Withanolide A is inherently de novo biosynthesized in roots of the medicinal plant Ashwagandha (Withania somnifera)

Abstract

Ashwagandha (Withania somnifera Dunal., Solanaceae) is one of the most reputed medicinal plants of Ayurveda, the traditional medical system. Several of its traditionally proclaimed medicinal properties have been corroborated by recent molecular pharmacological investigations and have been shown to be associated with its specific secondary metabolites known as withanolides, the novel group of ergostane skeletal phytosteroids named after the plant. Withanolides are structurally distinct from tropane/nortropane alkaloids (usually found in Solanaceae plants) and are produced only by a few genera within Solanaceae. W. somnifera contains many structurally diverse withanolides in its leaves as well as roots. To date, there has been little biosynthetic or metabolism-related research on withanolides. It is thought that withanolides are synthesized in leaves and transported to roots like the tropane alkaloids, a group of bioactive secondary metabolites in Solanaceae members known to be synthesized in roots and transported to leaves for storage. To examine this, we have studied incorporation of ¹⁴C from [2-¹⁴C]-acetate and [U-¹⁴C]-glucose into withanolide A in the in vitro cultured normal roots as well as native/orphan roots of W. somnifera. Analysis of products by thin layer chromatography revealed that these primary metabolites were incorporated into withanolide A, demonstrating that root-contained withanolide A is de novo synthesized within roots from primary isoprenogenic precursors. Therefore, withanolides are synthesized in different parts of the plant (through operation of the complete metabolic pathway) rather than imported.