Plant immune response

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The plant immune response is a natural protective mechanism of plants, analogous to the immune response in animals. Since plants do not have adaptive immunity mediated by antibodies, the plant immune response is entirely part of the innate immune response . The existence of pathogen-associated molecular patterns in plants and animals shows that this assignment is also correct in evolutionary terms .

In contrast to vertebrates, plants do not have antibodies . Nevertheless, they are usually resistant to diseases that can be caused by pathogens such as bacteria , nematodes , fungi or viruses . Various defense mechanisms are used here. Saponins , a group of triterpenes, are formed by plants before infection. They offer protection against fungi by binding to the sterols in their plasma membrane and thereby destroying them. Other defense measures do not take place until the infection occurs.

Induced Defense

Overview of the induced defense mechanisms of plants against pathogens

As part of the induced defense , molecules from the pathogen (e.g. proteins , sterols, polysaccharide fragments) bind to a receptor protein in the plant's plasma membrane. The binding of such an elicitor activates the receptor and sets two signal cascades in motion. On the one hand, the NADPH oxidase , which is also located in the plasma membrane, is activated so that atmospheric oxygen is reduced to superoxide anions. Superoxide anions are then converted into hydroxyl radicals and hydrogen peroxide . These three reactive oxygen species start radical chain reactions with organic molecules, which result in lipid peroxidation, enzyme inactivation and the breakdown of nucleic acids, which affects pathogens and infected plant cells. The second signal cascade initiated by the plant receptor leads to calcium-dependent activation of nitric oxide synthase and thus to the formation of nitric oxide , which, together with hydrogen peroxide (H 2 O 2 ), can trigger the following defense mechanisms depending on the plant species and pathogen:

Hypersensitive reaction

As part of the hypersensitive reaction, cells that surround the infection site die off more quickly, so that the removal of nutrients means that the pathogen is no longer given the opportunity to spread. The dead cells are macroscopically recognizable as necrotic lesions. The hypersensitive response is a form of programmed cell death that may cause a. leads to the increased formation of nucleases and proteases, which hydrolytically break down nucleic acids or proteins. Many plant species respond to pathogen infestation by synthesizing lignin or callose , which represent a physical barrier against the spread of pathogens. A similar reaction is a cross - linking of proline-rich proteins in the cell wall caused by hydrogen peroxide , which leads to their densification. Infecting fungi can also be fended off with the synthesis of chitinases, whereby the enzymes hydrolyze chitin as part of the fungal cell wall. In addition, the expression of glucanases or other fungus-attacking enzymes can be increased. The synthesis of the antimicrobial phytoalexins has been extensively researched on various plants. These secondary metabolites are different substances such as B. Isoflavones in the case of legumes or sesquiterpenes in the case of nightshades. After a pathogen infection, the plant can show an increased resistance to a range of different pathogens, even in areas not previously infected. This systemically acquired resistance is based u. a. on the hydrolytic enzymes already mentioned. Salicylic acid probably leads to the synthesis of a lipid (or lipid derivative). The signal molecule is transported in the phloem and causes the resistance in parts of the plant that have not yet been infected. The methylated form of salicylic acid ( methyl salicylate ) is volatile, so that increased resistance can also be induced in neighboring plants.

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