Bacillus velezensis P15 triggers the dynamic defense response of cherry tomato against Alternaria rot during postharvest.

Postharvest disease caused by Alternaria alternata is one of the most serious threats to the production and food security of tomatoes worldwide. Biocontrol microbes offer a sustainable alternative of chemical fungicides without residuals on agricultural commodities. However, there is a lack of effective biocontrol microbes for the tomato Alternaria rot disease currently. In this study, we demonstrated that Bacillus velezensis P15 can efficiently inhibit A. alternata both in vitro and in vivo, with a prominently higher biocontrol efficacy on tomato than chemical fungicides mancozeb. The metabolites of B. velezensis P15 suppressed the growth of A. alternata on PDA in a concentration-dependent manner, causing hyphal surface deformation, ultrastructural damage to hyphal cells, and earlier sporulation. Multiple biosynthetic gene clusters responsible for producing various antimicrobial compounds, such as macrolactin H, difficidin, fengycin, bacillibactin and bacilysin, were further identified in the genome of strain P15. Beyond disease suppression, B. velezensis P15 also improved fruit quality and alleviated membrane lipid peroxidation of tomato by modulating its antioxidant enzyme activities during postharvest, showing superior performance than mancozeb. Time-series transcriptomic analysis revealed that both strain P15 and mancozeb can suppress disease development through the regulation of host transcriptional reprogramming. However, in addition to the host immune responses commonly induced by these two treatments, strain P15 specifically activated a set of plant defense-related genes, including antioxidant enzymes, phytohormones, and pathogenesis-related proteins. Collectively, these findings highlight B. velezensis P15 as a promising biocontrol agent against postharvest Alternaria rot in tomatoes.