Element of the brassinosteroid metabolic pathway that regulates typhasterol (TY), castasterone (CS) and brassinolide (BL) levels (PubMed:17521414). Involved in the control of organ elongation (PubMed:17521414, PubMed:23893742). Plant hormone biosynthesis; brassinosteroid biosynthesis. Monomer. Mainly present in cell elongating-containing tissues. Strongly expressed in roots and flowers, also observed in petioles, stems, leaves and siliques. First observed after seed germination, mainly in the root cap, and in elongation and maturation zones, and, to a lower extent, in the apical meristem zone. Later present in roots, with higher levels in light conditions than in darkness. Weak levels in young flowers. Progressive accumulation in developing siliques, at both ends. In rosette leaves, mainly localized in vascular tissues and hydathodes. Down-regulated in the dark. Abnormal organs elongation leading to long inflorescences, leaves and petioles, especially in the light. The ben1-1D (bri1-5 enhanced 1-1dominant) activation-tagging mutant suppresses the bri1-5 weak mutant allele of the brassinosteroid receptor gene BRI1. Belongs to the NAD(P)-dependent epimerase/dehydratase family.