Pathogenesis-related (PR) proteins are proteins produced in plants in the event of a pathogen attack. They are induced as part of systemic acquired resistance. Infections activate genes that produce PR proteins. Some of these proteins are antimicrobial, attacking molecules in the cell wall of a bacterium or fungus. Others may function as signals that spread “news” of the infection to nearby cells. Infections also stimulate the cross-linking of molecules in the cell wall and the deposition of lignin, responses that set up a local barricade that slows spread of the pathogen to other parts of the plant.
According to this definition, any host protein induced by any type of infectious agent or comparable condition would qualify as a PR. Hence, the term pathological situations is taken to refer to all types of infected states, not just to resistant, hypersensitive responses in which PRs are most common; they also include parasitic attack by nematodes, phytophagous insects, and herbivores.
Although the term pathogenesis-related proteins by itself may give the impression that (all) proteins common to pathological processes in a given plant species can be considered PRs, proteins that are constitutively present in low, yet readily detectable amounts, but which are increased substantially by pathogen infection, such as oxidative and hydrolytic enzymes or general enzymes of aromatic biosynthesis, have been excluded. However, if specific isoforms of such enzymes do not normally occur but are induced as a result of infection, such isoenzymes do belong to the PRs. The major criterion for inclusion among the PRs is that the protein concerned is newly expressed upon infection, although not necessarily in all pathological conditions. These considerations imply that the characteristics of the induction of PRs take priority over other identifying characteristics, such as chemical properties and cellular localization.18 Proteins that are induced exclusively during disease development in compatible host–pathogen combinations have hardly been considered as PRs. Rather, the “classical” PRs are a collective set of novel proteins associated with host defense mainly in incompatible interactions and, thus, are related primarily to a special type of pathogenesis, i.e., one culminating in the impediment of further pathogen progress. In compatible host–pathogen interactions, pathogenesis, symptom expression, and reduction of growth are generally more severe, and enzyme activities may be increased to high levels, but novel host proteins have been observed only occasionally and remain poorly characterized. Proteins expressed under such conditions seem to be rather specific, being induced under the influence of particular pathogens but not others. This situation contrasts with the massive, apparently coordinated, induction of a common set of PRs in virtually any incompatible interaction, which in some cases can amount to 10% of the total soluble protein of the leaf. For this reason, PRs have been implicated in the resistance response and, more specifically, in the acquired resistance that is induced accompanying necrotic lesion formation in various plant species. When plants are locally infected with a pathogen inducing hypersensitive necrosis, they acquire systemically enhanced resistance to subsequent infection by various types of pathogens, and this systemic acquired resistance (SAR) is associated with the induction of PRs in tissues distant from the original inoculation site.1,7,8,19 Such observations suggested that PRs are defense proteins functioning in limiting pathogen multiplication and/or spread. Among the many metabolic alterations characteristic of a hypersensitive response, induction of PRs is a relatively late event,14 and their contribution to resistance against the initial infection is likely to be limited. In contrast, PRs induced in noninfected, distant leaves as a result of primary infection might afford an enhanced level of protection, resulting in reduced size or number of lesions, or even in reduced systemic symptom severity, upon further infection. The “related situations” in which PRs were found to be induced, seem to prove the point: application of chemicals that mimic the effect of pathogen infection or induce some aspects of the host response, as well as wound responses that give rise to proteins that are also induced during infections, can induce both PRs and acquired resistance. This is the case for salicylic acid, for example, which has since been shown to be an essential intermediate in the signal–transduction pathway leading to the induction of PRs in tobacco.
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- An important common function of most PRs is their antifungal effects
- Some PRs also exhibit antibacterial, insecticidal or antiviral action.
- Function as signals that spread “news” of the infection to nearby cells.
- Infections also stimulate the cross-linking of molecules in the cell wall and the deposition of lignin, responses that set up a local barricade that slows spread of the pathogen to other parts of the plant
- Chitinase activity
- Peroxidase, ribonuclease and lysozyme activities
- Their hydrolytic, proteinase-inhibitory and membrane-permeabilizing ability.
- They inactivate the proteins secreted by the parasites in the invaded plant tissues
- Loon LC (1985). "Pathogenesis-related proteins". Plant Molecular Biology. 4 (2-3): 111–116. doi:10.1007/BF02418757.
- Campbell, N.A. and Reece, J.B. (2005). Biology (7th ed). San Francisco: Benjamin Cummings.
- Van Huijsduijnen RAMH; Alblas SW; De Rijk RH; Bol JF (1986). "Induction by Salicylic Acid of Pathogenesis-related Proteins and Resistance to Alfalfa Mosaic Virus Infection in Various Plant Species". Journal of General Virology. 67 (10): 2135–2143. doi:10.1099/0022-1317-67-10-2135.
- Waters EJ, Shirley NJ, Williams PJ (1996). "Nuisance Proteins of Wine Are Grape Pathogenesis-Related Proteins". Journal of Agricultural and Food Chemistry. 44 (1): 3–5. doi:10.1021/jf9505584.
- Muthukrishnan S, Datta SP (1999). Pathogenesis-related proteins in plants. Boca Raton: CRC Press. p. 291. ISBN 0-8493-0697-3.
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