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

Abstract

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.

The Role of Secretion Systems, Effectors, and Secondary Metabolites of Beneficial Rhizobacteria in Interactions With Plants and Microbes

“…Recently, it was reported that R. etli Mim1 T6SS mutant produced plants with lower dry weight and smaller nodules than the wild-type strain, suggesting for the first time a positive role of T6SS in Rhizobium-legume symbiosis (Salinero-Lanzarote et al, 2019). The rhizobacterium Azorhizobium caulinodans ORS571 utilizes its T6SS to outcompete other strains during infection of its host Sesbania rostrata (Lin et al, 2018). However, the researchers could not show involvement in inter-bacterial competition in vitro.…”

Legume–rhizobium dance: an agricultural tool that could be improved?

“…Interbacterial competition activity mediated by T6SS‐secreted toxins has been described for some soil bacteria as Agrobacterium tumefaciens or Pseudomonas putida (Bondage et al ., 2016; Bernal et al ., 2017). Despite the available information on T6SS and related toxin‐immunity protein pairs in different rhizobial strains (Bondage et al ., 2016; Bernal et al ., 2018; Salinero‐Lanzarote et al ., 2019; Zalguizuri et al ., 2019), a role for rhizobial putative T6SS toxins in interbacterial competition has not been demonstrated thus far (Lin et al ., 2018). Another type of interbacterial toxins is bacteriocins (Holtsmark et al ., 2008).…”

“…In addition to the T3SS, other bacterial protein secretion systems have been described as having a role in the symbiotic process: type IV and type VI secretion system (T4SS and T6SS respectively) (Nelson and Sadowsky, 2015;Lin et al, 2018;Salinero-Lanzarote et al, 2019). In M. japonicum R7A, T4SS has an analogous role to that attributed to T3SS in M. japonicum MAFF303099 (Hubber et al, 2004).…”

Rhizobial Exopolysaccharides and Type VI Secretion Systems: A Promising Way to Improve Nitrogen Acquisition by Legumes

“…Since then, a few researchers have studied the role of T6SS in symbiosis beyond mentioning its presence. In two such studies, it was found that mutations in T6SS genes do not affect symbiotic efficiency but symbiotic competitiveness (De Campos et al, 2017;Lin et al, 2018), and in another study, a positive effect on symbiosis was demonstrated (Salinero-Lanzarote et al, 2019).…”