Polyketide Synthase Gene Expression in Relation to Chloromonilicin and Melanin Production in Monilinia fructicola

Monilinia fructicola is a fungal pathogen of worldwide significance that causes brown rot of stone fruits. There are only few reports related to the production of biologically active polyketides by this pathogen. In this study, we examined an atypical M. fructicola strain TW5-4 that shows strong antimicrobial activity against various plant pathogens. TW5-4 also displays sparse growth in culture, low virulence, and higher levels of melanin compared with its albino mutant, TW5-4WM, and a wild-type strain Mf13-81. Antifungal compounds were extracted from TW5-4 and purified by thin-layer chromatography following visualization with an on-the-chromatogram inhibition assay. The principal antifungal compound was identified by linear ion trap mass spectrometry, high-resolution electro-spray ionization mass spectrometry, and proton nuclear magnetic resonance analyses as the polyketide chloromonilicin. Multiple M. fructicola polyketide synthase (PKS) sequences were then cloned by degenerate PCR and inverse PCR. Sequence analyses support presence of a 10-member PKS gene family in the M. fructicola genome. Analyses of PKS gene expression found no strong correlation between chloromonilicin production in culture and transcript levels of any of the PKS gene family members in mycelium of strains TW5-4, TW5-4WM, and Mf13-81. However, MfPKS12, a homolog of BcPKS12 involved in biosynthesis of 1,8-dihydroxynaphthalene (DHN)-melanin in Botrytis cinerea, was strongly expressed in mycelia of TW5-4 and Mf13-81. An MfPKS12-silenced mutant accumulated significantly less melanin in mycelia, had lower resistance to polyethylene glycol-induced osmotic stress, and displayed reduced virulence on nectarine fruit. The results suggest that DHN-melanin is required for tolerance to osmotic stress and full virulence in M. fructicola.