MELANIN SYNTHESIS IS ASSOCIATED WITH CHANGES IN HYPHOPODIAL TURGOR, PERMEABILITY, AND WALL RIGIDITY IN GAEUMANNOMYCES-GRAMINIS VAR. GRAMINIS

Mycelia of Gaeumannomyces graminis var. graminis form large cells call ed hyphopodia with deeply lobed, melanized walls. Like appressoria pro duced by other pathogens, hyphopodia develop on hydrophobic surfaces, but it is not clear that hyphopodia function as platforms for host pen etration. In appressoria, melanin synthesis is linked to the generatio n of enormous turgor pressures that provide the necessary force for pl ant penetration. In the present study, hyphopodial turgor was measured in a wild-type strain of G. graminis var. graminis, a mutant exhibiti ng constitutive synthesis of melanin (referred to as the dark mutant), and a melanin-deficient strain (thr), These experiments demonstrate t hat hyphopodia of the wild-type strain generate higher pressures than the dark mutant and that nonmelanized thr hyphopodia generate minuscul e Internal pressures. Melanization of the wall is also associated with an increase in its rigidity. These data correlate with differences in wall permeability consistent with a recent model for turgor generatio n by appressoria. (C) 1998 Academic Press.