Leucine metabolism regulates TRI 6 expression and affects deoxynivalenol production and virulence in F usarium graminearum

Summary TRI 6 is a positive regulator of the trichothecene gene cluster and the production of trichothecene mycotoxins [deoxynivalenol ( DON )] and acetylated forms such as 15‐ Acetyl‐DON ) in the cereal pathogen F usarium graminearum . As a global transcriptional regulator, TRI 6 expression is modulated by nitrogen‐limiting conditions, sources of nitrogen and carbon, pH and light. However, the mechanism by which these diverse environmental factors affect TRI 6 expression remains underexplored. In our effort to understand how nutrients affect TRI 6 regulation, comparative digital expression profiling was performed with a wild‐type F . graminearum and a Δ tri6 mutant strain, grown in nutrient‐rich conditions. Analysis showed that TRI 6 negatively regulates genes of the branched‐chain amino acid ( BCAA ) metabolic pathway. Feeding studies with deletion mutants of MCC , encoding methylcrotonyl‐ CoA ‐carboxylase, one of the key enzymes of leucine metabolism, showed that addition of leucine specifically down‐regulated TRI 6 expression and reduced 15‐ ADON accumulation. Constitutive expression of TRI 6 in the Δ mcc mutant strain restored 15‐ ADON production. A combination of cellophane breach assays and pathogenicity experiments on wheat demonstrated that disrupting the leucine metabolic pathway significantly reduced disease. These findings suggest a complex interaction between one of the primary metabolic pathways with a global regulator of mycotoxin biosynthesis and virulence in F . graminearum .