Abstract
BACKGROUND: Saikosaponin-d (SSd) is an important active component of Bupleurum scorzonerifolium Willd., a traditional Chinese medicinal herb. Thus far, the biosynthetic pathway and biosynthetic site of saikosaponins in Bupleurum are largely unknown. The cellular localization of SSd will help in understanding saikosaponin biosynthesis and regulation. RESULTS: In this study, we characterize for the first time the localization of SSd in B. scorzonerifolium tissues and cells using histochemistry and immunoelectron microscopy. The results show that the saikosaponin distribution in different plant organs changes as they mature. The number of SSd gold particles distinctly differed among the roots, stems, and leaves, with the particles mainly concentrated in the roots. The gold particles were mainly observed in vacuoles, with a few particles in the protoplasm; hence, SSd is mainly stored in vacuoles. CONCLUSIONS: We speculate that saikosaponins are mainly synthesized via the mevalonate pathway in the protoplasm in young organs, and then transported to the central vacuole by the endoplasmic reticulum (ER) or the fusion of vacuoles, to protect plants from self-poisoning with the accumulation of more saikosaponins.