THE 73-KB PIAA PLASMID INCREASES COMPETITIVE FITNESS OF PSEUDOMONAS-SYRINGAE SUBSPECIES SAVASTANOI IN OLEANDER

Pseudomonas syringae subsp. savastanoi causes tumors on olive and olea nder by producing the plant growth regulators indoleacetic acid (IAA) and cytokinins following infection of the plant. The contribution of I AA production to the ability of P. syringae subsp. savastanoi to grow and survive in oleander leaf tissue was studied. Bacterial strains dif fering only with respect to IAA production were characterized. Growth and survival of wild-type and two mutant strains of P. syringae subsp. savastanoi in oleander leaf tissue were monitored by weekly colony co unts and IAA plate assays. Growth rate of the three strains in culture and in planta did not differ significantly. However, the wild-type st rain reached a higher population density and maintained its maximum de nsity at least 9 weeks longer than either mutant population. An insert ion mutant containing the IAA plasmid (pIAA), but incapable of IAA pro duction, did not maintain a higher population density than a strain cu red of the IAA plasmid. The pIAA-cured strain maintained a higher popu lation density when coinoculated with an IAA-producing strain than whe n inoculated alone. These results suggest that IAA production may cont ribute to the fitness of P. syringae subsp. savastanoi in oleander tis sue and that the iaa operon alone may be responsible for the competiti ve advantage of cells harboring pIAA.