In vitro studies and phytocompound analysis in Lessertia frutescens (Fabaceae)

Abstract

The cancer bush (Lessertia frutescens L.) is an important leguminous perennial native to southern Africa and has been used for centuries in traditional medicine by the continent’s diverse cultural groups. Like many other legumes, the seeds of this species exhibit dormancy. Moreover, woody plants are typically difficult to propagate in in vitro culture systems. But in vitro shoot cultures are valuable in providing an alternative means of deriving desired secondary metabolites or phytocompounds, under controlled conditions. This study describes novel protocols for breaking seed dormancy, rapid and efficient in vitro propagation, bioreactor culture, and comprehensive phytochemical data following screening and analysis of in vitro and field extracts of L. frutescens. Experiments using physical, mechanical and chemical pre-sowing treatments were conducted to determine the germination response of this species. The results indicated that seeds of L. frutescens exhibited exogenous dormancy due to the inhibitory effect of the hard coat on germination. Seed dormancy was released by mechanical scarification in which 100 % germination was achieved. In vitro propagation studies using single node explants in Murashige and Skoog (MS) medium supplemented with combinations of different concentrations of benzyladenine and naphthaleneacetic acid revealed a maximum number of 10 shoots per explant in solid medium, and 12.9 shoots per explant in liquid medium inside a temporary immersion bioreactor. Indirect shoot organogenesis and plant regeneration using rachis and stem segments was achieved with the highest percentage of explants forming shoots (88.8 %) from rachis explants cultured onto MS medium supplemented with thidiazuron. Direct shoot organogenesis from hypocotyl and cotyledon segments was also achieved in L. frutescens. The highest shoot regeneration using hypocotyls (83 %) was obtained in MS medium supplemented with kinetin whilst the highest shoot regeneration using cotyledons (46 %) was obtained in MS medium supplemented with kinetin in combination with benzyladenine. Successful rooting (up to 80 %) and acclimatization (up to 90 % survival rate) was attained. Spectrophotometric and gravimetric methods indicated that saponins were the most abundant, followed by phenolics, flavonoids and then alkaloids in in vitro leaf extracts then in field leaf extracts and seed extracts, respectively. After qualitative analysis these extracts were also found to contain tannins, phlobatannins and cardiac glycosides of medicinal interest. By using gas and liquid chromatography the presence of the medicinally important L-canavanine, gamma amino-butyric acid and D-pinitol was verified in in vitro leaf, field leaf and seed extracts. In vitro leaves had higher quantities of all compounds, except for D-pinitol. Phytocompound analysis of shoots derived from several of the cytokinin-enhanced media showed that these organs contained higher quantities of L-canavanine compared to the control. This study, therefore, highlights the potential techno-economic production of medicinal phytocompounds from in vitro leaves of L. frutescens following large scale production using the protocols described in this study.