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Module 1: Plant Metabolism

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Pathogens and Secondary Metabolites

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I told you that phytoalexins are secondarymetabolites which are not synthesized all the time, but they are species specific, pathogenspecific response.Now, this is involves therefore, de novo synthesis of the enzymes which includes transcription,translation all the process starts after the pathogen has attacked.So, these phytoalexins are they are a very wide range of secondary metabolites, sometimeswhich are very broad range pertaining to a particular taxa or taxonomy or which are veryspecific to a particular kind of insect which is or the pathogen which is continuously attackingthat particular species.So, they remain generally undetectable in the plant before the infection.So, now what other mechanism by which the plant tries to protect itself?You remember I was talking about acquired immunity and innate immunity in plants.So, we will be today talking about the different acquired an innate immunities in plants.Now, some plants they recognize specific pathogen derived substances.They can recognize particular antigen epitope of a particular broad taxa; like for example,bacteria which are movable, so, they will have flagella.So, there are epitopes present or the receptor present on the plant cells which can recognizemotile bacteria.So, by this they are able to have a generalized defense against any such bacteria which ismotile in nature which comes and attacks the plant.Now, through evolution they recognize these specific components of their pathogens throughwhich they then develop these specific antibodies which can recognize the antigens and thenwhich can further trigger the secondary metabolism specific to that particular taxa or that species.Now, individual plants we know that they differ in their immune system there a sensitivityor their capability to resist the attack.That depends on two things - one is the array of secondary metabolites and the second ishow quickly they recognize and induce the secondary metabolism.Now, the resistance depends on how plants sense the presence of pathogens and initiatedefense responses.Now, the first line of resistance is therefore, provided by a system that can recognize broadcategories of these pathogens.Now, these are called as microbe-associated general molecular patterns MAMPs.Now, these elicitors are evolutionary conserved pathogen derived molecules such as basicallyfirst some structural component will come into contact their cell walls.So, therefore, you will see that through evolution with their contact they learn the plant.So, there is a memory in the plant way through which it is learning and creating its owndefense for future.So, structural elements for fungal cell wall for example, or the bacterial flagellum.So, one such is the flagellin receptor FLS2 which enables the plant to recognize all mobilebacteria that is one such receptor.Then another as I said is called pep13 which is an epitope for oomycete transglutaminase.Now, it recognizes transglutaminase is and then hence it will induce its defense response.So, this can recognize broad range of oomycetes.So, this form of defense strategy comes under innate immunity which through evolution theplant has learned; it is there within.Now, when a plant survives first infection then it can resist it has been observed thatat its subsequent resistance increases.So, their first infection is said to improve its resistance so, they are able to show betterresistance during further infections against the similar kind.Now, this phenomena is called as systemic acquired resistance.Now, what is involved in systemic acquired resistance?It develops over several days; it is not that suddenly after the infection, the plant developsafter days from the first point of infection.How does it happen?Now, there are increased levels of certain PR proteins like chitinases and other hydrolyticenzymes in the plants in the process of systemic acquired resistance.Now, one of the endogenous signals involved is likely to be salicylic acid.You remember salicylic acid was one of the molecules involved in the signal transductionpathway.So, now salicylic acid and its volatile components like methyl salicylate it they are responsesaid to be responsible or involved in this systemic acquired resistance.So, there is a phloem induced signal.So, once the pathogen attacks now these are involved which through the phloem signal toother parts of the plant and because methyl salicylate is a volatile component of it so,the nearing plants also would therefore, induce the systemic acquired resistance . By inducingwhat?By inducing the production of PR proteins.Now, another such signal is H 2 O 2.Now, H 2 O 2 is another compound that accumulates at the site of infection and plays a rolein SAR.So, this is how it is also linked to I was talking about reactive oxygen species.So, at the point of infection so, multiple it is not that only one defense will be playinga role, but there are multiple modes of defense which come together to protect the plant.So, it is that hypersensitive response that the same reactive oxygen species or H 2 O2 which is generated is then helping in the SAR also.Like salicylic acid H 2 O 2 is unlikely to function as a long distance signaling molecule.So, you need volatile components . So, this is a picture which can demonstratethe systemic acquired resistance.So, upon infection from the site of infection then it induces a flow in signal which furtheraccelerates the production of salicylic acid or methyl salicylate, and then it furtheraccelerates or induces the production of elicits the production of PR proteins.What are PR proteins?Pathogen related proteins . So, this is what is systemic acquired resistance.Now, interaction of plants with non pathogenic bacteria also induces resistance in the plantagainst pathogenic bacteria.So, this comes under induced systemic resistance in the plant.Many plants although it is a hypothesis which we are planning to also test that you haveheard about endophytes, you have heard about mycorrhiza now some of the bacteria they livein symbiosis with the plant.So, now, these organisms also first infect the plant.It is not only one give, there is a give and take process involved.It will be a mutual beneficial association always.So, imagine why would plant resist one and why would plant allow that particular organismto grow within.So, there has to be something beneficial from it then only the plant.So, that is what we are also testing in our endophyte.So, in lab we also work on endophyte cultures.Now, these are microorganisms which live and reside within the plant tissues and throughyears through evolution as the plant has grown that symbiosis has developed.So, why only a particular species or set of microorganisms the plant is allowing it togrow and propagate throughout the plant and it is not allowing the others.So, some of these now any any microbial even to enter and live, reside within the plantit will have to enter the plant so, it is kind of an infection.So, think in those terms.Now, the plant is allowing that infection in that process although it is so, there isalways a balance between the merits and the demerits.So, it is because of the mutual benefit which it considers to be much it gives more priorityover the demerit of infection it is allowing that organism, but what other benefit theplant is getting is this induced systemic resistance.So, because of that so, for example, the signaling cascade were now jasmonic acid is involvedor ethylene is involved.Now, salicylic acid is not involved; now jasmonic acid and ethylene are ethylene is a growthhormone also plays a role in developmental stages of the plant.Now, jasmonic acid is another molecule which plays a role in the signal cascade.Now, jasmonic acid and ethylene are the components which are involved in induced systemic resistancewhich will increase the production of the secondary metabolites which are continuouslybeing produced in the plant.So, there were some metabolites which are always present, no spatial or temporal inductionof their biosynthesis, but there are phytoalexins which are produced only as a result of infectionto the plant.So, this induce systemic resistance enhances the inherent capacity of the plant to protectitself.This form of systemic defense activation does not involve salicylic acid as the signalingmolecule as I said.It does not induce the accumulation of typical PR proteins now certain defensive measuresare immediately put in place by ISR; ISR was induced systemic resistance.So, this is actually innate.Now, other defense responses are initiated only after actual pathogen attack.So, that is induced resulting in a faster and stronger response.So, everything comes together, all different modes of actions defense are brought together.The advantage of this defensive strategy would be logically?Without reading also you should be able to tell me.Why do you think the plant is having first line of defense, second line of defense, havingdifferent modes of actions?It does not want to waste . In the end everything needs carbon and energyand we know how difficult it is for the plant to get the carbon.We have now studied about photorespiration, photosynthesis, the loss the plant goes through.So, the plant has to judiciously use its energy to protect and the priority is growth always,all these come for survival, but ultimately the aim is to propagate, to grow.So, the plant has to prioritize all these together for the ultimate aim.So, therefore, it would not like to waste its carbon and energy all the time in this.So, it will have different stages.So, this is again a picture which can demonstrate to you systemic resistance, induced systemicresistance were jasmonic acid and ethylene is involved.Again, it is through phloem it spreads to other parts of the plant .So, they have shown in the root if you can see that there is a symbiosis between a bacteria.So, it is maybe forming a root nodule.So, because of this the ethylene andyour jasmonic acid production is induced which further isinvolved in the signal cascade to enhance the secondary metabolism.So, you will always find that some plants are found to be very resistant which willhave a very high antimicrobial active; not all plants have the similar antimicrobialactivity.That is the very reason when some natural products are used as pesticides, not all;like neem for example.So, there is a difference in the capability of each plant.So, from where is that coming?It is coming from all these.The different array of secondary metabolites which a plant would produce would depend onits systemic and acquired resistance.So, this is the summary of what we have learned up till now.So, it is against the herbivore the plant has to protect itself, carnivore is involvedor or is involved to protect HIPV any guesses?. So, it is herbivore induce plant volatiles.So, secondary metabolic pathways, again to just to reiterate that there is a broad rangeof secondary metabolites including classes like isoprenoids, complex isoprenoids.So, as I told you people earlier that it begins with the primary metabolism, then major secondarymetabolic pathways where MEP pathway, shikimate pathway, mevalonate pathway, these are inDX5 pathway they may be involved for the backbone of those secondary metabolites.Then from these intermediates they can further come together major classes can come togetherto form further more complex metabolites.Now, why do you think the plant is finding a need?. For for combining further branching it outfurther branching it out hm.May be to stabilize the compound . Can be, what else?. It is like it has very basal structure likefungal structure and then just by changing the functional groups or adding some moreyou can change the entire property of the metabolic for different .Property in the sense, bioactivity.Bioactivity . If you see the chemistry is the mode of actionof these complex metabolites specially plant based products they are very complex largemolecules organic compounds.Now, their mode of action depends on a particular moiety or a functional group.It is not now every moiety there has a different role to play.So, it is kind of increasing the diversity of defense in the plant or maybe to protectthe plant itself.So, that is the very reason possibly why the secondary metabolism is so widespread in plants. So, secondary metabolites, synthesis and metabolismis in plant cells is by specialized proteins.Represents the plant developmental process majorly - secondary metabolism.It is an expression of cell specialization.Now, all these points are important to know, why?So that when you are developing a plant based bio-process for the production of a specificsecondary metabolite, you will be able to know that what is needed in that bio process.Whether a simple undifferentiated cell or homogeneous dispersed cell would work or youneed aggregation also to happen or you need organogenesis to also happen for that particularmetabolite.So, when you know these aspects and you know your secondary metabolites and how is it andwhen is it and where is it getting produced in nature you accordingly device your bioprocess development; like some particular secondary metabolites are not produced allthe time.They are only found when a particular tissue root they are only found in roots or theyare only found when the leaf start new leaf starts coming out like an onion I will betalking about those volatile components.So, then if you know what is happening in nature in order to produce that in vitro youwill have to create similar conditions.So, expression of cell specialization secondary metabolism is now triggered by the processof cell differentiation because it is a higher order function.So, it is generally associated with cell differentiation; the yields will be higher when the cell differentiates.Cell differentiation is a component of metabolic regulation in higher organism.Now, it includes all process when I say cell differentiation is from the cells to the tissuesthen to the organs and then to the entire organism, that is differentiation.It includes all processes which differentiate cell with the same genetic.So, not all genetic information although they are throaty potent in nature which means thegenetic machinery is available, but not all genetic information is utilized all the time.So, biosynthesis of secondary compounds is limited to therefore, particular developmentalstage most of the times or specialized cells .Now, where do they get accumulated?Now, secondary metabolites you know there are specific synthesis sites, storage sitesand generally synthesis is not the same as the storage site, why?. .But, they are getting synthesized, is it not?. They undergo.so that it does not get.The site where it produces might be far from the first line of consumption.So, it gets.That can be a reason.Yes, what else?There is also the secondary metabolites we know they the plant will judiciously produceit only when it is needed it will accelerate the machinery.So, there is and plant would not like to have it all the time because they may be toxic.So, it will there are turnover processes.So, the metabolism, the rate of synthesis and there is a rate of utilization made thisalso includes rate of transport if it needs to be stored somewhere.So, there are turnover involved in secondary metabolites process.So, therefore, generally and I say as I said one of the logical reasons can also be todrive the reaction forward . Yeah.So, you would not like to you would that is the reason why you will find sometimes addingraisings for adsorption of the product helps in increasing the rate of synthesis of theproduct.So, that can also be a reason.So, now, they experience turnover processes with characteristics half lives some of thesetoxic secondary metabolites.Now, degradation and synthesis often occurs simultaneously.The extent of accumulation depends upon cell synthetic capacity and capacity to metabolizethe component.So, when we say metabolize the components this also includes further modifying it fortransport to the required places now in or modifying it for detoxification.Individual plant organ differs in their significance in this process.So, you will find that storage and synthesis they occur in different sites.It is and sometimes the synthesis also is compartmentalized.Now, the concentration also varies according to geographical location, climate and soil.So, it is nature dependent . Now, spatial compartmentalization: synthesisis bound to particular organs, specific cell organelles or vesicles.So, synthesis need not be like for example, in camptothecin, there are reports and literaturewhich say that if you try to extract camptothecin you will find that you will be able to doit best and maximum yields from the bark, but when synthesis has been observed it hasbeen there are literature reports which indicate that the synthesis happens in the root.So, which is an example that, the site of synthesis may be completely different fromthe site of storage . So, example intermediates and berberine synthesistravel back and forth between the cytoplasm and specific vesicles.This is for an example for spatial compartmentation of the secondary metabolite.Now, storage there are different places and there are different forms in which the productcan be stored.They can be stored on the surface - surface in the form oftrichomes or even in the intracellularspaces empty spaces like for example, hazarder rector if you do we were working on hairyroots.So, if you do a cross section of the hairy roots you can do histochemical assay and findhazarder rector spots colored under the microscope and they they get stored in the intracellularspaces of the root matrix . Then, if you can see a trichomes - trichomesare hair like glandular structures on the plant leaf or the stem; then edioblasts, theseare specialized plant cells which are meant for storage only.So, they are only meant for storage of either pigments or wax or oils or your secondarymetabolites . Then glandular hairs, trichomes, cuticle, epidermis, there are different partsor tissues or specialized organelles where you will find these secondary metaboliteswill get preferentially stored . So, vacuole it is a common site for shortand long term storage.Vacuole inside a plant cell if it wants to protect itself then also it is stored in vacuole.Now, only two categories of secondary metabolites are stored in vacuoles.Toxic compounds are glucosidated for this purpose.So, when glucose moiety is added it is one of the bio transformations for detoxification.Sometimes they do not store the final product, but rather it precursors we have studied aboutit where the enzymes for the final intermediate the final product are in the vicinity suchthat the intermediate compound as soon as the cell ruptures that comes in contact withthe enzymes to give the final product which is toxic metabolites for the pathogen .So, vacuole functions as a trap for ions.Now, how does vacuole functions as the trap for ions?When we were studying about the different organelles and their functions we studiedabout it.It is something to do with the tonoplast.The molecule is facilitated it is generally not active process while entering.It enters, but going out is active or it is not allowed, how?We have studied this.How is it happening?Go back and read.Because of the ion comes on the.But, I said it is a non-active process while coming in generally, what facilitates that?It says that interior of vacuole is acidic.Right.Ions can come inside . Tonoplast is what?. Is it no there is no membrane across the vacuole?There is.There is.Membrane is made of?protein . Cell membrane.Hydropropane or lipids.So, what will allow easy passage?It has to be?Hydrophobic . Ok.It has to be hydrophobic . Think.some portions in a.I heard hydrophilic also.. So, you need to think .