Cooperative Interactions Between Bacillus and Lysobacter Enhance Consortium Stability and Fusarium Wilt Suppression in Cucumber

The rhizosphere microbiome plays a pivotal role in plant health by mediating interactions between hosts, beneficial microbes, and pathogens. However, the ecological mechanisms underlying microbial consortia that suppress soil-borne diseases remain largely unexplored. In this study, we investigated how the biocontrol bacterium Bacillus velezensis SQR9 influences the assembly of the cucumber rhizosphere bacterial community in the presence of the pathogenic fungus Fusarium oxysporum f. sp. cucumerinum (FOC). Inoculation with B. velezensis SQR9 significantly enriched the genus Lysobacter, a known biocontrol taxon. Meta-analysis revealed a positive correlation between Bacillus and Lysobacter abundances in healthy plant rhizospheres-a relationship absent in Fusarium wilt-diseased soils-suggesting a conserved ecological association linked to disease suppression. Mechanistic assays demonstrated that Lysobacter enzymogenes XL8, an antifungal bacterium isolated from the cucumber rhizosphere, formed synergistic biofilms with B. velezensis SQR9. Spent medium growth assays indicated that strain SQR9 facilitated the growth of L. enzymogenes XL8 through metabolic interactions. Targeted RT-qPCR and UHPLC-MS/MS analyses confirmed that treatment with spent medium of the partner strain enhanced the expression and production of antifungal metabolites bacillomycin D and heat-stable antifungal factor (HSAF), both antagonistic to F. oxysporum. Greenhouse trials confirmed that this dual-species consortium more effectively suppressed Fusarium wilt than single-species inoculations, as evidenced by reduced pathogen abundance and enhanced plant growth. Together, our findings underscore the importance of microbial metabolic cooperation and biofilm-mediated coexistence in shaping rhizosphere community assembly and function, providing ecological insights for the development of synthetic microbial consortia aimed at sustainable plant disease management.