New Lactones Produced by Streptomyces sp. SN5431 and Their Antifungal Activity against Bipolaris maydis

Bipolaris maydis causes southern corn leaf blight and inflicts huge losses on maize production. In order to search for new natural products from insect gut bacteria to control plant fungal disease, 86 actinomycetes were isolated from more than 50 insect guts, in which crude extract of strain SN5431 showed significant inhibition of the mycelial growth of B. maydis. The strain was identified and named as Streptomyces sp. SN5431. Six compounds were obtained from the crude extract of strain SN5431, which includes five new γ-butyrolactones named as tiuslactone A–E (1–5), and one new long chain ester named as tiusester (6). Their structures were determined using NMR and HRESIMS data and then combined with the spectroscopic data of known similar compounds. Tiuslactone B (2) showed powerful antifungal activity against B. maydis. These results indicated metabolites of insect gut bacteria have the potential to be the leading compounds for the control of corn leaf blight.