Withania somnifera (L.) Dunal, a highly reputed medicinal plant, synthesizes a large array of steroidal lactone triterpenoids called withanolides. Although its chemical profile and pharmacological activities have been studied extensively during the last two decades, limited attempts have been made to decipher the biosynthetic route and identification of key regulatory genes involved in withanolide biosynthesis. Cytochrome P450 reductase is the most imperative redox partner of multiple P450s involved in primary and secondary metabolite biosynthesis. We describe here the cloning and characterization of two paralogs of cytochrome P450 reductase from W. somnifera. The full length paralogs of WsCPR1 and WsCPR2 have open reading frames of 2058 and 2142 bp encoding 685 and 713 amino acid residues, respectively. Phylogenetic analysis demonstrated that grouping of dual CPRs was in accordance with class I and class II of eudicotyledon CPRs. The corresponding coding sequences were expressed in Escherichia coli as glutathione-S-transferase fusion proteins, purified and characterized. Recombinant proteins of both the paralogs were purified with their intact membrane anchor regions and it is hitherto unreported for other CPRs which have been purified from microsomal fraction. Southern blot analysis suggested that two divergent isoforms of CPR exist independently in Withania genome. Quantitative real-time PCR analysis indicated that both genes were widely expressed in leaves, stalks, roots, flowers and berries with higher expression level of WsCPR2 in comparison to WsCPR1. Similar to CPRs of other plant species, WsCPR1 was un-inducible while WsCPR2 transcript level increased in a time-dependent manner after elicitor treatments. High performance liquid chromatography of withanolides extracted from elicitor-treated samples showed a significant increase in two of the key withanolides, withanolide A and withaferin A, possibly indicating the role of WsCPR2 in withanolide biosynthesis. Present investigation so far is the only report of characterization of CPR paralogs from W. somnifera.
Background: Pharmacological investigations position withanolides as important bioactive molecules demanding their copious production. Results: Differential transcriptional and translational expression of three oxidosqualene cyclases leads to redirection of metabolic fluxes. Conclusion: Negative regulator channelizes substrate pool toward cycloartenol synthase at subdividing junction leading to enhanced withanolide production. Significance: Understanding the regulatory role of oxidosqualene cyclases on withanolide accumulation could serve as a prognostic tool for metabolic engineering.
Withania somnifera, a multipurpose medicinal plant is a rich reservoir of pharmaceutically active triterpenoids that are steroidal lactones known as withanolides. Though the plant has been well-characterized in terms of phytochemical profiles as well as pharmaceutical activities, limited attempts have been made to decipher the biosynthetic route and identification of key regulatory genes involved in withanolide biosynthesis. This scenario limits biotechnological interventions for enhanced production of bioactive compounds. Nevertheless, recent emergent trends vis-à-vis, the exploration of genomic, transcriptomic, proteomic, metabolomics, and in vitro studies have opened new vistas regarding pathway engineering of withanolide production. During recent years, various strategic pathway genes have been characterized with significant amount of regulatory studies which allude toward development of molecular circuitries for production of key intermediates or end products in heterologous hosts. Another pivotal aspect covering redirection of metabolic flux for channelizing the precursor pool toward enhanced withanolide production has also been attained by deciphering decisive branch point(s) as robust targets for pathway modulation. With these perspectives, the current review provides a detailed overview of various studies undertaken by the authors and collated literature related to molecular and in vitro approaches employed in W. somnifera for understanding various molecular network interactions in entirety.
Withania somnifera is a rich reservoir of pharmaceutically active steroidal lactones known as withanolides. The plant is well characterized in terms of its chemistry and pharmacology, but very little is known about the pathway involved in the biosynthesis of withanolides. The present investigation describes the cloning, characterization and expression of squalene epoxidase (SE) gene from W. somnifera. SE (SQE; EC. 1.14.99.7) is one of the rate limiting enzymes in the biosynthesis of triterpenoids, catalyzing the stereospecific epoxidation of squalene to 2,3-oxidosqualene. A full length SE gene (WsSQE) of 1,956 bp was cloned which contained an open reading frame of 1,596 bp, encoding a protein of 531 amino acids with a predicted molecular mass of 57.67 kDa and theoretical PI of 8.48. Full length WsSQE was cloned into pGEX4T-2 vector and expressed in E.coli. Phylogenetic analysis indicated a significant evolutionary relatedness of WsSQE with squalene epoxidases of other plant species and the degree of relatedness with deduced amino acid sequences showed a significant correlation with different plant species. Using genome walking approach, a promoter sequence of 513 bp of WsSQE was isolated which revealed several key cis-regulatory elements known to be involved in various biotic and abiotic plant stresses. Comparative expression analysis of tissue specific WsSQE done by quantitative-PCR demonstrated the highest transcript levels in leaves, as compared to stalk and root tissues. This is the first report of cloning and bacterial expression of SE from W. somnifera and may be of significant interest to understand the regulatory role of SE in the biosynthesis of withanolides.
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