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.

Molecular insights into AabZIP1-mediated regulation on artemisinin biosynthesis and drought tolerance in Artemisia annua

Artemisia annua is the main natural source of artemisinin production. In A. annua, extended drought stress severely reduces its biomass and artemisinin production while short-term water-withholding or abscisic acid (ABA) treatment can increase artemisinin biosynthesis. ABA-responsive transcription factor AabZIP1 and JA signaling AaMYC2 have been shown in separate studies to promote artemisinin production by targeting several artemisinin biosynthesis genes. Here, we found AabZIP1 promote the expression of multiple artemisinin biosynthesis genes including AaDBR2 and AaALDH1, which AabZIP1 does not directly activate. Subsequently, it was found that AabZIP1 up-regulates AaMYC2 expression through direct binding to its promoter, and that AaMYC2 binds to the promoter of AaALDH1 to activate its transcription. In addition, AabZIP1 directly transactivates wax biosynthesis genes AaCER1 and AaCYP86A1. The biosynthesis of artemisinin and cuticular wax and the tolerance of drought stress were significantly increased by AabZIP1 overexpression, whereas they were significantly decreased in RNAi-AabZIP1 plants. Collectively, we have uncovered the AabZIP1-AaMYC2 transcriptional module as a point of cross-talk between ABA and JA signaling in artemisinin biosynthesis, which may have general implications. We have also identified AabZIP1 as a promising candidate gene for the development of A. annua plants with high artemisinin content and drought tolerance in metabolic engineering breeding.

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.

Deciphering transcriptome profiles of tetraploid Artemisia annua plants with high artemisinin content

To investigate on the effects of autopolyploidization on growth and artemisinin biosynthesis in Artemisia annua, we performed a comprehensive transcriptomic characterization of diploid and induced autotetraploid A. annua. The polyploidization treatment not only enhanced photosynthetic capacity and endogenous contents of indole-3-acetic acid (IAA), abscisic acid (ABA) and jasmonic acid (JA), oxidative stress, but increased the average level of artemisinin in tetraploids from 42.0 to 63.6%. The obvious phenotypic alterations in tetraploids were observed including shorter stems, larger size of stomata and glandular secretory trichomes (GSTs), larger leaves, more branches and roots. A total of 8763 (8.85%) differentially expressed genes (DEGs) were identified in autotetraploids and mainly involved in carbohydrate metabolic processes, cell wall organization and defense responses. Both the up-regulated expression of DNA methylation unigenes and enhanced level of DNA methylation in autotetraploids indicated a possible role of DNA methylation on transcriptomic remodeling and phenotypic alteration. The up-regulated genes were enriched in response to extracellular protein biosynthesis, photosynthesis and hormone stimulus for cell enlargement and phenotypic alteration. The genomic shock induced by chromosome duplication stimulated the expression of transcripts related to oxidative stress, biosynthesis and signal transduction of ABA and JA, and key enzymes in artemisinin biosynthetic pathway, leading to the increased accumulation of artemisinin. This is the first transcriptomic research that identifies DEGs involved in the polyploidization of A. annua. The results provide novel information for understanding the complexity of polyploidization and for further identification of the factors and genes involve in artemisinin biosynthesis.

Branch Pathway Blocking in Artemisia annua is a Useful Method for Obtaining High Yield Artemisinin

There are many biosynthetic pathways competing for the metabolic flux with the artemisinin biosynthetic pathway in Artemisia annua L. To study the relationship between genes encoding enzymes at branching points and the artemisinin biosynthetic pathway, β-caryophyllene, β-farnesene and squalene were sprayed on young seedlings of A. annua. Transient expression assays indicated that the transcription levels of β-caryophyllene synthase (CPS), β-farnesene synthase (BFS) and squalene synthase (SQS) were inhibited by β-caryophyllene, β-farnesene and squalene, respectively, while expression of some artemisinin biosynthetic pathway genes increased. Thus, inhibition of these genes encoding enzymes at branching points may be helpful to improve the artemisinin content. For further study, the expression levels of four branch pathway genes CPS, BFS, germacrene A synthase (GAS) and SQS were down-regulated by the antisense method in A. annua. In anti-CPS transgenic plants, mRNA levels of BFS and ADS were increased, and the contents of β-farnesene, artemisinin and dihydroartemisinic acid (DHAA) were increased by 212, 77 and 132%, respectively. The expression levels of CPS, SQS, GAS, amorpha-4,11-diene synthase (ADS), amorphadiene 12-hydroxylase (CYP71AV1) and aldehyde dehydrogenase 1 (ALDH1) were increased in anti-BFS transgenic plants and, at the same time, the contents of artemisinin and DHAA were increased by 77% and 54%, respectively, and the content of squalene was increased by 235%. In anti-GAS transgenic plants, mRNA levels of CPS, BFS, ADS and ALDH1 were increased. The contents of artemisinin and DHAA were enhanced by 103% and 130%, respectively. In anti-SQS transgenic plants, the transcription levels of BFS, GAS, CPS, ADS, CYP71AV1 and ALDH1 were all increased. Contents of artemisinin and DHAA were enhanced by 71% and 223%, respectively, while β-farnesene was raised to 123%. The mRNA level of artemisinic aldehyde Δ11(13) reductase (DBR2) had changed little in almost all transgenic plants.

Cloning and characterization of AabHLH1, a bHLH transcription factor that positively regulates artemisinin biosynthesis in Artemisia annua

Amorpha-4,11-diene synthase (ADS) and Cyt P450 monooxygenase (CYP71AV1) in Artemisia annua L. are two key enzymes involved in the biosynthesis of artemisinin. The promoters of ADS and CYP71AV1 contain E-box elements, which are putative binding sites for basic helix-loop-helix (bHLH) transcription factors. This study successfully isolated a bHLH transcription factor gene from A. annua, designated as AabHLH1, from a cDNA library of the glandular secretory trichomes (GSTs) in which artemisinin is synthesized and sequestered. AabHLH1 encodes a protein of 650 amino acids containing one putative bHLH domain. AabHLH1 and ADS genes were strongly induced by ABA and the fungal elicitor, chitosan. The transient expression analysis of the AabHLH1-green fluorescent protein (GFP) reporter gene revealed that AabHLH1 was targeted to nuclei. Biochemical analysis demonstrated that the AabHLH1 protein was capable of binding to the E-box cis-elements, present in both ADS and CYP71AV1 promoters, and possessed transactivation activity in yeast. In addition, transient co-transformation of AabHLH1 and CYP71AV1Pro::GUS in A. annua leaves showed a significant activation of the expression of the GUS (β-glucuronidase) gene in transformed A. annua, but mutation of the E-boxes resulted in abolition of activation, suggesting that the E-box is important for the CYP71AV1 promoter activity. Furthermore, transient expression of AabHLH1 in A. annua leaves increased transcript levels of the genes involved in artemisinin biosynthesis, such as ADS, CYP71AV1 and HMGR. These results suggest that AabHLH1 can positively regulate the biosynthesis of artemisinin.

Transcriptome responses involved in artemisinin production in Artemisia annua L. under UV-B radiation

Artemisinin, an endoperoxide sesquiterpene lactone, is an effective antimalarial drug isolated from Artemisia annua L. In this study, a low dose (1.44 kJm(-2)d(-1)) of UV-B radiation (280-320 nm) for short-term (1h per day for 10 days) was applied to A. annua seedlings to stimulate artemisinin production. UV-B treatment not only induced the generation of reactive oxygen species (ROS), enhanced peroxidase activity and endogenous content of abscisic acid (ABA), but stimulated the biosynthesis of artemisinin in the seedlings. Here, transcriptomic changes during UV-B radiation in A. annua were detected using an Agilent GeneChip with 43,692 probe sets. In total, 358 transcripts were identified as differentially expressed under UV-B stress, of which 172 transcripts increased and 186 transcripts decreased in abundance. In terms of biological processes, gene ontology (GO) terms including primary carbohydrate and nitrogen compound metabolic processes were enriched in UV-B-repressed genes. The up-regulated genes were enriched in response to stress, ROS generation, hormone (ethylene, ABA) stimulus and cell cycle control. The expression of key enzymes such as amorpha-4,11-diene synthase (ADS) and cytochrome P450 dependent monooxygenase/hydroxylase (CYP71AV1), and related WRKY transcription factors was up-regulated significantly for artemisinin biosynthesis. This profile of global gene expression patterns during UV-B stress will be valuable for further identification of the enzymes involved in artemisinin biosynthesis.

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

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

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

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