Enhanced pigmentation effecting decreased protein secretion and pathogenicity in spontaneous melanin variants of Gaeumannomyces graminis var tritici

Gaeumannomyces graminis var. tritici is the phytopathogenic agent of take-all, a severe root disease of wheat and barley. It is a member of the Gaeumannomyces-Phialophora complex of closely related homothallic, filamentous ascomycetes that are differentiated by pathogenicity and the ability to undergo meiosis. The population of Gaeumannomyces and Phialophora in native grasslands and take-all decline soils, where disease severity is absent or depressed, is characterized by darkly pigmented isolates. The parameters dictating this population dynamic have not been identified and there is no established relationship between fungal pigmentation and fungal virulence.;The spontaneous melanin variants of a virulent Gaeumannomyces graminis var. tritici isolate demonstrated Phialophora-like characteristics, loss of virulence and the progressive loss of the ability to undergo meiosis. G. graminis var. tritici effects penetration of intact host tissue by enzymatic means requiring the secretion of lytic enzymes for the extracellular degradation of macromolecules. The relationship between the secretion of fungal proteins (cellulases, pectinases, proteinases (plant-cell-wall degrading enzymes) and a polyphenol oxidase) and the melanin variant phenotype (enhanced pigmentation) was assessed in the wild-type, two spontaneous melanin variants, and two meiotically derived white-derivatives of the melanin variants. The effect of DHN-melanin biosynthesis inhibition on protein secretion was determined in melanin variants treated with tricyclazole.;The correlation between the melanin variant phenotype and pathogenicity was assessed in Triticum aestivum by shoot and root mass assay, root stelar lesion assay and root histopathology. A laccase-like polyphenol oxidase, not previously reported in G. graminis var. tritici, was assessed for plant phenolic detoxification (p-coumaric and caffeic acids) and lignin degradative (Poly B-411 decolorization) capacity and was identified as a putative fungal virulence factor. Results indicated (1) an inverse correlation between fungal pigmentation and protein secretion, (2) an inverse correlation between fungal pigmentation and pathogenicity and (3) an inferred direct correlation between protein secretion and pathogenicity.;The presentation of Phialophora-like characteristics by the spontaneous melanin variants is the first report of a sexual/asexual conversion in Gaeumannomyces. This observed phenotypic switching no doubt contributes to classification difficulties among members of the Gaeumannomyces-Phialophora complex.