Role of melanin in appressorium function

Appressoria of certain pathogenic fungi appear darkly pigmented due to a discrete cell wall layer of melanin. We have studied the function of appressorial melanin using wild-type and melanin-deficient strains of the rice blast pathogenMagnaporthe grisea and the melanin biosynthesis inhibitor tricyclazole. All appressoria exhibited a single, circular, wall-less pore against the substratum. Pores in both melanized and unmelanized appressoria were circumscribed by a ring of material, the pore ring, which might function to seal the pore-substratum interface. Experimental evidence demonstrated that appressoria adhered tightly to surfaces and that melanin did not play a major role in that adhesion. Melanin-less appressoria were consistently and uniformly plasmolyzed by low solute concentrations, whereas melanized appressoria, if formed on a solid surface, were plasmolyzed only by much higher solute concentrations. In addition, the cell wall of living or heat-killed melanized, but never unmelanized, appressoria collapsed during plasmolysis due to cytorrhysis. Melanin, having a differential permeability to water and solute, allowed sealed appressoria to establish and maintain a high internal solute concentration that created a high internal hydrostatic pressure. It is that pressure, we suggest, that empowers the pathogen to penetrate the surface of a plastic coverslip as well as the host.