Dissecting the antibacterial functions of the T6SS-2 cluster in Xanthomonas oryzae for environment and plant protection

Abstract

Plant pathogens pose a serious threat to global food security, and the excessive use of chemical pesticides has raised significant ecological concerns. Harnessing microbial competition presents a promising green technology for crop protection. In this study, we examined the functions of the type VI secretion system (T6SS), a crucial microbial competition tool, in Xanthomonas oryzae pv. oryzae strain PXO99A. PXO99A exhibited significant antibacterial activities dependent on its T6SS-2 cluster. Using genome, secretome, and functional analysis, we systematically predicted T6SS-associated effectors, detected the expression of five putative effectors, and functionally validated the toxicity of a VasX_N family effector PXO_00500 and its immunity protein PXO_RS08595. We further show that PXO99A uses its T6SS-2 to kill a broad range of plant and animal pathogens in vitro and in planta. Inactivation of the T3SS functions abolished virulence but had little effect on the T6SS-2-dependent antibacterial activities. Finally, we demonstrated that the avirulent T3SS-defective mutant is effective in protecting tomatoes from Pseudomonas syringae co-infection. Collectively, these results highlight an effective biocontrol strategy for plant protection.

Importance: The growing concerns over the toxicity, environmental impact, and resistance associated with chemical pesticides underscore the urgent need for alternative pathogen management strategies. In this study, we introduce an innovative approach of "turning waste into treasure" by repurposing plant pathogens as biocontrol-like agents. By elucidating the virulence and antimicrobial functions of Xanthomonas oryzae, we demonstrate that an avirulent mutant can employ its T6SS to effectively combat a broad spectrum of human and plant pathogens. Furthermore, its ability to protect tomato plants underscores its significant potential for sustainable agricultural practices.

Keywords: animal and plant pathogens; biocontrol-like agent; type VI secretion system.