have carpel, stamen, stamen, carpel similarly if you do not haveB class gene, then you are going to have sepal here sepal here, carpel, carpel. This youcan see here you have this two whorls of the sepal and then two whorls of the carpel. Thirdimportant thing if you have C class mutant gene what happens that A class gene will nowcontinue expressing here. So, here you have A and E it will give sepal A, B, E it willgive petal A, B, E it will give petal and here you have only A and E it will give sepalbut another important thing here what happens that these structure basically continued.So, there is a flower within flower phenotype which tells that in this mutants the floraldeterminacy is lost. So, normally what happens in the wild type, when your final organ whichis carpel, when carpel is made the organogenesis is stopped and that is called the state ofdeterminacy, but C is ensuring that determinacy if you do not have C class genes. So, apartfrom the floral organ defect pattern, but you also see a loss of floral determinacybut important thing if you do not have E mutant here E gene; so, basically SEPALLATA 1 2 34. So, if you have mutant for all of them althoughA genes B genes and C genes are fine, but they cannot function which means that theiractivity is dependent on E class gene and in this case you can see that all the whorlsare going to have leaf like structure. So, this is clear that the activity and interactionbetween A, B, C and E is important for regulating floral identity.So, these are different organ patterning’s. You have inflorescence to floral meristemthese are the different steps. Then in the floral meristem, you have the concentric ringformation sepal, petal, stamen and carpel. Then at the same time here there is a symmetrybroken you have organ growth and then maturation of the organ. So, these are different stagesof the flower development and then you have different genes which are regulating differentstages.The final thing which is important is the floral determinacy. So, as I said that flowersare determinate in nature. So, they basically terminate once final organ which is carpelis made and you see here if you look the expression of WUSCHEL which you have already studiedin previous classes that WUSCHEL and CLAVATA they are very important regulator of thisstem cell activity. And in flower what happens that at early stagethere is a strong WUSCHEL expression and strong CLAVATA THREE expression. But once the floweris mature you can see that WUSCHEL and CLAVATA three expression is almost disappeared, butif you have agamous mutant which is basically C class gene mutants where you have the organdefect as well as determinacy defect. You can see even at the late stage, the expressionof WUSCHEL and CLAVATA THREE are maintained which suggest that the determinacy is lostbecause there is no stem cell termination mechanism. So, under normal condition whathappens at early stage you have WUSCHEL and CLAVATA on. At this stage if you look WUSCHELand CLAVATA and they are basically maintaining stem cell pool in the center and that is whythere is a indeterminate growth. But at the later stage when the this floweris in the process of organogenesis, LEAFY and WUSCHEL together activates AGAMOUS inthe center and this AGAMOUS is responsible for carpel and stamen identity. But at thesame time at the later stage when the identity of carpel is already established this WUSCHELactivate gene called KNU and this gene basically goes and repress the WUSCHEL and this is importantfactor. Once you replace the WUSCHEL there is no WUSCHEL there is no WUSCHEL means thereis no CLAVATA once the WUSCHEL-CLAVATA signaling pathway is lost, then the center region cannotmaintain the stem cell niche and that is why the growth terminates.Apart from the WUSCHEL and CLAVATA cytokinin is also playing very very important role inmaintaining the stem cell pools; and you know that this is the biosynthesis enzymes cytokininis having a positive role on the stem cell maintenance. At early stage this importantAuxin response factor which is ARF3, which is kept repressed, but at the later stagethis ARF3 is getting activated by Auxin. Auxin is very specifically getting activated inthe carpel at the later stage. So, carpel has Auxin once Auxin is there inthe carpel it activate ARF3, ARF3 goes and repress the cytokinin signaling pathway andeventually that repress the WUSCHEL expression domain. So, this complex mechanism all thisregulators together ensures that, at the end of floral organ patterning the meristems aregetting consumed up and there is no mechanism to maintain the dividing cells.Thank you very much we will stop here and in next class we will discuss about the cell-cellcommunication in plants. Thank you.