IDENTIFICATION OF TOMATO LEAF FACTORS THAT ACTIVATE TOXIN GENE-EXPRESSION IN PSEUDOMONAS-SYRINGAE PV. TOMATO DC3000

Coronatine is a non-host-specific chlorosis-inducing phytotoxin produc ed by the tomato and crucifer pathogen Pseudomonas syringae pv. tomato DC3000. How the chromosomal gene cluster controlling toxin synthesis in this strain is regulated in planta is unknown. Ice nucleation-activ e cor:inaZ marker-exchange derivatives of strain DC3000 were used to d etermine coronatine gene expression in various host and nonhost plants and in a minimal medium supplemented with selected tomato plant const ituents. Ice nucleation activity, which was first detected 4 h after i noculation, was highest in cabbage, tomato, and soybean and lowest in melon and cucumber. No correlation existed between bacterial populatio n size and expression level on the various plants. Crude tomato leaf e xtract and intercellular fluid were strong inducers of toxin synthesis . Based an highperformance liquid chromatography analyses and bioassay s, we concluded that the active components of both preparations were m alic and citric acids, with minor contributions coming from shikimic a nd quinic acid. Although several compounds including glucose and inosi tol activated the toxin genes when tested at high concentrations (3 to 5 mM), shikimic and quinic acids were the only ones with activity at concentrations below 0.1 mM. Neither acid could be used as a sole carb on source by strain DC3000. The signal activity of shikimic acid was e nhanced 10-fold by the addition of glucose. None of the plant phenolic s that we screened affected coronatine gene expression.