Abstract

The conidial germ tube of the rice blast fungus, Magnaporthe grisea, differentiates an appressorium, essentially required for penetration into the host plant. In our differential cDNA library of M. grisea strongly expressed during appressorium formation, unique and distinct clones were contained. From the library we have found and reported the novel CBP1 gene involved in sensing physically the surface on which the conidia attached. Additionally we found novel genes, B19, B48 and B59, specifically expressed in appressorium formation. Bioinformatic analyses suggested that these genes could encode sucrase/ferredoxin-, dual specificity phosphatase- and glycoside hydrolase (GH61)-like protein, respectively. These genes were disrupted by homologous recombination to elucidate the functions of the genes. The null mutants tested normally grew on YG medium, and sporulated on oatmeal medium. But the B19 and B48 mutants significantly decreased the ability to form the appressorium on polycarbonate plates, although the conidia normally germinated. Chemical inducers to trigger the appressorium formation, 1,16-hexadecandiol as a cutin monomer, cAMP and IBMX (an inhibitor of degradation of cAMP), restored the ability of the appressorium formation on the plate. Both B19 and B48 mutants decreased the ability of penetration and infectious growth in host cells, although they did not completely lose the ability to develop lesions in spray and punch-wounded inoculation. These results suggest that the genes set required for appressorium formation, penetration, and initial infectious growth by establishment of colonization in host cells may be quite different from the genes set after the establishment to develop the visible lesions, and that B19 and B48 genes may be involved in the earlier infection steps. Function(s) of B59 was also elucidated by the similar ways.

*Tokyo Univ. of Science, Noda, Chiba, JAPAN