PATHOGENICITY OF PSEUDOMONAS-SYRINGAE SUBSP SAVASTANOI MUTANTS DEFECTIVE IN PHYTOHORMONE PRODUCTION

The present study compares the pathogenicity on olive and oleander pla nts of three wild-type strains of Pseudomonas syringae subsp. savastan oi (ITM317 and PBa230 from olive and ITM519 from oleander) and three p hytohormone-deficient mutants of ITM519: ITM519-41 (Iaa(+)/cytokinins( -)), ITM519-7 (Iaa(-)/cytokinins(+)), ITM519-6 (Iaa(-)/cytokinins(-)). Mutants not producing IAA (ITM519-7 and ITM519-6) only induced necros is of the inoculated tissues (ITM513-6) or swellings on the stems attr ibuted to cytokinin production accompanied by necrosis (ITM519-7). By contrast, the Iaa(+)/ cytokinins(-) mutant (ITM519-41) induced attenua ted symptoms on stems and knots similar to those obtained with the par ent strain on oleander leaves. Olive strains induced necrosis of olean der leaves and were virulent and avirulent, respectively, on olive and oleander stems. Strain ITM519 and its three mutants were all able to multiply in oleander leaves at similar rates, reaching the same final populations. By contrast, the two olive strains multiply poorly, reach ing populations c.10(2)-fold lower. These results confirm that express ion of IAA genes alone is sufficient to initiate the development of kn ots on oleander while cytokinins are necessary for the full expression of the disease symptoms (determining knot size). The findings also in dicate that the plant tissues (stems and leaves) react differently to the various strains of the bacterium and, furthermore, suggest that, b esides phytohormones, other pathogenetic factors could be involved in this host-pathogen interaction. The necrotic reaction of oleander leav es heavily inoculated with olive strains was interpreted as a possible form of hypersensitivity reaction.