Artemisia annua glandular secretory trichomes: the biofactory of antimalarial agent artemisinin

Chemical structures, biosynthesis, bioactivities, and utilisation values for the diterpenes produced in tobacco trichomes

Cembranoids and labdanes are two important types of diterpenes in tobacco (Nicotiana genus) that are predominantly found in the leaf and flower glandular trichome secretions. This is the first systematic review of the biosynthesis, chemical structures, bioactivities, and utilisation values of cembranoid and labdane diterpenes in tobacco. A total of 131 natural cembranoid diterpenes have been reported in tobacco since 1962; these were summarised and classified according to their chemical structure characteristics as isopropyl cembranoids (1-88), seco-cembranoids (89-103), chain cembranoids (104-123), and polycyclic cembranoids (124-131). Forty natural labdane diterpenes reported since 1961 were also summarised and divided into epoxy side chain labdanes (132-150) and epoxy-free side chain labdanes (151-171). Tobacco cembranoid and labdane diterpenes are both formed via the methylerythritol 4-phosphate pathway and are synthesised from geranylgeranyl diphosphate. Their biosynthetic pathways and the four key enzymes (cembratrienol synthase, cytochrome P450 hydroxylase, copalyl diphosphate synthase, and Z-abienol cyclase) that affect their biosynthesis have been described in detail. A systematic summary of the bioactivity and utilisation values of the cembranoid and labdane diterpenes is also provided. The agricultural bioactivities associated with cembranoid and labdane diterpenes include antimicrobial and insecticidal activities as well as induced resistance, while the medical bioactivities include cytotoxic and neuroprotective activities. Further research into the cembranoid and labdane diterpenes will help to promote their development and utilisation as plant-derived pesticides and medicines.

Graphene enhances artemisinin production in the traditional medicinal plant Artemisia annua via dynamic physiological processes and miRNA regulation

We investigated the effects of graphene on the model herb Artemisia annua, which is renowned for producing artemisinin, a widely used pharmacological compound. Seedling growth and biomass were promoted when A. annua was cultivated with low concentrations of graphene, an effect which was attributed to a 1.4-fold increase in nitrogen uptake, a 15%-22% increase in chlorophyll fluorescence, and greater abundance of carbon cycling-related bacteria. Exposure to 10 or 20 mg/L graphene resulted in a ∼60% increase in H2O2, and graphene could act as a catalyst accelerator, leading to a 9-fold increase in catalase (CAT) activity in vitro and thereby maintaining reactive oxygen species (ROS) homeostasis. Importantly, graphene exposure led to an 80% increase in the density of glandular secreting trichomes (GSTs), in which artemisinin is biosynthesized and stored. This contributed to a 5% increase in artemisinin content in mature leaves. Interestingly, expression of miR828 was reduced by both graphene and H2O2 treatments, resulting in induction of its target gene AaMYB17, a positive regulator of GST initiation. Subsequent molecular and genetic assays showed that graphene-induced H2O2 inhibits micro-RNA (miRNA) biogenesis through Dicers and regulates the miR828-AaMYB17 module, thus affecting GST density. Our results suggest that graphene may contribute to yield improvement in A. annua via dynamic physiological processes together with miRNA regulation, and it may thus represent a new cultivation strategy for increasing yield capacity through nanobiotechnology.

Bioprospecting of Artemisia genus: from artemisinin to other potentially bioactive compounds

Species from genus Artemisia are widely distributed throughout temperate regions of the northern hemisphere and many cultures have a long-standing traditional use of these plants as herbal remedies, liquors, cosmetics, spices, etc. Nowadays, the discovery of new plant-derived products to be used as food supplements or drugs has been pushed by the exploitation of bioprospection approaches. Often driven by the knowledge derived from the ethnobotanical use of plants, bioprospection explores the existing biodiversity through integration of modern omics techniques with targeted bioactivity assays. In this work we set up a bioprospection plan to investigate the phytochemical diversity and the potential bioactivity of five Artemisia species with recognized ethnobotanical tradition (A. absinthium, A. alba, A. annua, A. verlotiorum and A. vulgaris), growing wild in the natural areas of the Verona province. We characterized the specialized metabolomes of the species (including sesquiterpenoids from the artemisinin biosynthesis pathway) through an LC-MS based untargeted approach and, in order to identify potential bioactive metabolites, we correlated their composition with the in vitro antioxidant activity. We propose as potential bioactive compounds several isomers of caffeoyl and feruloyl quinic acid esters (e.g. dicaffeoylquinic acids, feruloylquinic acids and caffeoylferuloylquinic acids), which strongly characterize the most antioxidant species A. verlotiorum and A. annua. Morevoer, in this study we report for the first time the occurrence of sesquiterpenoids from the artemisinin biosynthesis pathway in the species A. alba.

Advanced metabolic engineering strategies for increasing artemisinin yield in Artemisia annua L

Artemisinin, also known as 'Qinghaosu', is a chemically sesquiterpene lactone containing an endoperoxide bridge. Due to the high activity to kill Plasmodium parasites, artemisinin and its derivatives have continuously served as the foundation for antimalarial therapies. Natural artemisinin is unique to the traditional Chinese medicinal plant Artemisia annua L., and its content in this plant is low. This has motivated the synthesis of this bioactive compound using yeast, tobacco, and Physcomitrium patens systems. However, the artemisinin production in these heterologous hosts is low and cannot fulfil its increasing clinical demand. Therefore, A. annua plants remain the major source of this bioactive component. Recently, the transcriptional regulatory networks related to artemisinin biosynthesis and glandular trichome formation have been extensively studied in A. annua. Various strategies including (i) enhancing the metabolic flux in artemisinin biosynthetic pathway; (ii) blocking competition branch pathways; (iii) using transcription factors (TFs); (iv) increasing peltate glandular secretory trichome (GST) density; (v) applying exogenous factors; and (vi) phytohormones have been used to improve artemisinin yields. Here we summarize recent scientific advances and achievements in artemisinin metabolic engineering, and discuss prospects in the development of high-artemisinin yielding A. annua varieties. This review provides new insights into revealing the transcriptional regulatory networks of other high-value plant-derived natural compounds (e.g., taxol, vinblastine, and camptothecin), as well as glandular trichome formation. It is also helpful for the researchers who intend to promote natural compounds production in other plants species.

A comparative analysis of morphology, microstructure, and volatile metabolomics of leaves at varied developmental stages in Ainaxiang (Blumea balsamifera (Linn.) DC.)

Introduction

Ainaxiang (Blumea balsamifera (Linn.) DC.) is cultivated for the extraction of (-)-borneol and other pharmaceutical raw materials due to its abundant volatile oil. However, there is limited knowledge regarding the structural basis and composition of volatile oil accumulation in fresh B. balsamifera leaves.

Methods

To address this problem, we compare the fresh leaves' morphology, microstructure, and volatile metabonomic at different development stages, orderly defined from the recently unfolded young stage (S1) to the senescent stage (S4).

Results and discussion

Distinct differences were observed in the macro-appearance and microstructure at each stage, particularly in the B. balsamifera glandular trichomes (BbGTs) distribution. This specialized structure may be responsible for the accumulation of volatile matter. 213 metabolites were identified through metabolomic analysis, which exhibited spatiotemporal accumulation patterns among different stages. Notably, (-)-borneol was enriched at S1, while 10 key odor metabolites associated with the characteristic balsamic, borneol, fresh, and camphor aromas of B. balsamifera were enriched in S1 and S2. Ultra-microstructural examination revealed the involvement of chloroplasts, mitochondria, endoplasmic reticulum, and vacuoles in the synthesizing, transporting, and storing essential oils. These findings confirm that BbGTs serve as the secretory structures in B. balsamifera, with the population and morphology of BbGTs potentially serving as biomarkers for (-)-borneol accumulation. Overall, young B. balsamifera leaves with dense BbGTs represent a rich (-)-borneol source, while mesophyll cells contribute to volatile oil accumulation. These findings reveal the essential oil accumulation characteristics in B. balsamifera, providing a foundation for further understanding.

An updated review on distribution, biosynthesis and pharmacological effects of artemisinin: A wonder drug

Plant-based drugs have been used for centuries for treating different ailments. Malaria, one of the prevalent threats in many parts of the world, is treated mainly by artemisinin-based drugs derived from plants of genus Artemisia. However, the distribution of artemisinin is restricted to a few species of the genus; besides, its yield depends on ontogeny and the plant's geographical location. Here, we review the studies focusing on biosynthesis and distributional pattern of artemisinin production in species of the genus Artemisia. We also discussed various agronomic and in vitro methods and molecular approaches to increase the yield of artemisinin. We have summarized different mechanisms of artemisinin involved in its anti-malarial, anti-cancer, anti-inflammatory and anti-viral activities (like against Covid-19). Overall the current review provides a synopsis of a global view of the distribution of artemisinin, its biosynthesis, and pharmacological potential in treating various diseases like malaria, cancer, and coronavirus, which may provoke future research efforts in drug development. Nevertheless, long-term trials and molecular approaches, like CRISPR-Cas, are required for in-depth research.

Chromatin architecture of two different strains of Artemisia annua reveals the alterations in interaction and gene expression

MAIN CONCLUSION

The hierarchical architecture of chromatins affects the gene expression level of glandular secreting trichomes and the artemisinin biosynthetic pathway-related genes, consequently bringing on huge differences in the content of artemisinin and its derivatives of A. annua. The plant of traditional Chinese medicine "Qinghao" is called Artemisia annua L. in Chinese Pharmacopoeia. High content and the total amount of artemisinin is the main goal of A. annua breeding, nevertheless, the change of chromatin organization during the artemisinin synthesis process has not been discovered yet. This study intended to find the roles of chromatin structure in the production of artemisinin through bioinformatics and experimental validation. Chromosome conformation capture analysis was used to scrutinize the interactions among chromosomes and categorize various scales of chromatin during artemisinin synthesis in A. annua. To confirm the effect of the changes in chromatin structure, Hi-C and RNA-sequencing were performed on two different strains to find the correlation between chromatin structure and gene expression levels on artemisinin synthesis progress and regulation. Our results revealed that the frequency of intra-chromosomal interactions was higher in the inter-chromosomal interactions between the root and leaves on a high artemisinin production strain (HAP) compared to a low artemisinin production strain (LAP). We found that compartmental transition was connected with interactions among different chromatins. Interestingly, glandular secreting trichomes (GSTs) and the artemisinin biosynthetic pathway (ABP) related genes were enriched in the areas which have the compartmental transition, reflecting the regulation of artemisinin synthesis. Topologically associated domain boundaries were associated with various distributions of genes and expression levels. Genes associated with ABP and GST in the adjacent loop were highly expressed, suggesting that epigenetic regulation plays an important role during artemisinin synthesis and glandular secreting trichomes production process. Chromatin structure could show an important status in the mechanisms of artemisinin synthesis process in A. annua.

Development and Biomechanics of Grewia lasiocarpa E. Mey. Ex Harv. Trichomes Exudate

Grewia lasiocarpa E. Mey. Ex Harv., Malvaceae (forest raisin) is a tropical small tree or shrub valued for its ecological importance as well as its nutritional, antioxidant, antibacterial, and anti-cancer properties as well as its ecological and ornamental importance. Glandular and non-glandular trichomes are present on the fruits, stem bark and leaves of G. lasiocarpa and these trichomes are the first line of defense. They are important structures that plants use to combat biotic and abiotic stress. The development of G. lasiocarpa trichomes and the biomechanics of the exudates present in the glandular (capitate) trichome were investigated for the first time using advanced microscopy techniques [Scanning electron microscope (SEM) and Transmission electron microscope (TEM)]. The pressurized cuticular striations may play a role in the exudates' biomechanics, i.e., releasing secondary metabolites present in the capitate trichome, which was observed to be multidirectional. The presence of many glandular trichomes on a plant implies an increase in the amount of phytometabolites. A common precursor for the development of trichomes (non-glandular and glandular) was observed to be DNA synthesis associated with a periclinal cell division, thus the final fate of the cell is determined by cell cycle regulation, polarity, and expansion. The glandular trichomes of G. lasiocarpa are multicellular and polyglandular, while the non-glandular (glandless) trichomes are either single-celled or multicellular. Since, trichomes 'house' phytocompounds of medicinal, nutritional, and agronomical benefits; the molecular and genetic study of the glandular trichomes of Grewia lasiocarpa will be beneficial to humanity.

AaWIN1, an AP2/ERF protein, positively regulates glandular secretory trichome initiation in Artemisia annua

Exploring the genetic network of glandular trichomes and manipulating genes relevant to secondary metabolite biosynthesis are of great importance and value. Artemisinin, a key antimalarial drug ingredient, is synthesized and stored in glandular secretory trichomes (GSTs) in Artemisia annua. WIN/SHN proteins, a clade of AP2/ERF family, are known as regulators for cuticle biosynthesis. However, their function in glandular trichome development is less unknown. In this study, we identified a WIN/SHN gene from A. annua and named it as AaWIN1. AaWIN1 was predominantly expressed in buds, flowers and trichomes, and encoded a nuclear-localized protein. Overexpressing AaWIN1 in A. annua significantly increased the density of GST as well as the artemisinin content. Furthermore, AaGSW2 was reported to play an important role in promoting GST initiation, and the expression of AaGSW2 was induced in AaWIN1-overexpression lines. AaMIXTA1, a MYB protein positively regulating trichome initiation and cuticle biosynthesis, was confirmed to interact with AaWIN1. In addition, the ectopic expression of AaWIN1 resulted in slender and curled leaves, fewer trichomes, and rising expressions of cuticle biosynthesis genes in Arabidopsis thaliana. Taken together, based on phenotype observations, content measurements and gene expression detections, AaWIN1 was considered as a positive regulator for GST initiation in A. annua.

Profiling of phytohormone-specific microRNAs and characterization of the miR160-ARF1 module involved in glandular trichome development and artemisinin biosynthesis in Artemisia annua

MicroRNAs (miRNAs) play crucial roles in plant development and secondary metabolism through different modes of sequence-specific interaction with their targets. Artemisinin biosynthesis is extensively regulated by phytohormones. However, the function of phytohormone-responsive miRNAs in artemisinin biosynthesis remains enigmatic. Thus, we combined the analysis of transcriptomics, small RNAs, and the degradome to generate a comprehensive resource for identifying key miRNA-target circuits involved in the phytohormone-induced process of artemisinin biosynthesis in Artemisia annua. In total, 151 conserved and 52 novel miRNAs and their 4132 targets were determined. Based on the differential expression analysis, miR160 was selected as a potential miRNA involved in artemisinin synthesis. Overexpressing MIR160 significantly impaired glandular trichome formation and suppressed artemisinin biosynthesis in A. annua, while repressing its expression resulted in the opposite effect, indicating that miR160 negatively regulates glandular trichome development and artemisinin biosynthesis. RNA ligase-mediated 5' RACE and transient transformation assays showed that miR160 mediates the RNA cleavage of Auxin Response Factor 1 (ARF1) in A. annua. Furthermore, ARF1 was shown to increase artemisinin synthesis by activating AaDBR2 expression. Taken together, our results reveal the intrinsic link between the miR160-ARF1 module and artemisinin biosynthesis, and may expedite the innovation of metabolic engineering approaches for high and stable production of artemisinin in the future.

Metabolic engineering of the anthocyanin biosynthetic pathway in Artemisia annua and relation to the expression of the artemisinin biosynthetic pathway

Four types of cells were engineered from Artemisia annua to produce approximately 17 anthocyanins, four of which were elucidated structurally. All of them expressed the artemisinin pathway. Artemisia annua is the only medicinal crop to produce artemisinin for the treatment of malignant malaria. Unfortunately, hundreds of thousands of people still lose their life every year due to the lack of sufficient artemisinin. Artemisinin is considered to result from the spontaneous autoxidation of dihydroartemisinic acid in the presence of reactive oxygen species (ROS) in an oxidative condition of glandular trichomes (GTs); however, whether increasing antioxidative compounds can inhibit artemisinin biosynthesis in plant cells is unknown. Anthocyanins are potent antioxidants that can remove ROS in plant cells. To date, no anthocyanins have been structurally elucidated from A. annua. In this study, we had two goals: (1) to engineer anthocyanins in A. annua cells and (2) to understand the artemisinin biosynthesis in anthocyanin-producing cells. Arabidopsis Production of Anthocyanin Pigment 1 was used to engineer four types of transgenic anthocyanin-producing A. annua (TAPA1-4) cells. Three wild-type cell types were developed as controls. TAPA1 cells produced the highest contents of total anthocyanins. LC-MS analysis detected 17 anthocyanin or anthocyanidin compounds. Crystallization, LC/MS/MS, and NMR analyses identified cyanidin, pelargonidin, one cyanin, and one pelargonin. An integrative analysis characterized that four types of TAPA cells expressed the artemisinin pathway and TAPA1 cells produced the highest artemisinin and artemisinic acid. The contents of arteannuin B were similar in seven cell types. These data showed that the engineering of anthocyanins does not eliminate the biosynthesis of artemisinin in cells. These data allow us to propose a new hypothesis that enzymes catalyze the formation of artemisinin from dihydroartemisinic acid in non-GT cells. These findings show a new platform to increase artemisinin production via non-GT cells of A. annua.

Genome-wide identification and characterisation of bHLH transcription factors in Artemisia annua

BACKGROUND

A. annua (also named Artemisia annua, sweet wormwood) is the main source of the anti-malarial drug artemisinin, which is synthesised and stored in its trichomes. Members of the basic Helix-Loop-Helix (bHLH) family of transcription factors (TFs) have been implicated in artemisinin biosynthesis in A. annua and in trichome development in other plant species.

RESULTS

Here, we have systematically identified and characterised 226 putative bHLH TFs in A. annua. All of the proteins contain a HLH domain, 213 of which also contain the basic motif that mediates DNA binding of HLH dimers. Of these, 22 also contained a Myc domain that permits dimerisation with other families of TFs; only two proteins lacking the basic motif contained a Myc domain. Highly conserved GO annotations reflected the transcriptional regulatory role of the identified TFs, and suggested conserved roles in biological processes such as iron homeostasis, and guard cell and endosperm development. Expression analysis revealed that three genes (AabHLH80, AabHLH96, and AaMyc-bHLH3) exhibited spatiotemporal expression patterns similar to genes encoding key enzymes in artemisinin synthesis.

CONCLUSIONS

This comprehensive analysis of bHLH TFs provides a new resource to direct further analysis into key molecular mechanisms underlying and regulating artemisinin biosynthesis and trichome development, as well as other biological processes, in the key medicinal plant A. annua.

Towards greenhouse cultivation of Artemisia annua: The application of LEDs in regulating plant growth and secondary metabolism

Artemisinin is a sesquiterpene lactone produced in glandular trichomes of Artemisia annua, and is extensively used in the treatment of malaria. Growth and secondary metabolism of A. annua are strongly regulated by environmental conditions, causing unstable supply and quality of raw materials from field grown plants. This study aimed to bring A. annua into greenhouse cultivation and to increase artemisinin production by manipulating greenhouse light environment using LEDs. A. annua plants were grown in a greenhouse compartment for five weeks in vegetative stage with either supplemental photosynthetically active radiation (PAR) (blue, green, red or white) or supplemental radiation outside PAR wavelength (far-red, UV-B or both). The colour of supplemental PAR hardly affected plant morphology and biomass, except that supplemental green decreased plant biomass by 15% (both fresh and dry mass) compared to supplemental white. Supplemental far-red increased final plant height by 23% whereas it decreased leaf area, plant fresh and dry weight by 30%, 17% and 7%, respectively, compared to the treatment without supplemental radiation. Supplemental UV-B decreased plant leaf area and dry weight (both by 7%). Interestingly, supplemental green and UV-B increased leaf glandular trichome density by 11% and 9%, respectively. However, concentrations of artemisinin, arteannuin B, dihydroartemisinic acid and artemisinic acid only exhibited marginal differences between the light treatments. There were no interactive effects of far-red and UV-B on plant biomass, morphology, trichome density and secondary metabolite concentrations. Our results illustrate the potential of applying light treatments in greenhouse production of A. annua to increase trichome density in vegetative stage. However, the trade-off between light effects on plant growth and trichome initiation needs to be considered. Moreover, the underlying mechanisms of light spectrum regulation on artemisinin biosynthesis need further clarification to enhance artemisinin yield in greenhouse production of A. annua.

Allele-aware chromosome-level genome assembly of Artemisia annua reveals the correlation between ADS expansion and artemisinin yield

Artemisia annua is the major natural source of artemisinin, an anti-malarial medicine commonly used worldwide. Here, we present chromosome-level haploid maps for two A. annua strains with different artemisinin contents to explore the relationships between genomic organization and artemisinin production. High-fidelity sequencing, optical mapping, and chromatin conformation capture sequencing were used to assemble the heterogeneous and repetitive genome and resolve the haplotypes of A. annua. Approximately 50,000 genes were annotated for each haplotype genome, and a triplication event that occurred approximately 58.12 million years ago was examined for the first time in this species. A total of 3,903,467-5,193,414 variants (SNPs, indels, and structural variants) were identified in the 1.5-Gb genome during pairwise comparison between haplotypes, consistent with the high heterozygosity of this species. Genomic analyses revealed a correlation between artemisinin concents and the copy number of amorpha-4,11-diene synthase genes. This correlation was further confirmed by resequencing of 36 A. annua samples with varied artemisinin contents. Circular consensus sequencing of transcripts facilitated the detection of paralog expression. Collectively, our study provides chromosome-level allele-aware genome assemblies for two A. annua strains and new insights into the biosynthesis of artemisinin and its regulation, which will contribute to conquering malaria worldwide.

The truncated AaActin1 promoter is a candidate tool for metabolic engineering of artemisinin biosynthesis in Artemisia annua L

Malaria is a devastating parasitic disease with high levels of morbidity and mortality worldwide. Artemisinin, the active substance against malaria, is a sesquiterpenoid produced by Artemisia annua. To improve artemisinin content in the native A. annua plants, considerable efforts have been attempted, with genetic transformation serving as an effective strategy. Although, the most frequently-used cauliflower mosaic virus (CaMV) 35S (CaMV35S) promoter has proved to be efficient in A. annua transgenic studies, it appears to show weak activity in peltate glandular secretory trichomes (GSTs) of A. annua plants. Here, we characterized the 1727 bp fragment upstream from the translation start codon (ATG) of AaActin1, however, found it was inactive in tobacco. After removal of the 5' intron, the truncated AaActin1 promoter (tpACT) showed 69% and 50% activity of CaMV35S promoter in transiently transformed tobacco and stably transformed A. annua, respectively. β-glucuronidase (GUS) staining analysis showed that the tpACT promoter was capable of directing the constant expression of a foreign gene in peltate GSTs of transgenic A. annua, representing higher activity than CaMV35S promoter. Collectively, our study provided a novel promoter available for metabolic engineering of artemisinin biosynthesis in A. annua.

Exogenous hydrogen sulphide alleviates copper stress impacts in Artemisia annua L.: Growth, antioxidant metabolism, glandular trichome development and artemisinin biosynthesis

A supply of plant micronutrients (some of which are metals) is necessary to regulate many plant processes; their excess, however, can have detrimental consequences and can hamper plant growth, physiology and metabolism. Artemisia annua is an important crop plant used in the treatment of malaria. In this investigation, the physio-biochemical mechanisms involved in exogenous hydrogen sulphide-mediated (H₂ S) alleviation of copper (Cu) stress in A. annua were assessed.. Two different levels of Cu (20, 40 mg·kg^-1 ), one H₂ S treatment (200 µm) and their combinations were introduced while one set of plants was retained as control. Results showed that the presence of excess Cu in the soil reduced growth and biomass, photosynthetic parameters, chlorophyll content and fluorescence, gas exchange parameters and induced antioxidant enzyme activity. Copper stress enhanced the production of thiobarbituric acid reactive substances (TBARS) and increased Cu content in both roots and shoots of affected plants. Exogenous application of H₂ S restored the physio-biochemical characteristics of Cu-treated A. annua plants by reducing lipid peroxidation and enhancing the activity of antioxidant enzymes in Cu-stressed plants as compared with the controls. Hydrogen sulphide also reduced the Cu content in different plant parts, increased photosynthetic efficiency, trichome density, average area of trichomes and artemisinin content. Therefore, our results provide a comprehensive assessment of the defensive role of H₂ S in Cu-stressed A. annua.

Characterisation of the Mentha canadensis R2R3-MYB transcription factor gene McMIXTA and its involvement in peltate glandular trichome development

BACKGROUND

Mentha canadensis L. has important economic value for the production of essential oils, which are synthesised, secreted and stored in peltate glandular trichomes. As a typical multicellular secretory trichome, glandular trichomes are important biological factories for the synthesis of some specialised metabolites. However, little is known about the molecular mechanism of glandular trichome development in M. canadensis.

RESULTS

In this study, the R2R3-MYB transcription factor gene McMIXTA was isolated to investigate its function in glandular trichome development. Bioinformatics analysis indicated that McMIXTA belonged to the subgroup 9 R2R3-MYB, with a R2R3 DNA-binding domain and conserved subgroup 9 motifs. A subcellular localisation assay indicated that McMIXTA was localised in the nucleus. Transactivation analysis indicated that McMIXTA was a positive regulator, with transactivation regions located between positions N253 and N307. Yeast two-hybrid and bimolecular fluorescence complementation assays showed that McMIXTA formed a complex with McHD-Zip3, a trichome development-related HD-ZIP IV transcription factor. Overexpression of McMIXTA in Mentha × piperita L. caused an increase in peltate glandular trichomes density of approximately 25% on the leaf abaxial surface.

CONCLUSIONS

Our results demonstrated that the subgroup 9 R2R3-MYB transcription factor McMIXTA has a positive effect on regulating peltate glandular trichome development and the MIXTA/HD-ZIP IV complexes might be conserved regulators for glandular trichome initiation. These results provide useful information for revealing the regulatory mechanism of multicellular glandular trichome development.

Exogenous Strigolactone (GR24) Positively Regulates Growth, Photosynthesis, and Improves Glandular Trichome Attributes for Enhanced Artemisinin Production in Artemisia annua

Artemisia annua is a medicinal plant particularly known for the production of a sesquiterpene lactone artemisinin; a specialty metabolite known for its efficacy in the treatment of malaria by killing different strains of Plasmodium falciparum due to radicals released upon the cleavage of its endoperoxide motif. Considering these facts and the immense medicinal value of artemisinin, the enhancement of in planta production of artemisinin is highly desirable. As strigolactones are known to regulate various aspects of plant growth and development, the effects of foliar spray of different concentrations of synthetic strigolactone analog GR24 (0, 0.5, 1, 2, 4, and 8 µM) on A. annua were studied. As compared to the control group, the foliar application of GR24 had a positive impact on general growth, photosynthesis, and other physiological indices with 4 µM GR24 showing the best results. The results indicate that GR24 application increased the plant biomass and various attributes related to photosynthesis, like total chlorophyll content, chlorophyll fluorescence, stomatal conductance, internal CO2, and net photosynthetic rate. Moreover, the activity of various enzymes related to photosynthesis like carbonic anhydrase, nitrate reductase, and RuBisCO was escalated. The GR24 also improved certain attributes related to glandular trichomes, with a significant enhancement in content and yield of artemisinin as compared to untreated plants.

AaSPL9 affects glandular trichomes initiation by positively regulating expression of AaHD1 in Artemisia annua L

Glandular trichomes can secrete and store a large number of secondary metabolites in plants, some of which are of high medicinal and commercial value. For example, artemisinin, isolated from Artemisia annua L. plants, and its derivatives have great high medicinal value. Previous research indicated that artemisinin was synthesized in the glandular trichomes on the leaves of A. annua. It is an important study direction to improve artemisinin yield by promoting the initiation and development of glandular trichome. In this study, SQUAMOSA promoter-binding protein-like 9 (AaSPL9) was identified. In AaSPL9 overexpression transgenic plants, the glandular trichomes density was increased by 45-60 %, and the content of artemisinin was increased by 33-60 %, indicating that AaSPL9 positively regulate the glandular trichomes initiation. Yeast one-hybrid(Y1H), Dual-luciferase (Dual-Luc), Electrophoretic Mobility Shift Assay (EMSA) demonstrated that AaSPL9 activated the expression of AaHD1 by combining directly the GTAC-box of the AaHD1 promoter. Taken together, we identified AaSPL9, a positive transcription factor, regulating the glandular trichome initiation in A. annua, and revealed a novel molecular mechanism by which a SPL protein to promote glandular trichome initiation.

AaHY5 ChIP-seq based on transient expression system reveals the role of AaWRKY14 in artemisinin biosynthetic gene regulation

ChIP-seq (Chromatin immunoprecipitation with sequencing) is the gold standard for determining genome-wide in vivo transcription factor binding sites, the first step for targets prediction and network construction. For non-model plants, it is challenging to perform ChIP-seq due to the difficulty in generating stable transgenic plants. AaHY5 is a positive regulator in artemisinin biosynthesis, whose detailed mode of action remains elusive. Here, we established a protoplast transformation procedure for Artemisia annua by optimizing different conditions in protoplast isolation and transfection. We then performed AaHY5 ChIP-seq based on the established transient expression system. Combining RNA-seq data for various tissues, we identified four transcription factors (one MYB and three WRKY family members) in AaHY5 targets that potentially regulated artemisinin biosynthesis. The three WRKY transcription factors could be induced by light and the overexpression of AaHY5 and upregulate two artemisinin biosynthetic genes, ADS and CYP71AV1. Furthermore, AaWRKY14 showed transcriptional activation activity on artemisinin biosynthetic gene CYP71AV1. Together, AaWRKY14 was identified as a potential transcription factor linking AaHY5 and the artemisinin biosynthetic gene regulation.

Study of the genetic and phenotypic variation among wild and cultivated clary sages provides interesting avenues for breeding programs of a perfume, medicinal and aromatic plant

A road-map of the genetic and phenotypic diversities in both crops and their wild related species can help identifying valuable genetic resources for further crop breeding. The clary sage (Salvia sclarea L.), a perfume, medicinal and aromatic plant, is used for sclareol production and ornamental purposes. Despite its wide use in the field of cosmetics, the phenotypic and genetic diversity of wild and cultivated clary sages remains to be explored. We characterized the genetic and phenotypic variation of a collection of six wild S. sclarea populations from Croatia, sampled along an altitudinal gradient, and, of populations of three S. sclarea cultivars. We showed low level of genetic diversity for the two S. sclarea traditional cultivars used for essential oil production and for ornamental purposes, respectively. In contrast, a recent cultivar resulting from new breeding methods, which involve hybridizations among several genotypes rather than traditional recurrent selection and self-crosses over time, showed high genetic diversity. We also observed a marked phenotypic differentiation for the ornamental clary sage compared with other cultivated and wild clary sages. Instead, the two cultivars used for essential oil production, a traditional and a recent one, respectively, were not phenotypically differentiated from the wild Croatian populations. Our results also featured some wild populations with high sclareol content and early-flowering phenotypes as good candidates for future breeding programs. This study opens up perspectives for basic research aiming at understanding the impact of breeding methods on clary sage evolution, and highlights interesting avenues for clary breeding programs.

Chromatin Accessibility Is Associated with Artemisinin Biosynthesis Regulation in Artemisia annua

Glandular trichome (GT) is the dominant site for artemisinin production in Artemisia annua. Several critical genes involved in artemisinin biosynthesis are specifically expressed in GT. However, the molecular mechanism of differential gene expression between GT and other tissue types remains elusive. Chromatin accessibility, defined as the degree to which nuclear molecules are able to interact with chromatin DNA, reflects gene expression capacity to a certain extent. Here, we investigated and compared the landscape of chromatin accessibility in Artemisia annua leaf and GT using the Assay for Transposase-Accessible Chromatin using sequencing (ATAC-seq) technique. We identified 5413 GT high accessible and 4045 GT low accessible regions, and these GT high accessible regions may contribute to GT-specific biological functions. Several GT-specific artemisinin biosynthetic genes, such as DBR2 and CYP71AV1, showed higher accessible regions in GT compared to that in leaf, implying that they might be regulated by chromatin accessibility. In addition, transcription factor binding motifs for MYB, bZIP, C2H2, and AP2 were overrepresented in the highly accessible chromatin regions associated with artemisinin biosynthetic genes in glandular trichomes. Finally, we proposed a working model illustrating the chromatin accessibility dynamics in regulating artemisinin biosynthetic gene expression. This work provided new insights into epigenetic regulation of gene expression in GT.

Mulberry Protection through Flowering-Stage Essential Oil of Artemisia annua against the Lesser Mulberry Pyralid, Glyphodes pyloalis Walker

In the present study, the toxicity and physiological disorders of the essential oil isolated from Artemisia annua flowers were assessed against one of the main insect pests of mulberry, Glyphodes pyloalis Walker, announcing one of the safe and effective alternatives to synthetic pesticides. The LC₅₀ (lethal concentration to kill 50% of tested insects) values of the oral and fumigant bioassays of A. annua essential oil were 1.204 % W/V and 3.343 μL/L air, respectively. The A. annua essential oil, rich in camphor, artemisia ketone, β-selinene, pinocarvone, 1,8-cineole, and α-pinene, caused a significant reduction in digestive and detoxifying enzyme activity of G. pyloalis larvae. The contents of protein, glucose, and triglyceride were also reduced in the treated larvae by oral and fumigant treatments. The immune system in treated larvae was weakened after both oral and fumigation applications compared to the control groups. Histological studies on the midgut and ovaries showed that A. annua essential oil caused an obvious change in the distribution of the principal cells of tissues and reduction in yolk spheres in oocytes. Therefore, it is suggested that the essential oil from A. annua flowers, with wide-range bio-effects on G. pyloalis, be used as an available, safe, effective insecticide in the protection of mulberry.

An R2R3-MYB Transcription Factor Positively Regulates the Glandular Secretory Trichome Initiation in Artemisia annua L

Artemisia annua L. is known for its specific product "artemisinin" which is an active ingredient for curing malaria. Artemisinin is secreted and accumulated in the glandular secretory trichomes (GSTs) on A. annua leaves. Earlier studies have shown that increasing GST density is effective in increasing artemisinin content. However, the mechanism of GST initiation is not fully understood. To this end, we isolated and characterized an R2R3-MYB gene, AaMYB17, which is expressed specifically in the GSTs of shoot tips. Overexpression of AaMYB17 in A. annua increased GST density and enhanced the artemisinin content, whereas RNA interference of AaMYB17 resulted in the reduction of GST density and artemisinin content. Additionally, neither overexpression lines nor RNAi lines showed an abnormal phenotype in plant growth and the morphology of GSTs. Our study demonstrates that AaMYB17 is a positive regulator of GSTs' initiation, without influencing the trichome morphology.

Glandular trichomes of Robinia viscosa Vent. var. hartwigii (Koehne) Ashe (Faboideae, Fabaceae)-morphology, histochemistry and ultrastructure

MAIN CONCLUSION

Permanent glandular trichomes of Robinia viscosa var. hartwigii produce viscous secretion containing several secondary metabolites, as lipids, mucilage, flavonoids, proteins and alkaloids. Robinia viscosa var. hartwigii (Hartweg's locust) is an ornamental tree with high apicultural value. It can be planted in urban greenery and in degraded areas. The shoots, leaves, and inflorescences of this plant are equipped with numerous persistent glandular trichomes producing sticky secretion. The distribution, origin, development, morphology, anatomy, and ultrastructure of glandular trichomes of Hartweg's locust flowers as well as the localisation and composition of their secretory products were investigated for the first time. To this end, light, scanning, and transmission electron microscopy combined with histochemical and fluorescence techniques were used. The massive glandular trichomes differing in the distribution, length, and stage of development were built of a multicellular and multiseriate stalk and a multicellular head. The secretory cells in the stalk and head had large nuclei with nucleoli, numerous chloroplasts with thylakoids and starch grains, mitochondria, endoplasmic reticulum profiles, Golgi apparatus, vesicles, and multivesicular bodies. Many vacuoles contained phenolic compounds dissolved or forming various condensed deposits. The secretion components were transported through symplast elements, and the granulocrine and eccrine modes of nectar secretion were observed. The secretion was accumulated in the subcuticular space at the trichome apex and released through a pore in the cuticle. Histochemical and fluorescence assays showed that the trichomes and secretion contained lipophilic and polyphenol compounds, polysaccharides, proteins, and alkaloids. We suggest that these metabolites may serve an important function in protection of plants against biotic stress conditions and may also be a source of phytopharmaceuticals in the future.

Identification of the Volatile Compounds and Observation of the Glandular Trichomes in Opisthopappus taihangensis and Four Species of Chrysanthemum

Opisthopappus taihangensis (Ling) Shih, a wild relative germplasm of chrysanthemum, releases a completely different fragrance from chrysanthemum species. We aimed to identify the volatile compounds of the leaves of O. taihangensis and four other Chrysanthemum species using headspace solid-phase micro-extraction combined with gas chromatography-mass spectrometry (HS-SPME-GC/MS). In total, 70 compounds were detected, and terpenoids accounted for the largest percentage in these five species. Many specific compounds were only emitted from O. taihangensis and not from the other four species. In particular, 1,8-cineole could be responsible for the special leaf fragrance of O. taihangensis as it accounted for the largest proportion of the compounds in O. taihangensis but a small or no proportion at all in other species. The glandular trichomes (GTs) in the leaves are the main organs responsible for the emission of volatiles. To explore the relationship between the emissions and the density of the GTs on the leaf epidermis, the shape and density of the GTs were observed and calculated, respectively. The results showed that the trichomes have two shapes in these leaves: T-shaped non-glandular trichomes and capitate trichomes. Histochemical staining analyses indicated that terpenoids are mainly emitted from capitate glandular trichomes. Correlation analysis showed that the volatile amount of terpenoids is highly related to the density of capitate trichomes. In O. taihangensis, the terpenoids content and density of capitate trichomes are the highest. We identified the diversity of leaf volatiles from O. taihangensis and four other Chrysanthemum species and found a possible relationship between the content of volatile compounds and the density of capitate trichomes, which explained the cause of the fragrance of O. taihangensis leaves.

Arbuscular Mycorrhizal Fungus Rhizophagus irregularis Influences Artemisia annua Plant Parameters and Artemisinin Content under Different Soil Types and Cultivation Methods

Artemisinin extracted from Artemisia annua has been used efficiently in malaria treatment since 2005. In this study, the variations in plant parameters (plant biomass, glandular trichome density, essential oil total chemical content, artemisinin production, and polyphenol oxidase (PPO) activity) were tested under different soil types (Luvisol, Gleysol, Anthrosol and sterile peat) and cultivation conditions (potted plants in semi-open field, and open field experiments) for plants inoculated with arbuscular mycorrhizal fungus (AMF) Rizophagus irregularis. Under semi-open field conditions, the AMF colonization of A. annua plant roots varied, and presented the highest percentage in Luvisol and sterile peat. The increase in the root colonization rate positively influenced some plant parameters (biomass, glandular trichome density, artemisinin concentration, essential oil quantity and composition), but no effects on PPO enzyme activity were detected. AMF fungus R. irregularis significantly increased the artemisinin content and essential oil yield of plants cultivated in Luvisol, Gleysol, Anthrosol and in peat. These soil types can offer appropriate conditions for A. annua cultivation and artemisinin production even on a smaller scale. Under open field conditions, low (about 5%) AMF colonization was observed. No differences in artemisin contents were detected, but essential oil yield significantly increased compared to control plants. AMF treatment increased beta-farnesene and germacrene D concentrations in Artemisia plants in the open field experiment.

Genetic Control of Glandular Trichome Development

Plant glandular trichomes are epidermal secretory structures producing various specialized metabolites. These metabolites are involved in plant adaptation to its environment and many of them have remarkable properties exploited by fragrance, flavor, and pharmaceutical industries. The identification of genes controlling glandular trichome development is of high interest to understand how plants produce specialized metabolites. Our knowledge about this developmental process is still limited, but genes controlling glandular trichome initiation and morphogenesis have recently been identified. In particular, R2R3-MYB and HD-ZIP IV transcription factors appear to play essential roles in glandular trichome initiation in Artemisia annua and tomato. In this review, we focus on the results obtained in these two species and we propose genetic regulation models integrating these data.

Using β-ocimene to increase the artemisinin content in juvenile plants of Artemisia annua L

OBJECTIVE

The anti-malarial drug, artemisinin, is harvested from the leaves of adult Artemisia annua L. plants. As its concentration in juvenile plants is very low, the present study aimed to assess if the airborne signaling molecule, β-ocimene, could be used to enhance artemisinin accumulation in juvenile A. annua plants.

RESULTS

Application of exogenous β-ocimene increased artemisinin accumulation in A. annua. Treatment with 10 µM β-ocimene for 4 days resulted in juvenile plants accumulating artemisinin contents of up to 25 mg/g (2.5%) of dry weight. The expression levels of key genes encoding enzymes involved in both precursor biosynthetic pathways and artemisinin biosynthetic pathways induced by β-ocimene were upregulated. Glandular secretory trichome (GST) size and density increased by 49.2% and 38.2%, respectively, along with the upregulation of genes associated with GST development.

CONCLUSION

β-ocimene enhances artemisinin accumulation in juvenile A. annua plants by modulating artemisinin biosynthetic pathways and GST development.

Artemisia annua L. and photoresponse: from artemisinin accumulation, volatile profile and anatomical modifications to gene expression

Blue and yellow light affected metabolism and the morphology. Blue and red promote the DOXP/MEP pathway. ADS gene expression was increased in plants cultivated under blue, promoting artemisinin content. Artemisinin-based combination therapies are the most effective treatment for highly lethal malaria. Artemisinin is produced in small quantities in the glandular trichomes of Artemisia annua L. Our aim was to evaluate the effect of light quality in A. annua cultivated in vitro under different light qualities, considering anatomical and morphological changes, the volatile composition, artemisinin content and the expression of two key enzymes for artemisinin biosynthesis. Yellow light is related to the increase in the number of glandular trichomes and this seemed to positively affect the molecular diversity in A. annua. Yellow light-stimulated glandular trichome frequency without triggered area enhancement, whereas blue light stimulated both parameters. Blue light enhanced the thickness of the leaf epidermis. The B-promoting effect was due to increased cell size and not to increased cell numbers. Green and yellow light positively influenced the volatile diversity in the plantlets. Nevertheless, blue and red light seemed to promote the DOXP/MEP pathway, while red light stimulates MVA pathway. Amorpha-4,11-diene synthase gene expression was significantly increased in plants cultivated under blue light, and not red light, promoting artemisinin content. Our results showed that light quality, more specifically blue and yellow light, positively affected secondary metabolism and the morphology of plantlets. It seemed that steps prior to the last one in the artemisinin biosynthesis pathway could be strongly influenced by blue light. Our work provides an alternative method to increase the amount of artemisinin production in A. annua without the use of transgenic plants, by the employment of blue light.

The Dynamic Genetic-Hormonal Regulatory Network Controlling the Trichome Development in Leaves

Plant trichomes are outgrowths developed from an epidermal pavement cells of leaves and other organs. Trichomes (also called 'hairs') play well-recognized roles in defense against insect herbivores, forming a physical barrier that obstructs insect movement and mediating chemical defenses. In addition, trichomes can act as a mechanosensory switch, transducing mechanical stimuli (e.g., insect movement) into physiological signals, helping the plant to respond to insect attacks. Hairs can also modulate plant responses to abiotic stresses, such as water loss, an excess of light and temperature, and reflect light to protect plants against UV radiation. The structure of trichomes is species-specific and this trait is generally related to their function. These outgrowths are easily analyzed and their origin represents an outstanding subject to study epidermal cell fate and patterning in plant organs. In leaves, the developmental control of the trichomatous complement has highlighted a regulatory network based on four fundamental elements: (i) genes that activate and/or modify the normal cell cycle of epidermal pavement cells (i.e., endoreduplication cycles); (ii) transcription factors that create an activator/repressor complex with a central role in determining cell fate, initiation, and differentiation of an epidermal cell in trichomes; (iii) evidence that underlines the interplay of the aforesaid complex with different classes of phytohormones; (iv) epigenetic mechanisms involved in trichome development. Here, we reviewed the role of genes in the development of trichomes, as well as the interaction between genes and hormones. Furthermore, we reported basic studies about the regulation of the cell cycle and the complexity of trichomes. Finally, this review focused on the epigenetic factors involved in the initiation and development of hairs, mainly on leaves.

Artemisinin Biosynthesis in Non-glandular Trichome Cells of Artemisia annua

Artemisinin-based combination therapy (ACT) forms the first line of malaria treatment. However, the yield fluctuation of artemisinin has remained an unsolved problem in meeting the global demand for ACT. This problem is mainly caused by the glandular trichome (GT)-specific biosynthesis of artemisinin in all currently used Artemisia annua cultivars. Here, we report that non-GT cells of self-pollinated inbred A. annua plants can express the artemisinin biosynthetic pathway. Gene expression analysis demonstrated the transcription of six known pathway genes in GT-free leaves and calli of inbred A. annua plants. LC-qTOF-MS/MS analysis showed that these two types of GT-free materials produce artemisinin, artemisinic acid, and arteannuin B. Detailed IR-MALDESI image profiling revealed that these three metabolites and dihydroartemisinin are localized in non-GT cells of leaves of inbred A. annua plants. Moreover, we employed all the above approaches to examine artemisinin biosynthesis in the reported A. annua glandless (gl) mutant. The resulting data demonstrated that leaves of regenerated gl plantlets biosynthesize artemisinin. Collectively, these findings not only add new knowledge leading to a revision of the current dogma of artemisinin biosynthesis in A. annua but also may expedite innovation of novel metabolic engineering approaches for high and stable production of artemisinin in the future.

Expression of key genes affecting artemisinin content in five Artemisia species

Artemisinin, an effective anti-malarial drug is synthesized in the specialized 10-celled biseriate glandular trichomes of some Artemisia species. In order to have an insight into artemisinin biosynthesis in species other than A. annua, five species with different artemisinin contents were investigated for the expression of key genes that influence artemisinin content. The least relative expression of the examined terpene synthase genes accompanied with very low glandular trichome density (4 No. mm⁻²) and absence of artemisinin content in A. khorassanica (S2) underscored the vast metabolic capacity of glandular trichomes. A. deserti (S4) with artemisinin content of 5.13 mg g⁻¹ DW had a very high expression of Aa-ALDH1 and Aa-CYP71AV1 and low expression of Aa-DBR2. It is possible to develop plants with high artemisinin synthesis ability by downregulating Aa-ORA in S4, which may result in the reduction of Aa-ALDH1 and Aa-CYP71AV1 genes expression and effectively change the metabolic flux to favor more of artemisinin production than artemisinic acid. Based on the results, the Aa-ABCG6 transporter may be involved in trichome development. S4 had high transcript levels and larger glandular trichomes (3.46 fold) than A. annua found in Iran (S1), which may be due to the presence of more 2C-DNA (3.48 fold) in S4 than S1.

Overexpression of Artemisia annua Cinnamyl Alcohol Dehydrogenase Increases Lignin and Coumarin and Reduces Artemisinin and Other Sesquiterpenes

Artemisia annua is the only medicinal crop that produces artemisinin for malarial treatment. Herein, we describe the cloning of a cinnamyl alcohol dehydrogenase (AaCAD) from an inbred self-pollinating (SP) A. annua cultivar and its effects on lignin and artemisinin production. A recombinant AaCAD was purified via heterogeneous expression. Enzyme assays showed that the recombinant AaCAD converted p-coumaryl, coniferyl, and sinapyl aldehydes to their corresponding alcohols, which are key intermediates involved in the biosynthesis of lignin. Km, Vmax, and Vmax/Km values were calculated for all three substrates. To characterize its function in planta, AaCAD was overexpressed in SP plants. Quantification using acetyl bromide (AcBr) showed significantly higher lignin contents in transgenics compared with wild-type (WT) plants. Moreover, GC-MS-based profiling revealed a significant increase in coumarin contents in transgenic plants. By contrast, HPLC-MS analysis showed significantly reduced artemisinin contents in transgenics compared with WT plants. Furthermore, GC-MS analysis revealed a decrease in the contents of arteannuin B and six other sesquiterpenes in transgenic plants. Confocal microscopy analysis showed the cytosolic localization of AaCAD. These data demonstrate that AaCAD plays a dual pathway function in the cytosol, in which it positively enhances lignin formation but negatively controls artemisinin formation. Based on these data, crosstalk between these two pathways mediated by AaCAD catalysis is discussed to understand the metabolic control of artemisinin biosynthesis in plants for high production.

Seasonal and Differential Sesquiterpene Accumulation in Artemisia annua Suggest Selection Based on Both Artemisinin and Dihydroartemisinic Acid may Increase Artemisinin in planta

Commercial Artemisia annua crops are the sole source of artemisinin (ART) worldwide. Data on seasonal accumulation and peak of sesquiterpenes, especially ART in commercial A. annua, is lacking while current breeding programs focus only on ART and plant biomass, but ignores dihydroartemisinic acid (DHAA) and artemisinic acid (AA). Despite past breeding successes, plants richer in ART are needed to decrease prices of artemisinin-combination therapy (ACT). Our results show that sesquiterpene concentrations vary greatly along the growing season and that sesquiterpene profiles differ widely among chemotypes. Field studies with elite Brazilian, Chinese, and Swiss germplasms established that ART peaked in vegetative plants from late August to early September, suggesting that ART is related to the photoperiod, not flowering. DHAA peaks with ART in Chinese and Swiss plants, but decreases, as ART increases, in Brazilian plants, while AA remained stable through the season in these genotypes. Chinese plants peaked at 0.9% ART, 1.6% DHAA; Brazilian plants at 0.9% ART, with less than 0.4% DHAA; Swiss plants at 0.8% ART and 1% DHAA. At single-date harvests, seeded Swiss plants produced 0.55-1.2% ART, with plants being higher in DHAA than ART; Brazilian plants produced 0.33-1.5% ART, with most having higher ART than DHAA. Elite germplasms produced from 0.02-0.43% AA, except Sandeman-UK (0.4-1.1% AA). Our data suggest that different chemotypes, high in ART and DHAA, have complementary pathways, while competing with AA. Crossing plants high in ART and DHAA may generate hybrids with higher ART than currently available in commercial germplasms. Selecting for high ART and DHAA (and low AA) can be a valuable approach for future selection and breeding to produce plants more efficient in transforming DHAA into ART in planta and during post-harvest. This novel approach could change the breeding focus of A. annua and other pharmaceutical species that produce more than one desired metabolite in the same pathway. Obtaining natural variants with high ART content will empower countries and farmers who select, improve, and cultivate A. annua as a commercial pharmaceutical crop. This selection approach could enable ART to be produced locally where it is most needed to fight malaria and other parasitic neglected diseases.

A Review of Biotechnological Artemisinin Production in Plants

Malaria is still an eminent threat to major parts of the world population mainly in sub-Saharan Africa. Researchers around the world continuously seek novel solutions to either eliminate or treat the disease. Artemisinin, isolated from the Chinese medicinal herb Artemisia annua, is the active ingredient in artemisinin-based combination therapies used to treat the disease. However, naturally artemisinin is produced in small quantities, which leads to a shortage of global supply. Due to its complex structure, it is difficult chemically synthesize. Thus to date, A. annua remains as the main commercial source of artemisinin. Current advances in genetic and metabolic engineering drives to more diverse approaches and developments on improving in planta production of artemisinin, both in A. annua and in other plants. In this review, we describe efforts in bioengineering to obtain a higher production of artemisinin in A. annua and stable heterologous in planta systems. The current progress and advancements provides hope for significantly improved production in plants.

Overexpression of a type-I isopentenyl pyrophosphate isomerase of Artemisia annua in the cytosol leads to high arteannuin B production and artemisinin increase

We recently characterized a gene-terpene network that is associated with artemisinin biosynthesis in self-pollinated (SP) Artemisia annua, an effective antimalarial plant. We hypothesize that an alteration of gene expression in the network may improve the production of artemisinin and its precursors. In this study, we cloned an isopentenyl pyrophosphate isomerase (IPPI) cDNA, AaIPPI1, from Artemisia annua (Aa). The full-length cDNA encodes a type-I IPPI containing a plastid transit peptide (PTP) at its amino terminus. After the removal of the PTP, the recombinant truncated AaIPPI1 isomerized isopentenyl pyrophosphate (IPP) to dimethyl allyl pyrophosphate (DMAPP) and vice versa. The steady-state equilibrium ratio of IPP/DMAPP in the enzymatic reactions was approximately 1:7. The truncated AaIPPI1 was overexpressed in the cytosol of the SP A. annua variety. The leaves of transgenic plants produced approximately 4% arteannuin B (g g^-1 , dry weight, dw) and 0.17-0.25% artemisinin (g g^-1 , dw), the levels of which were significantly higher than those in the leaves of wild-type plants. In addition, transgenic plants showed an increase in artemisinic acid production of more than 1% (g g^-1 , dw). In contrast, isoprene formation was significantly reduced in transgenic plants. These results provide evidence that overexpression of AaIPPI1 in the cytosol can lead to metabolic alterations of terpenoid biosynthesis, and show that these transgenic plants have the potential to yield high production levels of arteannuin B as a new precursor source for artemisinin.

HOMEODOMAIN PROTEIN 1 is required for jasmonate-mediated glandular trichome initiation in Artemisia annua

Glandular trichomes are generally considered biofactories that produce valuable chemicals. Increasing glandular trichome density is a very suitable way to improve the productivity of these valuable metabolites, but little is known about the regulation of glandular trichome formation. Phytohormone jasmonate (JA) promotes glandular trichome initiation in various plants, but its mechanism is also unknown. By searching transcription factors regulated by JA in Artemisia annua, we identified a novel homeodomain-leucine zipper transcription factor, HOMEODOMAIN PROTEIN 1 (AaHD1), which positively controls both glandular and nonglandular trichome initiations. Overexpression of AaHD1 in A. annua significantly increased glandular trichome density without harming plant growth. Consequently, the artemisinin content was improved. AaHD1 interacts with A. annua jasmonate ZIM-domain 8 (AaJAZ8), which is a repressor of JA, thereby resulting in decreased transcriptional activity. AaHD1 knockdown lines show decreased sensitivity to JA on glandular trichome initiation, which indicates that AaHD1 plays an important role in JA-mediated glandular trichome initiation. We identified a new transcription factor that promotes A. annua glandular trichome initiation and revealed a novel molecular mechanism by which a homeodomain protein transduces JA signal to promote glandular trichome initiation. Our results also suggested a connection between glandular and nonglandular trichome formations.