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The Hypersensitive Response
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The hypersensitive response (HR) is a complex, early defense response that causes necrosis and cell death to restrict the growth of a pathogen. Resistance (R) genes detect the pathogen and change the membrane potential and ion permeability of the plasma membrane. In phase one of the response, the R genes trigger an increase in extra cellular pH and K+ (Orlandi et al. 1992), while eliciting an influx of calcium and hydrogen ions into the cell. The outward K+ and inward Ca+2 and H+ ion flux are dependent and trigger the HR, resulting in cell death and formation of local lesions, which contain antimicrobial compounds. In phase two cells undergoing the HR produce reactive oxygen species (ROS; oxidative burst), including super oxide anions, hydrogen peroxide, and hydroxyl radicals (Baker et al. 1993). Lipid peroxidation and lipid damage may be partially responsible for some of these cell changes and probably affect membrane function. Phenolics and phytoalexins, such as glyceollin (in soybean), and other compounds are synthesized cells surrounding the lesion. Callose, lignin, HGRP are deposited and pathogen related (PR) proteins are induced and include 1,3-glucanase and chitinases. The HR precedes the secondary resistance response, the systemic acquired response (SAR) and is characterized by necrotic lesions around the infection site; biochemical changes include generation of active oxygen species (oxidative burst), cell death, overproduction of lignin-related materials, and the induction of certain pathogen related (PR) proteins. Several enzymes may be involved in generation of ROS. Copper amine oxidase (EC 1.4.3.6) is found in the apoplast and is loosely associated with the cell wall. catalyzes the oxidative deamination of polyamines, especially putrescine, releasing hydrogen peroxide and ammonia (Koyanagi et al. 2000). In pea it is a copper-containing, extra cellular, homodimer glycoprotein. Other enzymes thought to play a role in ROS production include xanthine oxidase; NADPH oxidase, oxalate oxidase, peroxidases, amine oxidases (copper containing and flavin containing) Refs Orlandi EW, SW Hutcheson, CJ Baker. 1992 Physiological and Molec Plant Path 40:173-180 Baker JC, N Mock, J Glazener, E Orlandi 1993 Physiol Molec Plant Path. 43:81-94 Koyanagi T, K Matsumura, S Kuroda, K Tanizawa. 2000. Biosci Biotechnol Biochem 64:717-222 |