Secreted proteins from Bacillus subtilis K1 induce apple resistance against gray mold

The antagonistic Bacillus subtilis K1, previously isolated in our laboratory, exhibited significant inhibitory activity against Botrytis cinerea. To investigate the underlying molecular mechanisms, genomic bioinformatics analysis predicted 93 potential secreted proteins. Preliminary screening via gene cloning and an Agrobacterium-mediated transient expression system in tobacco identified a gene, GM001344, which elicited a strong hypersensitive response on tobacco. The GM001344-His recombinant protein was successfully purified using a prokaryotic expression system. Apple fruit inoculation assays showed that pretreatment with GM001344 protein 24 h prior to B. cinerea challenge markedly reduced both disease incidence (63.5% vs. 100% in control) and disease severity index (38.43 vs. 76.75 in control), demonstrating effective disease control. GM001344 enhanced apple defenses through boosted activities of key defense-related enzymes and reduced malondialdehyde (MDA) content. Furthermore, GM001344 treatment improved postharvest fruit quality. After 6 d of treatment, treated fruit maintained significantly higher firmness, total soluble solid and ascorbic acid compared to the control. In conclusion, the secreted protein GM001344 from K1 exhibits elicitor activity, effectively inducing systemic resistance against gray mold in apple and delaying quality deterioration. This study provides promising candidate protein and a preliminary theoretical basis for the development of novel biopesticides based on protein elicitors.