Pathogenicity of Pseudomonas syringae subsp. savastanoi mutants defective in phytohormone production.
A study was undertaken of the pathogenicity on olive and oleander [Nerium oleander] of 3 wild-type strains of P. syringae subsp. [pv.] savastanoi, 2 from olive and 1 from N. oleander, and of 3 mutants of the N. oleander strain deficient in either cytokinins or IAA or both. The IAA- mutants induced only either necrosis of the inoculated tissues (cytokinins-) or swellings on the stems (cytokinins+). In contrast, the IAA+/cytokinins- mutant induced attenuated symptoms on stems, and knots similar to those obtained with the parent strain on N. oleander leaves. Olive strains induced necrosis on N. oleander leaves and were virulent and avirulent, respectively, on olive and N. oleander stems. The N. oleander strain and its 3 mutants multiplied in N. oleander leaves at similar rates, reaching the same final populations, whereas the 2 olive strains multiplied poorly, reaching populations c. 102-fold lower. Results confirmed that expression of IAA genes alone is sufficient to initiate development of knots on N. oleander, while cytokinins are necessary for the full expression of disease symptoms.