Biosynthesis and metabolism of brassinosteroids.

Brassinosteroids (BRs) are steroid hormones that regulate the growth and development of plants. Detailed study of the biosynthesis of brassinolide, a C28 BR, revealed that two parallel routes, the early and late C-6 oxidation pathways, are connected at multiple steps and also are linked to the early C-22 oxidation pathway. Thus, BR biosynthetic pathways are highly networked. Furthermore, the biosynthesis of C27 BRs was shown to proceed in a similar way to that of C28 BRs. Information on enzymes and genes involved in the BR biosynthesis, as well as their regulation, has been obtained using BR-deficient and BR-insensitive mutants. In addition, the biosynthesis of sterols, which were recently recognized not only as precursors of BRs and membrane constituents, but also as modulators of plant development, is discussed. Various metabolic reactions of BRs including epimerization, oxidation, and conjugation are also summarized.