Cuticular waxes relieve self-inhibition of germination and appressorium formation by the conidia of Magnaporthe grisea

We tested whether germination and appressorium formation by the conidia of Magnaporthe grisea requires a hydrophobic surface. On specially cleaned cover glass surfaces conidia of hi. grisea germinated but did not form appressoria. Addition of rice leaf wax, other plant surface waxes or synthetic n-C-22 fatty acid, fatty alcohol or alkane induced appressorium formation. The requirement for the presence of hydrophobic material to induce appressorium formation was dependent on the conidial population density. As the conidial concentration decreased from 10(5) ml(-1) to 10(4) ml(-1), the need for the lipid decreased to the point of not requiring any lipid to obtain nearly complete appressorium formation, suggesting possible presence of self-inhibitors on the conidia. This suggestion was supported by the observation that the surface lipids washed out of the conidia inhibited germination and appressorium formation in a dose-dependent manner. This inhibition could be reversed by plant surface wax. These results suggest that conidia of M. grisea carry lipophilic self-inhibitors which prevent germination at the site of sporulation, and that the plant cuticle can relieve this self-inhibition when conidia land on the plant surface. (C) 1997 Academic Press Limited.