Cloning and functional characterization of 3-hydroxy-3-methylglutaryl coenzyme A reductase gene from Withania somnifera: an important medicinal plant.

Withania somnifera (L.) Dunal is one of the most valuable medicinal plants synthesizing a large number of pharmacologically active secondary metabolites known as withanolides, the C28-steroidal lactones derived from triterpenoids. Though the plant has been well characterized in terms of phytochemical profiles as well as pharmaceutical activities, not much is known about the biosynthetic pathway and genes responsible for biosynthesis of these compounds. In this study, we have characterized the gene encoding 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR; EC 1.1.1.34) catalyzing the key regulatory step of the isoprenoid biosynthesis. The 1,728-bp full-length cDNA of Withania HMGR (WsHMGR) encodes a polypeptide of 575 amino acids. The amino acid sequence homology and phylogenetic analysis suggest that WsHMGR has typical structural features of other known plant HMGRs. The relative expression analysis suggests that WsHMGR expression varies in different tissues as well as chemotypes and is significantly elevated in response to exposure to salicylic acid, methyl jasmonate, and mechanical injury. The functional color assay in Escherichia coli showed that WsHMGR could accelerate the biosynthesis of carotenoids, establishing that WsHMGR encoded a functional protein and may play a catalytic role by its positive influence in isoprenoid biosynthesis.

Cloning and characterization of 2-C-methyl-D-erythritol-4-phosphate pathway genes for isoprenoid biosynthesis from Indian ginseng, Withania somnifera.

Withania somnifera (L.) is one of the most valuable medicinal plants used in Ayurvedic and other indigenous medicines. Pharmaceutical activities of this herb are associated with presence of secondary metabolites known as withanolides, a class of phytosteroids synthesized via mevalonate (MVA) and 2-C-methyl-D-erythritol-4-phosphate pathways. Though the plant has been well characterized in terms of phytochemical profiles as well as pharmaceutical activities, not much is known about the genes responsible for biosynthesis of these compounds. In this study, we have characterized two genes encoding 1-deoxy-D-xylulose-5-phosphate synthase (DXS; EC 2.2.1.7) and 1-deoxy-D-xylulose-5-phosphate reductase (DXR; EC 1.1.1.267) enzymes involved in the biosynthesis of isoprenoids. The full-length cDNAs of W. somnifera DXS (WsDXS) and DXR (WsDXR) of 2,154 and 1,428 bps encode polypeptides of 717 and 475 amino acids residues, respectively. The expression analysis suggests that WsDXS and WsDXR are differentially expressed in different tissues (with maximal expression in flower and young leaf), chemotypes of Withania, and in response to salicylic acid, methyl jasmonate, as well as in mechanical injury. Analysis of genomic organization of WsDXS shows close similarity with tomato DXS in terms of exon-intron arrangements. This is the first report on characterization of isoprenoid biosynthesis pathway genes from Withania.

Sterol glycosyltransferases-identification of members of gene family and their role in stress in Withania somnifera.

Sterol glycosyltransferases (SGTs) catalyze the transfer of sugar molecules to diverse sterol molecules, leading to a change in their participation in cellular metabolism. Withania somnifera is a medicinal plant rich in sterols, sterol glycosides and steroidal lactones. Sterols and their modified counterparts are medicinally important and play a role in adaptation of the plant to stress conditions. We have identified 3 members of SGT gene family through RACE (Rapid Amplification of cDNA Ends) in addition to sgtl1 reported earlier. The amino acid sequence deduced from the ORF's showed homology (45-67 %) to the reported plant SGTs. The expression of the genes was differentially modulated in different organs in W. somnifera and in response to external stimuli. Salicylic acid and methyl jasmonate treatments showed up to 10 fold increase in the expression of sgt genes suggesting their role in defense. The level of expression increased in heat and cold stress indicating the role of sterol modifications in abiotic stress. One of the members, was expressed in E. coli and the enzyme assay showed that the crude enzyme glycosylated stigmasterol. W. somnifera expresses a family of sgt genes and there is a functional recruitment of these genes under stress conditions. The genes which are involved in sterol modification are important in view of medicinal value and understanding stress.