Reduced pathogen fitness of Pseudomonas syringae pv. tomato Tn5 mutants defective in coronatine production

Pseudomonas syringae pv. tomato strain PT23 produces necrotic lesions on tomato which are surrounded by a diffuse yellow chlorosis. Mitchell et al. (Physiological Plant Pathology 23, 315–322) previously found that some strains of P. syringae pv. tomato produce the chlorosis-inducing phytotoxin, coronatine. In the present study, Tn5 mutagenesis was used to isolate nontoxigenic mutants of PT23. One thousand Tn5 mutants of PT23 were screened for loss of chlorosis induction on tomato leaves; 10 mutants were recovered which produced little or no chlorosis. Organic acids were extracted from two of the mutants and analysed by high performance liquid chromatography and a bean leaf bioassay; the two mutants were found to be defective in coronatine production. The necrotic lesions induced by the two mutants reached a maximum diameter (0·2 mm) 4 days after inoculation, but lesions induced by the wild-type strain continued to expand to a maximum diameter of 0·62 mm. An increase in bacterial populations was correlated with the increase in lesion size. Therefore, coronatine synthesis appears to play an important role in lesion expansion and multiplication of P. syringae pv. tomato in tomato leaves.