The dark side of the mycelium: Melanins of phytopathogenic fungi

Melanins are darkly pigmented polymers that protect organisms against environmental stress. Even when not directly involved in pathogenesis, fungal melanin is likely required by melanizing phytopathogens for survival in the environment. However, some phytopathogenic fungi that produce melanized appressoria for host invasion require appressorial melanogenesis for pathogenicity. Much less is known about the role melanins play in pathogenesis during infection by other phytopathogens that do not rely on appressoria for host penetration. Here we focus on one such phytopathogenic fungus, Gaeumannomyces graminis var. tritici, the etiologic agent of the devastating root disease of cereals, take-all. This fungus is lightly pigmented in culture, but requires melanin biosynthesis for pathogenesis, perhaps to produce melanized, ectotrophic macrohyphae on roots. However, the constitutively melanized, asexual Phialophora anamorph of G. graminis var. tritici is nonpathogenic. In addition, melanization of G. graminis var. graminis is not required to produce root disease on its rice host. Explanations for these apparent contradictions are suggested, as are other functions for the melanins of phytopathogenic fungi.