Isopropylmalate isomerase MoLeu1 orchestrates leucine biosynthesis, fungal development, and pathogenicity in Magnaporthe oryzae

The biosynthesis of branched-chain amino acids (BCAAs) is conserved in fungi and plants, but not in animals. The Leu1 gene encodes isopropylmalate isomerase that catalyzes the conversion of α-isopropylmalate into β-isopropylmalate in the second step of leucine biosynthesis in yeast. Here, we identified and characterized the functions of MoLeu1, an ortholog of yeast Leu1 in the rice blast fungus Magnaporthe oryzae. The transcriptional level of MoLEU1 was increased during conidiation and in infectious stages. Cellular localization analysis indicated that MoLeu1 localizes to the cytoplasm at all stages of fungal development. Targeted gene deletion of MoLEU1 led to leucine auxotrophy, and phenotypic analysis of the generated ∆Moleu1 strain revealed that MoLeu1-mediated leucine biosynthesis was required for vegetative growth, asexual development, and pathogenesis of M. oryzae. We further observed that invasive hyphae produced by the ∆Moleu1 strain were mainly limited to the primary infected host cells. The application of exogenous leucine fully restored vegetative growth and partially restored conidiation as well as pathogenicity defects in the ∆Moleu1 strain. In summary, our results suggested that MoLeu1-mediated leucine biosynthesis crucially promotes vegetative growth, conidiogenesis, and pathogenicity of M. oryzae. This study helps unveil the regulatory mechanisms that are essential for infection-related morphogenesis and pathogenicity of the rice blast fungus.