Cystathionine gamma-synthase is essential for methionine biosynthesis in Fusarium graminearum.

Methionine (Met) plays an important role in various cellular processes in both eukaryotes and prokaryotes. Cystathionine gamma-synthase encoded by STR2 gene is a key enzyme in Met biosynthesis in Saccharomyces cerevisiae. In this study, we identified FgMETB, a homologue of S. cerevisiae STR2, from Fusarium graminearum using the Protein Basic Local Alignment Search Tool (BLASTP) program. The FgMETB deletion mutants were unable to grow on fructose gelatin agar (FGA) medium containing SO(4)(2-) as sole sulphur source. In addition, more than 90 % conidia of the mutants were not able to germinate in 2 % sucrose solution within 6 or 12 h of incubation. Supplementation of 1 mM Met or 0.5 mg ml(-1) homocysteine, but not 1 mM cysteine or 0.5 mg ml(-1) glutathione, rescued the defect of mycelial growth and spore germination of FgMETB deletion mutants. These results indicated that the enzyme encoded by FgMETB is involved in conversion of cysteine into homocysteine. Inoculation tests showed that the FgMETB deletion mutant exhibited decreased virulence significantly on wheat heads, which is consistent with a low level of deoxynivalenol (DON) production of the mutant in wheat kernels. Fungicide sensitivity assays revealed FgMETB deletion mutants showed increased sensitivity to the sterol demethylation inhibitor tebuconazole, but did not change their sensitivities to other fungicides. Taken together, results of this study indicated that FgMETB plays a critical role in the regulation of various cellular processes in F. graminearum.