Characterization and Function of Avirulence Genes from Pseudomonas Syringae pv. Tomato

Avirulence genes in plant pathogens did not evolve for that purpose but are a consequence of the occurrence in plants of surveillance mechanisms that detect highly specific features of only certain pathogen biotypes. Thus, while the functions of avirulence genes in pathogens are generally not known, it is assumed that their role in recognition by plants is gratuitous. If recognition of an avirulent pathogen occurs, the plant undergoes a complex active defense reaction called the hypersensitive response (HR) that restricts further pathogen development. The plant surveillance system is controlled by plant disease resistance genes that are thought to encode specific receptors. They recognize pathogen metabolites called specific elicitors. The formation of these elicitors is poorly understood, but recent evidence suggests that they are either the primary protein products of pathogen avirulence genes or cellular metabolites arising from their catalytic activity. We also know little about the structure and function of plant disease resistance genes, although recent progress has occurred in attempts to clone them (e.g., Bennetzen et al 1988). In contrast, several pathogen avirulence genes have been cloned and sequenced (Keen and Staskawicz 1988).