Whereas animals generally repair tissues that have been damaged by pathogens, plants do not. Instead, plants seal off and sacrifice damaged tissues so that the rest of the plant does not become infected. Plants have the option of discarding damaged tissues because most plants, unlike most animals, can replace damaged parts by growing new ones.
Before we look at the details of the defensive process, we consider a key response by plant cells to invasion by pathogens: the rapid deposition of additional polymers on the inside of the cell wall. These macromolecules not only reinforce the mechanical barrier formed by the cell wall, but also block the plasmodesmata, limiting the ability of viral pathogens to move from cell to cell. The polysaccharides also serve as a base on which lignin, a polymer that provides strength and rigidity, may be laid down. Lignin enhances the mechanical barrier, and the toxicity of lignin precursor chemicals makes the cell inhospitable to some pathogens. These lignin building blocks are only one example of the toxic substances that plants use as chemical defenses.
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Induced responses to pathogens are controlled by receptors. Plant pathogens cause the host plant to activate various chemical defense responses. A wide range of molecules called elicitors have been identified that trigger these defenses. These molecules vary in character, from peptides made by bacteria to cell wall fragments from fungi. Elicitors can also be derived from fragments of plant cell wall components broken down by pathogens.
The responses of plants to elicitors can be described in terms of the “plant immune system.” Two forms of immunity are recognized (see Figure 38.2):
General immunity is triggered by general elicitors called pathogen associated molecular patterns (PAMPs). PAMPs are usually molecules that are produced by entire classes of pathogens, such as flagellin (found in bacterial flagella) or chitin (found in fungal cell walls). Thus, general immunity is an overall response rather than a response that is triggered by a specific pathogen in a particular plant. PAMPs are recognized by transmembrane receptors called pattern recognition receptors (PRRs), which activate signaling pathways that lead to general immunity.
Specific immunity is triggered by specific elicitors called effectors. Effectors include a wide variety of specific pathogen-
Understanding the pathways of pathogen infection is important for agricultural scientists, because pathogens can severely reduce crop production, as you saw in the opening investigation of this chapter. One approach to understanding is to describe the plant genes whose expression is changed after pathogen infection, as mutations in these genes may lead to resistance. Also, comparative genomics can be used: if a gene that confers resistance is found in one species, it may exist in close relatives, and may even be used to create a transgenic crop that is resistant to the pathogen. An example is described in Investigating Life: A Gene for Resistance to Wheat Rust.