A symbiotic relation found between Pseudomonas savastanoi and Pantoea agglomerans in the knots formed on olive

We report a case of symbiosis that can be interpreted in different ways (mutualism, commensalism, or inquinilism), of Pantoea agglomerans (ex Erwinia herbicola) with Pseudomonas savastanoi in the knots (or tubercles) formed by the latter on olive. P. agglomerans is known as one of the most common components of the saprophytic prokaryote microflora on both the phylloplane and the rhizoplane of many plant species. Nevertheless, some sets of strains of this bacterium were also described as primary pathogens on some agricultural crops, e.g. the pv. gypsophilae on Gypsophila paniculata, the pv. milletiae on Wisteria sinensis, and the pv. betae on Beta vulgaris, all of which cause galls on their respective hosts, while for other strains of this bacterium it has been supposed that they operate as secondary pathogens in some pathogenic processes, either because they enhance the predisposition of a host to infection, or because they change the virulence of certain plant-pathogenic bacteria. In the present case it was ascertained first of all that P. agglomerans occurs in intact tubercles of olive knot sometimes in even greater numbers that the primary pathogen itself, and that it occurs in a high proportion of tubercles. When some isolates of this bacterium were inoculated on healthy olive, they multiplied in olive tissues and remained vital for a long time but did not lead to the formation of tubercles. By contrast, when P. agglomerans was co-inoculated with P. savastanoi in ratios of 1:1 or 1:100, its multiplication was abundant, and equal or indeed even superior to that of P. savastanoi. Moreover, the tubercles that formed at these co-inoculation sites were larger than those formed by P. savastanoi when inoculated alone. It thus appears that the presence of P. agglomerans at plant sites where tubercles of olive knot are forming and developing hinders, at least in the initial phases of bacterial reproduction, the multiplication of P. savastanoi (possibly as a result of competition for space or for nutrients) but at the same time makes the tubercles larger, so that more space and more nutrients are available for both bacteria. This last could be explained by the fact, ascertained in our laboratory, that the olive isolates of P. agglomerans produce indolacetic acid in culture.