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4 Host resistance


Hello! welcome to the fourth topic that is host resistance for the week in the last class we have seen how cultural and biological practices help in reducing plant disease development. Today we are going to talk about the host components or the plant components that are responsible for reduction in plant disease development. So plants defense responses may be Constitutive or Inducible. Constitutive means it is the pre-existing structural or biochemical components of the plants that are fighting against the invading pathogens and they are responsible for lowering down the initial disease development. At the same time some of the structural and by chemical components that are induced after arrival of the pathogen. So we will see how this particular structural and inducible components help in reduction of plant diseases.

So Plant Defense can definitely be grouped into two that is Passive and Active. Passive means it is the constituted a defense mechanism whereas, active means inducible defense mechanisms. The constitutive or passive defense mechanisms include structural and biochemical the same time active also include structural and biochemical responses. In case of passive structural components these are the different types of waxes that are present on the surface of the plant leaves, cuticle thickness, cell wall, stomata shape and activity along with lenticels. These are the pre-existing structural defense they serve as a barrier for invasion of the pathogen into a particular plant species. Similarly constitutive biochemical mechanisms that are preformed include preformed inhibitors such as phenolics and hydrolytic enzymes. They are also already present in the plant when the MAC or the pathogen they arrive on the plant system. Active or Inducible defense structural components include Cork layer, Abscission layer, Gums and Papillae. These all are structural barriers that are induced only after arrival of the pathogen. At the same time the biochemical components that are basically arrived or induced after pathogen arrival includes Hypersensitivity, Phytoalexins, Systemic Acquired Resistance and Antimicrobial compounds. So these particular biochemical components they also help in reduction of plant disease development. So pathogens may be biotrophic in nature, that means they always need a living host for their own survival and to cause disease. For example the powdery mildews and white rust of cruciferous these are biotrophs and they require a living host for their growth and development. Some pathogens they rather kill their hosts at the very beginning and that is what how they acquire nutrition after killing of those cells. So these type of pathogens are known as necrotrophs. They first kill the cells and the nutrients that are released from these dead cells they utilize these nutrients for their growth and development.

At the same time some of the pathogens are hemibiotrophic in nature for example Collectotrichum Lindemuthianum that causes the anthranose disease in common bean and the early growth stage of the pathogen they don’t kill or the host cells rather, they grow as like a biotrophs but in the later stage they kill the host cells for derive wing nutrient from those host cells. So these type of pathogens are known as hemibiotrophs. When a pathogen is capable of causing disease the outcome may be of two types – Compatible and Incompatible. Compatible means when the pathogen is able to establish itself in the host plant and cause disease, we call the interaction to be a compatible one but, when the opposite happens that means even after arrival of the pathogen on the host if it does not get established on the host and does not able to cause disease on the host plant we call it Incompatible response. So this compatibility and incompatibility it is an inherent characteristics of the plant host which carry different set of genes and these genes are responsible for the outcome of the host and the pathogen interaction. In case of immunity when there is no disease development at all in the plant even after arrival of the pathogen then we call the interaction to be an immune interaction, where there is no symptom development takes place. The other responses include highly resistant that means some disease symptoms appears but these are minimal and these development of these type of disease symptoms does not lead to economic damage to the crop plants. At the same time it can be highly susceptible where significant defense symptoms are developed and susceptible interaction leads to heavy economic loss for the growers. So all these interactions are dependent on the genetic constituents’ of the pathogen as well as of the host.

Normally, when the pathogen arrived in the host surface then some of the components of the pathogen they may be lipopolysaccharides, they may be certain proteins, or they may be certain other polysaccharides. So these are sensed by a receptor proteins that are present on the plant cells and when this sense of pathogen factors by the host receptors then there is a development of Basal resistance and this resistance is basically broad-spectrum and if this broad-spectrum defense is able to lower down a variety of diseases from originating from different types of pathogens. The pathogen component which are sensed by the plant receptor systems are known as Pathogens associated molecular patterns or microbe-associated molecular patterns (MAMPs) if they are non-pathogenic in nature. But once they are sensed by the plant receptors there is a defense response generated which is broad-spectrum and effective against a number of plant pathogens. This is what we call it as Basal resistance. We can just see how the interaction takes place in this particular picture. Here these are the microbial components we call it Microbe Associated Molecular Pattern and these are plant component, these are receptors which are able to sense these MAMP and once their interaction is positive then there is development of a danger signal and this signal is responded in a expression in a way where by expressing defense genes in the host plant system. This expression of Defense genes lead to horizontal resistance against a number of pathogen microbes. At the same time plant also releases certain enzymes that causes break down of the pathogenic surface components and these surface components can also be sensed by certain receptors in the plant and this also has a similar impact on the host where the DANGER signal is perceived by the plant and there is development of horizontal resistance. So this is how the Basal Immunity development takes place. But pathogens are able to overcome this Basal developmental resistance and plant respond to this by development of hypersensitive response (HR) in the plant cells. Hypersensitive response is a volunteer or suicidal process of certain cells where the Pathogen is infecting or pathogen is trying to establish themselves on those particular cells. So here you can see these are hypersensitive response that are generated in the plant against a particular pathogen. Normally, when the Basal immunity is or Basal defense is generated then the Basal defense is overcome by the pathogen through secretion of certain proteinaceous molecules known as effectors and these effectors interfere the danger signal and the signal breaks down due to this effector interference and that is why the Basal defense is overcome by the pathogens. And then the plant responds to this breakdown of Basal defense through HR response or hypersensitive response by recognizing this effector molecules that have been introduced into the plant system by the pathogenic microbes. So Structural Defenses include Plant cell wall because cell wall is the primary layer that has to be invaded by the microbial pathogen and cell wall consists of Cross-linking of several glycans and hemicelluloses including celluloses and all this complex polysaccharide cross-linking leads to development of a resistance barrier against the invading pathogens. There may be lignin that are present in the plant cell wall along with Cutin, suberin and waxes. All these components also strengthen the plant cell wall against the invading pathogens and many a times these Pathogens are unable to break down these barriers and they are unable to establish themselves into the invading cells. Then plants can also respond to invading pathogens through Callose deposition.

So these Callose are basically developed at the point of entry side of the microbial pathogens there where a variety of complex polysaccharides and antimicrobial compounds are clubbed together to form a barrier initially to check the development or penetration of the cell membrane by the invading pathogenic hyphae. If this particular callose layer is broken down by the pathogen then plant starts adopting another mechanism through development of papillae. This papillae is again a complex of several polysaccharides and this is another physical barrier that are synthesized in below the callose layer and it’s also help in defending a plant system against the invading pathogen. Chemical Defense include the secondary metabolite production in the plant system. So some of the chemicals may be general effective against a large number of pathogens whereas, some of the chemicals may be specific against some of the pathogens. So general chemicals which are responsible for defending plant againsta wide variety of pathogens are the first line of defense which are basically Potent toxins and they don’thave rather a specificity towards any particular pathogen rather they are broad-spectrum effective against a number of pathogens. Whereas, the Specialist they are basically responsible for checking the infection of certain specific pathogens. So they are specific in nature in that way. There are different types of proteins and enzymes that are also synthesized in the plant when the pathogen starts infecting the host cells and this includes defensins, amylase inhibitors, lectins and proteinase inhibitors and the cumulative effect of these enzymes lead to check of the progress of the pathogen which may include a penetration by fungi, bacteria, nematode and certain insect herbivores. So ultimately it is the genetic background of the plant that leads to this different type of resistance responses and when we consider the Genetics of Resistance then it can be categorized into two broad group that is vertical and as well as horizontal. In case of vertical resistance it is also known as qualitative resistance which is specific against a particular race or a particular pathogen whereas horizontal or resistance is also known as quantitative resistance which is non specific in nature and it may be effective against a large number of races of the pathogen. Normally, vertical resistance is governed by a few major genes whereas the horizontal resistance is governed by the number of minor genes which are effective against a large number of races. Although, vertical resistance give very good control against a particular race but there is always a danger that this particular type of resistance may be broken down by a different race that evolves during the course of time. Whereas, horizontal or nonspecific resistance is more durable in a sense that this is dealing with a multiple races of the pathogen and that is high it allows a little amount of disease but overall progress of the disease is checked through the combination effect of all these minor genes. So in this particular talk we have seen that how plant respond to the infection by a particular microbial pathogen and what are the different types of structural and biochemical strategies plant adopt to manage this or to check the growth and development of the pathogen, and what are the genetic basis of this responses, how plant are able to basically suppress these microbial pathogens to lower down or to protect itself from these external threats.

Thank you.



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