A Novel Elicitor Protein from Endophytic Bacillus Alleviates Grape Gray Mold Rot

Microbe-derived elicitors can trigger plant immune responses, but elicitors from endophytic Bacillus species remain largely unexplored. Based on preliminary findings that endophytic Bacillus subtilis K1 can elicit grape immune responses, this study aimed to investigate the elicitor proteins produced by K1 and the effect on the control of gray mold in grapes. A novel elicitor protein PeBs1 screened from the genome of K1 could induce a strong hypersensitive response in Nicotiana benthamiana by Agrobacterium-mediated transformation, accompanied by significant reactive oxygen species accumulation and callose deposition. The PeBs1 gene encodes a 15.9 kDa protein consisting of 181 amino acids, which is evolutionarily conserved within the genus Bacillus. Exogenous infiltration with 10 μM purified PeBs1 protein also triggered typical hypersensitive response symptoms in N. benthamiana. Application of the purified PeBs1 to grape berries significantly reduced the incidence of gray mold and boosted the activities of defense enzymes, including peroxidase (POD), polyphenol oxidase (PPO), catalase (CAT), and phenylalanine ammonia-lyase (PAL), while also lowering the malondialdehyde (MDA) content. Meanwhile, PeBs1 treatment markedly increased the contents of total phenols and total flavonoids in grape berries, and significantly up-regulated the transcriptional levels of defense-related genes (VvPAL, VvPPO, VvPR1, VvPR2). This study suggests that PeBs1 from endophytic Bacillus may act as an immune activator to enhance grape resistance against gray mold. The findings provide a basis for exploring potential strategies to manage grape postharvest diseases.