Novel Antimycin Analogues with Agricultural Antifungal Activities from the Sponge-Associated Actinomycete Streptomyces sp. NBU3104

Phytopathogenic fungi could affect the growth of agricultural products and result in serious economic losses. To develop novel and potent fungicides, secondary metabolites of an oceanic mesophotic zone Streptomyces sp. NBU3104 was isolated by metabolomics and genomics, which led to the discovery of eight novel antimycins I-P (1-8), including antimycin I (1), six rare acetylated actimycins J-N (2-6), P (8), and an unusual deformylated antimycin O (7). The chemical structures of these metabolites were identified using nuclear magnetic resonance (NMR) spectroscopic analysis, high-resolution electrospray ionization mass spectrometry (HRESIMS) data, and the known reported metabolites in the literature. Their absolute configurations were elucidated by comparison of coupling constant and experimental electronic circular dichroism (ECD) spectra. Among them, compound 1 exhibited excellent inhibitory activities against phytopathogenic fungi, such as Candida albicans, Penicillium expansum, Penicillium citrinum, and Botrytis cinerea. Furthermore, compound 1 could effectively control gray mold of apple in vivo (minimum inhibitory concentration (MIC) = 8 μg/mL). The structure-activity relations of antimycins I-P (1-8) suggested that the aldehyde group in 3-formamidosalicylate unit moiety should be the key factor in their antifungal activities.