Stereochemical structure-activity relationship of brassinazole, a potent inhibitor of brassinosteroid biosynthesis

This master’s thesis investigates the stereochemical structure–activity relationship of brassinazole (BRZ), a triazole-type compound widely used as a chemical genetics tool to inhibit brassinosteroid (BR) biosynthesis in plants. Brassinosteroids are essential plant steroid hormones regulating growth, flowering, and stress responses. Despite BRZ’s extensive use in over 1,000 studies, its stereoisomer-specific activity had not been systematically characterized.

In this work, BRZ was synthesized and resolved into its stereoisomers (±)-BRZ1 and (±)-BRZ2, followed by separation into enantiopure compounds. These stereoisomers were structurally analyzed using NMR, IR, ORD, and CD spectroscopy. Their biological activities were assessed through hypocotyl elongation assays in Arabidopsis thaliana and Lepidium sativum (cress), alongside Western blot and sterol composition analyses via GC-MS.

The results reveal that (-)-BRZ1 is the most potent and specific inhibitor of BR biosynthesis, showing nanomolar activity without affecting sterol pathways. In contrast, (-)-BRZ2 and (+)-BRZ2 inhibited sterol biosynthesis and exhibited lower specificity. This study demonstrates that the biological effects of BRZ strongly depend on stereochemistry, and recommends using enantiopure (-)-BRZ1 for future plant hormone research to minimize off-target effects.

By establishing the first detailed correlation between BRZ stereochemistry and plant biological activity, this thesis advances chemical genetics tools for plant hormone research and highlights the importance of stereoisomer-specific analysis in agrochemical and plant biotechnology studies