Differential Expression of Type VI Secretion Systems (T6SS) Suggests a Role of T6SS-b in Early Symbiotic Interaction.

STM815, a rhizobial strain of the family, is able to nodulate a broad range of legumes including the agriculturally important (common bean). harbors two type VI Secretion Systems (T6SS-b and T6SS-3) in its genome that contribute to its high interbacterial competitiveness and in infecting the roots of several legumes. In this study, we show that T6SS-b is found in the genomes of several soil-dwelling plant symbionts and that its expression is induced by the presence of citrate and is higher at 20/28°C compared to 37°C. Conversely, T6SS-3 shows homologies to T6SS clusters found in several pathogenic strains, is more prominently expressed with succinate during stationary phase and at 37°C. In addition, T6SS-b expression was activated in the presence of germinated seeds as well as in and root nodules. Phenotypic analysis of selected deletion mutant strains suggested a role of T6SS-b in motility but not at later stages of the interaction with legumes. In contrast, the T6SS-3 mutant was not affected in any of the free-living and symbiotic phenotypes examined. Thus, T6SS-b is potentially important for the early infection step in the symbiosis with legumes.