Overexpression of the Artemisia orthologue of ABA receptor, AaPYL9, enhances ABA sensitivity and improves artemisinin content in Artemisia annua L

Molecular regulation of the key specialized metabolism pathways in medicinal plants

The basis of modern pharmacology is the human ability to exploit the production of specialized metabolites from medical plants, for example, terpenoids, alkaloids, and phenolic acids. However, in most cases, the availability of these valuable compounds is limited by cellular or organelle barriers or spatio-temporal accumulation patterns within different plant tissues. Transcription factors (TFs) regulate biosynthesis of these specialized metabolites by tightly controlling the expression of biosynthetic genes. Cutting-edge technologies and/or combining multiple strategies and approaches have been applied to elucidate the role of TFs. In this review, we focus on recent progress in the transcription regulation mechanism of representative high-value products and describe the transcriptional regulatory network, and future perspectives are discussed, which will help develop high-yield plant resources.

The Light- and Jasmonic Acid-Induced AaMYB108-like Positive Regulates the Initiation of Glandular Secretory Trichome in Artemisia annua L

The plant Artemisia annua L. is famous for producing "artemisinin", which is an essential component in the treatment of malaria. The glandular secretory trichomes (GSTs) on the leaves of A. annua secrete and store artemisinin. Previous research has demonstrated that raising GST density can effectively raise artemisinin content. However, the molecular mechanism of GST initiation is not fully understood yet. In this study, we identified an MYB transcription factor, the AaMYB108-like, which is co-induced by light and jasmonic acid, and positively regulates glandular secretory trichome initiation in A. annua. Overexpression of the AaMYB108-like gene in A. annua increased GST density and enhanced the artemisinin content, whereas anti-sense of the AaMYB108-like gene resulted in the reduction in GST density and artemisinin content. Further experiments demonstrated that the AaMYB108-like gene could form a complex with AaHD8 to promote the expression of downstream AaHD1, resulting in the initiation of GST. Taken together, the AaMYB108-like gene is a positive regulator induced by light and jasmonic acid for GST initiation in A. annua.

GhPYL9-5D and GhPYR1-3 A positively regulate Arabidopsis and cotton responses to ABA, drought, high salinity and osmotic stress

BACKGROUND

Abscisic acid (ABA) receptor pyrabactin resistance 1/PYR1-like/regulatory components of ABA receptor proteins (PYR/PYL/RCARs) have been demonstrated to play pivotal roles in ABA signaling and in response to diverse environmental stimuli including drought, salinity and osmotic stress in Arabidopsis. However, whether and how GhPYL9-5D and GhPYR1-3A, the homologues of Arabidopsis PYL9 and PYR1 in cotton, function in responding to ABA and abiotic stresses are still unclear.

RESULTS

GhPYL9-5D and GhPYR1-3A were targeted to the cytoplasm and nucleus. Overexpression of GhPYL9-5D and GhPYR1-3A in Arabidopsis wild type and sextuple mutant pyr1pyl1pyl2pyl4pyl5pyl8 plants resulted in ABA hypersensitivity in terms of seed germination, root growth and stomatal closure, as well as seedling tolerance to water deficit, salt and osmotic stress. Moreover, the VIGS (Virus-induced gene silencing) cotton plants, in which GhPYL9-5D or GhPYR1-3A were knocked down, showed clearly reduced tolerance to polyethylene glycol 6000 (PEG)-induced drought, salinity and osmotic stresses compared with the controls. Additionally, transcriptomic data revealed that GhPYL9-5D was highly expressed in the root, and GhPYR1-3A was strongly expressed in the fiber and stem. GhPYL9-5D, GhPYR1-3A and their homologs in cotton were highly expressed after treatment with PEG or NaCl, and the two genes were co-expressed with redox signaling components, transcription factors and auxin signal components. These results suggest that GhPYL9-5D and GhPYR1-3A may serve important roles through interplaying with hormone and other signaling components in cotton adaptation to salt or osmotic stress.

CONCLUSIONS

GhPYL9-5D and GhPYR1-3A positively regulate ABA-mediated seed germination, primary root growth and stomatal closure, as well as tolerance to drought, salt and osmotic stresses likely through affecting the expression of multiple downstream stress-associated genes in Arabidopsis and cotton.

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.

Trihelix Transcriptional Factor GhGT26 of Cotton Enhances Salinity Tolerance in Arabidopsis

Cotton (Gossypium hirsutum L.), the most important textile crop worldwide, often encounters abiotic stress during its growing season and its productivity is significantly limited by adverse factors. Trihelix transcription factors (also known as GT factors) are important proteins involved in the morphological development and responses to abiotic stress in plants. However, their functions and molecular mechanisms in the cotton toward abiotic stress response remain unclear. In this study, a member (GhGT26) of the cotton Trihelix family was functionally characterized in the model plant Arabidopsis. This protein containing a SANT domain belongs to the GT-1 subgroup of trihelix proteins. GhGT26 was widely expressed in tissues (with the highest level in flower) and responded to high salt and ABA treatments at the transcriptional level. Using the Arabidopsis protoplast assay system, we found that the GhGT26 protein was located in the cell nuclei. The EMSA assay revealed that the GhGT26 protein could bind to the Site1-type GT cis elements (GT-3a) and MYB elements MRE3 and MRE4. The overexpression of GhGT26 improved plant tolerance to salt stress in transgenic Arabidopsis plants. Although ABA inhibits root elongation, the statistical analysis revealed that the root lengths of GhGT26-overexpressing Arabidopsis were the same as the wild plants after ABA treatment. Our results demonstrate that GhGT26 positively regulates salt stress via ABA-independent pathways. This evidence suggests that the GhGT26 may participate in the regulation of stress tolerance in cotton.

MADS-box gene AaSEP4 promotes artemisinin biosynthesis in Artemisia annua

The plant Artemisia annua is well known for its production of artemisinin, a sesquiterpene lactone that is an effective antimalarial compound. Although remarkable progress has been made toward understanding artemisinin biosynthesis, the effect of MADS-box family transcription factors on artemisinin biosynthesis is still poorly understood. In this study, we identified a MADS transcription factor, AaSEP4, that was predominantly expressed in trichome. AaSEP4 acts as a nuclear-localized transcriptional activator activating the expression of AaGSW1 (GLANDULAR TRICHOME-SPECIFIC WRKY1). Dual-luciferase and Yeast one-hybrid assays revealed that AaSEP4 directly bound to the CArG motif in the promoter region of AaGSW1. Overexpression of AaSEP4 in A. annua significantly induced the expression of AaGSW1 and four artemisinin biosynthesis genes, including amorpha-4,11-diene synthase (ADS), cytochrome P450 monooxygenase (CYP71AV1), double-bond reductase 2 (DBR2) and aldehyde dehydrogenase 1 (ALDH1). Furthermore, the results of high-performance liquid chromatography (HPLC) showed that the artemisinin content was significantly increased in the AaSEP4-overexpressed plants. In addition, RT-qPCR results showed that AaSEP4 was induced by methyl jasmonic acid (MeJA) treatment. Taken together, these results explicitly demonstrate that AaSEP4 is a positive regulator of artemisinin biosynthesis, which can be used in the development of high-artemisinin yielding A. annua varieties.

Enhancing artemisinin content in and delivery from Artemisia annua: a review of alternative, classical, and transgenic approaches

This review analyses the most recent scientific research conducted for the purpose of enhancing artemisinin production. It may help to devise better artemisinin enhancement strategies, so that its production becomes cost effective and becomes available to masses. Malaria is a major threat to world population, particularly in South-East Asia and Africa, due to dearth of effective anti-malarial compounds, emergence of quinine resistant malarial strains, and lack of advanced healthcare facilities. Artemisinin, a sesquiterpene lactone obtained from Artemisia annua L., is the most potent drug against malaria and used in the formulation of artemisinin combination therapies (ACTs). Artemisinin is also effective against various types of cancers, many other microbes including viruses, parasites and bacteria. However, this specialty metabolite and its derivatives generally occur in low amounts in the source plant leading to its production scarcity. Considering the importance of this drug, researchers have been working worldwide to develop novel strategies to augment its production both in vivo and in vitro. Due to complex chemical structure, its chemical synthesis is quite expensive, so researchers need to devise synthetic protocols that are economically viable and also work on increasing the in-planta production of artemisinin by using various strategies like use of phytohormones, stress signals, bioinoculants, breeding and transgenic approaches. The focus of this review is to discuss these artemisinin enhancement strategies, understand mechanisms modulating its biosynthesis, and evaluate if roots play any role in artemisinin production. Furthermore, we also have a critical analysis of various assays used for artemisinin measurement. This may help to develop better artemisinin enhancement strategies which lead to decreased price of ACTs and increased profit to farmers.

Genome-wide identification and characterization of ABA receptor PYL gene family in rice

BACKGROUND

Abscisic acid (ABA), a key phytohormone that controls plant growth and stress responses, is sensed by the pyrabactin resistance 1(PYR1)/PYR1-like (PYL)/regulatory components of the ABA receptor (RCAR) family of proteins. Comprehensive information on evolution and function of PYL gene family in rice (Oryza sativa) needs further investigation. This study made detailed analysis on evolutionary relationship between PYL family members, collinearity, synteny, gene structure, protein motifs, cis-regulatory elements (CREs), SNP variations, miRNAs targeting PYLs and expression profiles in different tissues and stress responses.

RESULTS

Based on sequence homology with Arabidopsis PYL proteins, we identified a total of 13 PYLs in rice (BOP clade) and maize (PACCMAD clade), while other members of BOP (wheat - each diploid genome, barley and Brachypodium) and PACCMAD (sorghum and foxtail millet) have 8-9 PYLs. The phylogenetic analysis divided PYLs into three subfamilies that are structurally and functionally conserved across species. Gene structure and motif analysis of OsPYLs revealed that members of each subfamily have similar gene and motif structure. Segmental duplication appears be the driving force for the expansion of PYLs, and the majority of the PYLs underwent evolution under purifying selection in rice. 32 unique potential miRNAs that might target PYLs were identified in rice. Thus, the predicted regulation of PYLs through miRNAs in rice is more elaborate as compared with B. napus. Further, the miRNAs identified to in this study were also regulated by stresses, which adds additional layer of regulation of PYLs. The frequency of SAPs identified was higher in indica cultivars and were predominantly located in START domain that participate in ABA binding. The promoters of most of the OsPYLs have cis-regulatory elements involved in imparting abiotic stress responsive expression. In silico and q-RT-PCR expression analyses of PYL genes revealed multifaceted role of ABARs in shaping plant development as well as abiotic stress responses.

CONCLUSION

The predicted miRNA mediated regulation of OsPYLs and stress regulated expression of all OsPYLs, at least, under one stress, lays foundation for further validation and fine tuning ABA receptors for stress tolerance without yield penalty in rice.

Transcriptome analyses revealed the ultraviolet B irradiation and phytohormone gibberellins coordinately promoted the accumulation of artemisinin in Artemisia annua L

Background

Artemisinin-based combination therapy has become the preferred approach for treating malaria and has successfully reduced malaria-related mortality. Currently, the main source of artemisinin is Artemisia annua L., and thus, it is of strategic importance to enhance artemisinin contents in A. annua plants. Phytohormones and illumination are known to be important external environmental factor that can have notable effects on the production of secondary metabolite. The activities of different hormones can be influenced to varying degrees by light, and thus light and hormones may jointly regulate various processes in plants. Here, we performed transcriptome and metabolome analyses revealed that ultraviolet B irradiation and phytohormone gibberellins coordinately promoted the accumulation of artemisinin in Artemisia annua.

Methods

Artemisinin analysis was performed by ultra-high performance liquid chromatography-tandem quadrupole mass spectrometry (UPLC-ESI-QqQ-MS/MS). RNA sequencing, GO and KEGG enrichment analysis were applied to analyzing the differentially expressed genes (DEGs) under ultraviolet B irradiation and gibberellins treatments. Weighted gene co-expression network (WGCNA) analyzed the genes in artemisinin‑related modules and identified candidate hub genes in these modules.

Results

In this study, we found that cross-talk between UV-B and GA induced processes leading to modifications in artemisinin accumulation. A total of 14,762 genes differentially expressed (DEGs) among different treatments were identified by transcriptome analysis. UV-B and GA treatments enhanced the accumulation of artemisinin by up-regulating the expression of the key artemisinin biosynthesis genes ADS and CYP71AV1. According to the high degree value and high expression level, a total of 84 co-expressed transcription factors were identified. Among them, MYB and NAC TFs mainly involved in regulating the biosynthesis of artemisinin. Weighted gene co-expression network analysis revealed that GA + UV in blue modules was positively correlated with artemisinin synthesis, suggesting that the candidate hub genes in these modules should be up-regulated to enhance artemisinin synthesis in response to GA + UV treatment.

Conclusion

Our study demonstrated the co-regulation of artemisinin biosynthetic pathway genes under ultraviolet B irradiation and phytohormone gibberellins treatment. The co-expression was analysis revealed that the selected MYB and NAC TFs might have regulated the artemisinin biosynthesis gene expression with ADS and CYP71AV1 genes. Weighted gene co-expression network analysis revealed that GA + UV treatment in blue modules was positively correlated with artemisinin synthesis. We established the network to distinguish candidate hub genes in blue modules might be up-regulated to enhance artemisinin synthesis in response to GA + UV treatment.

AaCOI1, Encoding a CORONATINE INSENSITIVE 1-Like Protein of Artemisia annua L., Is Involved in Development, Defense, and Anthocyanin Synthesis

Artemisia annua is an important medicinal plant producing the majority of the antimalarial compound artemisinin. Jasmonates are potent inducers of artemisinin accumulation in Artemisisa annua plants. As the receptor of jasmonates, the F-box protein COI1 is critical to the JA signaling required for plant development, defense, and metabolic homeostasis. AaCOI1 from Artemisia annua, homologous to Arabidopsis AtCOI1, encodes a F-box protein located in the nuclei. Expressional profiles of the AaCOI1 in the root, stem, leaves, and inflorescence was investigated. The mRNA abundance of AaCOI1 was the highest in inflorescence, followed by in the leaves. Upon mechanical wounding or MeJA treatment, expression of AaCOI1 was upregulated after 6 h. When ectopically expressed, driven by the native promoter from Arabidopsis thaliana, AaCOI1 could partially complement the JA sensitivity and defense responses, but fully complemented the fertility, and the JA-induced anthocyanin accumulation in a coi1-16 loss-of-function mutant. Our study identifies the paralog of AtCOI1 in Artemisia annua, and revealed its implications in development, hormone signaling, defense, and metabolism. The results provide insight into JA perception in Artemisia annua, and pave the way for novel molecular breeding strategies in the canonical herbs to manipulate the anabolism of pharmaceutic compounds on the phytohormonal level.

AaABCG40 Enhances Artemisinin Content and Modulates Drought Tolerance in Artemisia annua

The phytohormone Abscisic acid (ABA) regulates plant growth, development, and responses to abiotic stresses, including senescence, seed germination, cold stress and drought. Several kinds of researches indicate that exogenous ABA can enhance artemisinin content in A. annua. Some transcription factors related to ABA signaling are identified to increase artemisinin accumulation through activating the artemisinin synthase genes. However, no prior study on ABA transporter has been performed in A. annua. Here, we identified a pleiotropic drug resistance (PDR) transporter gene AaPDR4/AaABCG40 from A. annua. AaABCG40 was expressed mainly in roots, leaves, buds, and trichomes. GUS activity is primarily observed in roots and the vascular tissues of young leaves in proAaABCG40: GUS transgenic A. annua plants. When AaABCG40 was transferred into yeast AD12345678, yeasts expressing AaABCG40 accumulated more ABA than the control. The AaABCG40 overexpressing plants showed higher artemisinin content and stronger drought tolerance. Besides, the expression of CYP71AV1 in OE-AaABCG40 plants showed more sensitivity to exogenous ABA than that in both wild-type and iAaABCG40 plants. According to these results, they strongly suggest that AaABCG40 is involved in ABA transport in A. annua.

Overexpression of ABA Receptor PYL10 Gene Confers Drought and Cold Tolerance to Indica Rice

Abscisic acid (ABA) plays versatile functions in regulating plant development and tolerance to various biotic and abiotic stresses. Towards elucidating the functions of one of the ABA receptors (ABARs) in rice, OsPYL10 was cloned from drought tolerant rice cv. Nagina 22 and was overexpressed under stress inducible RD29A promoter in a mega rice variety MTU1010 by using Agrobacterium mediated genetic transformation. Four single copy transgenic lines selected based on Southern blot analysis were used for physiological and molecular analysis. PYL10 receptor appears to regulate its ligand ABA accumulation as PYL10 overexpressing transgenics accumulated 2-3.3-fold higher levels of ABA than that of WT in flag leaf at anthesis under non-stress conditions. The enhanced accumulation of ABA was associated with enhanced expression of genes for ABA biosynthesis viz., ZEP1, NCED1, NCED2, NCED3, and NCED4 in transgenics than in WT plants. At seedling stage, PYL10 transgenics showed significantly higher survival rate under cold stress as compared with WT plants. qRT-PCR analysis showed that expression levels of cold responsive genes viz., DREB1F, MYB3R2, TPP1, COR410, DEHYDRIN, and LEA3 were significantly higher in PYL10 overexpressing transgenic lines as compared to WT plants under cold stress. PYL10 transgenic and WT plants grown in the same pot were subjected to -80 kPa drought stress and recovery treatments at vegetative and reproductive stages. At vegetative stage drought stress, three overexpressing lines showed significantly higher grain yield (40-58%) and at reproductive stage drought stress one of these overexpression lines showed two-fold higher grain yield than that of WT plants. Excised leaf water loss analysis showed that PYL10 transgenic lost about 20% less water than WT plants. At reproductive stage, OsPYL10 transgenic maintained higher RWC, membrane stability index, chlorophyll content, and accumulated lower amount of MDA and H2O2 as compared with WT plants. qRT-PCR analysis showed that expression levels of RAB16, Dehydrin, LEA3, and ABA45 were higher in PYL10 transgenics as compared with WT plants under drought stress. Thus, overall results showed that OsPYL10 overexpression has potential to improve both drought and cold stress tolerance of indica rice.

AaPP2C1 negatively regulates the expression of genes involved in artemisinin biosynthesis through dephosphorylating AaAPK1

Artemisinin is biosynthesized in Artemisia annua and widely used for the treatment of malaria. Abscisic acid (ABA)-responsive kinase 1 (AaAPK1), a member of the SnRK2 family, is involved in the regulation of artemisinin biosynthesis through the phosphorylation of AabZIP1, which directly transactivates genes involved in artemisinin biosynthesis. Through diverse assays - including yeast two-hybrid and bimolecular fluorescence complementation assays - we report that the ABA-responsive protein phosphatase AaPP2C1 physically interacts with AaAPK1. In addition, phos-tag mobility shift assays indicate that AaPP2C1 dephosphorylates AaAPK1. Moreover, dual-luciferase assays demonstrate that the presence of AaPP2C1 reduces the transactivation of artemisinin biosynthesis genes by AabZIP1. These results further refine the post-translational regulatory network of artemisinin biosynthesis, showing that AaPP2C1 is negatively involved through dephosphorylation of AaAPK1.

Characterization of the ABA Receptor VlPYL1 That Regulates Anthocyanin Accumulation in Grape Berry Skin

ABA plays a crucial role in controlling several ripening-associated processes in grape berries. The soluble proteins named as PYR (pyrabactin resistant)/PYL (PYR-like)/RCAR (regulatory component of ABA receptor) family have been characterized as ABA receptors. Here, the function of a grape PYL1 encoding gene involved in the response to ABA was verified through heterologous expression. The expression level of VlPYL1 was highest in grape leaf and fruit tissues of the cultivar Kyoho, and the expression of VlPYL1 was increased during fruit development and showed a reduction in ripe berries. Over-expression of VlPYL1 enhances ABA sensitivity in Arabidopsis. Using the transient overexpression technique, the VlPYL1 gene was over-expressed in grape berries. Up-regulation of the VlPYL1 gene not only promoted anthocyanin accumulation but also induced a set of ABA-responsive gene transcripts, including ABF2 and BG3. Although tobacco rattle virus (TRV)-induced gene silencing (VIGS) was not successfully applied in the "Kyoho" grape, the application of the transient overexpression technique in grape fruit could be used as a novel tool for studying grape fruit development.

ARTEMISININ BIOSYNTHESIS PROMOTING KINASE 1 positively regulates artemisinin biosynthesis through phosphorylating AabZIP1

The plant Artemisia annua produces the anti-malarial compound artemisinin. Although the transcriptional regulation of artemisinin biosynthesis has been extensively studied, its post-translational regulatory mechanisms, especially that of protein phosphorylation, remain unknown. Here, we report that an ABA-responsive kinase (AaAPK1), a member of the SnRK2 family, is involved in regulating artemisinin biosynthesis. The physical interaction of AaAPK1 with AabZIP1 was confirmed by multiple assays, including yeast two-hybrid, bimolecular fluorescence complementation, and pull-down. AaAPK1, mainly expressed in flower buds and leaves, could be induced by ABA, drought, and NaCl treatments. Phos-tag mobility shift assays indicated that AaAPK1 phosphorylated both itself and AabZIP1. As a result, the phosphorylated AaAPK1 significantly enhanced the transactivational activity of AabZIP1 on the artemisinin biosynthesis genes. Substituting the Ser37 with Ala37 of AabZIP1 significantly suppressed its phosphorylation, which inhibited the transactivational activity of AabZIP1. Consistent overexpression of AaAPK1 significantly increased the production of artemisinin, as well as the expression levels of the artemisinin biosynthesis genes. Our study opens a window into the regulatory network underlying artemisinin biosynthesis at the post-translational level. Importantly, and for the first time, we provide evidence for why the kinase gene AaAPK1 is a key candidate for the metabolic engineering of artemisinin biosynthesis.

AaABF3, an Abscisic Acid-Responsive Transcription Factor, Positively Regulates Artemisinin Biosynthesis in Artemisia annua

Artemisinin is well known for its irreplaceable curative effect on the devastating parasitic disease, Malaria. This sesquiterpenoid is specifically produced in Chinese traditional herbal plant Artemisia annua. Earlier studies have shown that phytohormone abscisic acid (ABA) plays an important role in increasing the artemisinin content, but how ABA regulates artemisinin biosynthesis is still poorly understood. In this study, we identified that AaABF3 encoded an ABRE (ABA-responsive elements) binding factor. qRT-PCR analysis showed that AaABF3 was induced by ABA and expressed much higher in trichomes where artemisinin is synthesized and accumulated. To further investigate the mechanism of AaABF3 regulating the artemisinin biosynthesis, we carried out dual-luciferase analysis, yeast one-hybrid assay and electrophoretic mobility shift assay. The results revealed that AaABF3 could directly bind to the promoter of ALDH1 gene, which is a key gene in artemisinin biosynthesis, and activate the expression of ALDH1. Functional analysis revealed that overexpression of AaABF3 in A. annua enhanced the production of artemisinin, while RNA interference of AaABF3 resulted in decreased artemisinin content. Taken together, our results demonstrated that AaABF3 played an important role in ABA-regulated artemisinin biosynthesis through direct regulation of artemisinin biosynthesis gene, ALDH1.

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.

Molecular cloning, characterization, and promoter analysis of the isochorismate synthase (AaICS1) gene from Artemisia annua

Isochorismate synthase (ICS) is a crucial enzyme in the salicylic acid (SA) synthesis pathway. The full-length complementary DNA (cDNA) sequence of the ICS gene was isolated from Artemisia annua L. The gene, named AaICS1, contained a 1710-bp open reading frame, which encoded a protein with 570 amino acids. Bioinformatics and comparative study revealed that the polypeptide protein of AaICS1 had high homology with ICSs from other plant species. Southern blot analysis suggested that AaICS1 might be a single-copy gene. Analysis of the 1470-bp promoter of AaICS1 identified distinct cis-acting regulatory elements, including TC-rich repeats, MYB binding site (MBS), and TCA-elements. An analysis of AaICS1 transcript levels in multifarious tissues of A. annua using quantitative real-time polymerase chain reaction (qRT-PCR) showed that old leaves had the highest transcription levels. AaICS1 was up-regulated under wounding, drought, salinity, and SA treatments. This was corroborated by the presence of the predicted cis-acting elements in the promoter region of AaICS1. Overexpressing transgenic plants and RNA interference transgenic lines of AaICS1 were generated and their expression was compared. High-performance liquid chromatography (HPLC) results from leaf tissue of transgenic A. annua showed an increase in artemisinin content in the overexpressing plants. These results confirm that AaICS1 is involved in the isochorismate pathway.

Metabolomics-assisted biotechnological interventions for developing plant-based functional foods and nutraceuticals

Today, the dramatic changes in types of food consumed have led to an increased burden of chronic diseases. Therefore, the emphasis of food research is not only to ensure quality food that can supply adequate nutrients to prevent nutrition related diseases, but also to ensure overall physical and mental-health. This has led to the concept of functional foods and nutraceuticals (FFNs), which can be ideally produced and delivered through plants. Metabolomics can help in getting the most relevant functional information, and thus has been considered the greatest -OMICS technology to date. However, metabolomics has not been exploited to the best potential in plant sciences. The technology can be leveraged to identify the health promoting compounds and metabolites that can be used for the development of FFNs. This article reviews (i) plant-based FFNs-related metabolites and their health benefits; (ii) use of different analytic platforms for targeted and non-targeted metabolite profiling along with experimental considerations; (iii) exploitation of metabolomics to develop FFNs in plants using various biotechnological tools; and (iv) potential use of metabolomics in plant breeding. We have also provided some insights into integration of metabolomics with latest genome editing tools for metabolic pathway regulation in plants.

Endosperm-specific OsPYL8 and OsPYL9 act as positive regulators of the ABA signaling pathway in rice seed germination

Pyrabactin resistance-like (PYL) proteins were identified as receptors of the plant hormone ABA. The PYL family consists of multiple members that are differently expressed in various tissues, exhibit distinct biochemical properties and have diverse biological functions. In the present study, we explored the expression patterns of the rice (Oryza sativa L.) OsPYL family genes and determined that OsPYL8 and OsPYL9 are specifically expressed in the endosperms. Sequence analysis and deletion experiments revealed that the OsPYL8 and OsPYL9 promoters contain multiple motifs involved in endosperm-specific expression. Transgenic rice plants overexpressing OsPYL8 or OsPYL9 showed hypersensitivity to ABA during seed germination, suggesting that both OsPYL8 and OsPYL9 act as positive regulators of the ABA signalling pathway in the seed. OsPYL8 and OsPYL9 interact with OsPP2C51 and OsPP2C68, whose expression is induced in the endosperms by ABA. Our results provided a foundation for future studies on OsPYL8- and OsPYL9-mediated ABA signalling in the rice endosperms.

Molecular characterization of an AtPYL1-like protein, BrPYL1, as a putative ABA receptor in Brassica rapa

Abscisic acid (ABA)-induced physiological changes are conserved in many land plants and underlie their responses to environmental stress and pathogens. The PYRABACTIN RESISTANCE1/PYR1-LIKE/REGULATORY COMPONENTS OF ABA RECEPTORS (PYLs)-type receptors perceive the ABA signal and initiate signal transduction. Here, we show that the genome of Brassica rapa encodes 24 putative AtPYL-like proteins. The AtPYL-like proteins in Brassica rapa (BrPYLs) can also be classified into 3 subclasses. We found that nearly all BrPYLs displayed high expression in at least one tissue. Overexpression of BrPYL1 conferred ABA hypersensitivity to Arabidopsis. Further, ABA activated the expression of an ABA-responsive reporter in Arabidopsis protoplasts expressing BrPYL1. Overall, these results suggest that BrPYL1 is a putative functional ABA receptor in Brassica rapa.

Plant Metabolic Engineering Strategies for the Production of Pharmaceutical Terpenoids

Pharmaceutical terpenoids belong to the most diverse class of natural products. They have significant curative effects on a variety of diseases, such as cancer, cardiovascular diseases, malaria and Alzheimer's disease. Nowadays, elicitors, including biotic and abiotic elicitors, are often used to activate the pathway of secondary metabolism and enhance the production of target terpenoids. Based on Agrobacterium-mediated genetic transformation, several plant metabolic engineering strategies hold great promise to regulate the biosynthesis of pharmaceutical terpenoids. Overexpressing terpenoids biosynthesis pathway genes in homologous and ectopic plants is an effective strategy to enhance the yield of pharmaceutical terpenoids. Another strategy is to suppress the expression of competitive metabolic pathways. In addition, global regulation which includes regulating the relative transcription factors, endogenous phytohormones and primary metabolism could also markedly increase their yield. All these strategies offer great opportunities to enhance the supply of scarce terpenoids drugs, reduce the price of expensive drugs and improve people's standards of living.

Overexpression of a Novel NAC Domain-Containing Transcription Factor Gene (AaNAC1) Enhances the Content of Artemisinin and Increases Tolerance to Drought and Botrytis cinerea in Artemisia annua

The NAC (NAM, ATAF and CUC) superfamily is one of the largest plant-specific transcription factor families. NAC transcription factors always play important roles in response to various abiotic stresses. A NAC transcription factor gene AaNAC1 containing a complete open reading frame (ORF) of 864 bp was cloned from Artemisia annua. The expression of AaNAC1 could be induced by dehydration, cold, salicylic acid (SA) and methyl jasmonate (MJ), suggesting that it might be a key regulator of stress signaling pathways in A. annua. AaNAC1 was shown to be localized to the nuclei by transforming tobacco leaf epidermal cells. When AaNAC1 was overexpressed in A. annua, the content of artemisinin and dihydroartemisinic acid was increased by 79% and 150%, respectively. The expression levels of artemisinin biosynthetic pathway genes, i.e. amorpha-4,11-diene synthase (ADS), artemisinic aldehyde Δ11(13) reductase (DBR2) and aldehyde dehydrogenase 1 (ALDH1), were increased. Dual luciferase (dual-LUC) assays showed that AaNAC1 could activate the transcription of ADS in vivo. The transgenic A. annua exhibited increased tolerance to drought and resistance to Botrytis cinerea. When AaNAC1 was overexpressed in Arabidopsis, the transgenic Arabidopsis were markedly more tolerant to drought. The transgenic Arabidopsis showed increased resistance to B. cinerea. These results indicate that AaNAC1 can potentially be used in transgenic breeding for improving the content of artemisinin and drought tolerance in A. annua.

Artemisinin-based antimalarial research: application of biotechnology to the production of artemisinin, its mode of action, and the mechanism of resistance of Plasmodium parasites

Malaria is a worldwide disease caused by Plasmodium parasites. A sesquiterpene endoperoxide artemisinin isolated from Artemisia annua was discovered and has been accepted for its use in artemisinin-based combinatorial therapies, as the most effective current antimalarial treatment. However, the quantity of this compound produced from the A. annua plant is very low, and the availability of artemisinin is insufficient to treat all infected patients. In addition, the emergence of artemisinin-resistant Plasmodium has been reported recently. Several techniques have been applied to enhance artemisinin availability, and studies related to its mode of action and the mechanism of resistance of malaria-causing parasites are ongoing. In this review, we summarize the application of modern technologies to improve the production of artemisinin, including our ongoing research on artemisinin biosynthetic genes in other Artemisia species. The current understanding of the mode of action of artemisinin as well as the mechanism of resistance against this compound in Plasmodium parasites is also presented. Finally, the current situation of malaria infection and the future direction of antimalarial drug development are discussed.

A Putative PP2C-Encoding Gene Negatively Regulates ABA Signaling in Populus euphratica

A PP2C homolog gene was cloned from the drought-treated cDNA library of Populus euphratica. Multiple sequence alignment analysis suggested that the gene is a potential ortholog of HAB1. The expression of this HAB1 ortholog (PeHAB1) was markedly induced by drought and moderately induced by ABA. To characterize its function in ABA signaling, we generated transgenic Arabidopsis thaliana plants overexpressing this gene. Transgenic lines exhibited reduced responses to exogenous ABA and reduced tolerance to drought compared to wide-type lines. Yeast two-hybrid analyses indicated that PeHAB1 could interact with the ABA receptor PYL4 in an ABA-independent manner. Taken together; these results indicated that PeHAB1 is a new negative regulator of ABA responses in poplar.

A basic leucine zipper transcription factor, AabZIP1, connects abscisic acid signaling with artemisinin biosynthesis in Artemisia annua

Artemisinin is a sesquiterpenoid especially synthesized in the Chinese herbal plant, Artemisia annua, which is widely used in the treatment of malaria. Artemisinin accumulation can be enhanced by exogenous abscisic acid (ABA) treatment. However, it is not known how ABA signaling regulates artemisinin biosynthesis. A global expression profile and phylogenetic analysis as well as the dual-LUC screening revealed that a basic leucine zipper family transcription factor from A. annua (namely AabZIP1) was involved in ABA signaling to regulate artemisinin biosynthesis. AabZIP1 had a higher expression level in the inflorescences than in other tissues; ABA treatment, drought, and salt stress strongly induced the expression of AabZIP1. Yeast one-hybrid assay and electrophoretic mobility shift assay (EMSA) showed that AabZIP1 bound to the ABA-responsive elements (ABRE) in the promoter regions of the amorpha-4,11-diene synthase (ADS) gene and CYP71AV1, which are two key structural genes of the artemisinin biosynthetic pathway. A mutagenesis assay showed that the C1 domain in the N-terminus of AabZIP1 was important for its transactivation activity. Furthermore, the activation of ADS and CYP71AV1 promoters by AabZIP1 was enhanced by ABA treatment in transient dual-LUC analysis. The AabZIP1 variant with C1 domain deletion lost the ability to activate ADS and CYP71AV1 promoters regardless of ABA treatment. Notably, overexpression of AabZIP1 in A. annua resulted in significantly increased accumulation of artemisinin. Our results indicate that ABA promotes artemisinin biosynthesis, likely through 1 activation of ADS and CYP71AV1 expression by AabZIP in A. annua. Meanwhile, our findings reveal the potential value of AabZIP1 in genetic engineering of artemisinin production.

Type 2C phosphatase 1 of Artemisia annua L. is a negative regulator of ABA signaling

The phytohormone abscisic acid (ABA) plays an important role in plant development and environmental stress response. Additionally, ABA also regulates secondary metabolism such as artemisinin in the medicinal plant Artemisia annua L. Although an earlier study showed that ABA receptor, AaPYL9, plays a positive role in ABA-induced artemisinin content improvement, many components in the ABA signaling pathway remain to be elucidated in Artemisia annua L. To get insight of the function of AaPYL9, we isolated and characterized an AaPYL9-interacting partner, AaPP2C1. The coding sequence of AaPP2C1 encodes a deduced protein of 464 amino acids, with all the features of plant type clade A PP2C. Transcriptional analysis showed that the expression level of AaPP2C1 is increased after ABA, salt, and drought treatments. Yeast two-hybrid and bimolecular fluorescence complementation assays (BiFC) showed that AaPYL9 interacted with AaPP2C1. The P89S, H116A substitution in AaPYL9 as well as G199D substitution or deletion of the third phosphorylation site-like motif in AaPP2C1 abolished this interaction. Furthermore, constitutive expression of AaPP2C1 conferred ABA insensitivity compared with the wild type. In summary, our data reveals that AaPP2C1 is an AaPYL9-interacting partner and involved in the negative modulation of the ABA signaling pathway in A. annua L.

Transgenic approach to increase artemisinin content in Artemisia annua L

Artemisinin, the endoperoxide sesquiterpene lactone, is an effective antimalarial drug isolated from the Chinese medicinal plant Artemisia annua L. Due to its effectiveness against multi-drug-resistant cerebral malaria, it becomes the essential components of the artemisinin-based combination therapies which are recommended by the World Health Organization as the preferred choice for malaria tropica treatments. To date, plant A. annua is still the main commercial source of artemisinin. Although semi-synthesis of artemisinin via artemisinic acid in yeast is feasible at present, another promising approach to reduce the price of artemisinin is using plant metabolic engineering to obtain a higher content of artemisinin in transgenic plants. In the past years, an Agrobacterium-mediated transformation system of A. annua has been established by which a number of genes related to artemisinin biosynthesis have been successfully transferred into A. annua plants. In this review, the progress on increasing artemisinin content in A. annua by transgenic approach and its future prospect are summarized and discussed.