| Summary: | BACKGROUND: Malayamycin is a novel perhydrofuropyran C-nucleoside isolated from Streptomyces malaysiensis that shows promising antifungal activity, fully controlling a range of diseases when applied to plants at 100 µg mL−1. The goal of this study was to determine the mode of action. RESULTS: Malayamycin exhibited in vitro antifungal activity against Stagonospora nodorum (Berk) Castell & Germano, the cause of stagonospora nodorum blotch of wheat. Growth in liquid minimum medium was merely delayed at 50 µg mL−1, but sporulation was suppressed by more than 50% by 10 µg mL−1 of malayamycin. When applied to wheat seedlings 36 h prior to infection, 10 µg mL−1 of malayamycin reduced lesion size and significantly reduced pycnidiation to only 5% of the non-treated level. A transcription factor gene, Mrg1 (malayamycin response gene) whose expression was upregulated by application of malayamycin, was identified. Both Mrg1 knockout and overexpression strains were created. These strains were fully pathogenic, suggesting that the expression of Mrg1 did not affect pathogenicity. Interestingly, a strain that expressed Mrg1 50 times more than wild type showed a significant reduction in sporulation. However, all the tested knockout and overexpression strains retained sensitivity to malayamycin. CONCLUSIONS: Malayamycin is a new type of antifungal compound that acts primarily by inhibiting sporulation. Although Mrg1 may be involved in the sporulation process, it is not the major contributor for sporulation inhibition caused by malayamycin treatment.
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