Isolation, identification, and biocontrol mechanism of a potential biocontrol agent, Bacillus velezensis XZ3-6 against apple canker disease

Apple Valsa canker (AVC), caused by the ascomycete Valsa mali, is a severe disease threatening the apple industry. In this study, antagonistic strains exhibiting inhibitory activity against AVC were isolated from the rhizosphere soil of Tibetan poplar trees. The most effective strain, XZ3-6, was identified through morphological and whole-genome analysis. Its stability under stress conditions (pH, salt concentrations, temperature, and UV radiation,) was assessed. Additionally, transcriptomic analysis and resistant-related enzymes were performed on XZ3-6-treated apple twigs to elucidate the molecular mechanisms of disease resistance. Results showed that strain XZ3-6, identified as Bacillus velezensis, exhibited significant inhibitory effects against Cytospora mali QH2 in both in vitro and in vivo experiments, and achieving an inhibition rate of 83.33% in vivo. Strain XZ3-6 demonstrated considerable tolerance and inhibitory stability under various stress conditions. Transcriptomic analysis revealed that XZ3-6 significantly induced the expression of antioxidant enzymes and upregulated genes related to the MAPK signalling pathway, plant hormone signal transduction, secondary metabolite synthesis. Furthermore, XZ3-6 significantly enhanced the expression of genes involved in glutathione metabolism, carotenoid biosynthesis, α-linolenic acid metabolism, and other related genes involved in the plant's antioxidant and disease resistance systems. This study elucidates the molecular mechanisms by which B. velezensis XZ3-6 enhances apple disease resistance through the induction of antioxidant systems and signalling pathways, providing new insights and a potential biocontrol agent for the management of AVC.