Withanolide biosynthesis recruits both mevalonate and DOXP pathways of isoprenogenesis in Ashwagandha Withania somnifera L. (Dunal)

Withanolides production by the endophytic fungus Penicillium oxalicum associated with Withania somnifera (L.) Dunal

Withanolides are steroidal lactones with diverse bioactive potential and their production from plant sources varies with genotype, age, culture conditions, and geographical region. Endophytic fungi serve as an alternative source to produce withanolides, like their host plant, Withania somnifera (L.) Dunal. The present study aimed to isolate endophytic fungi capable of producing withanolides, characterization and investigation of biological activities of these molecules. The methanolic fungal crude extract of one of the fungal isolates WSE16 showed maximum withanolide production (219 mg/L). The fungal isolate WSE16 was identified as Penicillium oxalicum based on its morphological and internal transcribed spacer (ITS) sequence analysis and submitted in NCBI (accession number OR888725). The methanolic crude extract of P. oxalicum was further purified by column chromatography, and collected fractions were assessed for the presence of withanolides. Fractions F3 and F4 showed a higher content of withanolides (51.8 and 59.1 mg/L, respectively) than other fractions. Fractions F3 and F4 exhibited antibacterial activity against Staphylococcus aureus with an IC50 of 23.52 and 17.39 µg/ml, respectively. These fractions also showed antioxidant activity (DPPH assay with IC50 of 39.42 and 38.71 µg/ml, superoxide anion scavenging assay with IC50 of 41.10 and 38.84 µg/ml, and reducing power assay with IC50 of 42.61 and 41.40 µg/ml, respectively) and acetylcholinesterase inhibitory activity (IC50 of 30.34 and 22.05 µg/ml, respectively). The withanolides present in fraction 3 and fraction 4 were identified as (20S, 22R)-1a-Acetoxy-27-hydroxywitha-5, 24-dienolide-3b-(O-b-D-glucopyranoside) and withanamide A, respectively, using UV, FTIR, HRMS, and NMR analysis. These results suggest that P. oxalicum, an endophytic fungus isolated from W. somnifera, is a potential source for producing bioactive withanolides.

Enhanced production of withaferin A from the hairy root culture of Withania somnifera via synergistic effect of Methyl jasmonate and β-cyclodextrin

Due to low amounts of withanolides produced in some plants and high demand for various applications, their biotechnological production is widely researched. The effects of two explant types (i.e., leaf and stem from the in vitro seedlings of three genotypes of Withania somnifera) and four Rhizobium strains (i.e., LBA 9402, A4, ATCC 15834, and C58C1) to improve hairy root formation efficiency was studied. Furthermore, the combined effects of β-cyclodextrin (β-CD) and methyl jasmonate (MeJA) on withaferin A production after 48 h exposure time was examined. Four hairy roots having the maximum percentage of induced roots and mean number of induced roots to analyze their growth kinetics and identified G3/ATCC/LEAF culture having the maximum specific growth rate (μ = 0.036 day-1) and growth index (GI = 9.18), and the shortest doubling time (Td = 18.82 day) were selected. After 48 h exposure of G3/ATCC/LEAF culture to different elicitation conditions, maximum amounts of withaferin A were produced in samples co-treated with 0.5 mM β-CD + 100 μM MeJA (9.57 mg/g DW) and 5.0 mM β-CD + 100 μM MeJA (17.45 mg/g DW). These outcomes represented a 6.8-fold and 12.5-fold increase, respectively, compared to the control. Similarly, combined β-CD/MeJA elicitation increased gene expression levels of HMGR, SQS, SMT-1, and SDS/CYP710A involved in withanolides biosynthetic pathway, of which just SMT-1 had significant correlation with withaferin A production. These results demonstrated the superiority of G1-leaf explant and ATCC 15834 for hairy root induction, and revealed synergistic effect of MeJA and β-CD on withaferin A production.

Differential regulation of key triterpene synthase gene under abiotic stress in Withania somnifera L. Dunal and its co-relation to sterols and withanolides

Withania somnifera (Ashwagandha), is one of the most reputed Indian medicinal plants, having immense pharmacological activities due to the occurrence of withanolides. The withanolides are biosynthesized through triterpenoid biosynthetic pathway with the involvement of WsCAS leading to cyclization of 2, 3 oxidosqualene, which is a key metabolite to further diversify to a myriad of phytochemicals. In contrast to the available reports on the studies of WsCAS in withanolide biosynthesis, its involvement in phytosterol biosynthesis needs investigation. Present work deals with the understanding of role of WsCAS triterpenoid synthase gene in the regulation of biosynthesis of phytosterols & withanolides. Docking studies of WsCAS protein revealed Conserved amino acids, DCATE motif, and QW motif which are involved in efficient substrate binding, structure stabilization, and catalytic activity. Overexpression/silencing of WsCAS leading to increment/decline of phytosterols confers its stringent regulation in phytosterols biosynthesis. Differential regulation of WsCAS on the metabolic flux towards phytosterols and withanolide biosynthesis was observed under abiotic stress conditions. The preferential channelization of 2, 3 oxidosqualene towards withanolides and/or phytosterols occurred under heat/salt stress and cold/water stress, respectively. Stigmasterol and β-sitosterol showed major contribution in high/low temperature and salt stress, and campesterol in water stress management. Overexpression of WsCAS in Arabidopsis thaliana led to the increment in phytosterols in general. Thus, the WsCAS plays important regulatory role in the biosynthetic pathway of phytosterols and withanolides under abiotic stress conditions.

Enhancement of specialized metabolites using CRISPR/Cas gene editing technology in medicinal plants

Plants are the richest source of specialized metabolites. The specialized metabolites offer a variety of physiological benefits and many adaptive evolutionary advantages and frequently linked to plant defense mechanisms. Medicinal plants are a vital source of nutrition and active pharmaceutical agents. The production of valuable specialized metabolites and bioactive compounds has increased with the improvement of transgenic techniques like gene silencing and gene overexpression. These techniques are beneficial for decreasing production costs and increasing nutritional value. Utilizing biotechnological applications to enhance specialized metabolites in medicinal plants needs characterization and identification of genes within an elucidated pathway. The breakthrough and advancement of CRISPR/Cas-based gene editing in improving the production of specific metabolites in medicinal plants have gained significant importance in contemporary times. This article imparts a comprehensive recapitulation of the latest advancements made in the implementation of CRISPR-gene editing techniques for the purpose of augmenting specific metabolites in medicinal plants. We also provide further insights and perspectives for improving metabolic engineering scenarios in medicinal plants.

A critical assessment of the whole plant-based phytotherapeutics from Withania somnifera (L.) Dunal with respect to safety and efficacy vis-a-vis leaf or root extract-based formulation

Withania somnifera (L.) Dunal, popularly known as Ashwagandha or Indian ginseng, is well acclaimed for its health-enhancing effects, including its potent immunomodulatory, anti-inflammatory, neuroprotective, and anti-tumorigenic properties. The prime biological effectors of these attributes are a diverse group of ergostane-based steroidal lactones termed withanolides. Withanones and withanosides are distributed differentially across the plant body, whereas withanolides and withanones are known to be more abundant in leaves, while withanosides are found exclusively in the roots of the plants. Standardized W. somnifera extract is Generally Recognized as Safe (GRAS)-affirmed, however, moderate to severe toxic manifestations may occur at high dosages. Withaferin A, which also happens to be the primary bioactive ingredient for the effectiveness of this plant. There have been contrasting reports regarding the distribution of withaferin A in W. somnifera. While most reports state that the roots of the plant have the highest concentrations of this phytochemical, several others have indicated that leaves can accumulate withaferin A in proportionately higher amounts. A comprehensive survey of the available reports suggests that the biological effects of Ashwagandha are grossly synergistic in nature, with many withanolides together mediating the desired physiological effect. In addition, an assorted formulation of withanolides can also neutralize the toxic effects (if any) associated with withaferin A. This mini-review presents a fresh take on the recent developments regarding the safety and toxicity of the plant, along with a critical assessment of the use of roots against leaves as well as whole plants to develop therapeutic formulations. Going by the currently available scientific evidence, it is safe to infer that the use of whole plant formulations instead of exclusively root or leaf recipes may present the best possible option for further exploration of therapeutic benefits from this novel medicinal plant.HighlightsTherapeutic potential of withanolides owes to the presence of α,β unsaturated ketone which binds to amines, alcohols, and esters and 5β, 6β epoxy group which react with side chain thiols of proteins.At concentrations above NOAEL (no observed adverse effect level), the same mechanisms contribute towards toxicity of the molecule.Although withanosides are found exclusively in roots, whole plants have higher contents of withanones and withanolides.Whole plant-based formulations have other metabolites which can nullify the toxicity associated with roots.Extracts made from whole plants, therefore can holistically impart all therapeutic benefits as well as mitigate toxicity.

Biodiversity, Biochemical Profiling, and Pharmaco-Commercial Applications of Withania somnifera: A Review

Withania somnifera L. Dunal (Ashwagandha), a key medicinal plant native to India, is used globally to manage various ailments. This review focuses on the traditional uses, botany, phytochemistry, and pharmacological advances of its plant-derived constituents. It has been reported that at least 62 crucial and 48 inferior primary and secondary metabolites are present in the W. somnifera leaves, and 29 among these found in its roots and leaves are chiefly steroidal compounds, steroidal lactones, alkaloids, amino acids, etc. In addition, the whole shrub parts possess various medicinal activities such as anti-leukotriene, antineoplastic, analgesic, anti-oxidant, immunostimulatory, and rejuvenating properties, mainly observed by in vitro demonstration. However, the course of its medical use remains unknown. This review provides a comprehensive understanding of W. somnifera, which will be useful for mechanism studies and potential medical applications of W. somnifera, as well as for the development of a rational quality control system for W. somnifera as a therapeutic material in the future.

Withaferin A: A Pleiotropic Anticancer Agent from the Indian Medicinal Plant Withania somnifera (L.) Dunal

Cancer represents the second most deadly disease and one of the most important public health concerns worldwide. Surgery, chemotherapy, radiation therapy, and immune therapy are the major types of treatment strategies that have been implemented in cancer treatment. Unfortunately, these treatment options suffer from major limitations, such as drug-resistance and adverse effects, which may eventually result in disease recurrence. Many phytochemicals have been investigated for their antitumor efficacy in preclinical models and clinical studies to discover newer therapeutic agents with fewer adverse effects. Withaferin A, a natural bioactive molecule isolated from the Indian medicinal plant Withania somnifera (L.) Dunal, has been reported to impart anticancer activities against various cancer cell lines and preclinical cancer models by modulating the expression and activity of different oncogenic proteins. In this article, we have comprehensively discussed the biosynthesis of withaferin A as well as its antineoplastic activities and mode-of-action in in vitro and in vivo settings. We have also reviewed the effect of withaferin A on the expression of miRNAs, its combinational effect with other cytotoxic agents, withaferin A-based formulations, safety and toxicity profiles, and its clinical potential.

Anti-inflammatory withajardins from the leaves of Athenaea velutina

Withajardins, uncommon modified withanolide-type steroids, have been isolated exclusively from plants of the Solanaceae family so far. Two undescribed withajardins and the known tuboanosigenin were isolated from the hexane/EtOAc 1:1 extract from Athenaea velutina leaves. Their structures were established by an extensive analysis of 1D and 2D-NMR and HRMS data. The absolute configuration was determined by X-ray diffraction (withajardin L and tuboanosigenin) and circular dichroism (CD) analyses (withajardin M). The anti-inflammatory activity of compounds was evaluated through the inhibition of the lipopolysaccharide (LPS)-induced nitric oxide (NO), TNF-α, and IL-6 release in RAW264.7 cells. The cell viability effects to RAW 264.7 cells showed IC₅₀ values of 74.4-354.4 μM. The compounds attenuated LPS-induced release of NO and decreased pro-inflammatory cytokines TNF-α and IL-6 in RAW264.7 cells.

Optimization of Elicitation Conditions to Enhance the Production of Potent Metabolite Withanolide from Withania somnifera (L.)

This study aimed at optimizing conditions for increased withanolide production in Withania somnifera. The elicitors used for the foliar spray on the aerial parts of the plant were salicylic acid, jasmonic acid, and chitosan for the enhancement of withanolides in Withania somnifera under different environmental regimes. Three different elicitors, i.e., chitosan, jasmonic acid and salicylic acid, were applied on the plants through foliar route every 15th day for 6 months, and later plants were used for sample preparation. Further, the elicitors were used in different concentration, i.e., jasmonic acid (50, 200 and 400 ppm), chitosan (10, 50 and 100 ppm) and salicylic acid (0.5, 1 and 2 ppm). The elicitors were sprayed on the foliar parts of the plant between 10:00-11:00 a.m. on application days. For elicitor spray, a calibrated sprayer was used. The withanolide A/withaferin A was quantified through HPLC. It was found that in an open environment, maximum withaferin A content, i.e., 0.570 mg/g (DW), was recorded with jasmonic acid (50 ppm) treatment in comparison to control (0.067 mg/g DW). Thus, there was an 8.5-fold increase in the withaferin A content. Maximum withanolide A content of 0.352 mg/g (DW) was recorded when chitosan (50 ppm) was sprayed, while in the control, withanolide A content was recorded to be 0.031 mg/g (DW); thus, chitosan application increased the production of withanolide A by 11.3-fold. Under controlled conditions, maximum withaferin A content of 1.659 mg/g (DW) was recorded when plants were sprayed with chitosan (100 ppm), which was 8.1 times greater than the control content of 0.203 mg/g (DW). Maximum withanolide A content of 0.460 mg/g (DW) was recorded when chitosan (100 ppm) was applied, whereas in the control, withanolide A content was found to be 0.061 mg/g (DW). Thus, foliar spraying of elicitors in very low concentrations can serve as a low-cost, eco-friendly, labor-intensive and elegant alternative approach that can be practiced by farmers for the enhancement, consistent production and improved yield of withanolide A/withaferin A. This can be a suitable way to enhance plant productivity, thus increasing the availability of withanolide A and withaferin A for the health and pharma industry.

Cloning and homologous characterization of geranylgeranyl pyrophosphate synthase (GGPPS) from Withania somnifera revealed alterations in metabolic flux towards gibberellic acid biosynthesis

Overexpression of a novel geranylgeranyl pyrophosphate synthase gene (WsGGPPS) in planta resulted in increased levels of gibberellic acid and decrease in withanolide content. Withania somnifera (L.) Dunal, the herb from family Solanaceae is one of the most treasured medicinal plant used in traditional medicinal systems owing to its unique stockpile of pharmaceutically active secondary metabolites. Phytochemical and pharmacological studies in this plant were well established, but the genes affecting the regulation of biosynthesis of major metabolites were not well elucidated. In this study cloning and functional characterization of a key enzyme in terpenoid biosynthetic pathway viz. geranylgeranyl pyrophosphate synthase (EC 2.5.1.29) gene from Withania somnifera was performed. The full length WsGGPPS gene contained 1,104 base pairs that encode a polypeptide of 365 amino acids. The quantitative expression analysis suggested that WsGGPPS transcripts were expressed maximally in flower tissues followed by berry tissues. The expression levels of WsGGPPS were found to be regulated by methyl jasmonate (MeJA) and salicylic acid (SA). Amino acid sequence alignment and phylogenetic studies suggested that WsGGPPS had close similarities with GGPPS of Solanum tuberosum and Solanum pennellii. The structural analysis provided basic information about three dimensional features and physicochemical parameters of WsGGPPS protein. Overexpression of WsGGPPS in planta for its functional characterization suggested that the WsGGPPS was involved in gibberellic acid biosynthesis.

Isolation and structure elucidation of a steroidal moiety from Withania somnifera and in silico evaluation of antimalarial efficacy against artemisinin resistance Plasmodium falciparum kelch 13 protein

According to the 2021 Malaria report, 241 million clinical episodes with 627000 deaths penalty was estimated across the worldwide. However, mutation in the propeller domain of Plasmodium falciparum kelch 13 protein resulted in longer parasite clearance time following an artemisinin-based treatment and had a greater survival rate of ring-stage parasites even after a brief exposure to a high dose of artesunate. Clinical manifestations become more complex and worse with the emerging trend of drug resistance against artemisinin derivatives and the poor effectiveness of malaria vaccination drive. Steroidal lactone (withanolide) moiety (C-28) isolated from methanolic leaf extract Withania somnifera show a greater affinity towards Pfkelch 13 protein in comparison to the artemisinin derivatives (artesunate, artemether). The isolated compound was characterized to be withaferin A with a percentage yield of 29.01% w/w in chloroform fraction, 1.75% w/w in methanolic extract, and 0.29% w/w in raw leaf powder. Structure-based analysis shows that withaferin A (docking score -8.253, -9.802) has a higher affinity for two distinct binding pockets I and II of the Plasmodium falciparum kelch 13 protein than artesunate (docking score -4.470, -3.656). Further, Gibbs binding free energy signifies thermodynamic stability of the docked complex of withaferin A (-43.25, -43.76 Kcal/mol) in comparison to artesunate docked complex (-8.49, -5.75 Kcal/mol). The pharmacokinetic profile of withaferin A shows more drug-likeness characteristics without violating Jorgensen's rule of three, and Lipinski's rule of five. Hence above experimental findings suggest withaferin A could be a suitable therapeutic adjunct for preclinical evaluation of antimalarial potentiality in artemisinin-resistant malaria.

Critical factors influencing in vitro propagation and modulation of important secondary metabolites in Withania somnifera (L.) dunal

Withania somnifera (L.) Dunal is a valuable medicinal plant in the Solanaceae family. It is commonly known as Ashwagandha and is widely distributed around the globe. It has multiple pharmacological properties owing to the existence of diverse secondary metabolites viz., withanolide A, withanolide D, withaferin A, and withanone. It is in great demand in the herbal industry because of its extensive use. In this background, the major challenge lies in the rapid multiplication of elite cultivars of W. somnifera in order to produce genetically and phytoconstituents uniform plant material for pharmaceutical industries. Thus it is necessary to explore various biotechnological approaches for the clonal mass propagation and synthesis of pharmaceutically important constituents in W. somnifera. Though there are several studies on in vitro propagation on W. somnifera, yet many factors that critically influence the in vitro response and withanolides production need to be fine-tuned in the pretext of the existing knowledge. The current review focuses on the advancements and prospects in biotechnological interventions to meet the worldwide demands for W. somnifera and its bioactive compounds. This update on in vitro studies on W. somnifera will be useful to many researchers, entrepreneurs, and herbal industries looking for its in vitro mass multiplication and scientific utilization.

Natural withanolides, an update

Covering: March 2010 to December 2020. Previous review: Nat. Prod. Rep., 2011, 28, 705This review summarizes the latest progress and perspectives on the structural classification, biological activities and mechanisms, metabolism and pharmacokinetic investigations, biosynthesis, chemical synthesis and structural modifications, as well as future research directions of the promising natural withanolides. The literature from March 2010 to December 2020 is reviewed, and 287 references are cited.

Genome mining methods to discover bioactive natural products

Covering: 2016 to 2021With genetic information available for hundreds of thousands of organisms in publicly accessible databases, scientists have an unprecedented opportunity to meticulously survey the diversity and inner workings of life. The natural product research community has harnessed this breadth of sequence information to mine microbes, plants, and animals for biosynthetic enzymes capable of producing bioactive compounds. Several orthogonal genome mining strategies have been developed in recent years to target specific chemical features or biological properties of bioactive molecules using biosynthetic, resistance, or transporter proteins. These "biosynthetic hooks" allow researchers to query for biosynthetic gene clusters with a high probability of encoding previously undiscovered, bioactive compounds. This review highlights recent case studies that feature orthogonal approaches that exploit genomic information to specifically discover bioactive natural products and their gene clusters.

Antifungal Activity of Isolated Compounds from the Leaves of Combretum erythrophyllum (Burch.) Sond. and Withania somnifera (L.) Dunal against Fusarium Pathogens

Crop diseases caused by Fusarium pathogens, among other microorganisms, threaten crop production in both commercial and smallholder farming. There are increasing concerns about the use of conventional synthetic fungicides due to fungal resistance and the associated negative effects of these chemicals on human health, livestock and the environment. This leads to the search for alternative fungicides from nature, especially from plants. The objectives of this study were to characterize isolated compounds from Combretum erythrophyllum (Burch.) Sond. and Withania somnifera (L.) Dunal leaf extracts, evaluate their antifungal activity against Fusarium pathogens, their phytotoxicity on maize seed germination and their cytotoxicity effect on Raw 264.7 macrophage cells. The investigation led to the isolation of antifungal compounds characterized as 5-hydroxy-7,4′-dimethoxyflavone, maslinic acid (21-hydroxy-3-oxo-olean-12-en-28-oic acid) and withaferin A (4β,27-dihydroxy-1-oxo-5β,6β-epoxywitha-2-24-dienolide). The structural elucidation of the isolated compounds was established using nuclear magnetic resonance (NMR) spectroscopy, mass spectroscopy (MS) and, in comparison, with the available published data. These compounds showed good antifungal activity with minimum inhibitory concentrations (MIC) less than 1.0 mg/mL against one or more of the tested Fusarium pathogens (F. oxysporum, F. verticilloides, F. subglutinans, F. proliferatum, F. solani, F. graminearum, F. chlamydosporum and F. semitectum). The findings from this study indicate that medicinal plants are a good source of natural antifungals. Furthermore, the isolated antifungal compounds did not show any phytotoxic effects on maize seed germination. The toxicity of the compounds A (5-hydroxy-7,4′-dimethoxyflavone) and AI (4β,27-dihydroxy-1-oxo-5β,6β-epoxywitha-2-24-dienolide) was dose-dependent, while compound B (21-hydroxy-3-oxo-olean-12-en-28-oic acid) showed no toxicity effect against Raw 264.7 macrophage cells.

Ashwagandha: Advances in plant biotechnological approaches for propagation and production of bioactive compounds

ETHNOPHARMACOLOGICAL RELEVANCE

Withania somnifera is one of the most extensively delved Ayurvedic medicine. Apart from rejuvenation and increasing longevity, it has several other properties such as immunomodulation, anti-cancer, anti-stress and neuroprotection. Because of its prevailing use and increasing demand, it becomes prudent to scientifically evaluate and document both its propagation and production of desired phytoconstituents.

AIM OF THE STUDY

This review aims to highlight the research progress achieved on various biotechnological and tissue culture aspects of Withania somnifera and to cover up-to-date information regarding in-vitro propagation and production of withanolides.

MATERIALS AND METHODS

Significant published studies were identified for the years 2000-2018 using Elsevier-Science Direct, Pubmed and Google scholar and several research studies in our laboratory. Following keywords such as "plant extracts", "in vitro cultures", "callus and suspension culture", "micropropagation", "hairy root cultures" were used. Further, "Withania somnifera", "secondary metabolites specially withanolides", "molecular techniques" and "in vitro conservation" were used to cross-reference the keywords.

RESULTS

Ashwagandha comprises a broad spectrum of phytochemicals with a wide range of pharmacological properties. W. somnifera seeds have reduced viability and germination rates; thus, its regular cultivation method fails to achieve commercial demands mainly for the production of desired phytoconstituents. Cultivation of plant cells/tissues under in vitro conditions and development of various biotechnological strategies will help to build an attractive alternative to provide adequate quality and quantity raw materials. Recently, a large number of in vitro protocols has developed for W. somnifera not only for its propagation but for the production of secondary metabolites as well. Present work highlights a variety of biotechnological strategies both for prompt propagation and production of different bioactive secondary metabolites.

CONCLUSION

The present review focuses on the development and opportunities in various biotechnological approaches to accomplish the global demand of W. somnifera and its secondary metabolites. This review underlines the advances in plant biotechnological approaches for the propagation of W. somnifera and production of its bioactive compounds.

Analysis of plant secondary metabolism using stable isotope-labelled precursors

INTRODUCTION

Analysis of biochemical pathways typically involves feeding a labelled precursor to an organism, and then monitoring the metabolic fate of the label. Initial studies used radioisotopes as a label and then monitored radioactivity in the metabolic products. As analytical equipment improved and became more widely available, preference shifted the use stable 'heavy' isotopes like deuterium (² H)-, carbon-13 (¹³ C)- and nitrogen-15 (¹⁵ N)-atoms as labels. Incorporation of the labels could be monitored by mass spectrometry (MS), as part of a hyphenated tool kits, e.g. Liquid chromatography (LC)-MS, gas chromatography (GC)-MS, LC-MS/MS. MS offers great sensitivity but the exact location of an isotope label in a given metabolite cannot always be unambiguously established. Nuclear magnetic resonance (NMR) can also be used to pick up signals of stable isotopes, and can give information on the precise location of incorporated label in the metabolites. However, the detection limit for NMR is quite a bit higher than that for MS.

OBJECTIVES

A number of experiments involving feeding stable isotope-labelled precursors followed by NMR analysis of the metabolites is presented. The aim is to highlight the use of NMR analysis in identifying the precise fate of isotope labels after precursor feeding experiments. As more powerful NMR equipment becomes available, applications as described in this review may become more commonplace in pathway analysis.

CONCLUSION AND PROSPECTS

NMR is a widely accepted tool for chemical structure elucidation and is now increasingly used in metabolomic studies. In addition, NMR, combined with stable isotope feeding, should be considered as a tool for metabolic flux analyses.

Effects of moderate high temperature and UV-B on accumulation of withanolides and relative expression of the squalene synthase gene in Physalis peruviana

Physalis peruviana L. (Cape gooseberry) is a source for a variety of phytocompounds such as withanolides, withanone, withaferin A, and withanolide A. These withanolides are high-value drug candidates due to their various pharmacological properties. To meet the increasing demands for these compounds, plant cell technology offers a reliable alternative. Exogenous addition of elicitors is considered the most effective strategy for enhanced production of secondary metabolites. In this study, we investigated changes in withanolide accumulation and characterized the gene expression level changes of squalene synthase enzyme in P. peruviana shoot cultures exposed to mild nonlethal heat stress (45°C for 2 and 5 h) and UV-B radiation (313 nm for 15 min and 3 h). We demonstrated significant changes in withanolide content with 7.86- and 12.5-fold increases for 2- and 5-hmild high-temperature exposure times, respectively. Exposure to UV-B also changed the withanolide content by 7.22- and 7-fold increases for 15 min and 3 h exposure times, respectively. The relative expression level of squalene synthase gene showed consistent results with1.80- and 10.13-fold increases in withanolide for 2- and 5-h mild high-temperature exposure times, and 1.34- and 2.01-fold increases with 15 min and 3 h UV-B exposure times, respectively.

Brassinosteroid Regulates 3-Hydroxy-3-methylglutaryl CoA Reductase to Promote Grape Fruit Development

Brassinosteroids (BRs) are known to regulate plant growth and development. However, only little is known about their mechanism in the regulation of berry development in grapes. This study demonstrates that BR treatment enhances the accumulation of fruit sugar components, reduces the content of organic acids (e.g., tartaric acid), promotes coloration, and increases the anthocyanin content in grape berries at the onset of the veraison, half veraison, and full veraison stages at the rate of 0.0998, 0.0560, and 0.0281 mg·g^-1_, respectively. In addition, BR treatment was also found to accelerate the biosynthesis of terpenoid aroma components, such as α-pinene, d-limonene, and γ-terpinene, which influence the aromatic composition of grapes. BRs can negatively regulate the expression of VvHMGR, a key gene involved in the mevalonate (MVA) pathway, and reduce the activity of 3-hydroxy-3-methylglutaryl CoA reductase (HMGR). Inhibiting the expression of HMGR promoted the accumulation of anthocyanins and fruit coloration. Meanwhile, after the inhibition, the contents of auxin indole-3-acetic acid (IAA), abscisic acid (ABA), and brassinosteroid (BR) increased, while gibberellin (GA₃) and zeatin riboside (ZR) decreased, and its aromatic composition also changed. Therefore, it may be concluded that BRs inhibited HMGR activity and cooperated with VvHMGR to regulate the formation of color, aroma, and other quality characteristics in fruits.

The 3-hydroxy-3-methylglutaryl-coenzyme A reductase 5 gene from Malus domestica enhances oxidative stress tolerance in Arabidopsis thaliana

3-Hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR) is the first rate-limiting enzyme regulating the synthesis of terpenoids upstream of the mevalonate (MVA) pathway. In higher plants, members of the HMGR genes families play an important role in plant growth and development and in response to various environmental stresses. In the present study, a novel HMGR gene, designated MdHMGR5, was isolated from apple (Malus domestica L.) and characterized. Expression of MdHMGR5 enhanced the activity of HMGR enzyme in transgenic Arabidopsis thaliana L. plants. Under oxidative stress, transgenic A. thaliana plants over-expressing MdHMGR5 had a higher germination rate, a longer main root length, higher chlorophyll and proline content, and higher activities of antioxidant enzymes. On the other hand, malondialdehyde (MDA) content, relative conductivity and reactive oxygen species (ROS) production rate were significantly lower than in wild type plants. These results indicated that over-expression of MdHMGR5 enhanced plant tolerance to oxidative stress by scavenging ROS in transgenic plants. Over-expression of MdHMGR5 also affected the expression levels of genes in mevalonic acid and 2C-methyl-D-erythritol 4-phosphate (MVA and MEP) pathways of A. thaliana plants. These results indicate that over-expression of MdHMGR5 enhances tolerance to oxidative stress by maintaining photosynthesis and scavenging ROS in transgenic A. thaliana plants.

Interspecies comparative features of trichomes in Ocimum reveal insights for biosynthesis of specialized essential oil metabolites

Ocimum species commonly referred to as "Tulsi" are well-known for their distinct medicinal and aromatic properties. The characteristic aroma of Ocimum species and cultivars is attributed to their specific combination of volatile phytochemicals mainly belonging to terpenoid and/or phenylpropanoid classes in their essential oils. The essential oil constituents are synthesized and sequestered in specialized epidermal secretory structures called as glandular trichomes. In this comparative study, inter- and intra-species diversity in structural attributes and profiles of expression of selected genes related to terpenoid and phenylpropanoid biosynthetic pathways have been investigated. This is performed to seek relationship of variations in the yield and phytochemical composition of the essential oils. Microscopic analysis of trichomes of O. basilicum, O. gratissimum, O. kilimandscharicum, and O. tenuiflorum (green and purple cultivars) revealed substantial variations in density, size, and relative proportions of peltate and capitate trichomes among them. The essential oil yield has been observed to be controlled by the population, dominance, and size of peltate and capitate glandular trichomes. The essential oil sequestration in leaf is controlled by the dominance of peltate glandular trichome size over its number and is also affected by the capitate glandular trichome size/number with variations in leaf area albeit at lower proportions. Comprehension and comparison of results of GC-MS analysis of essential oils showed that most of the Ocimum (O. basilicum, O. tenuiflorum, and O. gratissimum) species produce phenylpropanoids (eugenol, methyl chavicol) as major volatiles except O. kilimandscharicum, which is discrete in being monoterpenoid-rich species. Among the phenylpropanoid-enriched Ocimum (O. basilicum, O. gratissimum, O. tenuiflorum purple, O. tenuiflorum green) as well, terpenoids were important constituents in imparting characteristic aroma. Further, comparative abundance of transcripts of key genes of phenylpropanoid (PAL, C4H, 4CL, CAD, COMT, and ES) and terpenoid (DXS and HMGR) biosynthetic pathways was evaluated vis-à-vis volatile oil constituents. Transcript abundance demonstrated that richness of their essential oils with specific constituent(s) of a chemical group/subgroup was manifested by the predominant upregulation of phenylpropanoid/terpenoid pathway genes. The study provides trichomes as well as biosynthetic pathway-based knowledge for genetic improvement in Ocimum species for essential oil yield and quality.

From Central to Specialized Metabolism: An Overview of Some Secondary Compounds Derived From the Primary Metabolism for Their Role in Conferring Nutritional and Organoleptic Characteristics to Fruit

Fruit flavor and nutritional characteristics are key quality traits and ones of the main factors influencing consumer preference. Central carbon metabolism, also known as primary metabolism, contributes to the synthesis of intermediate compounds that act as precursors for plant secondary metabolism. Specific and specialized metabolic pathways that evolved from primary metabolism play a key role in the plant's interaction with its environment. In particular, secondary metabolites present in the fruit serve to increase its attractiveness to seed dispersers and to protect it against biotic and abiotic stresses. As a consequence, several important organoleptic characteristics, such as aroma, color, and fruit nutritional value, rely upon secondary metabolite content. Phenolic and terpenoid compounds are large and diverse classes of secondary metabolites that contribute to fruit quality and have their origin in primary metabolic pathways, while the delicate aroma of ripe fruits is formed by a unique combination of hundreds of volatiles that are derived from primary metabolites. In this review, we show that the manipulation of primary metabolism is a powerful tool to engineer quality traits in fruits, such as the phenolic, terpenoid, and volatile content. The enzymatic reactions responsible for the accumulation of primary precursors are bottlenecks in the transfer of metabolic flux from central to specialized metabolism and should be taken into account to increase the yield of the final products of the biosynthetic pathways. In addition, understanding the connection and regulation of the carbon flow between primary and secondary metabolism is a key factor for the development of fruit cultivars with enhanced organoleptic and nutritional traits.

Tracing the biosynthetic origin of limonoids and their functional groups through stable isotope labeling and inhibition in neem tree (Azadirachta indica) cell suspension

BACKGROUND

Neem tree serves as a cornucopia for triterpenoids called limonoids that are of profound interest to humans due to their diverse biological activities. However, the biosynthetic pathway that plant employs for the production of limonoids remains unexplored for this wonder tree.

RESULTS

Herein, we report the tracing of limonoid biosynthetic pathway through feeding experiments using ¹³C isotopologues of glucose in neem cell suspension. Growth and development specific limonoid spectrum of neem seedling and time dependent limonoid biosynthetic characteristics of cell lines were established. Further to understand the role of mevalonic acid (MVA) and methylerythritol phosphate (MEP) pathways in limonoid biosynthesis, Ultra Performance Liquid Chromatography (UPLC)- tandem mass spectrometry based structure-fragment relationship developed for limonoids and their isotopologues have been utilized. Analyses of labeled limonoid extract lead to the identification of signature isoprenoid units involved in azadirachtin and other limonoid biosynthesis, which are found to be formed through mevalonate pathway. This was further confirmed by treatment of cell suspension with mevinolin, a specific inhibitor for MVA pathway, which resulted in drastic decrease in limonoid levels whereas their biosynthesis was unaffected with fosmidomycin mediated plastidial methylerythritol 4-phosphate (MEP) pathway inhibition. This was also conspicuous, as the expression level of genes encoding for the rate-limiting enzyme of MVA pathway, 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase (HMGR) was comparatively higher to that of deoxyxylulose-phosphate synthase (DXS) of MEP pathway in different tissues and also in the in vitro grown cells. Thus, this study will give a comprehensive understanding of limonoid biosynthetic pathway with differential contribution of MVA and MEP pathways.

CONCLUSIONS

Limonoid biosynthesis of neem tree and cell lines have been unraveled through comparative quantification of limonoids with that of neem tree and through ¹³C limonoid isotopologues analysis. The undifferentiated cell lines of neem suspension produced a spectrum of C-seco limonoids, similar to parental tissue, kernel. Azadirachtin, a C-seco limonoid is produced in young tender leaves of plant whereas in the hard mature leaves of tree, ring intact limonoid nimocinol accumulates in high level. Furthermore, mevalonate pathway exclusively contributes for isoprene units of limonoids as evidenced through stable isotope labeling and no complementation of MEP pathway was observed with mevalonate pathway dysfunction, using chemical inhibitors.

Metabolites Identification of Bioactive Compounds Daturataturin A, Daturametelin I, N-Trans-Feruloyltyramine, and Cannabisin F From the Seeds of Datura metel in Rats

Datura metel L. is a widely used traditional herbal medicine, and withanolides and amides are the two groups of main bioactive constituents in Datura metel seeds. This study aimed to elucidate the metabolism of four representative bioactive compositions containing daturataturin A (1), daturametelin I (2), N-trans-feruloyltyramine (3), and cannabisin F (4) in rats. After separately oral administration of 20 mg/kg withanolides (1, 2) and amides (3, 4) to rats, a total of 12, 24, and 21 metabolites were detected in the plasma, urine, and fecal samples, respectively. Among them, three hydroxylated metabolites, 1-M3, 2-M2, and 3-M5, were detected in plasma and rat liver microsome incubation system in high abundance. Two metabolites of 1 and 2 were unambiguously identified by comparing with reference standards. Particularly, the methylated metabolite 27α-methoxy-(22R)-22,26-epoxy-27-[(β-D-glucopyranosyl)oxy]ergosta-2,4,6,24-tetraene-1,26-dione (daturametelin L) is a new compound. The withanolides could readily get hydroxylation or methylation metabolism. Meanwhile, the phase II metabolism (glucuronidation or sulfation) was the major reaction for the amides. This is the first study on in vivo metabolism of these active compounds in seeds of Datura metel.

Overexpression of LiDXS and LiDXR From Lily (Lilium 'Siberia') Enhances the Terpenoid Content in Tobacco Flowers

Lilium, the famous and significant cut flower, emits a variety of volatile organic compounds, which mainly contain monoterpenes, such as myrcene, (E)-β-ocimene, and linalool. To understand the molecular mechanism of monoterpene synthesis in Lilium, we cloned two potential genes in the methylerythritol 4-phosphate pathway, namely LiDXS and LiDXR, from the strong-flavored oriental Lilium 'Siberia' using a homology-based PCR strategy. The expression levels of LiDXS and LiDXR were consistent with the emission and accumulation of monoterpenes in different floral organs and during the floral development, indicating that these two genes may play key roles in monoterpene synthesis. Subcellular localization demonstrated that LiDXS and LiDXR are expressed in the chloroplasts. Ectopic expression in transgenic tobacco suggested that the flowers of LiDXS and LiDXR transgenic lines accumulated substantially more diterpene, sclareol, compared to the plants transformed with empty vector. Surprisingly, increased content of the monoterpene, linalool and sesquiterpene, caryophyllene, were detected in the LiDXR transgenic lines, whereas the emission of caryophyllene, increased in one of the LiDXS transgenic tobacco lines, indicating that these two genes play significant roles in the synthesis of floral volatiles in the transgenic plants. These results demonstrate that LiDXR can contribute to monoterpene biosynthesis in Lilium 'Siberia'; however, the role of LiDXS in the biosynthesis of monoterpenes needs further study.

HMG-CoA reductase from Camphor Tulsi (Ocimum kilimandscharicum) regulated MVA dependent biosynthesis of diverse terpenoids in homologous and heterologous plant systems

Ocimum kilimandscharicum is unique in possessing terpenoids whereas other Ocimum species are renowned for phenylpropanoids as major constituents of essential oil. The key enzyme of MVA/terpenoid metabolic pathway viz 3-hydroxy-3-methylglutaryl Co-A reductase (OkHMGR) of 1.7-Kb ORF encoding ~60-kDa protein was cloned from O. kilimandscharicum and its kinetic characteristics revealed the availability of HMG-CoA as a control point of MVA-pathway. Transcript profiling of the OkHMGR elucidated tissue-specific functions of the gene in flower and leaf tissues in accumulation of terpenoidal essential oil. OkHMGR was differentially regulated in response to exposure to methyl-jasmonate, salicylic-acid, and stress conditions such-as salt and temperature stress, demonstrating its key role in managing signaling and stress-responses. To elucidate its functional role, OkHMGR was transiently over-expressed in homologous and heterologous plants such as O. sanctum, O. basilicum, O. gratissimum, Withania somnifera and Artemisia annua. The over-expression and inhibition dual strategy revealed that the additional OkHMGR in-planta could afford endogenous flow of isoprenoid units towards synthesis of terpenoids. The present study provides in-depth insight of OkHMGR in regulation of biosynthesis of non-plastidal isoprenoids. This is first report on any gene of MVA/isoprenoid pathway from under-explored Camphor Tulsi belonging to genus Ocimum. Studies also suggested that OkHMGR could be a potential tool for attempting metabolic engineering for enhancing medicinally important terpenoidal metabolites in plants.

Comparative transcriptomics uncovers differences in photoautotrophic versus photoheterotrophic modes of nutrition in relation to secondary metabolites biosynthesis in Swertia chirayita

Swertia chirayita is a high-value medicinal herb exhibiting antidiabetic, hepatoprotective, anticancer, antiediematogenic and antipyretic properties. Scarcity of its plant material has necessitated in vitro production of therapeutic metabolites; however, their yields were low compared to field grown plants. Possible reasons for this could be differences in physiological and biochemical processes between plants grown in photoautotrophic versus photoheterotrophic modes of nutrition. Comparative transcriptomes of S. chirayita were generated to decipher the crucial molecular components associated with the secondary metabolites biosynthesis. Illumina HiSeq sequencing yielded 57,460 and 43,702 transcripts for green house grown (SCFG) and tissue cultured (SCTC) plants, respectively. Biological role analysis (GO and COG assignments) revealed major differences in SCFG and SCTC transcriptomes. KEGG orthology mapped 351 and 341 transcripts onto secondary metabolites biosynthesis pathways for SCFG and SCTC transcriptomes, respectively. Nineteen out of 30 genes from primary metabolism showed higher in silico expression (FPKM) in SCFG versus SCTC, possibly indicating their involvement in regulating the central carbon pool. In silico data were validated by RT-qPCR using a set of 16 genes, wherein 10 genes showed similar expression pattern across both the methods. Comparative transcriptomes identified differentially expressed transcription factors and ABC-type transporters putatively associated with secondary metabolism in S. chirayita. Additionally, functional classification was performed using NCBI Biosystems database. This study identified the molecular components implicated in differential modes of nutrition (photoautotrophic vs. photoheterotrophic) in relation to secondary metabolites production in S. chirayita.

Induction of Terpene Biosynthesis in Berries of Microvine Transformed with VvDXS1 Alleles

Terpenoids, especially monoterpenes, are major aroma-impact compounds in grape and wine. Previous studies highlighted a key regulatory role for grapevine 1-deoxy-D-xylulose 5-phosphate synthase 1 (VvDXS1), the first enzyme of the methylerythritol phosphate pathway for isoprenoid precursor biosynthesis. Here, the parallel analysis of VvDXS1 genotype and terpene concentration in a germplasm collection demonstrated that VvDXS1 sequence has a very high predictive value for the accumulation of monoterpenes and also has an influence on sesquiterpene levels. A metabolic engineering approach was applied by expressing distinct VvDXS1 alleles in the grapevine model system "microvine" and assessing the effects on downstream pathways at transcriptional and metabolic level in different organs and fruit developmental stages. The underlying goal was to investigate two potential perturbation mechanisms, the former based on a significant over-expression of the wild-type (neutral) VvDXS1 allele and the latter on the ex-novo expression of an enzyme with increased catalytic efficiency from the mutated (muscat) VvDXS1 allele. The integration of the two VvDXS1 alleles in distinct microvine lines was found to alter the expression of several terpenoid biosynthetic genes, as assayed through an ad hoc developed TaqMan array based on cDNA libraries of four aromatic cultivars. In particular, enhanced transcription of monoterpene, sesquiterpene and carotenoid pathway genes was observed. The accumulation of monoterpenes in ripe berries was higher in the transformed microvines compared to control plants. This effect is predominantly attributed to the improved activity of the VvDXS1 enzyme coded by the muscat allele, whereas the up-regulation of VvDXS1 plays a secondary role in the increase of monoterpenes.

Transcription factor repertoire in Ashwagandha (Withania somnifera) through analytics of transcriptomic resources: Insights into regulation of development and withanolide metabolism

Transcription factors (TFs) are important regulators of cellular and metabolic functions including secondary metabolism. Deep and intensive RNA-seq analysis of Withania somnifera using transcriptomic databases provided 3532 annotated transcripts of transcription factors in leaf and root tissues, belonging to 90 different families with major abundance for WD-repeat (174 and 165 transcripts) and WRKY (93 and 80 transcripts) in root and leaf tissues respectively, followed by that of MYB, BHLH and AP2-ERF. Their detailed comparative analysis with Arabidopsis thaliana, Capsicum annum, Nicotiana tabacum and Solanum lycopersicum counterparts together gave interesting patterns. However, no homologs for WsWDR representatives, LWD1 and WUSCHEL, were observed in other Solanaceae species. The data extracted from the sequence read archives (SRA) in public domain databases were subjected to re-annotation, re-mining, re-analysis and validation for dominant occurrence of WRKY and WD-repeat (WDR) gene families. Expression of recombinant LWD1 and WUSCHEL proteins in homologous system led to enhancements in withanolide content indicating their regulatory role in planta in the biosynthesis. Contrasting expression profiles of WsLWD1 and WsWUSCHEL provided tissue-specific insights for their participation in the regulation of developmental processes. The in-depth analysis provided first full-spectrum and comparative characteristics of TF-transcripts across plant species, in the perspective of integrated tissue-specific regulation of metabolic processes including specialized metabolism.

Over-expression of DXS gene enhances terpenoidal secondary metabolite accumulation in rose-scented geranium and Withania somnifera: active involvement of plastid isoprenogenic pathway in their biosynthesis

Rose-scented geranium (Pelargonium spp.) is one of the most important aromatic plants and is well known for its diverse perfumery uses. Its economic importance is due to presence of fragrance rich essential oil in its foliage. The essential oil is a mixture of various volatile phytochemicals which are mainly terpenes (isoprenoids) in nature. In this study, on the geranium foliage genes related to isoprenoid biosynthesis (DXS, DXR and HMGR) were isolated, cloned and confirmed by sequencing. Further, the first gene of 2-C-methyl-d-erythritol-4-phosphate (MEP) pathway, 1-deoxy-d-xylulose-5-phosphate synthase (GrDXS), was made full length by using rapid amplification of cDNA ends strategy. GrDXS contained a 2157 bp open reading frame that encoded a polypeptide of 792 amino acids having calculated molecular weight 77.5 kDa. This study is first report on heterologous expression and kinetic characterization of any gene from this economically important plant. Expression analysis of these genes was performed in different tissues as well as at different developmental stages of leaves. In response to external elicitors, such as methyl jasmonate, salicylic acid, light and wounding, all the three genes showed differential expression profiles. Further GrDXS was over expressed in the homologous (rose-scented geranium) as well as in heterologous (Withania somnifera) plant systems through genetic transformation approach. The over-expression of GrDXS led to enhanced secondary metabolites production (i.e. essential oil in rose-scented geranium and withanolides in W. somnifera). To the best of our knowledge, this is the first report showing the expression profile of the three genes related to isoprenoid biosynthesis pathways operated in rose-scented geranium as well as functional characterization study of any gene from rose-scented geranium through a genetic transformation system.

Addressing Challenges to Enhance the Bioactives of Withania somnifera through Organ, Tissue, and Cell Culture Based Approaches

Withania somnifera is a highly valued medicinal plant in traditional home medicine and is known for a wide range of bioactivities. Its commercial cultivation is adversely affected by poor seed viability and germination. Infestation by various pests and pathogens, survival under unfavourable environmental conditions, narrow genetic base, and meager information regarding biosynthesis of secondary metabolites are some of the other existing challenges in the crop. Biotechnological interventions through organ, tissue, and cell culture provide promising options for addressing some of these issues. In vitro propagation facilitates conservation and sustainable utilization of the existing germplasms and broadening the genetic base. It would also provide means for efficient and rapid mass propagation of elite chemotypes and generating uniform plant material round the year for experimentation and industrial applications. The potential of in vitro cell/organ cultures for the production of therapeutically valuable compounds and their large-scale production in bioreactors has received significant attention in recent years. In vitro culture system further provides distinct advantage for studying various cellular and molecular processes leading to secondary metabolite accumulation and their regulation. Engineering plants through genetic transformation and development of hairy root culture system are powerful strategies for modulation of secondary metabolites. The present review highlights the developments and sketches current scenario in this field.

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.

Withania coagulans tryptophan decarboxylase gene cloning, heterologous expression, and catalytic characteristics of the recombinant enzyme

Tryptophan decarboxylase (EC 4.1.1.28) catalyzes pyridoxal 5'-phosphate (PLP)-dependent decarboxylation of tryptophan to produce tryptamine for recruitment in a myriad of biosynthetic pathways of metabolites possessing indolyl moiety. A recent report of certain indolyl metabolites in Withania species calls for a possible predominant functional role of tryptophan decarboxylase (TDC) in the genome of Withania species to facilitate production of the indolyl progenitor molecule, tryptamine. Therefore, with this metabolic prospection, we have identified and cloned a full-length cDNA sequence of TDC from aerial tissues of Withania coagulans. The functional WcTDC gene comprises of 1506 bp open reading frame (ORF) encoding a 502 amino acid protein with calculated molecular mass and pI value of 56.38 kDa and 8.35, respectively. The gene was expressed in Escherichia coli, and the recombinant enzyme was affinity-purified to homogeneity to discern its kinetics of catalysis. The enzyme (WcTDC) exhibited much higher K_m value for tryptophan than for pyridoxal 5'-phosphate and was dedicated to catalyze decarboxylation of only tryptophan or, to a limited extent, of its analogue (like 5-hydroxy tryptophan). The observed optimal catalytic functionality of the enzyme on the slightly basic side of the pH scale and at slightly higher temperatures reflected adaptability of the plant to hot and arid regions, the predominant natural habitat of the herb. This pertains to be the first report on cloning and characterization of heterologously expressed recombinant enzyme from W. coagulans and forms a starting point to further understanding of withanamide biosynthesis.

Prolonged exposure to salt stress affects specialized metabolites-artemisinin and essential oil accumulation in Artemisia annua L.: metabolic acclimation in preferential favour of enhanced terpenoid accumulation accompanying vegetative to reproductive phase transition

Artemisia annua accumulates substantial quantities of unique and highly useful antimalarial sesquiternoid artemisinin and related phytomolecules as well as its characteristic essential oil in its glandular trichomes. The phytomolecules are mainly produced in its leaves and inflorescences. Artemisia annua plants were grown under NaCl salinity (50, 100 and 200 mM) stress conditions imposed throughout the entire life cycle of the plant. Results revealed that specialized metabolites like artemisinin, arteannuin-B, artemisinic acid + dihydroartemisinic acid and essential oil accumulation were positively modulated by NaCl salinity stress. Interestingly, total content of monoterpenoids and sesquiterpenoids of essential oil was induced by NaCl salinity treatment, contrary to previous observations. Production of camphor, the major essential oil constituent was induced under the influence of treatment. The metabolic acclimation and manifestations specific to terpenoid pathway are analysed vis-a-vis vegetative to reproductive periods and control of the modulation. WRKY and CYP71AV1 play a key role in mediating the responses through metabolism in glandular trichomes. The distinctness of the salinity induced responses is discussed in light of differential mechanism of adaptation to abiotic stresses and their impact on terpenoid-specific metabolic adjustments in A. annua. Results provide potential indications of possible adaptation of A. annua under saline conditions for agrarian techno-economic benefaction.

Plant secondary metabolism linked glycosyltransferases: An update on expanding knowledge and scopes

The multigene family of enzymes known as glycosyltransferases or popularly known as GTs catalyze the addition of carbohydrate moiety to a variety of synthetic as well as natural compounds. Glycosylation of plant secondary metabolites is an emerging area of research in drug designing and development. The unsurpassing complexity and diversity among natural products arising due to glycosylation type of alterations including glycodiversification and glycorandomization are emerging as the promising approaches in pharmacological studies. While, some GTs with broad spectrum of substrate specificity are promising candidates for glycoengineering while others with stringent specificity pose limitations in accepting molecules and performing catalysis. With the rising trends in diseases and the efficacy/potential of natural products in their treatment, glycosylation of plant secondary metabolites constitutes a key mechanism in biogeneration of their glycoconjugates possessing medicinal properties. The present review highlights the role of glycosyltransferases in plant secondary metabolism with an overview of their identification strategies, catalytic mechanism and structural studies on plant GTs. Furthermore, the article discusses the biotechnological and biomedical application of GTs ranging from detoxification of xenobiotics and hormone homeostasis to the synthesis of glycoconjugates and crop engineering. The future directions in glycosyltransferase research should focus on the synthesis of bioactive glycoconjugates via metabolic engineering and manipulation of enzyme's active site leading to improved/desirable catalytic properties. The multiple advantages of glycosylation in plant secondary metabolomics highlight the increasing significance of the GTs, and in near future, the enzyme superfamily may serve as promising path for progress in expanding drug targets for pharmacophore discovery and development.

A systematic approach to expound the variations in taxane production under different dissolved oxygen conditions in Taxus chinensis cells

Our results provide an evidence that the changes in taxane production caused by dissolved oxygen shifts could be associated with the global variations in the cell central carbon metabolism. Taxol is an important taxane synthesized by the Taxus plant. A two-stage culture of Taxus in vitro has been considered as an attractive alternative approach to produce Taxol and its precursors. To investigate the consequences of dissolved oxygen (DO) shifts for cell primary and secondary metabolism, we conducted metabolomic and transcriptomic profiling analyses under low dissolved oxygen (LDO), medium dissolved oxygen (MDO), and high dissolved oxygen (HDO) conditions in a suspension culture of Taxus chinensis cells. Under LDO, the results indicate a significant increase in the production of Taxol and its main precursors by 3.4- to 1.4-fold compared with those under MDO and HDO on 9th day. Multiple acyl taxanes (MAT) are abundant taxanes in the cells, and exhibited only a slight increase under the same conditions. Metabolomic analysis based on 209 primary metabolites indicated that several pathways in central carbon metabolism were involved, including the enhancement of the glycolysis pathway of glucose-6-phosphate to fructose-6-phosphate and pyruvate and the mevalonate pathway of terpene biosynthesis, and decline in the tricarboxylic acid pathway under LDO. These results indicate the mechanism by which related taxanes accumulate through enhancing the supplies of substrates and expression levels of hydroxylases. Excess acetyl-CoA supply induced by high oxygen stress was found to be correlated with high productivity of MAT. Our results provide an evidence that the changes in taxane production caused by DO shifts could be associated with the global variations in the cell central carbon metabolism.

RNAi and Homologous Over-Expression Based Functional Approaches Reveal Triterpenoid Synthase Gene-Cycloartenol Synthase Is Involved in Downstream Withanolide Biosynthesis in Withania somnifera

Withania somnifera Dunal, is one of the most commonly used medicinal plant in Ayurvedic and indigenous medicine traditionally owing to its therapeutic potential, because of major chemical constituents, withanolides. Withanolide biosynthesis requires the activities of several enzymes in vivo. Cycloartenol synthase (CAS) is an important enzyme in the withanolide biosynthetic pathway, catalyzing cyclization of 2, 3 oxidosqualene into cycloartenol. In the present study, we have cloned full-length WsCAS from Withania somnifera by homology-based PCR method. For gene function investigation, we constructed three RNAi gene-silencing constructs in backbone of RNAi vector pGSA and a full-length over-expression construct. These constructs were transformed in Agrobacterium strain GV3101 for plant transformation in W. somnifera. Molecular and metabolite analysis was performed in putative Withania transformants. The PCR and Southern blot results showed the genomic integration of these RNAi and overexpression construct(s) in Withania genome. The qRT-PCR analysis showed that the expression of WsCAS gene was considerably downregulated in stable transgenic silenced Withania lines compared with the non-transformed control and HPLC analysis showed that withanolide content was greatly reduced in silenced lines. Transgenic plants over expressing CAS gene displayed enhanced level of CAS transcript and withanolide content compared to non-transformed controls. This work is the first full proof report of functional validation of any metabolic pathway gene in W. somnifera at whole plant level as per our knowledge and it will be further useful to understand the regulatory role of different genes involved in the biosynthesis of withanolides.

Comparative transcriptome analysis of different chemotypes elucidates withanolide biosynthesis pathway from medicinal plant Withania somnifera

Withania somnifera is one of the most valuable medicinal plants synthesizing secondary metabolites known as withanolides. Despite pharmaceutical importance, limited information is available about the biosynthesis of withanolides. Chemo-profiling of leaf and root tissues of Withania suggest differences in the content and/or nature of withanolides in different chemotypes. To identify genes involved in chemotype and/or tissue-specific withanolide biosynthesis, we established transcriptomes of leaf and root tissues of distinct chemotypes. Genes encoding enzymes for intermediate steps of terpenoid backbone biosynthesis with their alternatively spliced forms and paralogous have been identified. Analysis suggests differential expression of large number genes among leaf and root tissues of different chemotypes. Study also identified differentially expressing transcripts encoding cytochrome P450s, glycosyltransferases, methyltransferases and transcription factors which might be involved in chemodiversity in Withania. Virus induced gene silencing of the sterol ∆7-reductase (WsDWF5) involved in the synthesis of 24-methylene cholesterol, withanolide backbone, suggests role of this enzyme in biosynthesis of withanolides. Information generated, in this study, provides a rich resource for functional analysis of withanolide-specific genes to elucidate chemotype- as well as tissue-specific withanolide biosynthesis. This genomic resource will also help in development of new tools for functional genomics and breeding in Withania.

Virus-induced gene silencing of Withania somnifera squalene synthase negatively regulates sterol and defence-related genes resulting in reduced withanolides and biotic stress tolerance

Withania somnifera (L.) Dunal is an important Indian medicinal plant that produces withanolides, which are triterpenoid steroidal lactones having diverse biological activities. To enable fast and efficient functional characterization of genes in this slow-growing and difficult-to-transform plant, a virus-induced gene silencing (VIGS) was established by silencing phytoene desaturase (PDS) and squalene synthase (SQS). VIGS of the gene encoding SQS, which provides precursors for triterpenoids, resulted in significant reduction of squalene and withanolides, demonstrating its application in studying withanolides biosynthesis in W. somnifera leaves. A comprehensive analysis of gene expression and sterol pathway intermediates in WsSQS-vigs plants revealed transcriptional modulation with positive feedback regulation of mevalonate pathway genes, and negative feed-forward regulation of downstream sterol pathway genes including DWF1 (delta-24-sterol reductase) and CYP710A1 (C-22-sterol desaturase), resulting in significant reduction of sitosterol, campesterol and stigmasterol. However, there was little effect of SQS silencing on cholesterol, indicating the contribution of sitosterol, campesterol and stigmasterol, but not of cholesterol, towards withanolides formation. Branch-point oxidosqualene synthases in WsSQS-vigs plants exhibited differential regulation with reduced CAS (cycloartenol synthase) and cycloartenol, and induced BAS (β-amyrin synthase) and β-amyrin. Moreover, SQS silencing also led to the down-regulation of brassinosteroid-6-oxidase-2 (BR6OX2), pathogenesis-related (PR) and nonexpressor of PR (NPR) genes, resulting in reduced tolerance to bacterial and fungal infection as well as to insect feeding. Taken together, SQS silencing negatively regulated sterol and defence-related genes leading to reduced phytosterols, withanolides and biotic stress tolerance, thus implicating the application of VIGS for functional analysis of genes related to withanolides formation in W. somnifera leaves.

A Decade of Molecular Understanding of Withanolide Biosynthesis and In vitro Studies in Withania somnifera (L.) Dunal: Prospects and Perspectives for Pathway 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.

Light and auxin responsive cytochrome P450s from Withania somnifera Dunal: cloning, expression and molecular modelling of two pairs of homologue genes with differential regulation

Cytochrome P450s (CYPs) catalyse a wide variety of oxygenation/hydroxylation reactions that facilitate diverse metabolic functions in plants. Specific CYP families are essential for the biosynthesis of species-specialized metabolites. Therefore, we investigated the role of different CYPs related to secondary metabolism in Withania somnifera, a medicinally important plant of the Indian subcontinent. In this study, complete complementary DNAs (cDNAs) of four different CYP genes were isolated and christened as WSCYP93Id, WSCYP93Sm, WSCYP734B and WSCYP734R. These cDNAs encoded polypeptides comprising of 498, 496, 522 and 550 amino acid residues with their deduced molecular mass of 56.7, 56.9, 59.4 and 62.2 kDa, respectively. Phylogenetic study and molecular modelling analysis of the four cloned WSCYPs revealed their categorization into two CYP families (CYP83B1 and CYP734A1) belonging to CYP71 and CYP72 clans, respectively. BLASTp searches showed similarity of 75 and 56 %, respectively, between the two CYP members of CYP83B1 and CYP734A1 with major variances exhibited in their N-terminal regions. The two pairs of homologues exhibited differential expression profiles in the leaf tissues of selected chemotypes of W. somnifera as well as in response to treatments such as methyl jasmonate, wounding, light and auxin. Light and auxin regulated two pairs of WSCYP homologues in a developing seedling in an interesting differential manner. Their lesser resemblance and homology with other CYP sequences suggested these genes to be more specialized and distinct ones. The results on chemotype-specific expression patterns of the four genes strongly suggested their key/specialized involvement of the CYPs in the biosynthesis of chemotype-specific metabolites, though their further biochemical characterization would reveal the specificity in more detail. It is revealed that WSCYP93Id and WSCYP93Sm may be broadly involved in the oxygenation reactions in the plant and, thereby, control various pathways involving such metabolic reactions in the plant. As a representative experimental validation of this notion, WSCYP93Id was heterologouly expressed in Escherichia coli and catalytic capabilities of the recombinant WSCYP93Id protein were evaluated using withanolides as substrates. Optimized assays with some major withanolides (withanone, withaferin A and withanolide A) involving spectrophotometric as well as high-pressure liquid chromatography (HPLC)-based evaluation (product detection) of the reactions showed conversion of withaferin A to a hydroxylated product. The genes belonging to other CYP group are possibly involved in some specialised synthesis such as that of brassinosteroids.

Metabolic shift from withasteroid formation to phenylpropanoid accumulation in cryptogein-cotransformed hairy roots of Withania somnifera (L.) Dunal

Cotransformed hairy roots containing a gene that encodes a fungal elicitor protein, β-cryptogein, were established in Withania somnifera, a medicinal plant widely used in Indian systems of medicine. To find out whether β-cryptogein protein endogenously elicits the pathway of withasteroid biosynthesis, withaferin A and withanolide A contents along with transcript accumulation of farnesyl pyrophosphate (FPP) synthase, 3-hydroxy-3-methyl-glutaryl-CoA reductase (HMGR), and sterol glycosyltransferase (SGT) were analyzed in both cryptogein-cotransformed and normal hairy roots of W. somnifera. It was observed that the withaferin A and withanolide A contents were drastically higher in normal hairy roots than cryptogein-cotransformed ones. Similar trends were also observed on the levels of transcript accumulation. Subsequently, the enzyme activity of phenylalanine ammonia lyase (PAL), one of the key enzymes of phenylpropanoid pathway, was measured in both cryptogein-cotransformed and normal hairy roots of W. somnifera along with the levels of PAL transcript accumulation. Upliftment of PAL activity was observed in cryptogein-cotransformed hairy roots as compared to the normal ones, and the PAL expression also reflected a similar trend, i.e., enhanced expression in the cryptogein-cotransformed lines. Upliftment of wall-bound ferulic acid accumulation was also observed in the cryptogein-cotransformed lines, as compared to normal hairy root lines. Thus, the outcome of the above studies suggests a metabolic shift from withanolide accumulation to phenylpropanoid biosynthesis in cryptogein-cotransformed hairy roots of W. somnifera.

Transcriptome analysis reveals in vitro cultured Withania somnifera leaf and root tissues as a promising source for targeted withanolide biosynthesis

Background

The production of metabolites via in vitro culture is promoted by the availability of fully defined metabolic pathways. Withanolides, the major bioactive phytochemicals of Withania somnifera, have been well studied for their pharmacological activities. However, only a few attempts have been made to identify key candidate genes involved in withanolide biosynthesis. Understanding the steps involved in withanolide biosynthesis is essential for metabolic engineering of this plant to increase withanolide production.

Results

Transcriptome sequencing was performed on in vitro adventitious root and leaf tissues using the Illumina platform. We obtained a total of 177,156 assembled transcripts with an average unigene length of 1,033 bp. About 13% of the transcripts were unique to in vitro adventitious roots but no unique transcripts were observed in in vitro-grown leaves. A putative withanolide biosynthetic pathway was deduced by mapping the assembled transcripts to the KEGG database, and the expression of candidate withanolide biosynthesis genes -were validated by qRT PCR. The accumulation pattern of withaferin A and withanolide A varied according to the type of tissue and the culture period. Further, we demonstrated that in vitro leaf extracts exhibit anticancer activity against human gastric adenocarcinoma cell lines at sub G1 phase.

Conclusions

We report here a validated large-scale transcriptome data set and the potential biological activity of in vitro cultures of W. somnifera. This study provides important information to enhance tissue-specific expression and accumulation of secondary metabolites, paving the way for industrialization of in vitro cultures of W. somnifera.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1214-0) contains supplementary material, which is available to authorized users.