Exploration of the antimicrobial activities of biocontrol agent Streptomyces strain h114 against Penicillium digitatum in citrus

Biocontrol is considerably crucial in phytopathogens management during postharvest fruit storage. In this study, assays were performed to screen beneficial microbes through the microorganisms which were isolated from the citrus rhizosphere soil, and to investigate the potential antifungal action modes towards Penicillium digitatum, the perpetrator of citrus green mold. Strain h114 was selected for its high controlling effect against citrus green mold pathogen, and it was phylogenetically identified as a Streptomyces monashensis isolate by whole genome analysis. Thereafter, a total of 35 secondary metabolite gene clusters which were responsible for the biosynthesis of antibiotics were found in the genome. The culture filtrates (CFS) fermented from strain h114 with incubation for 36 h in beef extract-peptone broth exhibited the strongest antifungal activity towards P. digitatum than filtrates obtained from other time points. Despite the susceptibilities to long time treated by boiled water and high concentration of trypsin, the 36 h fermented CFS showed relatively stable antagonism in room conditions. Further, its xylene-extracted crude compounds also displayed a bioactive effect to P. digitatum. CFS-treated P. digitatum cells exhibited degradation of mitochondria, accompanied by reduced expression of developmental regulatory genes PdBrlA and PdWetA. Moreover, CFS induced the fruit defense activities such as superoxide dismutase and peroxidase, and alleviated the virulence of P. digitatum in citrus. After CFS application, the expansion diameters of P. digitatum on artificially-wounded citrus fruit were reduced by 71.5% at most, and the disease incidence diminished by 62.7–72.9% compared to that of P. digitatum treated fruit alone. Taken together, strain h114 might be a promising bio-fungicide in postharvest citrus fruit handling.