Callus culture and in vitro biosynthesis of cardiac glycosides from Calotropis gigantea (L.) Ait

Calotropis gigantea (L.) Ait., belonging to the family Asclepiadaceae, is a source of many cardiac glycosides (CGs) and their steroidal moieties (genins). These CGs have been reported to have anti-proliferative activity on tumor cell lines and are potential targets for cancer chemotherapy. However, the abundance of CGs in wild plants is particularly restricted and it is difficult to isolate the desired compound in required quantities. This study is the first attempt to standardize the induction and proliferation of callus from various explants of C. gigantea specifically for the production of CGs. Callus growth was accompanied by CG measurement using high-performance liquid chromatography-tandem mass spectrometry. Murashige and Skoog (MS) and modified Murashige and Skoog (MMS) media were optimized with various combinations and concentrations of auxin and cytokinin for induction and growth of calli from a range of explant sources. While leaves and stem explants resulted in greatest callus induction, MMS medium was found to be optimal. However, no CG was produced from callus grown on this medium. In contrast, the induction and proliferation of callus on MS medium were optimum at primary stages, but growth slowed during the third subculture. Therefore, calli were transferred to MMS medium to promote callus proliferation and production of CGs. As a result, three CGs and two genins were biosynthesized. Furthermore, the callus induction data in MS medium indicated that among different auxins, 2,4-dichlorophenoxyacetic acid was the best for callus induction compared to 1-naphthylacetic acid and indole-3-acetic acid. The data also revealed that the cytokinin/auxin ratio was critical rather than their independent presence for the induction of callus. Thus, the in vitro biosynthesis of targeted CGs may offer an alternative pathway for new source of anti-proliferative agents in required quantities.