The Methylcitrate Cycle is Required for Development and Virulence in the Rice Blast Fungus .

The methylcitrate cycle metabolizes propionyl-CoA, a toxic metabolite, into pyruvate. (syn. ) is a phytopathogenic fungus that causes a destructive blast disease in rice and wheat. We characterized the essential roles of the methylcitrate cycle in the development and virulence of using functional genomics. In , the transcript levels of and , which encode a 2-methylcitrate synthase and a 2-methylisocitrate lyase, respectively, were upregulated during appressorium formation and when grown on propionyl-CoA-producing carbon sources. We found that deletion of and inhibited fungal growth on media containing both glucose and propionate, and media using propionate or propionyl-CoA-producing amino acids (valine, isoleucine, methionine, and threonine) as the sole carbon or nitrogen sources. The Δ mutant formed sparse aerial hyphae and did not produce conidia on complete medium (CM), while the Δ mutant showed decreased conidiation. The aerial mycelium of Δ displayed a lowered NAD/NADH ratio, reduced nitric oxide content, and downregulated transcription of hydrophobin genes. Δ showed reduced appressorium turgor, severely delayed plant penetration, and weakened virulence. Addition of acetate recovered the growth of the wild type and Δ on medium containing both glucose and propionate and recovered the conidiation of both Δ and Δ on CM by reducing propionyl-CoA formation. Deletion of together with , an isocitrate lyase gene in the glyoxylate cycle, greatly reduced the mutant's virulence as compared with the single-gene deletion mutants (Δ and Δ). This experimental evidence provides important information about the role of the methylcitrate cycle in development and virulence of by detoxification of propionyl-CoA and 2-methylisocitrate.