Production of Candicidin Isomers Is Essential for Biocontrol of Cucumber Rhizoctonia rot By Streptomyces albidoflavus W68

ABSTRACT

Diseases caused by soil-borne fungal pathogens result in significant crop yield losses and quality reduction. Streptomyces albidoflavus strain W68 is effective in controlling several soil-borne fungal diseases. To identify antifungal substances critical for biocontrol activity of W68, the genome of W68 was sequenced and a linear chromosome of 6.80 Mb was assembled. A total of 21 secondary metabolite biosynthesis gene clusters (BGCs), accounting for 12.27% of the genome, were identified. Core gene deletion mutants for each of all 8 BGCs for non-ribosomal peptide synthetases and polyketide synthases, were created. Among them, only the mutant lacking gene ctg1-5755 (the gene was renamed as fscD_W68) in BGC 19, which shares 100% sequence similarity to the BGC for candicidin synthesis, showed obvious reduction in antifungal activity. A pot experiment revealed that biocontrol effects of the ΔfscD_W68 mutant in Rhizoctonia rot of cucumber were also significantly compromised relative to W68. LC-MS analysis revealed that W68 but not ΔfscD_W68 can produce candicidin isomers, indicating that the production of candicidin isomers is key for antifungal activity and biocontrol activity of S. albidoflavus W68.

Importance

This study provides the first report that candicidin-like secondary metabolites produced by microbial cells in natural soil environment can effectively control soil-borne fungal diseases, revealing a novel mechanism of microbial biocontrol agents. We demonstrated that the main antifungal activity and biocontrol activity of Streptomyces albidoflavus strain W68 is attributed to the production of candicidin isomers, suggesting gene clusters for candicidin-like compound biosynthesis might be used as molecular markers to screen and breed microbial strains for biocontrol agent development.