Agrobacterium tumefaciens Deploys a Superfamily of Type VI Secretion DNase Effectors as Weapons for Interbacterial Competition In Planta

The type VI secretion system (T6SS) is a widespread molecular weapon deployed by many Proteobacteria to target effectors/toxins into both eukaryotic and prokaryotic cells. We report that Agrobacterium tumefaciens, a soil bacterium that triggers tumorigenesis in plants, produces a family of type VI DNase effectors (Tde) that are distinct from previously known polymorphic toxins and nucleases. Tde exhibits an antibacterial DNase activity that relies on a conserved HxxD motif and can be counteracted by a cognate immunity protein, Tdi. In vitro, A. tumefaciens T6SS could kill Escherichia coli but triggered a lethal counterattack by Pseudomonas aeruginosa upon injection of the Tde toxins. However, in an in planta coinfection assay, A. tumefaciens used Tde effectors to attack both siblings cells and P. aeruginosa to ultimately gain a competitive advantage. Such acquired T6SS-dependent fitness in vivo and conservation of Tde-Tdi couples in bacteria highlights a widespread antibacterial weapon beneficial for niche colonization. [Display omitted] •A type VI secretion DNase effector (Tde) family identified in Agrobacterium tumefaciens•Tde exhibits antibacterial DNase activity and is counteracted by immunity protein Tdi•Tde confers a competitive advantage to A. tumefaciens during host colonization•Tde and Tdi superfamilies are broadly conserved in bacteria Type VI secretion systems (T6SSs) are weapons involved in interbacterial competition. Ma et al. report the discovery of a class of T6SS-dependent DNases that support the survival and fitness of the plant pathogen Agrobacterium tumefaciens against competitors such as Pseudomonas aeruginosa during plant colonization.