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 conidi a of Magnaporthe grisea requires a hydrophobic surface. On specially c leaned cover glass surfaces conidia of hi. grisea germinated but did n ot form appressoria. Addition of rice leaf wax, other plant surface wa xes or synthetic n-C-22 fatty acid, fatty alcohol or alkane induced ap pressorium formation. The requirement for the presence of hydrophobic material to induce appressorium formation was dependent on the conidia l 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 poi nt of not requiring any lipid to obtain nearly complete appressorium f ormation, suggesting possible presence of self-inhibitors on the conid ia. This suggestion was supported by the observation that the surface lipids washed out of the conidia inhibited germination and appressoriu m formation in a dose-dependent manner. This inhibition could be rever sed by plant surface wax. These results suggest that conidia of M. gri sea 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 P ress Limited.