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Module 1: Plant Metabolism

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Secondary Metabolism in Plant Cells

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. So,from today we are going to learn aboutsecondary metabolism in plant cells . Now comes the commercial applications of plantcell technology. So, before we move on to how we exploit secondary metabolism commercially,it is important for us to know about secondary metabolism in plants, its role and hence wewill take it extrapolated to commercial applications of it. So, what is secondary metabolism? nowwhatis metabolism to begin with? it is the sum of all chemical reactions that take placein an organism, so this forms metabolism whether primary or secondary. Now most of the carbonnitrogen or energy which is accumulated in the cell is majorly used for its growth anddevelopment, so that forms the primary metabolism. Now, some of thecarbon and energy is alsoutilized for specialized functions which majorly deal withmore organized function and differentiationin plants, which deals with its a survival in the nature for competition or defense relatedall that. So, most of the carbon nitrogen and energy it ends up in molecules requiredfor proper functioning of the cells and organism. Now for example, what are those?We know lipids carbohydrates nucleic acids, now these from the primary metabolites. Nowmost plants divert a significant proportion of this assimilated carbon pool and energy,is to synthesis organic molecule. So, these secondary metabolites are generally organiccompounds, so these organic compounds actually have no obvious role in general plant growthand development, but they have other roles which come under secondary metabolism ok.So, how are they different? At the biosynthetic level primary and secondary metabolites, generallythere are common pathways, majorly you will find what all pathways and primary metabolismdo we know about? Pentose phosphate pathway, glycolytic pathway then.T C A. T C A cycle. So, most of the secondary metabolismalso leads from some one or more of these intermediate of the primary metabolism andthen it gets diverted into secondary metabolite pathways. So, at the biosynthetic level primaryand secondary metabolite, they share many of the same intermediates, now they are derivedfrom the same core metabolic pathways which we know as a part of primary metabolism. Nowsecondary metabolites generally occur in relatively low amount in comparison to primary metabolites.What can be the reason, then I will ask why they are found in the stationary phase . So,now, try to reason outWhy do you think they will be produced in small amounts. So, two things; one is onlywhen it is needed the cell would spend its carbon pool and energy in this biosynthesisand only up to a desired level it will try to use . So, every time the cell would liketo conserve its carbon and energy pool. So, only therefore, you generally observedthat secondary metabolites first, they are low volume which means they are present invery low quantities in the plants and the second thing is, there is a wide array, youcannot even imagine how wide is the secondary metabolism in plants. It is not uniform; itcan vary from plant to plant in terms of genes, in terms ofstarting from family. Then geneseven till species you can find variations in secondary metabolites. So, for every suchmetabolites there will be a carbon required carbon flux. So, there is a wide array. So,the cell would like to judiciously use and priorities.So, secondary metabolites generally occur in relatively no quantities I said. Then theirproduction is widespread or restricted to particular families genera and even species.Now, what is the role of secondary metabolites in plants. Now secondary metabolites theyhave ecological functions in plants. So, what kind of functions do they carry out? So, theyprevent the plants being eaten by herbivores or they prevent the plant from being attackedby pathogens. These pathogens can be fungal pathogens, can be means of microbial origin,can be insects, can be predators or like herbivores, can be mammals, can be humans.So, they they need to have that wide spectrum of defense against all these and then theyalso need to prevent against environmental stresses . So, they serve asattractants andit is not only defense, but they also carry out higher order functions ; like for example,in pollinators attracting the pollinators or even attracting the predators of the pathogens.So, by using this volatile secondary metabolites which can attract these predators, so thatthey can get rid of the pathogens. Then they serve as attractants; like in theform of odor colour taste for pollinators and seed dispersal. Then sometimes in orderto prevent being eaten up, you will find some of the fruits are very bitter, so that humansdo not eat. Now they function as agents of plant plant competition. Sometimes you willsee that plants produce exudates in the soil which are toxic to the nearing beads or thenearing plants. This is the kind of overcoming the competition for the sunlight and for theresources in the soil . So, they increase the reproductive fitness,how do they increase? by warding off fungi, so that they remain healthy then bacteria,whatever pathogens may attack them, so that the status of the plant is quite healthy.So, the ability of plant to compete and survive, is therefore, profoundly affected by the ecologicalfunctions of their secondary metabolites. This is the general class, there are manyother different sub classes of secondary metabolites, was majority of them start from these threemajor classes of secondary metabolites in plants. What are these three major classes;one is terpenes, then phenolics and nitrogen containing compounds. Your high value compoundswhich are alkaloids your anticancer agents, taxon and all these, is a they or camptothecinor vincristine vinblastine, they come under alkaloids and these alkaloids are nitrogencontaining compounds. So, this is the general picture which showsthat its start from primary metabolites, primary metabolism and then some of these intermediateget diverted towards the formation of secondary metabolites. So, you will find that shikimicacid pathway or pentose phosphate pathway or MEP pathways, they may independently producesome of the secondary metabolites or some of these intermediate from two different pathwayscan come together to give rise to the new secondary metabolism pathway, biosynthesispathway . So, you will find different types of secondarymetabolites coumarins, anthocyanins, flavonoids, alkaloids, terpenes. So, there are n numbers.So, let us talk about terpenes, what are terpenes? They constituent the largest group of secondarymetabolites present in plants. Nowone of the most popular biopesticide which is azadirachtin,I gave an example also is a sesquiterpenoid. So, its structure comes from terpene and thenits a say sesqui means there are,because of the structure the number of carbons and thenumber of rings involved thethat terpenes and then its derivatives it is formed.So, the terpenes or terpenoids the constitute of the largest class of secondary metabolitesin plants. Now most of the diverse substances of this class are insoluble in water, so theyare not soluble in water. Then certain terpenes are well characterize, have well characterizedfunction in the plant growth or development and so can be considered as primary ratherthan secondary. So, some of these terpenes also can play a role in the growth and development.So, therefore they are classed as primary rather than secondary.For example some of the plant growth hormones like gibberellins. Gibberellin we have studiedit is one of the plant growth hormone. So, gibberellins are an important group of planthormones and it is a diterpene. Now brassinosteroids another class of plant hormones with growthregulating functions, with takes care of the primary aspect of the plant growth and development.So, it is also termed as primary metabolite, but it originate from triterpenes.So, generally you will observe that most of these high value terpenes they are plant defencerelated triterpenoids or terpene molecules. Now terpenes they act as toxins and feedingdeterrents to many herbivorous insect and mammals. That is the very reason why hazarddirection is used as a biopesticide, it acts as a feeding deterrent to the insect pest. Monoterpenes, it also depends on the modeof action. Suppose if it is a wide spectrum of, like a hazard direction has a wide spectrumof antifedenrt activity, so it can cater to a large range of insect pest. So, that isthe reason by neem is being used as against even mosquitoes. Then it is also used forpathogens or for bio pesticides in crops to remove insect pest, so it has a wide rangeof applications. Now, monoterpene esters are also called aspyrethroids, this is also a class of biopesticides, found in the leaves and flowers of chrysanthemumspecies which shows insecticidal behaviour. So, it is also a part from hazard directance,it is also used or hazard direct related liminoids, it is also used as biopesticides. Now youcan see and make out that it the commercial application is very closely associated toits application in nature, so that can give you a cue or an indication to what kind ofcommercial application of particular secondary metabolite may have. Now in conifers monoterpeneaccumulate in resin ducts, found in the needles and trunks that is for what? Once it is inthe bark region then it can prevent either the pathogenic attack or even the herbivory.So, many plants they contain a mixture of volatile monoterpenes and sesquiterpene andthese are called as essential oils and they are highly expensive. So, you will find thatthese essential oils are very expensive in market and they are not found in very large.I was looking for, we had gone to coimbatore last month and there in the shopI went toa tea plantation where a factory where they were showing.So, there was a shop in which they were selling these essential oils. So, there was a teatree oil which caught my attention, it was just two ml two ml bottles which were beingsold, so tiny one, and then I started asking what is the use of this. So, they were saidit is a very expensive, very good oil for acne treatment and then for, which means antimicrobialactivity and all. So, by the time I was about to reach the counterto buy the tiny stuff, because that is, was very , I was asking why it is so expensiveby in such. So, they started to telling me its very difficult, the tree itself is notfound, it is called tea tree oil. So, that got my tension because we work on plant biotechnology,so I thought it could be a good species to work with. So, because availability the demandis high and the availability is less. So, the minute I by the time I could reach thecounter to buy it the stock got over. So, they just had two bottles and those two someonepicked up before me. So, that really has got my attention thatessential oils are very expensive, they can have large commercial applications in cosmeticindustries, in as your antimicrobial agents, so all related. So, these are nothing thatwas one example of essential oils. So, they are frequently found in where, glandular hairsor. So, these are nothing, but a projections of the epidermis, epidermal hair or even intrichomes. trichomes are also found on the surface of the leaf on the epidermis projections,where these secondary metabolites get accumulated in the form of bulbs. Now why do you thinkthese essential oils are getting accumulated in on the surface.Insolubility. That is the reason the plant will put it onthe surface, how is insolubility related .The question was that why do you think these essential oils, generally you will find themon the surface of the leaf or on the surface epidermis dermal tissue nearprotection from . If it is volatile it will help on the surface,then if it is nonvolatile and it has a role to play in attracting, so if only becauseof the either the color should be responsible that should also show up or the odor shouldbe responsible that should also be on the surface, then if it is acting as a defensemechanism, then should it be inside or should it be on the surfacefirst level of ah. First level of defense, so that. Seethe plantis not moving can move our hands and legs like us. So, if at one point the attack is,it cannot move, it has to make sure that the rest of the parts of the plant are now gearedto overcome the pathogen attack. So, if the compound is on the surface it will be first.. Line of defense, it will be ruptured and thenthe secondary metabolite is released. So, that is the one of the reasons of having thesein the form of trichomes or glandular hairs. Now, triterpenes that different plant againstvertebrate herbivores include cardenolides and saponins. I do not expect you to remember,but at least the I would like to tell you what are the different wide range of secondarymetabolites and they can have varied commercial applications. Now cardenolides are what? Glycoside.Now glycosilation if you remember in the last classes I told you,addition of sugar moietyis sometimes to reduce the toxicity, one of the biotransformations, so glycosides.So, even even here cardenolides are glycosides, now compounds containing and attach sugaror sugar alcohols that taste bitter and are extremely toxic to higher animals. So, someof these glycosides they that taste is very better and which can act as a deterrent, feedingdetterent or can be very toxic to the feeders, the herbivores.Now, saponins are steroids and triterpene glycoside. So, as I said earlier that thereis wide range of secondary metabolism. So, some of these intermediates can be directlygetting produced to the final, used up in the final product in the straight biosyntheticpathway, but some may have one more involvement of various one, more than one secondary metabolicpathways; like for example, saponin, saponins are steroids and triterpene glycoside. So,there as triterpene moiety and there is a steroidal moiety, so presence of both lipids.So, which means what? it will be both water soluble water insoluble, so it can form then.. Micelles, so which can improve its diffusionthrough the membrane. Now I will talk about it later, some of the secondary metaboliteshow do you think the plant protect itself from the toxic action, it is said it is toxic.So, one is they should be modified and stored, detoxified and stored and when required, therequired final step to again bring it back to the original, whether it is enzymatic stepand so then the enzymes are stored in the vicinity so that as soon as it is rupturedand they has to generally in suppose vacuoles So, tonoplast; for example, I said in thiscase, if it is has to be stored in tonoplast then in the form of micelles it can easilypass through the phospholipid layer, but once it comes inside then its an acidic environment,so which will again transform and it will find it difficult to return back. So, theseare ways in which the plant would protect, but suppose this leaf where this was storedor stem part gets ruptured by the attack. So, now, this cell is ruptured, so everythingwill come out from the vacuole although the cell has died, the one which is damage now,but now whatever enzymes were there in the vicinity will also come and the final stepcan take place with that intermediate and the enzyme which can lead to the productionof the final toxic metabolite. So, it is not that it is very quick andyouwill also find it is not for the very wound response by the plant of the defense mechanism,is not to have immediate sometimes defense, which means the cell which has died on thatissue which has been affected may not come back again, may not be revived like what wedo as treatment, but the remaining the objective is to prevent its spread to the other partsof the plant or even to the neighboring plants, so that also forms a part of secondary metabolism.So, the other class as I said was phenols. Now plants the produce a large . So, whatdo they have they have, we know a final group attach with the functional of functional groupis what OH. OH.So, that forms the phenolic group. Now these compounds they are chemically heterogeneousand almost 10,000 different phenolic compounds are produced as different secondary metabolitesin plants. Now some are soluble in organic solvents, some are water soluble carbox inthe form of, when they are in the form of carboxylic acids or glycoside, so they becomewater soluble and some are insoluble polymers. So, why do you think they are these differentforms, and this is nothing, but biotransformations; functional groups getting added, derivatives,saturation, unsaturation which might change the chemical structure and its biologicalaction in the plant. So, phenolics play a variety of roles in theplant, what are the different roles? as defense against herbivores and pathogens, they canalso act as mechanical support. now I need to know how do you think this phenolic compoundcan act as a chemical support there are alsopolymers, large polymers whichare present in . right. So, they can also form a structuralcomponent of your polymers. Now these polymers in turn are utilized in your lignin; likefor example lignin, it is a bioplymer similarly other lines of defense where biopolymers areto be required. Now in attracting pollinators and fruit dispersers, now the colored pigmentsof plants usuallyprovide visual cues that help to attract pollinators and seed dispersers.Now, in absorbing harmful ultraviolet radiation some of these compounds may also absorb theU V radiation, in reducing growth of the nearby competing plants. In in vitro culture sometimeswe find that I was, I referred to this that some of the species, because of large exudationof phenolic compounds it becomes very difficult to grow them in vitro; that is because ofthe toxic nature of the phenolic compounds which get released in the medium.So, phenolic pigments are of two principle types; one is carotenoids and flavonoids.Now, carotenoids these are isoprenoid alcohols; for example, there are different colors yellow,orange, red, terpenoid compounds that serve as accessory pigments in photosynthesis. So,it is not only green colour or your chloroplast,sometimes you will find that there is a transformationfrom chloroplast to chromoplast and lycopene . Sometimes in in vitro cultures in orderto enhance the production of ah, there is an example of lycopene in some elicitors wereadded, but it was also observed that once the lycopene. So, it is works the other wayround backward direction. Once the lycopene synthesis started happening,the chloroplast started getting transformed reduced and the proplastid which is the previousstageof farming different types of plastid, they started getting converted to chromoplast,so that also is nothing, but. Now these sometimes for u v range which is the lower wavelength,which are not visible to the eye, but some of these secondary metabolites can capturelight energy which is in this shorter wavelength area. So, the flavonoids also include a widerange of coloured substances. The most widespread group of pigmented flavonoids is anthocyanins.These are commercial very important molecule, secondary metabolites. Now which are responsiblefor most of the red pink or purple colour fruits and flowers. Now the two other groupsof flavonoids that found in flowers are flevones and flevonols. Now these are those secondarymetabolites which are also use, not only for the helping in pollination asor they haveother defense related activities, but they can also help in light absorption, becausethey can absorb light energy in the shorter wavelength.So, isoflavonoid which are found mostly in legumes and have several different biologicalactivities. Rotenone can be used effectively as insecticide or for pesticide which is forkilling the fishes. So, isoflavones may also be responsible for the anticancer benefitof food prepared from soybeans. Now, phenolic polymers, now these phenolicpolymers they have defensive properties besides lignin are called as tannins. So, that iswhat we may say that they are utilized in the formation of defense. Now they are generaltoxins that can reduce the growth and survival of many herbivores. Now tannins they act asfeeding repellents to great variety of animals. Unripe fruits for example, now the plant wouldnot like anybody to eat the unripe fruit until unless the seed is ready for the next stageSo, it then produces more amounts, where will the plant produce more amount of such compounds?It will produce inside the fruit, so that it can deter the herbivores. You will findthat I also spoke about that hazard in the plant is found maximally where, in the seeds,what can be the reason. So, as to prevent it will act as a feeding deterrent. It isbitter in taste, you will find the neem leafs are very bitter and the seed is also verybitter, but the ripe seeds they are full of sugar.So, the release of secondary compounds by one plant that have an effect on neighbouringplants is called allelopathy. If a plant can reduce the growth of nearby plants by releasingchemicals into the soil, it may increase its access to its competition. So, it would liketo reduce the growth of the nearby plant, so secondary metabolism also plays the rolethere. Now how do you think that in all this the plant will protect itself; one is thestorage, what else all the compounds will be stored . There are so many compounds inthe plant is preparing. Vacuoles are how much, can vacuole old, thousandsare getting produced. So, everything is not getting stored in just vacuoles, some arein the open spaces also in the plant, some particular tissue or you will find that itis and cortex cells, directly accumulating there, you can visualize them in an tissuesection, but then how do you think the plant protect itself. If suppose it is acting, letus take a camptothecin because we are studying competition, so we know about it. So, in thatI will take that as an example. So, camptothecin, it is a topoisomerase inhibitors that is whyit is a very good candidate for anticancer activity.So, its cytotoxic in nature. Now it is also present in plants, so thenhow do the plants protect itself. The topoisomerase of the plant gets mutated such that the modeof action that is why I said. See the mode of action in cells, in mammalian cells elseis the topoisomerase is different than in the plants, that is how plant protect itselffrom the cytotoxic effect of camptothecin. So, alkaloids; So, a large variety of theplant secondary metabolites they have nitrogen as a part of their structures. So, nitrogencontaining organic compounds they form alkaloids. Now the well-known anti herbivore defense,hence these are called as alkaloids. Now most nitrogen secondary metabolites are synthesizedfrom common amino acids. So, most of these alkaloids they are formed from amino acids. So, alkaloids are usually synthesized from one of the few common amino acids. What arethese? Lysine, tyrosine or tryptophan. The alkaloids are a large family of more than15,000 nitrogen containing secondary metabolites. Now can you see 10,000 phenols 15,000 alkaloidsand the largest group was triterpenes much more than that. So, imagine the wide arrayof secondary metabolites the plants are producing. So, how much carbon and energy is gettingutilize apart from growth in to secondary metabolites. So, no wonder the plant has tojudiciously prioritized the carbon flux into these wide array of secondary metabolites.What else do you think, how does the plantprioritize then. The enzymes which are involved in the biosynthesisof these, they in turn get impacted by these external factors, may be temperature, lightor elicitors. So, that is how the plant is all the time managing its resource, maybethe enzyme is there, may be enzyme is not at all there. We will see it later as I goon that there are a class ofsecondary metabolites which are called as phytoalexins. Phytoalexinsare not present, they are synthesized de novo when needed, but there the trick is, the synthesisis very fast and there are constitutive secondary metabolites which are present all the time.So, these are different ways in which the plant is prioritizing .