Identification of genes encoding heat shock protein 40 family and the functional characterization of two Hsp40s, MHF16 and MHF21, in Magnaporthe oryzae

Magnaporthe oryzae, the causal agent of the rice blast disease, poses a worldwide threat to stable rice production. The large-scale functional characterization of genes controlling the pathogenicity of M. oryzae is currently under way, but little is known about heat shock protein 40 (Hsp40) function in the rice blast fungus or any other filamentous plant pathogen. We identified 25 genes encoding putative Hsp40s in the genome of M. oryzae using a bioinformatic approach, which we designated M. oryzae heat shock protein forty (MHF1-25). To elucidate the roles of these genes, we characterized the functions of MHF16 and MHF21, which encode type M and type H Hsp40 proteins, respectively. MHF16 and MHF21 expression was not significantly induced by heat shock, but it was down-regulated by cold shock. Knockout mutants of these genes (Delta mhf16 and Delta mhf21) were viable, but conidiation was severely reduced. Moreover, sectoring was observed in the Delta mhf16 mutant when it was grown on oatmeal agar medium. Conidial germination, appressorium formation, and pathogenicity in rice were not significantly affected in the mutants. The defects in conidiation and colony morphology were fully complemented by reintroduction of wild type MHF16 and MHF21 alleles, respectively. These data indicate that MHF16 and MHF21 play important roles in conidiation in the rice blast fungus.