BACKGROUND Fusarium fujikuroi is a plant pathogen that causes rice bakanae disease. Prochloraz is an imidazole‐class sterol, 14α‐demethylase inhibitor (DMI), which has been in use for several years as a foliar spray to control Fusarium spp. on agriculturally important monocot crops. F. fujikuroi is highly resistant to prochloraz treatment, and the aim of this study was to clarify the mechanism by which F. fujikuroi renders itself resistant to prochloraz. RESULTS Recently, prochloraz‐resistant strains were identified over a vast geographical area in the agricultural regions of Zhejiang Province, China. It was found that 21.13% and 3.96% of the strains examined were highly resistant (HR) to prochloraz during 2017 to 2018. The HR strains contained a point mutation (S312T) in the FfCYP51B protein, while the strains identified with prochloraz susceptibility had no such point mutation in FfCYP51A/B/C. To confirm whether the mutations in FfCYP51B confer resistance to prochloraz, we exchanged the CYP51B locus between the sensitive strain and the resistant strain by homologous double exchange. The transformed mutants with a copy of the resistant fragment exhibited resistance to prochloraz, and the transformed mutants with a copy of the sensitive fragment exhibited sensitivity to prochloraz. Furthermore, qRT‐PCR analysis of Ffcyp51a/b/c gene expression revealed that Ffcyp51a and Ffcyp51b were significantly up‐regulated in the prochloraz‐resistant strains relative to the sensitive strains in F. fujikuroi. Contrary to our expectation, docking of prochloraz into the modeled binding pocket of FfCYP51B indicated that the affinity between prochloraz and the FfCYP51B increased after the amino acid at codon 312 changed to Thr. CONCLUSION The point mutation S312T in FfCYP51B and overexpression of Ffcyp51a and Ffcyp51b together lead to the prochloraz‐resistant phenotype in F. fujikuroi.

Background Fusarium fujikuroi is the pathogenic agent of rice bakanae disease and has developed serious resistance to prochloraz, a 14α‐demethylase inhibitor (DMI). Prochloraz resistance in F. fujikuroi is caused by cooperation between FfCyp51B with Cyp51A and shows cross‐resistance only to prothioconazole but not to tebuconazole, difenoconazole, propiconazole, metconazole, hexaconazole, and triadimefon. This study aimed to analyze the functions of the three Cyp51s in F. fujikuroi, especially their role in determining sensitivity to DMIs. Results The respective deletion of FfCyp51A, Cyp51B, and Cyp51C had no obvious effect on morphology, conidium germination, or pathogenicity. The involvement of growth, growth and ergosterol biosynthesis, and conidium production and ergosterol biosynthesis was observed for FfCyp51A, Cyp51B, and Cyp51C, respectively. Compared with the sensitive isolate of F. fujikuroi, the effect on sensitivity to the tested DMIs was divided into four groups: (i) both of Cyp51A and Cyp51B positively regulate the sensitivity to prochloraz and prothioconazole; (ii) Cyp51B positively regulate the sensitivity to tebuconazole and metconazole, but negatively regulate the sensitivity to difenoconazole; (iii) Cyp51A and Cyp51B play opposite roles in the sensitivity to triadimefon. Therefore, deletion of Cyp51A in F. fujikuroi confers a higher sensitivity to triadimefon, while deletion of Cyp51B results in a triadimefon‐resistant mutant isolate; (iv) deletion of Cyp51B yielded a mutant isolate that was more resistant to propiconazole and hexaconazole. Conclusion Sophisticated interactions exist within the three Cyp51 genes to DMIs fungicides sensitivity in F. fujikuroi, and Cyp51B probably plays a more critical role than Cyp51A and Cyp51C. © 2022 Society of Chemical Industry.