Pseudomonas syringae phytotoxins: Mode of action, regulation, and biosynthesis by peptide and polyketide synthetases

Coronatine, syringomycin, syringopeptin, tabtoxin, and phaseolotoxin are th e most intensively studied phytotoxins of Pseudomonas syringae, and each co ntributes si,significantly to bacterial virulence in plants. Coronatine fun ctions partly as a mimic of methyl jasmonate, a hormone synthesized by plan ts undergoing biological stress. Syringomycin and syringopeptin form pol es in plasma membranes, a process that lends to electrolyte leakage. Tabtoxin and phaseolotoxin are strongly antimicrobial and function by inhibiting gl utamine synthetase and ornithine carbamoyltransferase,, respectively. Genet ic analysis has revealed the mechanisms responsible for toxin biosynthesis. Coronatine biosynthesis requires the cooperation of polyketide and peptide synthetases for the assembly of the coronafacic and coronamic acid moietie s, respectively. Tabtoxin is derived fi-om the lysine biosynthetic pathway whereas syringomycin syringopeptin, and phaseolotoxin? biosynthesis require s peptide synthetases. Activation of phytotoxin synthesis is controlled by diverse environmental factors including plant signal molecules and temperat ure. Genes involved in the regulation of phytotoxin synthesis have been loc ated within the coronatine and syringomycin gene clusters; however, additio nal regulatory genes are required for the synthesis of these and other phyt otoxins. Global regulatory genes such as gacS modulate phytotoxin productio n in certain pathovars, indicating the complexity of the regulatory circuit s controlling phytotoxin synthesis. The coronatine and syringomycin gene cl usters have been intensively characterized and show potential for construct ing modified polyketides and peptides. Genetic reprogramming of peptide and polyketide synthetases has been successful and portions of the coronatine and syringomycin gene clusters could be valuable resources in developing ne w antimicrobial agents.