Design and synthesis of hybrid-type molecular probes useful for elucidation of molecular mechanisms of physiological events caused by brassinosteroids

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Brassinosteroids(BRs) constitute a class of steroidal plant hormones, which are essential for plant growth and development. Many efforts have been made in a wide range of research fields in order to elucidate the real features of BR actions at molecular level in plants, using technologies based on molecular biology. In this project, I aimed the design and synthesis of BR-based hybrid-type molecular probes useful for elucidation of molecular mechanisms of physiological events caused by BRs. and made considerable achievements as follows.1.Starting with natural BRs, brassinolide(BL) and castasterone(CS), the synthesis of several BR-based hybrid-type molecular probes : i.e., phenyldiazirine/biotin double-labelled BR, biotin labeled BR and dexamethazone labeled BR, were completed.2.In early works on BR perception mechanism in plant, we elucidated that a transmembrane receptor kinase identified from Arabidopsis, BRI1,is a receptor of BRs. The photoaffinity labeling experiments using aryldiazirine/biotin double-labelled CS synthesized as a photoaffinity probe clarified that BR directly binds to BRI1 without other proteins or peptides. Furthermore, the minimal binding domain for BR consisting of 94 amino acids was identified, which was a new binding domain for steroid hormones.3.26-hydroxy-CS and -BL synthesized in this project as synthetic intermediates to BR-based hybrid-type molecular probes played a crucial role in the functional analysis of the Arabidopsis CYP72B1,BAS1 and SOB7 genes. It was elucidated that these genes inactivated BRs through 26-hydroxylation of active BRs, CS and BL.4.In connection with this project, four positional isomers of mono-O-β-D-glucopyranosides of BL and CS with tetrahydroxy groups were prepared. These glycosides played a crucial role in the functional analysis of the Arabidopsis UGT73C5 gene expressing a UPD-glycosyltransferase, UGT73C5. It was elucidated that UGT73C5 specifically glycosylated 23-hydroxyl function of CS and BL.