LXG Toxins of Bacillus Velezensis Mediate Contact-Dependent Inhibition in a T7SS-Dependent Manner to Enhance Rhizosphere Adaptability

Rhizosphere bacteria always compete intensely for ecological niches, employing various strategies to inhibit the growth of microbial competitors. One such strategy, contact-dependent inhibition (CDI), involves the direct delivery of toxic proteins into competing neighboring bacteria by a secretion system, leading to the inhibition of their growth. However, the ecological function of CDI competition in the natural environment remains unclear. In this study, we examined the role of the type VII secretion system (T7SS) substrate LXG domain-containing protein in the rhizobacterium <i>Bacillus velezensis</i> SQR9 and found that SQR9 encodes LXG toxins mediate contact-dependent inhibition against other <i>Bacillus</i> strains in biofilms. Transcriptional analysis revealed that the expression of these lxg genes is induced by root exudates and positively correlates with that of the T7SS gene cluster. We further confirmed that the survival of the mutants deficient of the LXG toxins was significantly decreased in natural soil. These findings highlight the critical role of T7SS and its substrate LXG toxins in competition of <i>Bacillus</i> species in the rhizosphere, providing new insights into the ecological importance of CDI in natural environments.