Bacillus velezensis XRD006: genomic characteristics and biocontrol effects on diseases of postharvest green walnuts

Abstract:

[Objective] To investigate Bacillus velezensis XRD006 in terms of the inhibitory effects on diseases of postharvest green walnuts, the fresh-keeping effect on green walnuts, genetic characteristics, secondary metabolites, and antifungal mechanism. [Methods] The activities of XRD006 against pathogens of postharvest green walnuts were determined by the inhibition experiments. In vivo fungal inhibition and storage quality experiments were conducted to investigate the inhibitory effects of the strain on pathogens and the effect of the strain on the storage quality of postharvest green walnuts. The genomic characteristics and potential antifungal genes of XRD006 were investigated by whole genome sequencing. The secondary metabolites of XRD006 were predicted by antiSMASH, and the collinearity and differences of the secondary metabolite gene clusters between strains XRD006, FZB42, and SQR9 were analyzed by comparative genomics. The secondary metabolites of XRD006 were identified by high performance liquid chromatography (HPLC) and mass spectrometry. The antagonistic ability of the strain was tested by the oxford cup method. [Results] The inhibition rates of XRD006 against Colletotrichum aenigma, Colletotrichum siamense, Botryosphaeria dothidea, and Fusarium fujikuroi of postharvest green walnuts were 49.22%, 50.61%, 53.83%, and 58.71%, respectively. In vivo antifungal experiments showed that XRD006 effectively inhibited the infection and growth of pathogenic fungi on the fruits. The fermentation supernatant of XRD006 significantly retarded the weight loss, inhibited the microbial growth, and reduced changes in peroxidase (POD) and polyphenol oxidase (PPO) activities while maintaining the kernel quality. The whole genome sequencing showed that the genome of XRD006 was 4 371 975 bp in length, containing 46.07% GC and 4 362 protein-coding genes (including antifungal and plant growth-promoting genes such as extracellular hydrolase and biofilm genes). The antiSMASH predicted that XRD006 had nine known and five unknown gene clusters of secondary metabolites. XRD006 was closely related to FZB42 and SQR9, and they shared eight secondary metabolite gene clusters, which showed varied location and coding genes. XRD006 produced two families of lipopeptides: iturin and fengycin. Compared with C13-iturin, C14-iturin, C15-iturin, and C17-fengycin C, C16-fengycin B of the fengycin family had the strongest inhibitory effect on C. siamense HT12. [Conclusion] B. velezensis XRD006 has good biocontrol effects on diseases of postharvest green walnuts and the potential for application in production.