Uses One Type VI Secretion System (T6SS-1) as a Powerful Weapon against Notorious Plant Pathogens.

Paraburkholderia sabiae LMG24235 is a nitrogen-fixing betaproteobacterium originally isolated from a root nodule of Mimosa caesalpiniifolia in Brazil. We show here that this strain effectively kills strains from several bacterial families (, , ) which include important plant pathogens in a contact-dependent manner. assembly of the first complete genome of using long sequencing reads and subsequent annotation revealed two gene clusters predicted to encode type VI secretion systems (T6SS), which we named T6SS-1 and T6SS-3 according to previous classification methods (G. Shalom, J. G. Shaw, and M. S. Thomas, Microbiology, 153:2689-2699, 2007, https://doi.org/10.1099/mic.0.2007/006585-0). We created with mutations in each of the two T6SS gene clusters that abrogated their function, and the T6SS-1 mutant was no longer able to outcompete other strains in a contact-dependent manner. Notably, our analysis revealed that T6SS-1 is essential for competition against several important plant pathogens including Burkholderia plantarii, Ralstonia solanacearum, Pseudomonas syringae, and Pectobacterium carotovorum. The 9-log reduction in P. syringae cells in the presence of was particularly remarkable. Importantly, in an assay, was able to protect potato tubers from bacterial soft rot disease caused by , and this protection was partly dependent on T6SS-1. Rhizobia often display additional beneficial traits such as the production of plant hormones and the acquisition of limited essential nutrients that improve plant growth and enhance plant yields. Here, we show that the rhizobial strain antagonizes important phytopathogens such as P. syringae, and R. solanacearum and that this effect is due to contact-dependent killing mediated by one of two T6SS systems identified in the complete, assembled genome sequence of . Importantly, co-inoculation of Solanum tuberosum tubers with also resulted in a drastic reduction of soft rot caused by in an model system. This result highlights the protective potential of against important bacterial plant diseases, which makes it a valuable candidate for application as a biocontrol agent. It also emphasizes the particular potential of rhizobial inoculants that combine several beneficial effects such as plant growth promotion and biocontrol for sustainable agriculture.