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Module 1: Plant Metabolism

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Secondary Metabolites and Plant Defenses

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We studied about secondary metabolism, its role in plants,what are the different functionsof secondary metabolites; for example, what are the different functions of secondary metabolitesin plants . Defence.One is defence . Specialised function.In specialised functions like ripening, attracting pollinators, completing the cycle and .to overcome competition To overcome competition for survival , ok.So, then we went on to see what are the major class of secondary metabolites in plants;what were the major class ofsecondary metabolites found in plants .Nitrogen So, nitrogen containing compounds terpenesand . phenolicsPhenolics.So,we then studied in detail about these three classes; now we were at alkaloids.So, alkaloids is one of the nitrogen containing compounds, there are almost 15000 differenttypes of alkaloids present.So, now,in these is we also have other nitrogen containing compounds which are not that wellknown commercially utilised, but have a function to play in the plant defence.So, talking about alkaloids, alkaloids they are generally used as antiherbivore in theplants system, they are toxic to humans and most nitrogen is secondary metabolites aresynthesized from amino acids.So, alkaloids their backbone will be amino acids.Majorly what three amino acids?Lysine, tyrosine or tryptophan . Now the alkaloids they as I said these are almost 15000 presentin the plants and the skeleton may also be.So, now,alkaloids like for example camptothecin it belongs to monoterpene indole alkaloids.So, which means that it is structural moiety also has the nitrogen containing moiety aswell as the terpene moiety.So, there are many such secondary metabolites, as I mentioned in the previous class thatwhat were the different pathways ; from primary metabolism then it went on to secondary metabolism,shikimate pathway was involved, MEP pathway was involved, mevalonate pathway was involved.So, either singly these pathways are leading to products or combining; the two pathwaysare combining to give intermediate subsequently leading to different types of secondary metabolites,so which means that the array is so widespread.So, apart from the alkaloids which we know than there are other alkaloids where are argininefor example, is involved.So, this is nicotine, your cocaine, morphine, caffeine these are also parts of alkaloidsthey are used as deterrence for mammals, animals, for humans.So, they act as either as feeding deterrence; they directly act as toxins or they will disturbthe digestion process of the pathogen or the herbivore .Now, for example, your nicotinic acid the backbone,in case of nicotine it is obtainedfrom ornithine; ornithine is a precursor as an intermediate in arginine biosynthesis.Now ornithine is a non proteinaceous amino acid.Now B vitamin which is nicotinic acid is a precursor of the pyridine moiety of this alkaloidnicotine.So, what is pyridine moiety?The nitrogen containing ring, so it is C 5 H 5 N.So, now this is pyridine moiety which is obtained from the nicotinic acid.Now alkaloids were thought to be nitrogenous.So, initially it was thought that because they were nitrogen containing compounds.So, people were of the belief that either they are acting as nitrogen sources for theplant, compounds which are stored have stored nitrogen; or they are nitrogenous base likeurea or uric acid in mammals or they might be acting as growth regulators.Later it was found that they are performing higher functions of defence in the plants, by either toxicity function or by feeding deterrence.Now, nitrogen containing other secondary metabolites they are cyanogenic glycosides and glucosinolates.Now cyanogenic glycosides which means that; sometimes for the plants as were discussingfor it to become non toxic they are joint with a as conjugates, with a sugar moiety,so for example, this cyanogenic glycoside.Now, after hydrolysis or the enzymes which are present in the vicinity, once thecellgets ruptured; these conjugates will be exposed to the enzymes which are in the vicinity.And the final break down to such toxic compounds which can be volatile like; for example, hydrogencyanide, which gets released, which can either directly damage the pathogen, or which canhelp to trigger the signal cascade mechanism in the plant itself .So, two groups of these substances cyanogenic glycosides and glucosinolates.Now they are themselves not toxic, but are readily broken down to give off poison; forexample, I was saying hydrogen cyanide ; now which may be volatile when the plant is crushed.Now, they are stored in the intact plant separately from the enzymes that will hydrolyse themas I said to perform the final toxic breakdown product . Now cya nogenic glycosides theyrelease hydrogen cyanide which is a well-known toxic compound . Now the presence of cyanogenicglycosides deters feeding by insects and other herbivores like snails and slugs.So, this is likeit flows through generations if, so it is kind ofacquired deterrence.Now, the second class of plant glycosides I was talking about is glucosinolates, whichare also called as mustard oil glycosides . They are present in plants like your cabbage,or yourradish which has a very different taste, strong taste.So, it acts like a feeding deterrent for the feeders . So, glucosinolate break down iscatalysed by a hydrolytic enzyme which is thioglucosidase or myrosinase; that cleavesglucose from it is bond with the sulfur atom So, it is generally present as conjugates;the conjugates may not be toxic, but the final break down may lead to the impactful compound. So, they generally lead tobad smell or a strong smell and taste in the fruits or inthe plant parts.Now, non protein amino acids, many plants they contain unusual amino acids which arecalled as non-protein amino acids; which get incorporated in the proteins.So, for example, canavanine, it can be mistakenly or sometimes these non-amino acids, non-proteinamino acids they can also replace the amino acid in the metabolism of that herbivore.So, which may then disturb the metabolism of the herbivore; for example, it is writtenhere canavanine which can replace arginine.So, this is what I was talking about primary metabolism leading to secondary metabolism;in secondary metabolism the major pathways involved shikimic acid, mevalonic acid, pathwayMEP pathway, which is methylerythritol phosphate pathway.So, now we were also talking out the two different types of defence which are present in plants;one is constitutive defence and the other is induced defence.In order to conserve the resourses the plant would not like to use all at once.So, there is first line of defence and only when needed there is a stronger much strongersecond line of defence; once it has been found that the second line of defence is needed.And what caters to this and what induces the second line of defence, we will see to that.So, where your signal cascade pathways are involved . So, let us first talk of the constitutivedefence.What is constituted defence in the plants ? Now this is species specific, the secondarymetabolites which may be present inherently in that plant, may not be present in the otherspecies . So, they may exist as stored compounds, conjugated compounds to reduce toxicity, oras precursors of the active compound which can easily be activated if the plant is damaged.Now most of the defensive secondary metabolites are constitutive defence.For example if you find that a particular secondary metabolite is present in lower amountsthroughout the plant and irrespective of the season; then it is a constitutive line ofdefence, where the machinery the enzymes needed to produce that are always present .Now, induced defence on the other hand, it is initiated only after the actual damagehas occurred; which means de novo synthesis happens; which means that, the proteins whichare required to produce they are not present.But these compounds are only obtained de novo; which means transcription translation beginsonly after the damage has happened.But the key here, whether the plant would be able to survive; which species is strongerthan the other to survive such, depends on how fast the plant is producing these compounds. So, they include the production of defensive proteins such as lectins, and protease inhibitorsas well as production of toxic secondary metabolites.In principle induced defences require a small investment; obviously, because they are notproduced all the time, so it is like conserving the resource of the plant, the carbon andthe energy.Now, how do the plants recognize?Plants recognize specific components in case of insects; they recognize specific componentsof insects which may also include saliva.So, now, the plant response to the damage by insect herbivores, it involves both thewound response and also the response towards insect bearing components.Now, the recognition of certain insect derive compounds which we class as elicitors.Now these elicitors once recognize they lead to induction of the signal cascade in theplants . Repeated mechanical wounding can also induce responses similar to that causedby the insect herbivory . Now molecules likes insect saliva can act as enhances or elicitors;elicitors are nothing, but stimulus to the defence, second line of defence .Now plants recognise these elicitors and activate a complex signal transduction pathway.One of the products of this complex signal transduction pathway is jasmonic acid . Nowthis jasmonic acid in turn once produced through the phloem flows to the different parts ofthe plant, sometimes it gets conjugated as methyl jasmonate and methylated and becomesvolatile; such that this becomes a signal for the nearing plants , and they all flowthrough the phloem to different parts of the plant.So, insect derived elicitors can trigger the signalling pathways systematically, initiatingdefensive responses in distant regions of the plant or even the nearing plants.So, the major signalling pathway as I said is the jasmonic acid formation pathway whichis called as octadecanoid pathway.Now in octadecanoid pathway it leads to the production of the plant hormone which we knowas jasmonic acid.Now jasmonic acid levels steeply rise upon the pathogen attack, herbivore attack, andtrigger the production of many proteins which are called as pathogen related proteins.Now these proteins form the part of signal cascade mechanism.So, either these proteins will directly befunctioning as catenaneses , or hydraulic enzymes, orproteases which may damage the pathogen; or they may induce the proteins in the biosyntheticpathway of the second line of defence; such that the production of much stronger and muchspecific secondary metabolites begins in the plants .So, two organelles which are involved in jasmonic acid formation; they are chloroplast and peroxisome,just for information.Now, jasmonic acid it is known to induce the transcription of host of genes involved inthe defensive mechanism.So, it is like umbrella or a big.So, it is affect is very widespread, a number of like transcription factors.So, a number of enzymes,expression get affected by induced by jasmonic acid . Amongst thegenes it induces those that encode key enzymes in all the major pathways for secondary metabolitebiosynthesis.So, therefore, you will notice that when plant cell bio processor optimised; jasmonic acid,salicylic acid which are part of these signal cascades are very well known elicitors usedin plant biotechnology for enhancing the yield of the secondary metabolite; because theirmode of action is so widespread that they are general elicitors, they are not speciesspecific.So, in most of the cases if you add they work . Several other signalling compounds likeethylene; ethylene is also a plant hormone, but it also acts as a signalling moleculeto induce secondary metabolism . So, ethylene, salicylic acid, methyl salicylate;so methylatedthese components can become volatile which may act as volatile signals to the otherparts of the plant itself or the nearing plants . The concerted action of these signallingcompounds is necessary for the full activation of induced defences.So, it is not just that only one at a time would happen . So,it is not that only jasmonicacid would be produced.The plant then because everything is a part of that signal cascade.So, everything gets produced and everything in turn then induces a different line of defence.So, therefore, the plant is able to have such strong multiple mode of action based defenceagainst the pathogens.Now, among the diverse components of plant defensive arsenals, proteins that interferewith the herbivore digestions arealso induced by jasmonic acid; like for example, legumesynthesise, alpha amylase inhibitors.Nowthis blocks the action of alpha amylase.Alpha amylase will be for starch so; obviously, it would like to block and those which arepathogensto this kind of plant where the starch is stored, would certainly have alpha amylaseto utilise that as carbon source.So, therefore, they will produce alpha amylase inhibitors, so as to deter the feeding . Otherplant species lectins, defensive proteins that bind to carbohydrates or carbohydratescontaining proteins.Now in this what happens; these once they bind, they will bind to your epithelial cellsof the digestive system and therefore, disturb the digestion of the food that is also oneof the ways for feeding deterrents.Now the best known anti digestive proteins in plants are the protease inhibitors.So, that is like alpha amylase inhibitors, it is a protease inhibitor which the plantsrelease . Some of the plants you must have heard,ourparents say stay away from it; I do not know whether you have heard.So, that is because these toxins are nothing, but secondary metabolites which may eitherdirectly act as toxins or may disturb the digestion of your system .The induction and release of volatiles in response to insect herbivore damage.Now the combination of molecules emitted is often specific for each insect herbivore,there can be so many different types of pathogens.So, either the plant should have a very generalized strong mode of action, but you will observethat sometimes the insects are also very specific to a particular variety of the plant; likefor example, your cotton Bt cotton came.So, it is always against a particular pest, which is generally known to attack that plant.So, you needthat pest specific toxin.So, plants are also known to secrete or produce insect specific toxins and some are general. Now typically in due includes representatives from these three major classes; where terpenes,phenolics, and alkaloids are involved . Now, there are other forms of secondary metaboliteswhich are called as green leaf volatiles.Now green leaf volatiles are mixtures of terpenoids and fatty acids.Now you will also find that many of these are present in near the surface, leaf surfaceor will be present in the membranes of the cells.Because that is the first line which will be broken; anything which would try to attackwould first try to lyse the cell through piercing through the cuticle, which is one of the barriers;then through the cell wall, so once it crosses cell wall, then comes the cell membrane.So, therefore, these secondary metabolites which can deter or which can be toxins aresometimes present on these surfaces; including your cuticle, leaves surface, or your bark,or even your cell membranes like saponins are known to be present near the membranes. So, green leaf volatiles they act as how;they attract the natural and enemies of the pathogens.So, because they are volatiles they will send cues to the predator of these pathogens, whowill then find out that through this cues that where the host is and this is how theplants.So, which means how intelligent is the machinery and how widespread is the plant defence inwhat different ways it protects, it is very interesting.They attract natural enemies which are predators or parasites of the attacking insect herbivorethat utilize the volatiles as cues to find their prey or host, as I said .Like there is an example it is given here that moths when theythey lay eggs on the leaves.So, now, in order to deter further laying of eggs by other moths; moths they producesuch volatile compounds which will then give signals to the other moths not to, in thesedifferent ways not to come to that leaf or that plant for further egg laying . So, manyof these compounds remain attached to the surface of the leaf and serve as feeding deterrentsbecause of their taste; something the taste would be so bad that they will that will deterthe feeding even to us, we do not prefer some fruits which are bad tasting .So, plants are continuously exposed to diverse array of pathogens.Now to be successful, these pathogens have developed various strategies, so both thesides the fight is going on; the plant is increasing its defence and the pathogens arealso clever enough to create ways, new ways of breaking this line of defence.So, some penetrate through the cuticle, the cell wall directly by secreting lytic enzymeswhich can digest these mechanical barriers.So, if you know why is something happening in nature; obviously, you can get cues ofwhat can be the potential applications of these.So, if you know that these pathogens are capable of producing lytic enzymes which can breakdown the plant cell wall; obviously, they would have enzymes which might be capableof lignin degradation, or cellulose degradation, hemicellulose degradation which can give youcues for its other applications.So, it is very important to understand, why are something happening in literature, literatureI mean nature . Now, so which digest these mechanical barriers?Now other enter the plant through natural openings, stomata we know is one of the naturalopenings; it is also written that lenticels.Now lenticels, if you see some of these plants they are they have a very rough surface; yougo and type it on Google you will be able to see, these are open spaces, specially intree species.Because in tree species there is many layers of cortex and all, so gas exchange is difficult. So, there are open spaces kept in the plant in the trunk which can lead to this gas exchange.Now these open spaces although they are meant for gas exchange they become also an openingfor pathogens to enter.So, there is always a balancing out between the merit and the demerit . A third categoryinvades the plant through wounding sites, whenever there is a wound this may enter;now, for example, those caused by the insect herbivores.Additional, many viruses these may enter, so some time some of the insects they; supposefor even nectar, they pierce through the vascular bundle to get the food.Now these may in turn, then these viruses may travel through while they have piercedthese vascular bundles for food, the virus will enter and then spread throughout theplant.Now phloem feeders this is one of the examples; like for example, whiteflies and aphids theydeposit pathogens directly into the vascular system and from which they can easily gettransmitted to the entire plant.So, now, plant has to cater to this problem also .Now, several classes of secondary metabolites have strong antimicrobial activity.So, generally because most of it isfungus, then single microorganism, bacterial speciesthen, so there is majority of secondary metabolites you will find that; they will be having antimicrobialactivity . Now saponins, is a group of triterpenes thatdisrupts fungal membranes by binding to sterol; sterols is a part of the phospholipid membranes.So, it disrupts the sterols in the membranes of fungal cultures . Now genetic approaches,now have demonstrated for example, here; in oat cell lines which were mutated to reducethe production of saponins, it was found that they were they became more susceptible tofungal attack; than the mutant lines which had higher amount of saponins .Now, after being infected by a pathogen, plant deploy a broad spectrum of defences againstthe invading microbes.Now the common defence is called as the hypersensitive response.What is this?As soon as the pathogen attacks a part of the plant, the nearing cells they die immediately. Now once they die, is to prevent any nutrients to be provided to that pathogen which hasattack, so; which means that, it tries to deprive that pathogen of the nutrients.So, there is a sacrifice involved.So, nearing the wounding site these cells die.And how do they die?There is a surge of as soon as the attack happens, there is a surge of toxic gases likenitric oxide accumulation or there will be higher accumulation of oxygen radicals, reactiveoxygen species which will cause the death of the cells also and may also in turn helpin direct damage to the pathogen also . So, cells in the vicinity of the infectionsynthesize a burst of toxic compounds formed by the reduction of molecular oxygen, reactiveoxygen species.Now active oxygen species may contribute to host cell death, as part of hypersensitiveresponse or act to kill the pathogen directly.Now, another defensive response is formation of hydrolytic enzymes that attack the cellwall of the pathogen itself; like for example, chitinase in case of fungal cell walls, orcan even be your hydrolases, or your glucanases, orwhat else proteases.So, there can be different types of enzymes, lytic enzymes which are produced which candirectly harm the pathogen . So, these hydrolytic enzymes belong to a groupof proteins that are closely associated with the pathogen and they are called as pathogenrelated proteins, PR proteins.Now, response of the plants to bacterial and fungal invasion is the synthesis of phytoalexins.Now phytoalexins they are not always present as the constitutive defence; but they areproduced as a result of the damage caused by a particular pathogen.Now phytoalexins are chemically diverse group of secondary metabolites with strong antimicrobialactivity that accumulate around the site of infection.So, which means; now the speed matters, the speed at which the de novo synthesis of thesephytoalexins will be taking place as soon as the damage has happened.So, that determines the survival capability of a particular plant species against thatpathogen . So, that is why you will see that some plants survive and the others do not;because of the defence, how strong, how fast is the defence in a particular plant speciesagainst that particular pathogen . Now, phytoalexin production appears to becommon mechanism of resistance to pathogenic microbes in wide range of plants . Now forexample, in leguminous plants such as alfalfa and soyabean, isoflavonoids they are knownto act phytoalexins.In solanaceae plants which is; potato, tomato, or tobacco, various sesquiterpenes are producedas phytoalexins . Now, phytoalexins as I said are generallyundetectable when there is no attack; they begin to form at higher amounts or at verylarge amounts at fast rate once the attack has happened.So, the point of control for the activation of these biosynthetic pathways is usuallythe initiation of gene transcription.Now for this your, what is involved; it is de novo synthesis.So, a signal cascade mechanism is involved, where your jasmonic acid, or salicylic acid,your ethylene everything is involved . So, plants do not store any of the enzymatic machineryrequired for phytoalexin synthesis.After microbial invasion they begin transcribing and translating the appropriate mRNA for thatparticular synthesis .