Plants activate an array of co-ordinated defence responses to restrict microbial attack. Timely perception of foreign molecules appears to be critical for the success of these defences. However, the nature of the molecular events required for resistance is largely unknown. Recent isolation of disease resistance (R) genes, has revealed that R gene products have several features in common. This finding suggests that plants have evolved several common or similar signal transduction pathways to activate resistance to a range of unrelated microbes. R gene isolation and the genetic identification of other loci required for R function permits analysis of the structure and evolution of microbe-perception mechanisms. Numerous types of activated defence responses are recognized. A requirement for salicylic acid in resistance has been established in some systems. Roles for other events, like reactive oxygen species and the hypersensitive (host cell death) response, remain enigmatic. A thorough understanding of the components leading to pathogen recognition and the expression of resistance should permit the design of novel strategies to engineer broad-spectrum and durable plant disease control.
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