Antifungal Metabolites from Marine-Derived Streptomyces sp. AMA49 against Pyricularia oryzae

Marine-derived actinobacteria are considered as potential sources of bioactive metabolites including antifungal substances. Fifteen out of 155 marine-derived actinobacteria exhibited strong antifungal activity against the rice blast fungus Pyricularia oryzae. Their extracts were further determined for minimum inhibitory concentrations (MIC) and minimum fungicidal concentrations (MFC). Ethyl acetate extract from the strain AMA49 and its subfraction AMA49F1 strongly inhibited hyphal growth of various P. oryzae strains with MICs (8 to 16 mu g/ml) and MFCs (16 to 128 mu g/ml) comparable to propiconazole. Both extracts destroyed fungal membrane and organelles, completely inhibited conidial germination, appressorium formation, and were non-toxic to Galleria mellonella. High performance liquid chromatography/mass spectrometry identified oligomycin A and its derivatives as the active components of AMA49F1 besides several diketopiperazines. AMA49 was identified as a Streptomyces sp. based on morphological characteristics and 16S rDNA sequence analysis. The results suggest that the Streptomyces sp. strain AMA49 is a potential biocontrol agent against rice blast pathogen P. oryzae. This is the first report on the inhibitory effect of the marine-derived Streptomyces extract containing oligomycin A and its derivatives on mycelial growth, conidial germination and appressorium formation of P. oryzae.