The role of activated oxygen species in plant disease resistance

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