RNAi-mediated gene silencing of WsSGTL1 in W.somnifera affects growth and glycosylation pattern

Molecular Cloning and Functional Characterization of a Sterol 3-O-Glucosyltransferase Involved in Biosynthesis of Steroidal Saponins in Trigonella foenum-graecum

Fenugreek (Trigonella foenum-graecum), a pharmacologically important herb, is widely known for its antidiabetic, hypolipidemic, and anticancer effects. The medicinal properties of this herb are accredited to the presence of bioactive steroidal saponins with one or more sugar moieties linked to the C-3 OH position of disogenin or its C25-epimer yamogenin. Despite intensive studies regarding pharmacology and phytochemical profiles of this plant, enzymes and/or genes involved in synthesizing the glycosidic part of fenugreek steroidal saponins are still missing so far. This study reports the molecular cloning and functional characterization of a key sterol-specific glucosyltransferase, designated as TfS3GT2 here, from fenugreek plant. The recombinant TfS3GT2 was purified via expression in Escherichia coli, and biochemical characterization of the recombinant enzyme suggested its role in transferring a glucose group onto the C-3 hydroxyl group of diosgenin or yamogenin. The functional role of TfS3GT2 in the steroidal saponin biosynthesis was also demonstrated by suppressing the gene in the transgenic fenugreek hairy roots via the RNA interference (RNAi) approach. Down-regulation of TfS3GT2 in fenugreek generally led to reduced levels of diosgenin or yamogenin-derived steroidal saponins. Thus, Tf3SGT2 was identified as a steroid-specific UDP-glucose 3-O-glucosyltransferase that appears to be involved in steroidal saponin biosynthesis in T. foenum-graecum.

Third DWF1 paralog in Solanaceae, sterol Δ24-isomerase, branches withanolide biosynthesis from the general phytosterol pathway

A large part of chemodiversity of plant triterpenes is due to the modification of their side chains. Reduction or isomerization of double bonds in the side chains is often an important step for the diversification of triterpenes, although the enzymes involved are not fully understood. Withanolides are a large group of structurally diverse C28 steroidal lactones derived from 24-methylenecholesterol. These compounds are found in the Indian medicinal plant Withania somnifera, also known as ashwagandha, and other members of the Solanaceae. The pathway for withanolide biosynthesis is unknown, preventing sustainable production via white biotechnology and downstream pharmaceutical usages. In the present study, based on genome and transcriptome data we have identified a key enzyme in the biosynthesis of withanolides: a DWF1 paralog encoding a sterol Δ24-isomerase (24ISO). 24ISO originated from DWF1 after two subsequent duplication events in Solanoideae plants. Withanolides and 24ISO appear only in the medicinal plants in the Solanoideae, not in crop plants such as potato and tomato, indicating negative selection during domestication. 24ISO is a unique isomerase enzyme evolved from a reductase and as such has maintained the FAD-binding oxidoreductase structure and requirement for NADPH. Using phylogenetic, metabolomic, and gene expression analysis in combination with heterologous expression and virus-induced gene silencing, we showed that 24ISO catalyzes the conversion of 24-methylenecholesterol to 24-methyldesmosterol. We propose that this catalytic step is the committing step in withanolide biosynthesis, opening up elucidation of the whole pathway and future larger-scale sustainable production of withanolides and related compounds with pharmacological properties.

Withania somnifera: Advances and Implementation of Molecular and Tissue Culture Techniques to Enhance Its Application

Withania somnifera, commonly known as Ashwagandha an important medicinal plant largely used in Ayurvedic and indigenous medicine for over 3,000 years. Being a medicinal plant, dried powder, crude extract as well as purified metabolies of the plant has shown promising therapeutic properties. Withanolides are the principal metabolites, responsible for the medicinal properties of the plant. Availability and amount of particular withanolides differ with tissue type and chemotype and its importance leads to identification characterization of several genes/ enzymes related to withanolide biosynthetic pathway. The modulation in withanolides can be achieved by controlling the environmental conditions like, different tissue culture techniques, altered media compositions, use of elicitors, etc. Among all the in vitro techniques, hairy root culture proved its importance at industrial scale, which also gets benefits due to more accumulation (amount and number) of withanolides in roots tissues of W. somnifera. Use of media compostion and elicitors further enhances the amount of withanolides in hairy roots. Another important modern day technique used for accumulation of desired secondary metabolites is modulating the gene expression by altering environmental conditions (use of different media composition, elicitors, etc.) or through genetic enginnering. Knowing the significance of the gene and the key enzymatic step of the pathway, modulation in withanolide contents can be achieved upto required amount in therapeutic industry. To accomplish maximum productivity through genetic enginnering different means of Withania transformation methods have been developed to obtain maximum transformation efficiency. These standardized transformation procedues have been used to overexpress/silence desired gene in W. somnifera to understand the outcome and succeed with enhanced metabolic production for the ultimate benefit of human race.