Physiological and morphological evidence of brassinosteroid-biosynthesis inhibition by the fungicide imazalil

Dark germinated Arabidopsis thaliana Ler seedlings grown on medium with the commonly used imidazole‐type fungicide imazalil (IMA) resemble de‐etiolated (det) brassinosteroid‐deficient mutants. IMA hampers cell elongation in the hypocotyl, but stimulates radial expansion during dark growth. This phenotype could partially be restored by simultaneous addition of 24‐epibrassinolide (EBR). A complete restoration of the hypocotyl length could only be achieved by combining EBR and gibberellic acid. As Arabidopsis thaliana etr1‐3 de‐etiolates on IMA containing medium in the dark, in the same dose‐dependent manner as the wild type, its effects cannot be attributed to the induction of extra ethylene production. Studies with A. thaliana seedlings expressing CPD::GUS show that IMA up‐regulates the expression of CPD, which encodes a key cytochrome P450 enzyme in the brassinosteroid (BR)‐biosynthesis pathway. This effect is reverted by EBR, indicating that the up‐regulation of CPD by IMA might result from the lack of end product brassinolide. Together these data suggest that, in Arabidopsis, one of the effects of IMA is an inhibition of BR‐biosynthesis. IMA is an available and cheap agrochemical that might be a valuable tool for future brassinosteroid research.