Abstract
Brassinosteroids (BRs) are the only steroid hormones in plant which widely distribute across the plant kingdom. From a number of structure-activity relationship analyzes, the activity as BR was largely affected by the structure of A-ring, B-ring and the side chain. In this work we found that two B-ring modified BRs, Iso-carbabrassinolide (Iso-carbaBL) and 6-deoxobrassinolide (6-deoxoBL) show different BR activity between Arabidopsis and rice. The activity as BR was measured by the det2 hypocotyl elongation assay for Arabidopsis and the lamina joint inclination assay for rice. In Arabidopsis, the BR activity of Iso-carbaBL was strong as that of castasterone (CS), while in rice, Iso-carbaBL showed no BR activity. The BR activity of 6-deoxoBL is stronger than that of brassinolide (BL), the most active BR, in Arabidopsis, while the BR activity of 6-deoxoBL in rice was weaker than that of CS. We investigated the binding activity of Iso-carbaBL and 6-deoxoBL to Arabidopsis and rice BRI1 and feedback regulation of BR biosynthetic genes by these chemicals to clarify the function of these synthetic BRs as BR in Arabidopsis and rice.
