Extracellular matrix protein gene, EMP1, is required for appressorium formation and pathogenicity of the rice blast fungus, Magnaporthe grisea

Magnaporthe grisea, the causal fungus of rice blast, forms a specialized infection structure called an appressorium that is crucial for host plant penetration. A cDNA clone of M. grisea, showing strong sequence homology to FEM1 of Fusarium oxysporum and encoding an extracellular matrix protein, was isolated during an expressed sequence tag (EST) analysis of an appressorium cDNA library and named extracellular matrix protein 1 (EMP1). Sequence analysis of the corresponding genomic clone revealed that EMP1 contains an open reading frame of 685 nucleotides encoding 207 amino acids. The estimated molecular weight of the protein product was 20.5 kDa with a pI of 7.84. It contains an 18 amino acid N-terminal secretion signal sequence, as well as four potential N-glycosylation sites. At its C-terminus, the protein contains a 16 amino acid sequence with the characteristics of a glycosylphosphatidylinositol (GPI) anchor addition signal. Northern blot analysis showed that EMP1 transcripts accumulate during appressorium formation but not during vegetative growth. An EMP1 null mutant, emp1, generated by targeted gene disruption, exhibited reduced levels of appressorium formation and pathogenicity but no effect on mycelial growth rate or conidiation ability. These data suggest that EMPI plays important roles in appressorium formation and the pathogenicity of M. grisea.