Abstract: Brassinosteroids (BRs) are steroidal plant hormones with potent plant growth promoting activity. Because BR-deficient mutants of rice exhibit altered plant architecture and important agronomic traits, we conducted a systemic search for specific inhibitors of BR biosynthesis to manipulate the BR levels in plant tissues. Although previous studies have been conducted with BR biosynthesis inhibitors in dicots, little is known regarding the effects of BR biosynthesis inhibition in monocot plants. In this work, we used potent inhibitors of BR biosynthesis in Arabidopsis, and we performed a hydroponic culture of rice seedlings to evaluate the effects of BR biosynthesis inhibition. Among the test compounds, we found that 1-[[2-(4-Chlorophenyl)-4-(phenoxymethyl)-1,3-dioxolan-2- yl]methyl]-1H-1,2,4-triazole (1) is a potent inhibitor that could induce phenotypes in rice seedlings that were similar to those observed in brassinosteroid deficient plants. The IC 50 value for the retardation of plant growth in rice seedlings was approximately 1.27 ± 0.43 μM. The IC 50 value for reducing the bending angle of the lamina joint was approximately 0.55 ± 0.15 μM.
The chemical structures of the compounds are listed in Table 1. In our assays, we used hydroponicmethods to culture the rice seedlings and evaluate the biological activity of the analogues ofYCZ-series [16]. The common phenotypes of BR deficient mutants of rice plants are characterised by different degrees of dwarfism with erect leaves [6]. We chose plant height as a factor for the evaluationof the biological activity of the test compounds on rice seedlings. Another assay method that is knownto be very sensitive to the levels of BR in rice is based on the determination of the bending angle of thelamina joint of the rice seedlings. The rice lamina joint is a tissue that is found between the leaf blade(leaf lamina) and the leaf sheath that acts similar to a hinge to bend the leaf blade. The bending angleof the lamina joint is sensitive to BR [17]. Furthermore, BR biosynthesis-deficient mutants of rice plants exhibit erect leaves with small bending angles of the lamina joint [18–20]. Thus, determination of the bending angle of the lamina joint of chemically treated rice plants is a straightforward approach to speculate the effects of the compounds used in this study on BR biosynthesis inhibition