Abstract

We have already constructed the differential cDNA library of M. grisea by subtracting the cDNAs expressed in vegetative mycelia from the cDNAs in appressorium formation on polycarbonate plates. The library contains unique and distinct clones strongly expressed during appressorium formation (Kamakura, 1999). From the library we found and reported the novel CBP1 gene involved in sensing physically the surface on which the conidia attached (Kamakura, 2002). Additionally we found novel genes, B19 and B48, specifically expressed in appressorium formation. To elucidate the functions of the genes, these genes were disrupted by homologous recombination with vectors including the BSD as a marker gene in the center between the upstream sequences of genes and the downstream of ones. The null mutants of B19 and B48 normally grew on YG medium, and sporulated on oatmeal medium. But the 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. On rice leaf sheaths, the mutants decreased the ability of appressorium formation, penetration to host cells, and also infectious growth in the host cells. These results suggest that the B19 and B48 genes may be involved in the signal transduction in hydrophobicity and/or hardness sensing to initiate the differentiation of appressorium and also in the infection to rice plant, and that their products have potentiality to be the novel targets sites of fungicides to control rice blast disease.
*Tokyo University of Science.