Impact of external calcium and calcium sensors on ginsenoside Rb1 biosynthesis by Panax notoginseng cells

The effects of external calcium concentrations on biosynthesis of ginsenoside Rb1 and several calcium signal sensors were quantitatively investigated in suspension cultures of Panax notoginseng cells. It was observed that the synthesis of intracellular ginsenoside Rb1 in 3-day incubation was dependent on the medium Ca2+ concentration (0-13 mM). At an optimal Ca2+ concentration of 8 mM, a maximal ginsenoside Rb1 content of 1.88 +/- 0.03 mg g(-1) dry weight was reached, which was about 60% and 25% higher than that at Ca2+ concentrations of 0 and 3 mM, respectively. Ca2+ feeding experiments confirmed the Ca2+ concentration-dependent Rb1 biosynthesis. In order to understand the mechanism of the signal transduction from external Ca2+ to ginsenoside biosynthesis, the intracellular content of calcium and calmodulin (CaM), activities of calcium/calmodulin-dependent NAD kinase (CCDNK) and calcium-dependent protein kinase (CDPK), and activity of a new biosynthetic enzyme of ginsenoside Rb1, i.e., UDPG:ginsenoside Rd glucosyltransferase (UGRdGT), in the cultured cells were all analyzed. The intracellular calcium content and CCDNK activity were increased with an increase of external Ca2+ concentration within 0-13 mM. In contrast, the CaM content and activities of CDPK and UGRdGT reached their highest levels at 8 mM of initial Ca2+ concentration, which was also optimal to the ginsenoside Rb1 synthesis. A similar Ca2+ concentration-dependency of the intracellular contents of calcium and CaM and activities of CCDNK, CDPK, and UGRdGT was confirmed in Ca2+ feeding experiments. Finally, a possible model on the effect of external calcium on ginsenoside Rb1 biosynthesis via the signal transduction pathway of CaM, CDPK, and UGRdGT is proposed. Regulation of external Ca2+ concentration is considered a useful strategy for manipulating ginsenoside Rb1 biosynthesis by P. notoginseng cells.