INVESTIGATING THE ROLES OF LACTATE DEHYDROGENASES IN THE RICE BLAST FUNGUS MAGNAPORTHE ORYZAE

Abstract

Magnaporthe oryzae is a filamentous ascomycete fungus that causes rice blast, the most destructive disease of rice worldwide. Upon attachment of pathogen spores to the plant surface, a specialized cell called an appressorium differentiates from the germ tubes to facilitate fungal entry into plant tissues using mechanical force. The appressorium development is fuelled by nutrient reserves carried in spores mainly in the form of lipids and glycogen. Previous studies suggest that breakdown of lipids and glycogen, primarily restricted to the peroxisomes and the cytosol respectively, culminates in production of pyruvate. However, downstream metabolism of pyruvate and its coordination with mitochondrial activities remain elusive. In this study, we showed that D-lactate, interconvertible with pyruvate by activities of lactate dehydrogenases, is a central metabolite utilized during spore germination and appressorium development. Genome-wide analysis of five lactate dehydrogenase genes in M. oryzae demonstrated that a D-lactate dehydrogenase, namely MoDLD1, located on the mitochondrial inner membrane, is responsible for conversion of D-lactate to pyruvate. Targeted replacement of MoDLD1 resulted in failure of efficient appressorium formation, which was associated with inability to utilize lipids and glycogen in fungal spores, and consequently the loss of fungal pathogenicity. In summary, our findings reveal a novel metabolic pathway operated by MoDLD1 that bridges metabolite flow to the mitochondria, and contributes to the fungal development and virulence of M. oryzae.