SinI and SinR function differently in biofilm formation, rhizosphere colonization, and biocontrol efficacy between <i>Bacillus velezensis</i> and <i>B. subtilis</i>

<i>Bacillus</i> species, exemplified by <i>B. subtilis</i> as a model organism for Gram-positive bacteria, have been extensively studied, particularly regarding biofilm formation. Biofilms represent a form of quorum sensing in microbial communities, and the biocontrol efficacy of <i>Bacillus</i> species in the rhizosphere, against plant pathogens, hinges on their biofilm-forming capabilities. In <i>B. subtilis</i>, the regulatory proteins SinI and SinR are known to have opposing functions in biofilm formation, with SinI facilitating and SinR inhibiting biofilm development. Drawing from this foundational knowledge, we endeavored to knock out the <i>sinR</i> gene in <i>B. velezensis</i>, a biocontrol bacterium, to enhance biofilm formation and, consequently, its colonization of the rhizosphere and biocontrol efficacy. Unexpectedly, the deletion of <i>sinR</i> reduced the bacterium's proficiency in biofilm formation and its ability to colonize the rhizosphere, resulting in a decrease in biocontrol effectiveness. On the contrary, the knockout of <i>sinI</i> promoted biofilm formation, bolstered the strain's colonization capacity in the rhizosphere, and thus fortified its biocontrol efficacy. These findings underscore that SinI and SinR exert divergent, even antithetical effects in <i>Bacillus</i> species. Insights gleaned from <i>B. subtilis</i> research cannot be extrapolated to encompass all <i>Bacillus</i> species, at least not <i>B. velezensis</i>, indicating the need for species-specific investigations.