It is known that bacteria interact with other bacteria, with eukaryotes or with the environment, thanks to proteins that secrete outside the cytoplasm. So far, several secretion systems have been described and some act as nano-syringes by directly injecting proteins that are called effectors into other bacteria or into eukaryotic organisms.
One of these nano-syringes, called type VI secretion system (T6SS) is usually encoded by 13-14 genes grouped and conserved between different bacterial species. These genes have been identified in a quarter of the Gram-negative bacteria in which their genome is known. Among these bacteria are the rhizobia, of great agronomic and ecological importance because they are able to fix atmospheric nitrogen in association with legumes. Little is known about the importance of T6SS for rhizobia. In most of the bacteria in which T6SS has been studied, it secretes toxins with antibacterial function and has also been associated with the virulence of plant and animal pathogens. In our work we have shown for the first time that a rhizobial T6SS has a positive effect on symbiosis with legumes. We have also seen that the analyzed T6SS is active in free living and symbiotic conditions and that its expression increases in the presence of radicular exudates of several legumes. Now we are trying to identify and characterize the possible effectors that this system secretes and their importance in the rhizosphere and in the symbiosis with legumes.
Salinero-Lanzarote, A., Pacheco-Moreno, A., Domingo-Serrano, L., Durán, D., Ormeño-Orrillo, E., Martínez-Romero, E., Albareda, M., Palacios, J.M., Rey, L. 2019. The type VI secretion system of Rhizobium etli Mim1 has a positive effect in symbiosis. FEMS Microbiology Ecology fiz054. DOI: 10.1093/femsec/fiz054