Metabolic engineering of artemisinin biosynthesis in Artemisia annua L

Future antimalarials from Artemisia? A rationale for natural product mining against drug-refractory Plasmodium stages

Covering: up to 2023Infusions of the plants Artemisia annua and A. afra are gaining broad popularity to prevent or treat malaria. There is an urgent need to address this controversial public health question by providing solid scientific evidence in relation to these uses. Infusions of either species were shown to inhibit the asexual blood stages, the liver stages including the hypnozoites, but also the sexual stages, the gametocytes, of Plasmodium parasites. Elimination of hypnozoites and sterilization of mature gametocytes remain pivotal elements of the radical cure of P. vivax, and the blockage of P. vivax and P. falciparum transmission, respectively. Drugs active against these stages are restricted to the 8-aminoquinolines primaquine and tafenoquine, a paucity worsened by their double dependence on the host genetic to elicit clinical activity without severe toxicity. Besides artemisinin, these Artemisia spp. contain many natural products effective against Plasmodium asexual blood stages, but their activity against hypnozoites and gametocytes was never investigated. In the context of important therapeutic issues, we provide a review addressing (i) the role of artemisinin in the bioactivity of these Artemisia infusions against specific parasite stages, i.e., alone or in association with other phytochemicals; (ii) the mechanisms of action and biological targets in Plasmodium of ca. 60 infusion-specific Artemisia phytochemicals, with an emphasis on drug-refractory parasite stages (i.e., hypnozoites and gametocytes). Our objective is to guide the strategic prospecting of antiplasmodial natural products from these Artemisia spp., paving the way toward novel antimalarial "hit" compounds either naturally occurring or Artemisia-inspired.

Morphological characterization of intraspecific variation for trichome traits in tomato (Solanum lycopersicum)

Trichomes, the hairlike protuberances in plants, have been well known to act as the first line of defense against herbivores, and abiotic stresses, along with other structural defenses such as spines, thorns, and waxes. We previously reported the tremendous variation in trichome traits among different wild and cultivated Solanum species and demonstrated that trichomes types and density are traditionally miscalculated and often misnamed. However, intraspecific variation in trichome traits is poorly understood, although this has implications for stress tolerance and resistance breeding programs in economically important crop species and can also mediate ecological interactions at multiple trophic levels in their wild congeners. In this study, using tomato as a model, we characterized the trichomes from 10 commonly grown varieties using a minimal sample prep desktop scanning electron microscopy, and followed up with estimating their dimensions across the varieties and trichome types. We hypothesized that although trichome number may vary, the varieties will have similar trichome types, based on current literature. Our results show that there is significant variation for trichome number as well as dimensions of trichome types among these varieties. Furthermore, when we separately analyzed the number and dimensions of commonly found glandular and non-glandular trichomes, the results were consistent with broad assessment of trichomes, showing consistent variation among varieties, suggesting that trichome studies should not be limited to basic classification into glandular and non-glandular, and should accommodate the sub-types and their dimensions.

Integrated Transcriptomic and Metabolomic Analyses Reveal the Mechanisms Underlying Anthocyanin Coloration and Aroma Formation in Purple Fennel

The color and aroma are the significant traits of vegetables and fruits, but the metabolic and molecular mechanisms underlying anthocyanin accumulation and aroma formation remain almost unknown in fennel (Anethum foeniculum L.), which is a crucial vegetable crop and grown widely for aromatic leaves and bulbs. Here, ten major anthocyanins identified and quantified by ultra-high performance liquid chromatography coupled with quadrupole Orbitrap high-resolution mass spectrometry (UHPLC-Q-Orbitrap HRMS) were mainly responsible for the coloration of purple fennel leaf. With the application of GC-MS, it was found that the reduced volatile phenylpropanoids including isoeugenol, trans-isoeugenol, and apiol chiefly account for the characteristic aroma changes of the purple fennel. Moreover, the characteristic anthocyanin coloration and aroma formation in purple fennel were systematically studied with the integrated transcriptomics and metabolomics. The critical genes associated with the biosynthesis and regulation of anthocyanins and volatile phenylpropanoids were isolated and studied carefully in transiently transfected tobacco cells and transgenic tomato plants. Together with the results of UHPLC-Q-Orbitrap HRMS, RT-qPCR, and yeast two hybrid (Y2H), it is proved that the metabolic flux redirection of phenylpropanoid pathway primarily regulated by a functional MYB-bHLH-WD40 complex consisting of AfTT8, AfMYB7, and AfTTG1 accounts for the characteristic anthocyanin coloration and aroma formation in purple fennel leaf. The systematic understanding of the anthocyanin accumulation and aroma formation will assist in the improvement of fennel resource utilization and breeding.

Artemisinins in Combating Viral Infections Like SARS-CoV-2, Inflammation and Cancers and Options to Meet Increased Global Demand

Artemisinin is a natural bioactive sesquiterpene lactone containing an unusual endoperoxide 1, 2, 4-trioxane ring. It is derived from the herbal medicinal plant Artemisia annua and is best known for its use in treatment of malaria. However, recent studies also indicate the potential for artemisinin and related compounds, commonly referred to as artemisinins, in combating viral infections, inflammation and certain cancers. Moreover, the different potential modes of action of artemisinins make these compounds also potentially relevant to the challenges the world faces in the COVID-19 pandemic. Initial studies indicate positive effects of artemisinin or Artemisia spp. extracts to combat SARS-CoV-2 infection or COVID-19 related symptoms and WHO-supervised clinical studies on the potential of artemisinins to combat COVID-19 are now in progress. However, implementing multiple potential new uses of artemisinins will require effective solutions to boost production, either by enhancing synthesis in A. annua itself or through biotechnological engineering in alternative biosynthesis platforms. Because of this renewed interest in artemisinin and its derivatives, here we review its modes of action, its potential application in different diseases including COVID-19, its biosynthesis and future options to boost production.

Application of Hairy Root Culture for Bioactive Compounds Production in Medicinal Plants

Medicinal plants are rich sources of natural bioactive compounds used to treat many diseases. With the development of the health industry, the market demands for Chinese medicine have been rapidly increasing in recent years. However, over-utilization of herbal plants would cause serious ecological problems. Therefore, an effective approach should be developed to produce the pharmaceutically important natural drugs. Hairy root culture induced by Agrobacterium rhizogenes has been considered to be an effective tool to produce secondary metabolites that are originally biosynthesized in the roots or even in the aerial organs of mature plants. This review aims to summarize current progress on medicinal plant hairy root culture for bioactive compounds production. It presents the stimulating effects of various biotic and abiotic elicitors on the accumulation of secondary metabolites. Synergetic effects by combination of different elicitors or with other strategies are also included. Besides, the transgenic system has promising prospects to increase bioactive compounds content by introducing their biosynthetic or regulatory genes into medicinal plant hairy root. It offers great potential to further increase secondary metabolites yield by the integration of manipulating pathway genes with elicitors and other strategies. Then advances on two valuable pharmaceuticals production in the hairy root cultures are illustrated in detail. Finally, successful production of bioactive compounds by hairy root culture in bioreactors are introduced.

Canonical transient receptor potential channels and their modulators: biology, pharmacology and therapeutic potentials

Canonical transient receptor potential channels (TRPCs) are nonselective, high calcium permeability cationic channels. The TRPCs family includes TRPC1, TRPC2, TRPC3, TRPC4, TRPC5, TRPC6, and TRPC7. These channels are widely expressed in the cardiovascular and nervous systems and exist in many other human tissues and cell types, playing several crucial roles in the human physiological and pathological processes. Hence, the emergence of TRPCs modulators can help investigate these channels’ applications in health and disease. It is worth noting that the TRPCs subfamilies have structural and functional similarities, which presents a significant difficulty in screening and discovering of TRPCs modulators. In the past few years, only a limited number of selective modulators of TRPCs were detected; thus, additional research on more potent and more selective TRPCs modulators is needed. The present review focuses on the striking desired therapeutic effects of TRPCs modulators, which provides intel on the structural modification of TRPCs modulators and further pharmacological research. Importantly, TRPCs modulators can significantly facilitate future studies of TRPCs and TRPCs related diseases.

Enhancement of In Vitro Production of Volatile Organic Compounds by Shoot Differentiation in Artemisia spicigera

Callus initiation, shoot formation and plant regeneration were established for Artemisia spicigera, a traditional medicinal plant growing in Armenia, Middle-Anatolia and Iran, and producing valuable volatile organic compounds (VOCs) that are mostly represented by monoterpenoids. Optimal callus initiation and shoot production were obtained by culture of hypocotyl and cotyledon explants on MS medium comprising 0.5 mg L^-1 naphthalene acetic acid (NAA) and 0.5 mg L^-1 6-benzyladenine (BA). Consequently, the shoots were transferred onto the MS media supplemented with 1 mg L^-1 of indole-3-butyric acid (IBA) or 1 mg L^-1 of NAA. Both types of auxin induced root formation on the shoots and the resulting plantlets were successfully grown in pots. The production of VOCs in callus tissues and regenerated plantlets was studied by gas chromatography-mass spectrometry (GC-MS) analysis. Although the potential of undifferentiated callus to produce VOCs was very low, an increased content of bioactive volatile components was observed at the beginning of shoot primordia differentiation. Intriguingly, the volatiles obtained from in vitro plantlets showed quantitative and qualitative variation depending on the type of auxins used for the rooting process. The acquired quantities based on total ion current (TIC) showed that the regenerated plantlets using 1 mg L^-1 NAA produced higher amounts of oxygenated monoterpenes such as camphor (30.29%), cis-thujone (7.07%), and 1,8-cineole (6.71%) and sesquiterpene derivatives, namely germacrene D (8.75%), bicyclogermacrene (4.0%) and spathulenol (1.49%) compared with the intact plant. According to these findings, in vitro generation of volatile organic compounds in A. spicigera depends on the developmental stages of tissues and may enhance with the formation of shoot primordia and regeneration of plantlets.

Accumulation of high-value bioproducts in planta can improve the economics of advanced biofuels

Cellulosic biofuels have not yet reached cost parity with conventional petroleum fuels. One strategy to address this challenge is to generate valuable coproducts alongside biofuels. Engineering bioenergy crops to generate value-added bioproducts in planta can reduce input requirements relative to microbial chassis and skip costly deconstruction and conversion steps. Although rapid progress has been made in plant metabolic engineering, there has been no systematic analysis devoted to quantifying the impact of such engineered bioenergy crops on biorefinery economics. Here, we provide new insights into how bioproduct accumulation in planta affects biofuel selling prices. We present the range of bioproduct selling prices and accumulation rates needed to compensate for additional extraction steps and reach a target $2.50/gal minimum biofuel selling price.

Coproduction of high-value bioproducts at biorefineries is a key factor in making biofuels more cost-competitive. One strategy for generating coproducts is to directly engineer bioenergy crops to accumulate bioproducts in planta that can be fractionated and recovered at biorefineries. Here, we develop quantitative insights into the relationship between bioproduct market value and target accumulation rates by investigating a set of industrially relevant compounds already extracted from plant sources with a wide range of market prices and applications, including <$10/kg (limonene, latex, and polyhydroxybutyrate [PHB]), $10 to $100/kg (cannabidiol), and >$100/kg (artemisinin). These compounds are used to identify a range of mass fraction thresholds required to achieve net economic benefits for biorefineries and the additional amounts needed to reach a target $2.50/gal biofuel selling price, using cellulosic ethanol production as a test case. Bioproduct market prices and recovery costs determine the accumulation threshold; we find that moderate- to high-value compounds (i.e., cannabidiol and artemisinin) offer net economic benefits at accumulation rates of just 0.01% dry weight (dwt) to 0.02 dwt%. Lower-value compounds, including limonene, latex, and PHB, require at least an order-of-magnitude greater accumulation to overcome additional extraction and recovery costs (0.3 to 1.2 dwt%). We also find that a diversified approach is critical. For example, global artemisinin demand could be met with fewer than 10 biorefineries, while global demand for latex is equivalent to nearly 180 facilities. Our results provide a roadmap for future plant metabolic engineering efforts aimed at increasing the value derived from bioenergy crops.

Biotechnological approaches for artemisinin production in Artemisia

Abstract

Artemisinin and its analogues are naturally occurring most effective antimalarial secondary metabolites. These compounds also possess activity against various types of cancer cells, schistosomiasis, and some viral diseases. Artemisinin and its derivatives (A&D) are found in very low amounts in the only natural source i.e. Artemisia plant. To meet the global needs, plant sources have been exploited for the enhanced production of these natural products because their chemical synthesis is not profitable. The generally adopted approaches include non-transgenic (tissue and cell cultures) and transgenic together with the cell, tissue, and whole transgenic plant cultures. The genes targeted for the overproduction of A&D include the biosynthetic pathway genes, trichome development genes and rol genes, etc. Artemisinin is naturally produced in trichomes of leaves. At the same time, transgenic hairy roots are considered a good source to harvest artemisinin. However, the absence of trichomes in hairy roots suggests that artemisinin biosynthesis is not limited to trichomes. Moreover, the expression of the gene involved in trichome development and sesquiterpenoid biosynthesis (TFAR1) in transgenic and non-transgenic roots provokes researchers to look for new insight of artemisinin biosynthesis. Here we discuss and review precisely the various biotechnological approaches for the enhanced biosynthesis of A&D.

Graphical Abstract

Next-Generation Plant Metabolic Engineering, Inspired by an Ancient Chinese Irrigation System

Specialized secondary metabolites serve not only to protect plants against abiotic and biotic challenges, but have also been used extensively by humans to combat diseases. Due to the great importance of medicinal plants for health, we need to find new and sustainable ways to improve the production of the specialized metabolites. In addition to direct extraction, recent progress in metabolic engineering of plants offers an alternative supply option. We argue that metabolic engineering for producing the secondary metabolites in plants may have distinct advantages over microbial production platforms, and thus propose new approaches of plant metabolic engineering, which are inspired by an ancient Chinese irrigation system. Metabolic engineering strategies work at three levels: introducing biosynthetic genes, using transcription factors, and improving metabolic flux including increasing the supply of precursors, energy, and reducing power. In addition, recent progress in biotechnology contributes markedly to better engineering, such as the use of specific promoters and the deletion of competing branch pathways. We propose that next-generation plant metabolic engineering will improve current engineering strategies, for the purpose of producing valuable metabolites in plants on industrial scales.

Selection and Clonal Propagation of High Artemisinin Genotypes of Artemisia annua

Artemisinin, produced in the glandular trichomes of Artemisia annua L. is a vital antimalarial drug effective against Plasmodium falciparum resistant to quinine-derived medicines. Although work has progressed on the semi-synthetic production of artemisinin, field production of A. annua remains the principal commercial source of the compound. Crop production of artemisia must be increased to meet the growing worldwide demand for artemisinin combination therapies (ACTs) to treat malaria. Grower artemisinin yields rely on plants generated from seeds from open-pollinated parents. Although selection has considerably increased plant artemisinin concentration in the past 15 years, seed-generated plants have highly variable artemisinin content that lowers artemisinin yield per hectare. Breeding efforts to produce improved F₁ hybrids have been hampered by the inability to produce inbred lines due to self-incompatibility. An approach combining conventional hybridization and selection with clonal propagation of superior genotypes is proposed as a means to enhance crop yield and artemisinin production. Typical seed-propagated artemisia plants produce less than 1% (dry weight) artemisinin with yields below 25 kg/ha. Genotypes were identified producing high artemisinin levels of over 2% and possessing improved agronomic characteristics such as high leaf area and shoot biomass production. Field studies of clonally-propagated high-artemisinin plants verified enhanced plant uniformity and an estimated gross primary productivity of up to 70 kg/ha artemisinin, with a crop density of one plant m^-2. Tissue culture and cutting protocols for the mass clonal propagation of A. annua were developed for shoot regeneration, rooting, acclimatization, and field cultivation. Proof of concept studies showed that both tissue culture-regenerated plants and rooted cutting performed better than plants derived from seed in terms of uniformity, yield, and consistently high artemisinin content. Use of this technology to produce plants with homogeneously-high artemisinin can help farmers markedly increase the artemisinin yield per cultivated area. This would lead to increased profit to farmers and decreased prices of ACT.

Overexpression and Suppression of Artemisia annua 4-Hydroxy-3-Methylbut-2-enyl Diphosphate Reductase 1 Gene (AaHDR1) Differentially Regulate Artemisinin and Terpenoid Biosynthesis

4-Hydroxy-3-methylbut-2-enyl diphosphate reductase (HDR) catalyzes the last step of the 2-C-methyl-D-erythritol 4- phosphate (MEP) pathway to synthesize isopentenyl pyrophosphate (IPP) and dimethylallyl diphosphate (DMAPP). To date, little is known regarding effects of an increase or a decrease of a HDR expression on terpenoid and other metabolite profiles in plants. In our study, an Artemisia annua HDR cDNA (namely AaHDR1) was cloned from leaves. Expression profiling showed that it was highly expressed in leaves, roots, stems, and flowers with different levels. Green florescence protein fusion and confocal microscope analyses showed that AaHDR1 was localized in chloroplasts. The overexpression of AaHDR1 increased contents of artemisinin, arteannuin B and other sesquiterpenes, and multiple monoterpenes. By contrast, the suppression of AaHDR1 by anti-sense led to opposite results. In addition, an untargeted metabolic profiling showed that the overexpression and suppression altered non-polar metabolite profiles. In conclusion, the overexpression and suppression of AaHDR1 protein level in plastids differentially affect artemisinin and other terpenoid biosynthesis, and alter non-polar metabolite profiles of A. annua. Particularly, its overexpression leading to the increase of artemisinin production is informative to future metabolic engineering of this antimalarial medicine.

A new synthetic biology approach allows transfer of an entire metabolic pathway from a medicinal plant to a biomass crop

Artemisinin-based therapies are the only effective treatment for malaria, the most devastating disease in human history. To meet the growing demand for artemisinin and make it accessible to the poorest, an inexpensive and rapidly scalable production platform is urgently needed. Here we have developed a new synthetic biology approach, combinatorial supertransformation of transplastomic recipient lines (COSTREL), and applied it to introduce the complete pathway for artemisinic acid, the precursor of artemisinin, into the high-biomass crop tobacco. We first introduced the core pathway of artemisinic acid biosynthesis into the chloroplast genome. The transplastomic plants were then combinatorially supertransformed with cassettes for all additional enzymes known to affect flux through the artemisinin pathway. By screening large populations of COSTREL lines, we isolated plants that produce more than 120 milligram artemisinic acid per kilogram biomass. Our work provides an efficient strategy for engineering complex biochemical pathways into plants and optimizing the metabolic output.

Cellular engineering of Artemisia annua and Artemisia dubia with the rol ABC genes for enhanced production of potent anti-malarial drug artemisinin

BACKGROUND

Malaria is causing more than half of a million deaths and 214 million clinical cases annually. Despite tremendous efforts for the control of malaria, the global morbidity and mortality have not been significantly changed in the last 50 years. Artemisinin, extracted from the medicinal plant Artemisia sp. is an effective anti-malarial drug. In 2015, elucidation of the effectiveness of artemisinin as a potent anti-malarial drug was acknowledged with a Nobel prize. Owing to the tight market and low yield of artemisinin, an economical way to increase its production is to increase its content in Artemisia sp. through different biotechnological approaches including genetic transformation.

METHODS

Artemisia annua and Artemisia dubia were transformed with rol ABC genes through Agrobacterium tumefacienes and Agrobacterium rhizogenes methods. The artemisinin content was analysed and compared between transformed and untransformed plants with the help of LC-MS/MS. Expression of key genes [Cytochrome P450 (CYP71AV1), aldehyde dehydrogenase 1 (ALDH1), amorpha-4, 11 diene synthase (ADS)] in the biosynthetic pathway of artemisinin and gene for trichome development and sesquiterpenoid biosynthetic (TFAR1) were measured using Quantitative real time PCR (qRT-PCR). Trichome density was analysed using confocal microscope.

RESULTS

Artemisinin content was significantly increased in transformed material of both Artemisia species when compared to un-transformed plants. The artemisinin content within leaves of transformed lines was increased by a factor of nine, indicating that the plant is capable of synthesizing much higher amounts than has been achieved so far through traditional breeding. Expression of all artemisinin biosynthesis genes was significantly increased, although variation between the genes was observed. CYP71AV1 and ALDH1 expression levels were higher than that of ADS. Levels of the TFAR1 expression were also increased in all transgenic lines. Trichome density was also significantly increased in the leaves of transformed plants, but no trichomes were found in control roots or transformed roots. The detection of significantly raised levels of expression of the genes involved in artemisinin biosynthesis in transformed roots correlated with the production of significant amounts of artemisinin in these tissues. This suggests that synthesis is occurring in tissues other than the trichomes, which contradicts previous theories.

CONCLUSION

Transformation of Artemisia sp. with rol ABC genes can lead to the increased production of artemisinin, which will help to meet the increasing demand of artemisinin because of its diverse pharmacological and anti-malarial importance.

Molecular Farming in Artemisia annua, a Promising Approach to Improve Anti-malarial Drug Production

Malaria is a parasite infection affecting millions of people worldwide. Even though progress has been made in prevention and treatment of the disease; an estimated 214 million cases of malaria occurred in 2015, resulting in 438,000 estimated deaths; most of them occurring in Africa among children under the age of five. This article aims to review the epidemiology, future risk factors and current treatments of malaria, with particular focus on the promising potential of molecular farming that uses metabolic engineering in plants as an effective anti-malarial solution. Malaria represents an example of how a health problem may, on one hand, influence the proper development of a country, due to its burden of the disease. On the other hand, it constitutes an opportunity for lucrative business of diverse stakeholders. In contrast, plant biofarming is proposed here as a sustainable, promising, alternative for the production, not only of natural herbal repellents for malaria prevention but also for the production of sustainable anti-malarial drugs, like artemisinin (AN), used for primary parasite infection treatments. AN, a sesquiterpene lactone, is a natural anti-malarial compound that can be found in Artemisia annua. However, the low concentration of AN in the plant makes this molecule relatively expensive and difficult to produce in order to meet the current worldwide demand of Artemisinin Combination Therapies (ACTs), especially for economically disadvantaged people in developing countries. The biosynthetic pathway of AN, a process that takes place only in glandular secretory trichomes of A. annua, is relatively well elucidated. Significant efforts have been made using plant genetic engineering to increase production of this compound. These include diverse genetic manipulation approaches, such as studies on diverse transcription factors which have been shown to regulate the AN genetic pathway and other biological processes. Results look promising; however, further efforts should be addressed toward optimization of the most cost-effective biofarming approaches for synthesis and production of medicines against the malaria parasite.

A Genome-Wide Scenario of Terpene Pathways in Self-pollinated Artemisia annua

Scenarios of genes to metabolites in Artemisia annua remain uninvestigated. Here, we report the use of an integrated approach combining metabolomics, transcriptomics, and gene function analyses to characterize gene-to-terpene and terpene pathway scenarios in a self-pollinating variety of this species. Eighty-eight metabolites including 22 sesquiterpenes (e.g., artemisinin), 26 monoterpenes, two triterpenes, one diterpene and 38 other non-polar metabolites were identified from 14 tissues. These metabolites were differentially produced by leaves and flowers at lower to higher positions. Sequences from cDNA libraries of six tissues were assembled into 18 871 contigs and genome-wide gene expression profiles in tissues were strongly associated with developmental stages and spatial specificities. Sequence mining identified 47 genes that mapped to the artemisinin, non-amorphadiene sesquiterpene, monoterpene, triterpene, 2-C-methyl-D-erythritol 4-phosphate and mevalonate pathways. Pearson correlation analysis resulted in network integration that characterized significant correlations of gene-to-gene expression patterns and gene expression-to-metabolite levels in six tissues simultaneously. More importantly, manipulations of amorpha-4,11-diene synthase gene expression not only affected the activity of this pathway toward artemisinin, artemisinic acid, and arteannuin b but also altered non-amorphadiene sesquiterpene and genome-wide volatile profiles. Such gene-to-terpene landscapes associated with different tissues are fundamental to the metabolic engineering of artemisinin.

Expression analysis of biosynthetic pathway genes vis-à-vis podophyllotoxin content in Podophyllum hexandrum Royle

Podophyllum hexandrum Royle is known for its vast medicinal properties, particularly anticancer. It contains higher amount of podophyllotoxin (4.3 %), compared to Podophyllum peltatum (0.025 %) and other plant species; as a result, it has been used worldwide in the preparation of various drugs including anticancer, antimalarial, antiviral, antioxidant, antifungal, and so on. Currently, Etoposide (VP-16-213), Vumon® (Teniposide; VM-26), Etopophos®, Pod-Ben- 25, Condofil, Verrusol, and Warticon are available in the market. Due to highly complex synthesis and low cell culture yields of podophyllotoxin (0.3 %), the supply of raw material cannot be met due to increasing industrial demands. The knowledge on podophyllotoxin biosynthetic pathway vis-à-vis expression status of genes is fragmentary. Quantitative expression analysis of 21 pathway genes has revealed 9 genes, namely SD, PD, PCH, CM, CMT, CAD, CCR, C4H, and ADH, that showed increase in transcript abundance up to 1.4 to 23.05 folds, respectively, vis-à-vis podophyllotoxin content in roots (1.37 %) and rhizomes (3.05 %) of P. hexandrum. In silico analysis of putative cis-regulatory elements in promoter regions of overexpressed genes showed the presence of common Skn-1 motif and MBS elements in CMT, CAD, CCR, C4H, and ADH genes, thereby, suggesting their common regulation. The outcome of the study has resulted in the identification of suitable candidate genes which might be contributing to podophyllotoxin biosynthesis that can act as potential targets for any genetic intervention strategies aimed at its enhanced production.

Enzyme-Mediated Synthesis of Sesquiterpenes

This review article focuses mainly on the scientific developments concerning the enzyme-mediated synthesis of sesquiterpenes which have been reported in the academic and patent literature during the last twenty years. Nevertheless, this is not a comprehensive description of every single biotransformation involving sesquiterpenes. Only synthetic approaches that have represented a new and innovative perspective from a scientific standpoint are reported. More specifically, the review describes in depth how the use of metabolic engineering of the microbial biotransformations and of the isolated enzymes were exploited in order to perform chemo- and stereoselective chemical transformations of interest for sesquiterpenes synthesis.

Effects of overexpression of AaWRKY1 on artemisinin biosynthesis in transgenic Artemisia annua plants

The effective anti-malarial medicine artemisinin is costly because of the low content in Artemisia annua. Genetic engineering of A. annua is one of the most promising approaches to improve the yield of artemisinin. In this work, the transcription factor AaWRKY1, which is thought to be involved in the regulation of artemisinin biosynthesis, was cloned from A. annua var. Chongqing and overexpressed using the CaMV35S promoter or the trichome-specific CYP71AV1 promoter in stably transformed A. annua plants. The transcript level of AaWRKY1 was increased more than one hundred times under the CaMV35S promoter and about 40 times under the CYP71AV1 promoter. The overexpressed AaWRKY1 activated the transcription of CYP71AV1 and moreover the trichome-specific overexpression of AaWRKY1 improved the transcription of CYP71AV1 much more effectively than the constitutive overexpression of AaWRKY1, i.e. up to 33 times as compared to the wild-type plant. However the transcription levels of FDS, ADS, and DBR2 did not change significantly in transgenic plants. The significantly up-regulated CYP71AV1 promoted artemisinin biosynthesis, i.e. up to about 1.8 times as compared to the wild-type plant. It is demonstrated that trichome-specific overexpression of AaWRKY1 can significantly activate the transcription of CYP71AV1 and the up-regulated CYP71AV1 promotes artemisinin biosynthesis.

Engineering the MEP pathway enhanced ajmalicine biosynthesis

The 2-C-methyl-D-erythritol-4-phosphate (MEP) pathway genes encoding DXR and MECS from Taxus species and STR from Catharanthus roseus were used to genetically modify the ajmalicine biosynthetic pathway in hairy root cultures of C. roseus. As expected, the STR-overexpressed root cultures showed twofold higher accumulation of ajmalicine than the control. It was important to discover that overexpression of the single DXR or MECS gene from the MEP pathway also remarkably enhanced ajmalicine biosynthesis in transgenic hairy root cultures, and this suggested that engineering the MEP pathway by overexpression of DXR or MECS promoted the metabolic flux into ajmalicine biosynthesis. The transgenic hairy root cultures with co-overexpression of DXR and STR or MECS and STR had higher levels of ajmalicine than those with overexpression of a single gene alone such as DXR, MECS, and STR. It could be concluded that transgenic hairy root cultures harboring both DXR/MECS and STR possessed an increased flux in the terpenoid indole alkaloid biosynthetic pathway that enhanced ajmalicine yield, which was more efficient than cultures harboring only one of the three genes.

Sesquiterpene lactones: adverse health effects and toxicity mechanisms

Sesquiterpene lactones (STLs) present a wide range of biological activities, mostly based on their alkylating capabilities, which underlie their therapeutic potential. These compounds are the active constituents of a variety of plants, frequently used as herbal remedies. STLs such as artemisinin and its derivatives are in use as first-line antimalarials while others, such as parthenolide, have recently reached cancer clinical trials. However, the toxicological profile of these compounds must be thoroughly characterized, since the same properties that make STL useful medicines can also cause severe toxicity. STL-containing plants have long been known to induce a contact dermatitis in exposed farm workers, and also to cause several toxic syndromes in farm animals. More recently, concerns are been raised regarding the genotoxic potential of these compounds and the embryotoxicity of artemisinins. A growing number of STLs are being reported to be mutagenic in different in vitro and in vivo assays. As yet no systematic studies have been published, but the genotoxicity of STLs seems to depend not so much on direct DNA alkylation as on oxidative DNA damage and other partially elucidated mechanisms. As the medicinal use of these compounds increases, further studies of their toxic potential are needed, especially those focusing on the structural determinants of genotoxicity and embryotoxicity.

Studies on the expression of linalool synthase using a promoter-β-glucuronidase fusion in transgenic Artemisia annua

Artemisinin, an antimalarial endoperoxide sesquiterpene, is synthesized in glandular trichomes of Artemisia annua L. A number of other enzymes of terpene metabolism utilize intermediates of artemisinin biosynthesis, such as isopentenyl and farnesyl diphosphate, and may thereby influence the yield of artemisinin. In order to study the expression of such enzymes, we have cloned the promoter regions of some enzymes and fused them to β-glucuronidase (GUS). In this study, we have investigated the expression of the monoterpene synthase linalool synthase (LIS) using transgenic A. annua carrying the GUS gene under the control of the LIS promoter. The 652bp promoter region was cloned by the genome walker method. A number of putative cis-acting elements were predicted indicating that the LIS is driven by a complex regulation mechanism. Transgenic plants carrying the promoter-GUS fusion showed specific expression of GUS in T-shaped trichomes (TSTs) but not in glandular secretory trichomes, which is the site for artemisinin biosynthesis. GUS expression was observed at late stage of flower development in styles of florets and in TSTs and guard cells of basal bracts. GUS expression after wounding showed that LIS is involved in plant responsiveness to wounding. Furthermore, the LIS promoter responded to methyl jasmonate (MeJA). These results indicate that the promoter carries a number of cis-acting regulatory elements involved in the tissue-specific expression of LIS and in the response of the plant to wounding and MeJA treatment. Southern blot analysis indicated that the GUS gene was integrated in the A. annua genome as single or multi copies in different transgenic lines. Promoter activity analysis by qPCR showed that both the wild-type and the recombinant promoter are active in the aerial parts of the plant while only the recombinant promoter was active in roots. Due to the expression in TSTs but not in glandular trichomes, it may be concluded that LIS expression will most likely have little or no effect on artemisinin production.

Metabolic engineering of plants for artemisinin synthesis

Artemisinin, a natural compound from Artemisia annua, is highly effective in treating drug-resistant malaria. Because chemical synthesis of this natural terpenoid is not economically feasible, its only source remains as the native plant which produces only small quantities of it, resulting in a supply that is far short of demand. Extensive efforts have been invested in metabolic engineering for the biosynthesis of artemisinin precursors in microbes. However, the production of artemisinin itself has only been achieved in plants. Since, A. annua possesses only poorly developed genetic resources for traditional breeders, molecular breeding is the best alternative. In this review, we describe the efforts taken to enhance artemisinin production in A. annua via transgenesis and advocate metabolic engineering of the complete functional artemisinin metabolic pathway in heterologous plants. In both cases, we emphasize the need to apply state-of-the-art synthetic biology approaches to ensure successful biosynthesis of the drug.

Differentially expressed genes during contrasting growth stages of Artemisia annua for artemisinin content

Artemisia annua is the source of antimalarial phytomolecule, artemisinin. It is mainly produced and stored in the glandular secretory trichomes present in the leaves of the plant. Since, the artemisinin biosynthesis steps are yet to be worked out, in this investigation a microarray chip was strategized for the first time to shortlist the differentially expressing genes at a stage of plant producing highest artemisinin compared to the stage with no artemisinin. As the target of this study was to analyze differential gene expression associated with contrasting artemisinin content in planta and a genotype having zero/negligible artemisinin content was unavailable, it was decided to compare different stages of the same genotype with contrasting artemisinin content (seedling - negligible artemisinin, mature leaf - high artemisinin). The SCAR-marked artemisinin-rich (∼1.2%) Indian variety ‘CIM-Arogya’ was used in the present study to determine optimal plant stage and leaf ontogenic level for artemisinin content. A representative EST dataset from leaf trichome at the stage of maximal artemisinin biosynthesis was established. The high utility small scale custom microarray chip of A. annua containing all the significant artemisinin biosynthesis-related genes, the established EST dataset, gene sequences isolated in-house and strategically selected candidates from the A. annua Unigene database (NCBI) was employed to compare the gene expression profiles of two stages. The expression data was validated through semiquantitative and quantitative RT-PCR followed by putative annotations through bioinformatics-based approaches. Many candidates having probable role in artemisinin metabolism were identified and described with scope for further functional characterization.

New Perspectives on How to Discover Drugs from Herbal Medicines: CAM's Outstanding Contribution to Modern Therapeutics

With tens of thousands of plant species on earth, we are endowed with an enormous wealth of medicinal remedies from Mother Nature. Natural products and their derivatives represent more than 50% of all the drugs in modern therapeutics. Because of the low success rate and huge capital investment need, the research and development of conventional drugs are very costly and difficult. Over the past few decades, researchers have focused on drug discovery from herbal medicines or botanical sources, an important group of complementary and alternative medicine (CAM) therapy. With a long history of herbal usage for the clinical management of a variety of diseases in indigenous cultures, the success rate of developing a new drug from herbal medicinal preparations should, in theory, be higher than that from chemical synthesis. While the endeavor for drug discovery from herbal medicines is "experience driven," the search for a therapeutically useful synthetic drug, like "looking for a needle in a haystack," is a daunting task. In this paper, we first illustrated various approaches of drug discovery from herbal medicines. Typical examples of successful drug discovery from botanical sources were given. In addition, problems in drug discovery from herbal medicines were described and possible solutions were proposed. The prospect of drug discovery from herbal medicines in the postgenomic era was made with the provision of future directions in this area of drug development.

Terpenoid biosynthesis in trichomes--current status and future opportunities

Glandular trichomes are anatomical structures specialized for the synthesis of secreted natural products. In this review we focus on the description of glands that accumulate terpenoid essential oils and oleoresins. We also provide an in-depth account of the current knowledge about the biosynthesis of terpenoids and secretion mechanisms in the highly specialized secretory cells of glandular trichomes, and highlight the implications for metabolic engineering efforts.

Striking natural diversity in glandular trichome acylsugar composition is shaped by variation at the Acyltransferase2 locus in the wild tomato Solanum habrochaites

Acylsugars are polyesters of short- to medium-length acyl chains on sucrose or glucose backbones that are produced in secretory glandular trichomes of many solanaceous plants, including cultivated tomato (Solanum lycopersicum). Despite their roles in biotic stress adaptation and their wide taxonomic distribution, there is relatively little information about the diversity of these compounds and the genes responsible for their biosynthesis. In this study, acylsugar diversity was assessed for 80 accessions of the wild tomato species Solanum habrochaites from throughout the Andes Mountains. Trichome metabolites were analyzed by liquid chromatography-time of flight-mass spectrometry, revealing the presence of at least 34 structurally diverse acylsucroses and two acylglucoses. Distinct phenotypic classes were discovered that varied based on the presence of glucose or sucrose, the numbers and lengths of acyl chains, and the relative total amounts of acylsugars. The presence or absence of an acetyl chain on the acylsucrose hexose ring caused clustering of the accessions into two main groups. Analysis of the Acyltransferase2 gene (the apparent ortholog of Solyc01g105580) revealed differences in enzyme activity and gene expression correlated with polymorphism in S. habrochaites accessions that varied in acylsucrose acetylation. These results are consistent with the hypothesis that glandular trichome acylsugar acetylation is under selective pressure in some populations of S. habrochaites and that the gene mutates to inactivity in the absence of selection.

Characterization of cytochrome P450 monooxygenases isolated from trichome enriched fraction of Artemisia annua L. leaf

CYPs have major role in the biosynthesis and modification of secondary metabolites. Predicting the possible involvement of CYPs in secondary metabolism, 20 partial sequences were amplified from the cDNA of trichome enriched tissue of Artemisia annua. Seven CYPs were converted to full length and assigned to different families based on sequence homology. These were co-expressed with CPR in Saccharomyces cerevisiae and microsome fractions were assayed for conversion of sesquiterpenes, phenols and fatty acid substrates. CIM_CYP02(c73) and CIM_CYP05(c81) converted trans-cinnamic acid to p-coumaric acid; and capric acid, lauric acid to their hydroxylated products, respectively. Higher expression of CIM_CYP71AV1, CIM_CYP03(c72a), CIM_CYP06(c72b), CIM_CYP02(c73) and CIM_CYP04(c83) was observed in the mature leaf, whereas expression of CIM_CYP05(c81) was more in the seedling. CIM_CYP71AV1, CIM_CYP02(c73) and CIM_CYP04(c83) expressed more in the flower bud compared to the leaf, with minor expression in stem. All CYPs' expression increased progressively with time after wounding except for CIM_CYP07(c92). These results relate involvement of CIM_CYP02(c73) to phenyl-propanoid metabolism in the leaf and CIM_CYP05(c81) to fatty acid metabolism in the seedling. Expression of CIM_CYP71AV1 and CIM_CYP02(c73) significantly increased when sprayed with trans-cinnamic acid indicating a relationship between phenylpropanoid and artemisinic acid pathways.

Overexpression of the cytochrome P450 monooxygenase (cyp71av1) and cytochrome P450 reductase (cpr) genes increased artemisinin content in Artemisia annua (Asteraceae)

Finding an efficient and affordable treatment against malaria is still a challenge for medicine. Artemisinin is an effective anti-malarial drug isolated from Artemisia annua. However, the artemisinin content of A. annua is very low. We used transgenic technology to increase the artemisinin content of A. annua by overexpressing cytochrome P450 monooxygenase (cyp71av1) and cytochrome P450 reductase (cpr) genes. CYP71AV1 is a key enzyme in the artemisinin biosynthesis pathway, while CPR is a redox partner for CYP71AV1. Eight independent transgenic A. annua plants were obtained through Agrobacterium tumefaciens-mediated transformation, which was confirmed by PCR and Southern blot analyses. The real-time qPCR results showed that the gene cyp71av1 was highly expressed at the transcriptional level in the transgenic A. annua plants. HPLC analysis showed that the artemisinin content was increased in a number of the transgenic plants, in which both cyp71av1 and cpr were overexpressed. In one of the transgenic A. annua plants, the artemisinin content was 38% higher than in the non-transgenic plants. We conclude that overexpressing key enzymes of the biosynthesis pathway is an effective means for increasing artemisinin content in plants.

Glandular trichomes: what comes after expressed sequence tags?

Glandular trichomes cover the surface of many plant species. They exhibit tremendous diversity, be it in their shape or the compounds they secrete. This diversity is expressed between species but also within species or even individual plants. The industrial uses of some trichome secretions and their potential as a defense barrier, for example against arthropod pests, has spurred research into the biosynthesis pathways that lead to these specialized metabolites. Because complete biosynthesis pathways take place in the secretory cells, the establishment of trichome-specific expressed sequence tag libraries has greatly accelerated their elucidation. Glandular trichomes also have an important metabolic capacity and may be considered as true cell factories. To fully exploit the potential of glandular trichomes as breeding or engineering objects, several research areas will have to be further investigated, such as development, patterning, metabolic fluxes and transcription regulation. The purpose of this review is to provide an update on the methods and technologies which have been used to investigate glandular trichomes and to propose new avenues of research to deepen our understanding of these specialized structures.

Structure and dynamics of the isoprenoid pathway network

Isoprenoids are functionally and structurally the most diverse group of plant metabolites reported to date. They can function as primary metabolites, participating in essential plant cellular processes, and as secondary metabolites, of which many have substantial commercial, pharmacological, and agricultural value. Isoprenoid end products participate in plants in a wide range of physiological processes acting in them both synergistically, such as chlorophyll and carotenoids during photosynthesis, or antagonistically, such as gibberellic acid and abscisic acid during seed germination. It is therefore expected that fluxes via isoprenoid metabolic network are tightly controlled both temporally and spatially, and that this control occurs at different levels of regulation and in an orchestrated manner over the entire isoprenoid metabolic network. In this review, we summarize our current knowledge of the topology of the plant isoprenoid pathway network and its regulation at the gene expression level following diverse stimuli. We conclude by discussing agronomical and biotechnological applications emerging from the plant isoprenoid metabolism and provide an outlook on future directions in the systems analysis of the plant isoprenoid pathway network.

Transgenerational adaptation to heavy metal salts in Arabidopsis

Exposure to abiotic and biotic stress results in changes in plant physiology and triggers genomic instability. Recent reports suggest that the progeny of stressed plants also exhibit changes in genome stability, stress tolerance, and methylation. Here we analyzed whether exposure to Ni(2+), Cd(2+), and Cu(2+) salts leads to transgenerational changes in homologous recombination frequency and stress tolerance. We found that the immediate progeny of stressed plants exhibited an increased rate of recombination. However, when the progeny of stressed plants was propagated without stress, recombination reverted to normal levels. Exposure of plants to heavy metals for five consecutive generations (S1-S5) resulted in recombination frequency being maintained at a high level. Skipping stress following two to three generations of propagation with 50 mM Ni(2+) or Cd(2+) did not decrease the recombination frequency, suggesting plant acclimation to upregulated recombination. Analysis of the progeny of plants exposed to Cu(2+) and Ni(2+) indicated higher stress tolerance to the heavy metal parental plants were exposed to. Tolerance was higher in plants propagated with stress for three to five generations, which resulted in longer roots than plants propagated on heavy metals for only one to two generations. Tolerance was also more prominent upon exposure to a higher concentration of salts. The progeny of stressed plants were also more tolerant to NaCl and methyl methane sulfonate.