Premature termination codon mutations in osmosensor-like histidine kinase FgOs1 endow Fusarium graminearum with fludioxonil resistance

Fusarium graminearum is a devastating disease in cereal production, causing a loss in grain production. Fludioxonil, a phenylpyrrole fungicide, has been registered for disease management of many crops. However, the resistance mechanism of F. graminearum to fludioxonil has not been systematically analyzed. This study elucidates the molecular basis of fludioxonil resistance in F. graminearum by demonstrating that premature termination codon mutations in the osmosensor-like histidine kinase gene FgOs1 are causative factors. Two independent mutations were identified in fludioxonil-resistant mutants, leading to truncated FgOs1 proteins: a FgOs1^Q140STOP mutation locating at N-terminal and an FgOs1^R1183STOP mutation locating at the REC signaling domain. Homologous gene replacement and complementation assays confirmed that these mutations specifically confer high-level resistance to fludioxonil (resistance factor > 1000) without cross-resistance to tebuconazole, phenamacril, or carbendazim. Notably, FgOs1-mutated strains exhibited heightened sensitivity to osmotic and metal ion stresses, suggesting that the premature termination codon-induced protein truncation impairs osmoregulation and ion homeostasis pathways. Phylogenetic analysis revealed that the mutated residues are highly conserved across diverse fungal species, underscoring their functional importance. These findings uncover a previously unrecognized role of FgOs1 in antifungal resistance and provide critical targets for developing innovative strategies to manage fludioxonil resistance in F. graminearum.