Immunocytochemical localization of saikosaponin-d in vegetative organs of Bupleurum scorzonerifolium Willd

Abstract

BackgroundSaikosaponin-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.ResultsIn 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.ConclusionsWe 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.Electronic supplementary materialThe online version of this article (doi:10.1186/1999-3110-54-32) contains supplementary material, which is available to authorized users.

Identification and functional characterization of DzS3GT, a cytoplasmic glycosyltransferase catalyzing biosynthesis of diosgenin 3-O-glucoside in Dioscorea zingiberensis

“…Steroidal saponins in plants are often stored in the vacuole to prevent self-toxicity (Sirikantaramas et al 2008;Zhao et al 2013). In this study, we found that the DzS3GT enzyme is present mainly in the cytoplasm and is separated from steroidal saponins in different cellular compartments, which differed from the previously reported SGTs found in the plasma membrane, Golgi vesicles, the endoplasmic reticulum membrane, and occasionally the tonoplast Ullmann et al 1987;Warnecke et al 1997) that speculated to be involved in the cell wall synthesis.…”

“…In this study, we found that the DzS3GT enzyme is present mainly in the cytoplasm and is separated from steroidal saponins in different cellular compartments, which differed from the previously reported SGTs found in the plasma membrane, Golgi vesicles, the endoplasmic reticulum membrane, and occasionally the tonoplast Ullmann et al 1987;Warnecke et al 1997) that speculated to be involved in the cell wall synthesis. It is possible that the aglycones are glycosylated in cytoplasm allowing them to be easily transported into the vacuole to protect the plant from self-poisoning due to excessive levels of secondary metabolites (Gleadow and Møller 2014;Li and Hu 2009;Zhao et al 2013). Thus, the DzS3GT may play an important role in maintaining the stabilization of steroidal compound metabolism in D. zingiberensis.…”

“…The rhizomes may be the mainly storage place for saponins. It is possible for the reason that many secondary metabolites were synthesized in leaves and then transported to other tissues via stems (Yazaki et al 2008;Zhao et al 2013), while the water solubility of trillin is not good enough for transport or storage (Cheng et al 2012;Ghasemzadeh et al 2014). But this is not always the case.…”

“…Steroidal saponins are biosynthesized mainly via cycloartenol in a branch of the cytoplasmic mevalonate pathway (Kalinowska et al 2005) and often stored in the vacuole to prevent self-toxicity in plants ( Gupta et al 2004;Sirikantaramas et al 2008;Zhao et al 2013). In addition, the aglycones of steroidal saponins, e.g., diosgenin, are speculated to be synthesized in leaves and then transported to other tissues via stems (Li et al 2009;Kim et al 2014), but the glycosylation pathway is still unclear.…”

Validation of reference genes for expression analysis in three Bupleurum species

“…Previous histochemical studies on B. chinense DC., B. falcatum L. and B. scorzonerifolium Willd. have demonstrated that saikosaponins are mainly found in the epidermal areas of the roots [15,[32][33][34]. This was confirmed as plants with more lateral roots showed higher saikosaponin content than plants with less lateral roots.…”