Biosynthetic relationship between C-28-brassinosteroids and C-29-brassinosteroids in rice (Oryza sativa) seedlings

Abstract

A crude enzyme solution was prepared from young rice seedlings, and the metabolism of C-29-brassinosteroids identified from the seedlings was examined. When 28-homoteasterone was added as a substrate, 28-homotyphasterol, teasterone, and 26-nor-28-homoteasterone were characterized as enzyme products by GC-MS/SIM analysis. With 28-homotyphasterol, 28-homoteasterone, typhasterol, 28-homocastasterone, and 26-nor-28-homotyphasterol were formed and identified as products. When 28-homocastasterone was used, castasterone and 26-nor-28-homocastasterone were identified as products. Together with the reduced biological activity of C-29-brassinosteroids and their metabolites in the rice lamina inclination assay, these metabolic studies suggest a biosynthetic sequence, 28-homoteasterone <-> 28-homotyphasterol -> 28-homocastasterone for C-29-brassinosteroid biosynthesis is connected to the biosynthetic sequence teasterone <-> typhasterol -> castasterone for C-28-brassinosteroids by C-28 demethylation, i.e., in order to increase biological activity in the rice plant. Additionally, the C-29-brassinosteroids seem to bio-degrade their C-26 demethylated C-28-brassinosteroid analogs to reduce brassinosteroid activity in planta. In conclusion, the biosynthesis of C-29-brassinosteroids is a likely alternative route to the biologically-active brassinosteroid, castasterone, in rice. (C) 2014 Elsevier Ltd. All rights reserved.