Fusarium graminearum is a major pathogen of wheat and barley, causing fusarium head blight (FHB) and contaminating grain with harmful mycotoxins. Azole fungicides (demethylation inhibitors) are among the key tools available for managing F. graminearum infections. In an attempt to characterize how resistance to azole fungicides arises, we performed an experimental evolution study that imposed selection by exposing F. graminearum to increasing concentrations of prothioconazole (PTZ), tebuconazole (TBF), a combination of both fungicides (CMB), or a no-drug control. All evolved lineages exceeded the ancestral minimum inhibitory concentration before strain extinction, retained normal in vitro colony growth in the absence of drugs, and were able to successfully infect a fusarium head blight-susceptible wheat cultivar. However, most lineages lost their apparent resistance improvements after being revived from preservation at -80°C. One lineage (TBF1), however, showed stable enhanced resistance to tebuconazole, which was accompanied by a phenotype of precocious germination of macroconidia. Genomic analyses indicated no change in the cyp51 genes in any lineage but identified a single base insertion resulting in a premature stop codon in the aos1 gene (involved in SUMOylation) in the TBF1 genome. We created aos1 gene deletion strains, which phenocopied TBF1 for both tebuconazole resistance and altered macroconidial germination. This work suggests that adjustments to spore germination processes may influence sensitivity toward fungicides and also highlights the role of SUMOylation in spore dormancy.IMPORTANCEFusarium graminearum is a major crop pathogen that can acquire mutations over time to common fungicides. Historically, studies have focused their attention on a single gene, cyp51, as the primary cause of resistance. However, there may be other pathways to enhanced resistance. For example, changes in macroconidium germination rates and transitions between cell types offer a route to fungicide resistance that has not been adequately appreciated to date. This study highlights a novel pathway to azole resistance, providing new insights into how F. graminearum may circumvent chemical controls. Through laboratory evolution, a single base insertion arose within the aos1 gene, which caused phenotypes of altered macroconidium dormancy and reduced fungicide sensitivity. This research highlights the importance of experimental approaches that remain open to surprising evolutionary innovations and unexpected resistance mechanisms.
Wog et al. (Fri,) studied this question.