Pseudomonas viridiflava antagonizes the cabbage leaf spot pathogen Pseudomonas cannabina via interspecies contact‐dependent killing

BACKGROUND: Cabbage (Brassica oleracea) is a globally significant vegetable crop. However, frequent outbreaks of leaf spot disease severely constrain yield and quality. Development of efficient biocontrol bacterial resources can provide an option for managing the cabbage leaf spot disease. RESULTS: In this study, Pseudomonas cannabina CSPY1 was identified as the causative agent of leaf spot in Chinese cabbage. Multiple bacterial strains were subsequently isolated from the diseased tissues associated with cabbage leaf spot caused by Pseudomonas cannabina CSPY1. Using agar plate co-culture to mimic cell-to-cell contact, we identified Pseudomonas viridiflava CSBD2, which exhibited potent antagonism against Pseudomonas cannabina CSPY1. Whole-genome sequencing revealed that Pseudomonas viridiflava CSBD2 harbors multiple structurally intact type VI secretion system (T6SS) gene clusters. Furthermore, quantitative real-time polymerase chain reaction (qPCR) analysis demonstrated significant up-regulation of T6SS core genes in Pseudomonas viridiflava CSBD2 during co-cultured with the pathogen Pseudomonas cannabina CSPY1, suggesting that T6SS likely functions as a contact-dependent antibacterial mechanism mediating antagonism against CSPY1. Finally, through biocontrol tests in plants, Pseudomonas viridiflava CSBD2 demonstrated its effectiveness in protecting cabbage from Pseudomonas cannabina CSPY1 infection. CONCLUSION: Our findings demonstrate that the biocontrol agent Pseudomonas viridiflava CSBD2 employs a T6SS-mediated contact-dependent mechanism to antagonize Pseudomonas cannabina CSPY1. This study provides the theoretical foundation and promising microbial resource for ecosystem-friendly management of the cabbage leaf spot disease.