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The CPKA Gene of Magnaporthe grisea Is Essential for Appressorial Penetration

The rice blast fungus Magnaporthe grisea uses appressoria to penetrate into plant cells. Appressorium formation occurs following conidial germination on hydrophobic surfaces and may involve a cyclic AMP (cAMP)-dependent signaling mechanism. Recently, gene replacement mutants of CPKA, a gene encoding a proposed catalytic subunit of cAMP-dependent protein kinase A, were shown to be defective in appressorium formation, cAMP responsiveness, and lesion formation (T. K. Mitchell and R. A. Dean, Plant Cell, 7:1869--1878, 1995). Here we report a detailed phenotypic characterization of three cpkA mutants. cpkA mutants are dramatically reduced in pathogenicity toward healthy plants. However, the reduced pathogenicity does not appear to be due to a loss of appressorium formation. cpkA mutants are delayed in appressorium formation but form appressoria at the same level as wild-type strains over a 24-h period. Appressoria formed by cpkA mutants are fully melanized but are smaller than wild type and are defective in penetrating plant cells. cpkA mutants can produce infectious hyphae and cause lesion formation when inoculated through wounds. Finally, cpkA mutants are still responsive to exogenous cAMP for appressorium formation. These findings indicate an additional role for cAMP signaling involved in appressorial penetration and suggest the presence of additional cAMP-dependent protein kinase(s) involved in surface sensing in M. grisea.