Functional Exploration of the Bacterial Type VI Secretion System in Mutualism: Azorhizobium caulinodans ORS571-Sesbania rostrata as a Research Model

The bacterial type VI secretion system (T6SS) has been considered the armed force of bacteria because it can deliver toxin effectors to prokaryotic or eukaryotic cells for survival and fitness. Although many legume symbiotic rhizobacteria encode T6SS in their genome, the biological function of T6SS in these bacteria is still unclear. To elucidate this issue, we used Azorhizobium caulinodans ORS571 and its symbiotic host Sesbania rostrata as our research model. By using T6SS gene deletion mutants, we found that T6SS provides A. caulinodans with better symbiotic competitiveness when coinfected with a T6SS-lacking strain, as demonstrated by two independent T6SS-deficient mutants. Meanwhile, the symbiotic effectiveness was not affected by T6SS because the nodule phenotype, nodule size, and nodule nitrogen-fixation ability did not differ between the T6SS mutants and the wild type when infected alone. Our data also suggest that under several lab culture conditions tested, A. caulinodans showed no T6SS-dependent interbacterial competition activity. Therefore, instead of being an antihost or antibacterial weapon of the bacterium, the T6SS in A. caulinodans ORS571 seems to participate specifically in symbiosis by increasing its symbiotic competitiveness.