Accumulation of 6-deoxocathasterone and 6-deoxocastasterone in Arabidopsis, pea and tomato is suggestive of common rate-limiting steps in brassinosteroid biosynthesis

To gain a better understanding of brassinosteroid biosynthesis, the levels of brassinosteroids and sterols related to brassinolide biosynthesis in Ara bidopsiss, pea, and tomato plants were quantified by gas chromatography-sel ected ion monitoring. In these plants, the late C-6 oxidation pathway was f ound to be the predominant pathway in the synthesis of castasterone. Furthe rmore, all these plant species had similar BR profiles, suggesting the pres ence of common biosynthetic control mechanisms. The especially high levels of 6-deoxocathasterone and 6-deoxocastasterone may indicate that their resp ective conversions to 6-deoxoteasterone and castasterone are regulated in p lanta and hence are important rate-limiting steps in brassinosteroid biosyn thesis. Other possible rate-limiting reactions, including the conversion of campestanol to 6-deoxocathasteonre, are also discussed. Tomato differs fro m Arabidopsis and pea in that tomato contains 28-norcastasterone as a biolo gically active brassinosteroid, and that its putative precursors, cholester ol and its relatives are the major sterols. (C) 2001 Elsevier Science Ltd. All rights reserved.