RNA Silencing in the Phytopathogenic Fungus Magnaporthe oryzae

Systematic analysis of RNA silencing was carried out in the blast fungus Magnaporthe oryzae (formerly Magnaporthe grisea) using the enhanced green fluorescence protein (eGFP) gene as a model. To assess the ability of RNA species to induce RNA silencing in the fungus, plasmid constructs expressing sense, antisense, and hairpin RNAs were introduced into an eGFP-expressing transformant. The fluorescence of eGFP in the transformant was silenced much more efficiently by hairpin RNA of eGFP than by other RNA species. In the silenced transformants, the accumulation of eGFP mRNA was drastically reduced, but no methylation of the promoter or coding region was involved in it. In addition, we found small interfering RNAs (siRNAs) only in the silenced transformants. Interestingly, the siRNAs consisted of RNA molecules with at least three different sizes ranging from 19 to 23 nucleotides, and all of them contained both sense and antisense strands of the eGFP gene. To our knowledge, this is the first demonstration in which different molecular sizes of siRNAs have been found in filamentous fungi. Overall, these results indicate that RNA silencing operates in M. oryzae, which gives us a new tool for genome-wide gene analysis in this fungus.