Leucine biosynthesis is required for infection-related morphogenesis and pathogenicity in the rice blast fungus Magnaporthe oryzae

The rice blast fungus Magnaporthe oryzae causes one of the most devastating crop diseases world-wide and new control strategies for blast disease are urgently required. We have used insertional mutagenesis in M. oryzae to define biological processes that are critical for blast disease. Here, we report the identification of LEU2A by T-DNA mutagenesis, which putatively encodes 3-isopropylmalate dehydrogenase (3-IPMDH) required for leucine biosynthesis, implicating that synthesis of this amino acid is required for fungal pathogenesis. M. oryzae contains a further predicted 3-IPMDH gene (LEU2B), two 2-isopropylmalate synthase (2-IPMS) genes (LEU4 and LEU9) and an isopropylmalate isomerase (IPMI) gene (LEU1). Targeted gene deletion mutants of LEU1, LEU2A or LEU4 are leucine auxotrophs, and severely defective in pathogenicity. All phenotypes associated with mutants lacking LEU1, LEU2A or LEU4 could be overcome by adding exogenous leucine. The expression levels of LEU1, LEU2A or LEU4 genes were significantly down-regulated by deletion of the transcription factor gene LEU3, an ortholog of Saccharomyces cerevisiae LEU3. We also functionally characterized leucine biosynthesis genes in the wheat pathogen Fusarium graminearum and found that FgLEU1, FgLEU3 and FgLEU4 are essential for wheat head blight disease, suggesting that leucine biosynthesis in filamentous fungal pathogens may be a conserved factor for fungal pathogenicity and, therefore, a potential target for disease control.