THE ROLE OF ACTIVATED OXYGEN SPECIES IN PLANT-DISEASE RESISTANCE

One of the earliest responses of plant cells to pathogens is the produ ction of activated oxygen species. In several plant species, this oxid ative burst appears to be generated by a signalling pathway similar to that employed by mammalian neutrophils during immune responses. Simil ar to neutrophils, elicitor receptors in the plant plasma membrane lik ely activate G proteins and phospholipase C leading to increased intra cellular Ca2+ and protein phosphorylation and ultimately, formation of an active NADPH oxidase complex hi the plasma membrane. The NADPH oxi dase produces O-2(-) which can be rapidly converted to H2O2. Work from a number of laboratories supports the concept that H2O2 plays a centr al role in mediating diverse aspects of disease resistance. Potential functions of H2O2 include its participation in direct killing of patho gen cells as part of the hypersensitive response, oxidative cross-link ing of plant cell wall proteins to render the wall less digestible by microbial enzymes, and as an intracellular signal regulating defense-r elated gene expression.