Pathogenic properties of fungal melanins

Melanins are complex black polymers of resonance stabilized cyclic subunits (including indoles, phenols, hydroxynaphthalenes) and are notoriously intractable to chemical analysis. Cell walls of numerous fungi are melanized, rendering many spores, vegetative hyphae, and certain fruit bodies opaque. Melanin deposition protects the pigmented cell from physical and biological stresses, excludes poisons, and helps limit leakage of certain metabolites. By forming a physical barrier between the cell and its surroundings, the production of a melanized wall layer has a profound effect upon the interactions between pathogenic fungi and their hosts. In many plant and animal diseases caused by melanotic fungi, melanin biosynthesis is an important determinant of pathogenicity and virulence. The mode of action of melanin in these diseases is the subject of intensive research, and a variety of mechanisms that link fungal melanin and disease have been discovered.