Phenolic Compound 3-Methylcinnamic Acid Produced by <i>Bacillus cereus</i> BR3 Suppresses the Pathogenicity of Xanthomonas campestris pv <i>campestris</i>

Cabbage black rot, caused by <i>Xanthomonas campestris</i> pv <i>campestris</i> (<i>Xcc</i>), is a severe disease worldwide. The predominant method for controlling this disease relies on agrichemicals, which increase antibiotic resistance. This study showed that the secondary metabolites of <i>B. cereus</i> BR3 elicited induced systemic resistance in cabbage and impaired the type III secretion system (T3SS) of <i>Xcc</i>. Furthermore, the extract of strain BR3 significantly enhanced the activities of plant-defense-related enzymes, suppressed the hypersensitive response (HR) induced by <i>Xcc</i> in tobacco, and attenuated the pathogenicity of <i>Xcc</i> on cabbage. Additionally, several genes of <i>Xcc</i> T3SS were repressed by the BR3 extract. Activity-guided purification identified 3-methylcinnamic acid as an effective inhibitor against the T3SS of <i>Xcc</i>. Overall, this study suggests that 3-methylcinnamic acid produced by strain BR3 disrupts a critical virulence mechanism in <i>Xcc</i>, providing a promising alternative for controlling the black rot disease in cabbage.