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Divergent cAMP Signaling Pathways Regulate Growth and Pathogenesis in the Rice Blast Fungus Magnaporthe grisea

Kiichi Adachi and John E. Hamer
The Plant Cell
Vol. 10, No. 8 (Aug., 1998), pp. 1361-1373
DOI: 10.2307/3870646
Stable URL: http://www.jstor.org/stable/3870646
Page Count: 13
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Divergent cAMP Signaling Pathways Regulate Growth and Pathogenesis in the Rice Blast Fungus Magnaporthe grisea
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Abstract

cAMP is involved in signaling appressorium formation in the rice blast fungus Magnaporthe grisea. However, null mutations in a protein kinase A (PKA) catalytic subunit gene, CPKA, do not block appressorium formation, and mutations in the adenylate cyclase gene have pleiotropic effects on growth, conidiation, sexual development, and appressorium formation. Thus, cAMP signaling plays roles in both growth and morphogenesis as well as in appressorium formation. To clarify cAMP signaling in M. grisea, we have identified strains in which a null mutation in the adenylate cyclase gene (MAC1) has an unstable phenotype such that the bypass suppressors of the $\underline{{\rm M}}{\rm ac}1^{-}$ phenotype (sum) could be identified. sum mutations completely restore growth and sexual and asexual morphogenesis and lead to an ability to form appressoria under conditions inhibitory to the wild type. PKA assays and molecular cloning showed that one suppressor mutation (sum1-99) alters a conserved amino acid in cAMP binding domain A of the regulatory subunit gene of PKA (SUM1), whereas other suppressor mutations act independently of PKA activity. PKA assays demonstrated that the catalytic subunit gene, CPKA, encodes the only detectable PKA activity in M. grisea. Because CPKA is dispensable for growth, morphogenesis, and appressorium formation, divergent catalytic subunit genes must play roles in these processes. These results suggest a model in which both saprophytic and pathogenic growth of M. grisea is regulated by adenylate cyclase but different effectors of cAMP mediate downstream effects specific for either cell morphogenesis or pathogenesis.

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