DHN melanin biosynthesis in the plant pathogenic fungusBotrytis cinereais based on two developmentally regulated key enzyme (PKS)-encoding genes

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Botrytis cinereais the causal agent of gray mold disease in various plant species and produces grayish macroconidia and/or black sclerotia at the end of the infection cycle. It has been suggested that the pigmentation is due to the accumulation of 1,8-dihydroxynaphthalene (DHN) melanin. To unravel its basis and regulation, the putative melanogenic and regulatory genes were identified and functionally characterized. Unlike other DHN melanin-producing fungi,B. cinereaand other Leotiomycetes contain two key enzyme (PKS)-encoding enzymes.Bcpks12andbcpks13are developmentally regulated and are required for melanogenesis in sclerotia and conidia respectively. BcYGH1 converts the BcPKS13 product and contributes thereby to conidial melanogenesis. In contrast, enzymes acting downstream in conversion of the PKS products (BcBRN2, BcSCD1 and BcBRN1) are required for both, sclerotial and conidial melanogenesis, suggesting that DHN melanogenesis inB. cinereafollows a non-linear pathway that is rather unusual for secondary metabolic pathways. Regulation of the melanogenic genes involves three pathway-specific transcription factors (TFs) that are clustered withbcpks12orbcpks13and other developmental regulators such as light-responsive TFs. Melanogenic genes are dispensable in vegetative mycelia for proper growth and virulence. However, DHN melanin is considered to contribute to the longevity of the reproduction structures.

Many black fungi produce the dark pigment 1,8-dihydroxynaphthalene (DHN) melanin, often in a spatially and/or temporarily regulated manner. In the gray mold fungus Botrytis cinerea, DHN melanin is produced by a non-linear/ converging biosynthetic pathway. The fungus contains two versions of the key enzyme (PKS; polyketide synthase) that are differentially expressed in conidia and sclerotia providing there the precursor for the subsequent conversion to DHN by the same enzymes.

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