Biosynthesis and Metabolism of Dolichosterone in Arabidopsis thaliana

Abstract

Figure 1. Conversion of 6-deoxoDS to CS intermediated by DS in A. thaliana. The steroidal phytohormones, collectively referred to as the brassinosteroids (BRs), are essential chemical signals that regulate the growth and development of plants. Thus far, over 50 naturally-occurring BRs have been identified from the entire plant kingdom. The natural BRs can be classified as C27-, C28-, and C29-BRs based on their carbon skeletons, which basically carry the same carbon numbers as those of the sterols commonly detected in plants. The observed structural similarity between BRs and plant sterols suggests that BRs are biosynthesized from plant sterols, which have similar carbon structures. In fact, the results of feeding experiments and molecular genetic studies of BR-related mutants have shown that the most physiologically important C28-BRs, castasterone (Fig. 1, CS) and brassinolide (BL) which have a methyl at C-24, are biosynthesized from 24-methylcholesterol (campesterol) via two parallel pathwaysnamely, the early and late C-6 oxidation pathway-in various plants. We demonstrated recently that the C27-BRs, which harbor no alkyl at C-24, are biosynthesized from a C27-sterol, cholesterol, in young tomato plants in which the presence of C28-BRs biosynthesis has been confirmed. Additionally, we have determined that the end product of C27-BRs biosynthesis, 28-norCS, is converted into CS, thereby indicating that two BRs biosyntheses are connected biosynthetically to maintain the steady-state level of an active C28-BR, CS, in the plant. Our continuing interest in the physiological functions of multiple biosyntheses of BRs in plants prompted us to evaluate endogenous BRs in large numbers of Arabidopsis thaliana, in which