Abstract
The biosynthesis of brassinosteroids was investigated by feeding 2H-labeled teasterone (TE), 2H-labeled typhasterol (TY), and a mixture of 2H-labeled and 3H-labeled castasterone (CS) to seedlings of Catharanthus roseus, Nicotiana tabacum, and Oryza sativa. In all the seedlings examined, the conversion of TE to TY, and the conversion of TY to CS and to TE was demonstrated. In the seedlings of C. roseus, the conversion of CS to brassinolide (BL) and to 3-epicastasterone (3-epiCS) was observed, while the conversion of CS to only 3-epiCS was found in the seedlings of N. tabacum and O. sativa. These results demonstrate the presence of the biosynthetic pathway of TE →TY →CS →BL in seedlings of C. roseus, and the biosynthetic pathway of TE→TY→CS in seedlings of N. tabacum and O. sativa. A reversible conversion between TE and TY was also found. This is the first evidence of CS being the biosynthetic origin of 3-epiCS.
