Syringomycin Production Among Strains of Pseudomonas syringae pv. syringae: Conservation of the syrB and syrD Genes and Activation of Phytotoxin Production by Plant Signal Molecules. Neil B. Quigley. Department of Plant Pathology, Washington State University, Pullman 99164-6430 U.S.A. Dennis C. Gross. Department of Plant Pathology, Washington State University, Pullman 99164-6430 U.S.A. MPMI 7:78-90. Accepted 9 September 1993. Copyright 1994 The American Phytopathological Society.

The syrB and syrD genes of Pseudomonas syringae pv. syringae are predicted to encode proteins that function in the synthesis and export of syringomycin, respectively. Using portions of the syr genes as DNA probes, both genes were shown lo be conserved as single copies within a 15-kb or smaller DNA region among a broad spectrum of P. s. pv. syringae strains that produce syringomycin or one of its amino acid analogs, syringotoxin and syringostatin. Strains representative of P. viridiflava and six pathovars of P. syringae failed to hybridize with the gene probes, demonstrating that syr sequences are highly specific to P. s. pv. syringae and related nonpathogenic strains. Maximum parsimony analysis of restriction fragment length polymorphism profiles was used to evaluate relatedness among strains within the syrB and syrD gene region. A tree, conveying the smallest number of evolutionary changes among strains, revealed considerable diversity within the syr gene region; subclustcrs of strains were identified that appear lo share specific qualities relevant to the plant-pathogen interaction. Because both the syrB gene and syringomycin production can be induced in response to plant signal molecules, 42 strains containing homologous syr sequences were tested for signal-mediated induction of toxin production. Over 90% of the toxigenic strains produced larger quantities of toxin when the plant signal molecules, arbutin and D-fructose, were added to syringomycin-minimal medium; 13 of the strains produced =10-fold higher toxin levels. Some strains, such as 5D428, produced toxin only in the presence of these signals. This demonstrates that nearly all strains of P s. pv. syringae have a sensory mechanism for specific plant metabolites that modulate syringomycin, syringotoxin, or syringostatin production.