Elicitation of the hypersensitive response by Pseudomonas syringae

The hypersensitive response (HR), a rapid local necrosis accompanied by the production of antimicrobial compounds, is a manifestation of the primary defense mechanisms through which plants prevent colonization by potentially pathogenic microorganisms. The pathogen factors that trigger HR-associated defense mechanisms have yet to be established. Application of transposon mutagenesis and molecular cloning techniques have provided new insights into the bacterial elicitation of the HR. This review focuses on the interaction of Pseudomonas syringae with non-host plants as a model system for the bacterial induction of the HR and discusses (a) the genetics of Pseudomonas syringae pathogenicity and HR elicitation, (b) the environmental conditions controlling expression of Pseudomonas syringae pathogenicity/HR determinants, (c) the nature of the HR elicitor and (d) early plant responses to the pathogen.