Molecular and Genetic Analysis of Toxin Production by Pathovars of Pseudomonas Syringae

Plant pathogenic bacteria are equipped with an arsenal of virulence factors that first condition the host for colonization and then escalate the intensity of disease. Nonhost-specific toxins are generally acknowledged to be an element of virulence for many bacteria, and this is especially evident among the pathovars of Pseudomonas syringae. But considerable debate has focused on whether toxin production has a primary role in virulence because phytotoxic symptoms occur long after lesion formation (20, 75). Only recently with the advent of molecular genetic analysis of site-specific mutants of phytobacteria has the extent to which toxin production contributes to virulence been tested rigorously. Studies of non toxigenic (Tox-) mutants strongly implicate toxins in phytopathogenesis, but the functional roles of these genes associated with toxin production are unclear. Consequently, genetic research is beginning to probe the secrets of toxin biosynthesis and the regulatory network that links toxin production to expression of plant pathogenicity. The nonhost-specific toxins produced by pathovars of P. syringae are a