Abstract
Triadimefon (Bayleton), a widely used triazole-type fungicide, affects gibberellin (GA) biosynthesis and 14 alpha-demethylase in sterol biosynthesis. The present study revealed that the phenotype of Arabidopsis treated with triadimefon resembled that of a brassinosteroid (BR)-biosynthesis mutant, and that the phenotype was rescued by brassinolide (BL), the most active BR, partly rescued by GA, and fully rescued by the co-application of BL and GA, suggesting that triadimefon affects both BR and GA biosynthesis. The target sites of triadimefon were investigated using a rescue experiment, feeding triadimefon-treated Arabidopsis BR-biosynthesis intermediates, and a binding assay to expressed DWF4 protein, which is reported to be involved in the BR-biosynthesis pathway. The binding assay indicated that the dissociation constant for triadimefon was in good agreement with the activity in an in planta assay. In the triadimefon-treated Arabidopsis cells, the CPD gene in the BR-biosynthesis pathway was up-regulated, probably due to feedback regulation caused by BR deficiency. These results strongly suggest that triadimefon inhibits the reaction catalysed by DWF4 protein and induces BR deficiency in plants. As triadimefon treatment has proved to be beneficial to plants, this result suggests that BR-biosynthesis inhibitors can be applied to crops.