Biocontrol efficacy and mechanism of Bacillus subtilis PL5 against root rot of sweet potato: Insights into antifungal activity and metabolite function.

Fusarium solani causes sweet potato root rot disease under optimal temperature conditions. Biocontrol using antagonistic microorganisms offers a sustainable and eco-friendly management strategy through dual mechanisms: direct pathogen suppression and induction of systemic plant resistance. Bacillus subtilis PL5 culture broth significantly reduced the plaque diameter of F. solani, impaired spore germination, and curtailed germ tube length. Furthermore, it triggered reactive oxygen species (ROS) accumulation, initiating apoptosis and membrane integrity disruption. In vivo experiments demonstrated that spraying with B. subtilis PL5 culture broth mitigated root rot incidence in sweet potato. It enhanced the activities of nicotinamide adenine dinucleotide phosphate oxidase (NOX), superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and peroxidase (POD), thereby strengthening the antioxidative capacity of roots and balancing ROS levels, including hydrogen peroxide (H2O2) and superoxide anion radical (O2.-). Concurrently, B. subtilis PL5 culture broth modulated polyamines (PAs) metabolism, leading to significant elevations in putrescine (Put), spermidine (Spd), and spermine (Spm) levels, as well as enhanced activities of arginine decarboxylase (ADC), ornithine decarboxylase (ODC), and PA oxidase (PAO) and decline in diamine oxidase (DAO) activity. Natural disease occurrence studies showed that B. subtilis PL5 widely suppressed the disease progression in sweet potatoes. In addition, non-targeted metabolic and "Pearson" correlation analysis showed the metabolites in B. subtilis PL5 were closely related to the levels of ROS and PAs. Collectively, these findings elucidate the mechanism basis by which B. subtilis PL5 mediates pathogen suppression and enhances resistance, providing novel insights for developing sustainable sweet potato protection agent.