Function and regulation of the type VI secretion system in Pantoea agglomerans plant pathogenic bacteria.

Bacteria utilize protein secretion systems to interact with host organisms, competing bacteria and the environment. Among them, the type VI secretion system (T6SS) is a versatile machine deployed by Gram-negative bacteria and widely distributed in different bacterial species. The T6SS represents a contractile, phage-related nanoweapon, which is composed of several subcomplexes and translocates effector proteins into target cells by a single-step mechanism. In plant pathogen bacteria, the T6SS has been shown to play a role in interbacterial competition, and in a few examples, in plant pathogenesis. Pantoea agglomerans is a Gram‐negative facultative anaerobic bacterium widespread in nature on the surfaces of many different plants. Several strains of P. agglomerans have been naturally transformed into gall forming plant pathogens by acquiring a plasmid which contains pathogenicity determinants. We have recently found that the strain P. agglomerans pv. betae (Pab), which incites galls on both Beta vulgaris L. and Gypsophila paniculata, carries a functional T6SS. We are in the process of investigating functions, regulation, and effector repertoire of the Pab T6SS. Preliminary results showed that wild-type Pab bacteria are able to outcompete E. coli bacteria. Conversely, Pab strains mutated in T6SS structural genes are not able to outcompete E. coli bacteria and form reduced galls in infected beet roots supporting the hypothesis that the T6SS plays a role in both antibacterial and virulence functions.