Screening for
brassinosteroid (BR) biosynthesis inhibitors was performed to find chemicals that induce
dwarfism in Arabidopsis, mutants that resembled BR biosynthesis mutants that can be rescued by BR. Through this screening experiment, the compound
brassinazole was selected as the most potent chemical. In dark-grown Arabidopsis,
brassinazole-induced morphological changes were nearly restored to those of wild type by treatment with
brassinolide. The structure of
brassinazole is similar to pacrobutrazol, a
gibberellin biosynthesis inhibitor. However, in assays with cress (Lepidium sativum) plants,
brassinazole-treated plants did not show recovery after the addition of
gibberellin but showed good recovery after the addition of
brassinolide. These data demonstrate that
brassinazole is a specific BR biosynthesis inhibitor.
Brassinazole-treated cress also showed
dwarfism, with altered leaf morphology, including the downward curling and dark green color typical of Arabidopsis BR-deficient mutants, and this
dwarfism was reversed by the application of 10 nM
brassinolide. This result suggests that BRs are essential for plant growth, and that
brassinazole can be used to clarify the function of BRs in plants as a
complement to BR-deficient mutants. The
brassinazole action site was also investigated by feeding BR biosynthesis intermediates to cress grown in the light.