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1 Molecular Plant-Pathogen Interactions


Hello! I welcome you to the sixth week of this course. In the previous weeks we have seen what are the different pathogens? How they cause disease in plants? How disease is developed in plants and how they are managed under field conditions in different field crops ? In this particular week we’ll be talking about the modern tools and techniques that are being used for management of plant diseases. These tools and techniques are useful because without understanding the pathogen we cannot move ahead for managing them in a long run.

So the first talk of this week is designed to understand the interaction between plant and pathogen at molecular level because without understanding this we will not be able to understand when a pathogen is going to knock down a resistant varieties or whether a pathogen is developing resistance against a particular fungicide. So to understand this we need to understand in plant and pathogen interaction at molecular level. Plants are exposed to a diverse type of microbes both the above-ground parts and the underground parts are exposed to such microbes and the microbes are broadly categorized into two groups one group that is harmful or Pathogenic to the plant and the other group that is beneficial to the plants. So we will be talking about the harmful interaction of plant and pathogens. We know that plant disease is an exception rather the rule that is why under natural condition we normally see green plants or healthy plants in comparison to a significantly low amount of infected or diseased plants. The plants get infected only when the plant is unable to resist the pathogen attack. The majority of the plants are resistant to infection by a majority of microbes. Then what are the hallmarks of a successful pathogen that leads to disease development and how plant resists them. So let us see these things one by one.

The plant has Two-tiered immune system one is known as PTI that is PAMP Triggered Immunity which is general and basal resistance and this is effective against a wide variety of pathogens where at the same time the other immune system is the effectors triggered immunity which is basically specific to a particular pathogen and this is based on gene for gene interaction. Gene- for- gene hypothesis was given by H. H. Flor in 1946 while working with the rust pathogens of flax. Flor showed that the inheritance of both pathogens in the host and parasitic ability of the pathogen to cause disease is controlled by a pair of matching genes. In one plant the gene is called resistant gene and the other in the parasite is called the Avirulence gene. Plant producing a specific R-gene product are resistant toward the pathogen that produces the corresponding (Avr) gene. So this is very important to understand that how the plant and pathogen behave. So plant recognizes pathogen by a particular mechanism.

Say for example fungi and bacteria they have some structural components that are perceived by the plants through some receptors and the pathogen components are known as PAMPS. These PAMPS are perceived by pathogen receptors and once the pathogen is perceived by plants then there is a signal mechanism that generate and that leads to PAMP triggered immunity. But pathogens also secret effectors. These are basically proteins and these effectors molecules interfere with this particular signaling mechanism that leads to PTI rather these effectors break down. This PTI defense mechanism and the plant becomes susceptible. The effectors can also be recognized by another group of proteins that are synthesized in plants and these proteins are known R- proteins. So these effectors proteins are recognized R proteins in the plant then effectors trigger immunity response takes place in the plant and the plant that thereby able to defend themselves against invading pathogens. So PTI – as Basal resistance development what is the mechanism? So here you can see that PAMP (Pathogen Associated Molecular Pattern) when they are perceived by the receptors in the plant surface they generate a danger response and that leads to development of resistance against a variety of pathogens.

There is another term that is known as DAMP or damage associated molecular pattern. This is nothing but some plants secrets some enzymes that basically degrade the surface components of pathogens which are then fragmented into smaller units and these small units of pathogens surface fragments then are perceived by plant receptor MAC system and then again a danger trait is triggered and this leads to horizontal resistance against a variety of pathogens. So this is the basal resistance that is commonly known as PTI PAMP triggered immunity and it is effective against a large number of pathogens. Then is it the only way the pathogen on plant interact. If we just look into the evolution of plant and pathogens then we can see that once the PAMPS are responsible for triggering the immunity of the host plants which we call it as PTI PAMP Triggered Immunity then pathogen responds to that situation by secreting some effector molecules which make the plant susceptible have which we call it as effecter triggered susceptibility. Then plant respond to this effectors equation by synthesizing some R proteins which can recognize the effectors molecules and once this recognition takes place then the plant immune response again trigger which we call it as Effectors Triggered Immunity. So this particular Effectors Triggered Immunity is pathogen specific and it is very robust. Then pathogen react to this ETI response of plants by either secreting new molecules of effectors or modifying the existing effectors so that the existing R- proteins can be avoided. Then by this curse situation they can again interfere with that defense response of the host and make the plant susceptible. Then plant responds to these modified effectors proteins by modifying its R protein product and thereby the new effectors are recognized by the new R- protein and again the Effectors Triggered Immunity is evident and that is how plant defend itself under natural condition. So this is a ongoing process how plant and pathogen interact with each other if pathogen wins at one point of time then the plant develops strategies to overcome that situation and make it a winner. But again the question arises whether all type of pathogens are responded by plant in a similar way? The answer is certainly ‘NO’. Say for example the Necrotrophs pathogen like Botrytis cinerea and biotrophs like Tobacco Mosaic Virus ( TVM) or bacteria pseudomonas syringae are perceived by plants by different mechanisms. Once the biotrophs are perceived by plants with a particular receptor then there is generation of a signal that is mediated by Salicylic Acid (SA) . There are certain signature genes like NPR 1 that are expressed during this process and the ultimate result is Defense against these particular pathogens. At the same time necrotrophs when they encounter with the plant then the plant perceived the signal and the signal perception takes place through certain signaling molecules like jasmonic acid (JA) and the signature gene in this case is COI1 and the ultimate resultant is defense against the pathogen. At the same time if both the pathogen or both the mechanisms are operative normally, it is seen that the Salicylic Acid trigger can suppress the jasmonic acid (JA) trigger mechanism and the resultant effect we get is the defense through Salicylic Acid mechanism. So plant also does not respond to different types of pathogen in a similar way they respond to by biotrophic and necotrophic pathogen in a different manner. So in this talk we have seen that the molecular aspect of plant and microbe interaction. How plant responds to different types of microbes and how ultimately it leads to development of resistant response in the host.

Thank you.



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