Potential Mechanisms of Hexaconazole Resistance in Fusarium graminearum

Fusarium head blight (FHB) caused by Fusarium graminearum is a serious fungal disease that can dramatically impact wheat production. At present, disease control is mainly achieved by the use of chemical fungicides. Hexaconazole (IUPAC name: 2(2,4-dichlorophenyl)-1-(1,2,4-triazol-1-yl)hexan-2-ol) is a widely used triazole fungicide, but the sensitivity of F. graminearum to this compound has yet to be established. The current study found that the EC₅₀ values of 83 field isolates of F. graminearum ranged between 0.06 and 4.33 μg/ml, with an average EC₅₀ value of 0.78 μg/ml. Assessment of four hexaconazole-resistant laboratory mutants of F. graminearum revealed that their mycelial growth and pathogenicity were reduced compared with their parental isolates and that asexual reproduction was reduced by resistance to hexaconazole. Meanwhile, the mutants appeared to be more sensitive to abiotic stress associated with SDS and H₂O₂, while their tolerance to high concentrations of Congo red, and Na⁺ and K⁺ increased. Molecular analysis revealed numerous point mutations in the FgCYP51 target genes that resulted in amino acid substitutions, including L92P and N123S in FgCYP51A, as well as M331V, F62L, Q252R, A412V, and V488A in FgCYP51B, and S28L, S256A, V307A, D287G, and R515I in FgCYP51C, three of which (S28L, S256A, and V307A) were conserved in all of the resistant mutants. Furthermore, the expression of the FgCYP51 genes in resistant strains was found to be significantly (P < 0.05) reduced compared with their sensitive parental isolates. Positive cross-resistance was found between hexaconazole and metconazole and flutriafol, as well as with the diarylamine fungicide fluazinam, but not with propiconazole, and the phenylpyrrole fungicide fludioxonil, or with tebuconazole, which actually exhibited negative cross-resistance. These results provide valuable insight into resistant mechanisms to triazole fungicides in F. graminearum, as well as the appropriate selection of fungicide combinations for the control of FHB to ensure optimal wheat production.