Bacillus velezensis HY19 as a sustainable preservative in post-harvest citrus (Citrus reticulata Blanco L.) fruit management

Citrus fruit post-harvest decay has caused economic loss for fruit-dependent industries worldwide in recent decades. However, bio-preservatives offer a promising and sustainable solution. In this study, culture experiments were carried out to assess the ability of Bacillus velezensis HY19, which we previously identified, to inhibit common pathogenic fungi in postharvest citrus fruit. HY19 was capable of producing extracellular siderophores and hydrolases (protease, phosphatase, and cellulase). The cells and metabolites (fermentation broth and volatiles) of the bacterium exhibited strong suppression against Penicillium italicum, P. digitatum, and Colletotrichum gloeosporioides. LC-MS analysis of B. velezensis HY19 fermentation broth (BFB) revealed 13 antimicrobial substances (simvastatin, cinnamic acid, cyclopentanone, etc.), 3 antifungal pesticide intermediates (M-cresol, 4-hydroxybenzaldehyde, and L-menthyl acetoacetate), and P-salicylic acid. GC-MS detected 18 antimicrobial substances (2-nonanol, 2-nonanone, benzoic acid, etc.) in HY19's volatile organic compounds (VOCs). Transcriptome analysis of P. digitatum exposed to BFB stress revealed that numerous genes involved in diverse biological processes, such as DNA replication and cell wall biosynthesis, were differentially expressed. Moreover, BFB inhibited the expression of multiple genes responsible for the biosynthesis of N-glycan and proteins in P. digitatum. BFB enhanced antioxidant enzyme activities in citrus peels, reduced electrolyte leakage and reactive oxygen species (ROS) accumulation, and induced the disease resistance in citrus fruit, leading to the decrease in the infection of pathogenic fungi on citrus fruit. During storage at low-temperature (4–8 °C), BFB inhibited the disease incidence in citrus fruit for up to 90 days and achieved over 60% control efficacy. Additionally, BFB had no impact on fruit quality, making it a lower risk option. Considering the benefits of sustainability and environmentally friendly options, it could be concluded that BFB was an effective alternative for controlling citrus diseases and had the potential to be used as a fruit preservative.