Skip to main content
PMC home page
NIHPA Author Manuscripts logoLink to NIHPA Author Manuscripts
. Author manuscript; available in PMC: 2012 Jun 1.
Published in final edited form as: In Vitro Cell Dev Biol Plant. 2011 Jun;47(3):329–338. doi: 10.1007/s11627-011-9343-x

Trichomes + roots + ROS = artemisinin: regulating artemisinin biosynthesis in Artemisia annua L

Khanhvan T Nguyen 1, Patrick R Arsenault 2,3, Pamela J Weathers 4
PMCID: PMC3110715  NIHMSID: NIHMS282214  PMID: 21666770

Abstract

Artemisinin is a highly effective sesquiterpene lactone therapeutic produced in the plant, Artemisia annua. Despite its efficacy against malaria and many other infectious diseases and neoplasms, the drug is in short supply mainly because the plant produces low levels of the compound. This review updates the current understanding of artemisinin biosynthesis with a special focus on the emerging knowledge of how biosynthesis of the compound is regulated in planta.

Keywords: Artemisinin, Trichome, ROS, Reactive oxygen species, Jasmonic acid, Salicylic acid, Abscisic acid, Gibberellic acid, Cytokinin

Introduction

In 2008, there were an estimated 253 million cases of malaria resulting in 863,000 deaths (World Health Organization [WHO] 2009). Approximately 85% of the cases and 89% of the deaths occurred in Africa where many countries are undeveloped and poor. While artemisinin is very effective against malaria, it remains too expensive for many people in these countries. Its high cost is due mainly to the high demand for the drug and low production in planta; Artemisia annua yields are typically less than 1.2% of dry weight (Kindermans et al. 2007). Much work is being done to increase the artemisinin supply, including breeding high-yielding A. annua strains (Graham et al. 2010), producing transgenic A. annua and engineering the artemisinin biosynthetic pathway into Escherichia coli, yeast, and tobacco (Arsenault et al. 2008; Covello 2008; Zhang et al. 2010). Artemisinin is not only effective against malaria but other diseases as well, including: different types of cancer, other parasites like schistosomiasis, and some viruses such as hepatitis B (Efferth 2009). The drug also appears to be useful in treating some animal diseases (Ferreira and Gonzalez 2008). Even though there is a lot of effort toward increasing the artemisinin supply, the biosynthetic pathway is not entirely clear and only recently have studies begun to elucidate its regulation. In this review, we update recent earlier reviews (Weathers et al. 2006; Covello et al. 2007) but with a focus on what is known currently about the regulation of artemisinin biosynthesis in A. annua.

Biosynthesis of Artemisinin

Artemisinin (AN; Fig. 1) is a sesquiterpene lactone that stems from isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). These 5-carbon building blocks of all terpenes can originate from either the cytosolic mevalonate (MVA) pathway or the methylerythritol phosphate (MEP) pathway in the plastid (Fig. 1). In 2007, Towler and Weathers showed that, through inhibition with mevinolin or fosmidomycin, AN is produced using IPP/DMAPP from both the MVA and MEP pathways, respectively. This result was quantitatively confirmed by Schramek et al. (2010) using isotopologue profiling of A. annua plants with 13CO2; the farnesyl diphosphate which eventually yields AN is composed of one isoprene unit from the MEP pathway and two isoprene units from the MVA pathway.

Figure 1.

Figure 1

Open in a new tab

Artemisinin biosynthetic pathway. Art acid artemisinic acid, ADS amorpha-4, 11-diene synthase, Aldh1 aldehyde dehydrogenase 1, CYP cytochrome P 450 CYP71AV1, DBR2 double bond reductase 2, DXR 1-deoxyxylulouse 5-phosphate reductoisomerase, DXS 1-deoxyxylulose 5- phosphate synthase, FPS farnesyl diphosphate synthase, HMGR 3-hydroxy-3-methylglutaryl-CoA reductase, HMG-CoA 3-hydroxy-3-methylglutaryl-CoA, MEP nonmevalonate pathway (plastid pathway), RED1 dihydroartemisinic aldehyde reductase 1.

Although FPP can lead to a number of other compounds in A. annua (Weathers et al. 2006), the first step toward artemisinin biosynthesis is the cyclization of FPP to amorpha-4,11-diene by amorphadiene synthase (ADS; Chang et al. 2000; Mercke et al. 2000; Wallaart et al. 2001). Amorpha-4,11-diene is then oxidized twice by a cytochrome P450, CYP71AV1 (CYP), to form artemisinic alcohol and then artemisinic aldehyde (Ro et al. 2006; Teoh et al. 2006). From here, the exact pathway to artemisinin and other end products is not as clearly understood. It seems there are two possible routes from artemisinic aldehyde: one route to arteannuin B (AB) and one to AN. The route to AB occurs via oxidation of artemisinic aldehyde to artemisinic acid (AA) either by CYP or an aldehyde dehydrogenase (Aldh1), apparently culminating in AB production (Teoh et al. 2009; Fig. 1). Although Dhingra and Narasu (2001) suggested that AB could be converted to artemisitene (AT) and then to AN in a cell-free system, to our knowledge there has been no other work to validate enzymatic in planta conversion of AB to AN via AT. Indeed, Brown and Sy (2007) provide data pointing to a lack of conversion of AA to AN. Thus, it is currently thought that AB is one end product of two branches of the pathway to seco-cadinanes (including AN and AB) in A. annua.

In the other branch of the seco-cadinane pathway post-CYP, artemisinic aldehyde is converted to dihydroartemisinic aldehyde by the double bond artemisinic aldehyde reductase (DBR2; Zhang et al. 2008) and then converted to DHAA by Aldh1 (Teoh et al. 2009). DHAA is then converted to AN probably by more than one non-enzymatic, spontaneous photo-oxidation reactions (Wallaart et al. 2001; Brown and Sy 2007). Recently, Rydén et al. (2010) showed that a new enzyme, dihydroartemisinic aldehyde reductase (Red1), in the pathway appears to be competing for dihydroartemisinic aldehyde, the precursor to DHAA, thereby reducing potential AN production. Once better understood, inhibition of these alternate pathway enzymes offers potential targets for increasing production of AN.

There also exists in A. annua two well-defined chemotypes, which account for differences in flux of artemisinic metabolites within the pathway (Wallaart et al. 2000). One chemotype produces mainly AN and its precursor DHAA, while the other produces mainly AB and its AA precursor. These variants can apparently be maintained as separate genetic stocks (Delabays et al. 2001). It is possible that understanding the regulatory mechanisms will elucidate how such pathway variations occur within these genetic sub-types.

Morphological and Developmental Regulation of Artemisinin

Trichomes

Trichomes are appendages that develop from the epidermal cells of leaves, stems, and flowers and occur in a variety of shapes and sizes: unicellular or multicellular, branched or unbranched, and glandular or nonglandular. Glandular trichomes are important to artemisinin biosynthesis because they store many secondary metabolites including AN (Duke et al. 1994; Ferreira and Janick 1996a).

AN is produced and sequestered in the glandular secretory trichomes (GSTs) of A. annua which are composed of ten cells (Fig. 2): two basal cells, two stalk cells, four sub-apical cells, two apical cells, and a subcuticular space where AN accumulates (Ferreira and Janick 1996a, b; Olsson et al. 2009). GSTs are located mainly on the leaves and flowers of A. annua (Ferreira and Janick 1996a). Since trichomes are the sites of synthesis of AN, it should be expected that more trichomes would result in more AN produced, and indeed, there is a strong correlation between AN concentration and trichome density (Kapoor et al. 2007; Arsenault et al. 2010a; Graham et al. 2010). It was also found that younger leaves develop more trichomes and produce more AN than older leaves (Ferreira and Janick 1996a; Arsenault et al. 2010a). Interestingly, in mature plants, however, senescent leaves provide nearly half of the total AN in the plant (Lommen et al. 2007).

Figure 2.

Figure 2

Open in a new tab

Schematic (left) and photo (right)of Artemisia annua trichomes. The trichomes on the right are visualized using UV under a confocal microscope; bar = 50 μm. ADS amorphadiene 4,11 synthase, CYP cytochrome P 450 CYP71AV1, DBR2 double bond reductase 2, DXR 1-deoxyxylulouse 5-phosphate reductoisomerase, DXS 1-deoxyxylulose 5-phosphate synthase, FPS farnesyl diphosphate synthase.

Through microdissection of A. annua glandular trichomes and RT-PCR, transcripts of early genes in the AN biosynthesis pathway (ADS, CYP71AV1, DBR2) were found only in the apical cells of the trichome, while transcripts of DXR from the plastid-localized MEP pathway were found only in the sub-apical cells (Olsson et al. 2009). The apical and basal cells are not green, but the four subapical cells are, so the presence of DXR transcripts in these latter cells is consistent with functional chloroplasts. Furthermore, transcripts of FPS were found in both the apical and sub-apical cells suggesting that the nonplastid MVA pathway is functional in both cell types. It appears that different cells in the glandular trichomes play different roles in the AN pathway. Although no artemisinin was detected in glandless A. annua, it is still unclear if any of the genes in the AN pathway are expressed elsewhere in the leaves (Duke et al. 1994).

Lommen et al. (2006) suggested that upon maturation, trichomes collapse and release AN, which may inhibit the AN biosynthetic pathway. Arsenault et al. (2010a) later tested this hypothesis by spraying the foliage of soil-grown plants during the vegetative stage with either 100 μg/ml AN or AA in 70% ethanol and measuring the mRNA transcripts of HMGR, FPS, ADS and CYP. Compared to the control plants sprayed with only 70% ethanol carrier, plants sprayed with AN only showed decreased transcription of CYP. In contrast, plants sprayed with AA showed inhibition of both ADS and CYP. This suggested that the AN biosynthetic pathway is regulated through feedback inhibition and that ruptured trichomes may reduce production.

Maes et al. (2010) recently showed that trichome density significantly increased in response to jasmonic acid (JA) and 6-benzylaminopurine (BAP). Seedlings that were typically either high or low AN producers were sprayed and the associated soil was drenched with hormones every 2 d for 5 wk. When they compared trichome size between these two groups of plants, only the low producers showed a significant increase in trichome size. When artemisinic metabolites were measured, however, both the high and the low AN-producing plants showed significant increases in AA, AB, DHAA, and AN in response to JA. Overall, however, the largest percentage increases in AN and AB occurred in the low AN-producing plants. Together these results further substantiate that there may be upper limits to both trichome density as well as AN content.

Roots

In vitro-cultured shoots of A. annua that develop roots produce more AN than shoots without roots (Ferreira and Janick 1996b). Similarly, compared to unrooted shoots growing in liquid culture, only rooted shoots of A. annua responded to DMSO elicitation of AN production (Mannan et al. 2010). Furthermore, when DMSO was applied only to leaves of rooted plants, there was no change in AN production. Together, these results show that despite the fact that the roots produce neither AN nor its precursors, they play a critical role in AN biosynthesis and possibly in the perception of signals that affect its synthesis. How the roots affect AN production is still unclear, since negligible amounts of transcripts of ADS (Kim et al. 2008), CYP (Teoh et al. 2006), DBR2 (Zhang et al. 2008), and Aldh1 (Teoh et al. 2009) were found in the roots. Clearly more research is needed to elucidate how roots increase AN in the shoots of A. annua.

Shifting from vegetative to reproductive growth

As A. annua plants develop from vegetative to reproductive growth, AN levels increase (Ferreira et al. 1995; Lommen et al. 2006; Ma et al. 2008). Recently, Arsenault et al. (2010a) measured AN, AB, and their respective precursors, DHAA and AA, as well as transcripts of six genes in the AN biosynthetic pathway (HMGR, FPS, DXR, DXS, ADS, CYP) in two types of leaf tissue, flower buds, and full flowers in soil-grown A. annua in different developmental stages: vegetative, floral budding, and full flowering. When comparing the transcript levels of these six genes in similar leaf tissues at vegetative, budding and flowering stages, the highest levels of HMGR, DXR, and DXS were found in leaves at the budding stage and the highest level of FPS in leaves at the flowering stage. Although the highest level of FPS was coincident with the highest AN level detected, it is unclear how FPS plays a role in AN regulation since overexpression of the FPS gene in A. annua has provided conflicting results. While several studies showed a significant increase in AN when FPS was overexpressed (Chen et al. 2000; Han et al. 2006; Banyai et al. 2010), others reported no change from controls (Ma et al. 2008) or a decrease in AN yield in some transformants (Banyai et al. 2010). On further analysis, Banyai et al. (2010) observed some gene silencing when two copies of the gene were inserted. In cases where there was a single gene insertion, position of the Agrobacterium tumefaciens delivered transgene was suggested as a possible explanation. Despite these conflicting results using transgenic A. annua, untransformed plants increased expression of FPS under various stimuli and this generally correlated with increases in AN production (Arsenault et al. 2010a).

As plants transition to flowering, transcript levels of ADS and CYP showed a very different pattern than the previously mentioned four genes. Instead of the highest transcript levels of ADS and CYP appearing during budding or flowering, as one might expect, they were highest during vegetative growth and then declined significantly during floral development (Arsenault et al. 2010a). Similarly, levels of DHAA were highest in vegetative plants compared to budding and full flower plants. These data suggest that the key metabolic genes responsible for directing terpenoid biosynthesis toward AN may be negatively related to AN production, or that increased AN negatively regulates these genes in vivo. At least during the reproductive stage, when it is the highest, AN appears to inhibit the transcription of ADS and CYP, which are AN-specific biosynthetic genes. Indeed, this has been confirmed through exogenous application of AN (Arsenault et al. 2010a).

Artemisinin levels are highest either just before or after full flower, suggesting that the flowering process may be involved in regulation of AN biosynthesis (Ferreira et al. 1995; Ma et al. 2008; Arsenault et al. 2010a). This now appears unlikely as two studies that induced premature flowering using the early flowering gene CONSTANS (Wang et al. 2004) or the flowering promoting factor1 (Wang et al. 2007) showed no significant increase in AN levels compared to wild type controls. This suggested that the high level of AN found before or after the flowering stage is not directly linked to the act of flowering.

Environmental Regulation of Artemisinin

Abiotic

Plants can respond to their environment by altering shape, pigment content, and their secondary metabolites. Not surprisingly, some environmental factors also alter AN production. Lulu et al. (2008) showed that chilling can affect both AN content and transcript levels of ADS and CYP in in vitro cultures. Using qPCR and HPLC analysis of AN levels, they observed after 24 h 11- and 7-fold increases in ADS and CYP, respectively, in A. annua plant cultures chilled to 4°C for 30 min compared to non-chilled cultures. Although chilling also doubled AN levels, higher temperatures (42°C) did not seem to affect AN biosynthesis or transcript levels of either ADS or CYP. Yang et al. (2010) also showed marked increases in ADS, CYP, and DXS in response to chilling. In contrast to Lulu et al. (2008), the Yang group also showed a significant increase in both ADS and CYP in response to a 1-h dose of UV light.

Drought also affects AN levels in plants (Marchese et al. 2010). Pot-grown plants deprived of water for either 38 or 62 h showed increased AN levels in leaves. Only the shorter duration treatment was effective as it did not reduce leaf biomass. Yang et al. (2010) substantiated this response by showing ADS increased in shoots when plant roots were allowed to dry for 6 h. In contrast, water logging decreased ADS, but increased CYP transcription; DBR2 also increased. These results may be important for selecting optimal harvest time as there are two perceived benefits: increased AN content overall and reduced water content of plants, thereby reducing drying time.

Various minerals also affect AN yields. For example, boron at limited application levels can increase AN significantly with little or no effect on biomass (Aftab et al. 2010). Levels of H2O2 also increased with boron application, so the increase in AN may be explained by a ROS stress response. Phosphate was also shown to increase AN levels, but the mechanism of the stimulus is not known (Kapoor et al. 2007).

Biotic

Although pathogens and pathogen-related compounds like chitosan are known to be elicitors of secondary metabolites in many plants, only a few reports seem to focus on studies with whole A. annua plants. Chitosan is an elicitor of AN production and appears to function by inducing transcription of the AaWRKY1 transcription factor that seems to regulate the ADS promoter (Ma et al. 2009). In another example, Kapoor et al. (2007) showed that addition of mycorrhizae to roots of potted plants increased AN levels in leaves by as much as 300%, depending on the fungal species, with Glomus fasciculatum performing best. This gain also correlated closely with increased trichome density, a response observed in other species of mycorhizzal plants (Kapoor et al. 2007). In comparison to uninoculated plants, addition of phosphate to plants did not alter AN levels. This study shows that certain biotic factors can play a significant role in altering the morphology of the A. annua leaves by increasing trichome numbers and that the perception of a stimulating signal again occurs via the roots. Overall, there are clearly a number of different environmental effects which are beginning to be elucidated to show how they may alter regulation of the AN biosynthetic pathway.

Phytohormones and Other Signaling Molecules

GA, ABA, and BAP effects

Gibberellic acid (GA3), abscisic acid (ABA), and cytokinins are phytohormones known to affect AN production. GA3 and BAP are also known to stimulate trichome development (Maes et al. 2008). When the A. tumefaciens cytokinin biosynthetic gene, isopentenyl transferase was transferred and constitutively expressed in A. annua, the in vivo content of cytokinins increased along with chlorophyll and AN (Sa et al. 2001) suggesting that biosynthesis of AN is regulated by cytokinins. This is not surprising since trichome development is known to be stimulated by BAP. Although the level of AN was increased in these transgenic plants, root mass decreased by about 50%, indicating that either the source (bacterial gene) or amount of cytokinin produced were problematic.

In a later study, Maes et al. (2010) sprayed and soil-drenched two A. annua cultivars with either water, 50 μM BAP, or 100 μM GA3 and saw that both phytohormones stimulated filamentous trichome development, but only BAP stimulated glandular trichome development in both cultivars. Although the two cultivars showed an increase in glandular trichomes, the low-artemisinin-producing cultivar showed a greater increase than the high-artemisinin-producing cultivar (Maes et al. 2010). Also, compared to the control, only GA3 increased the trichome size of the low-artemisinin-producing but not the high-artemisinin-producing cultivar. This suggested that trichomes in the high-artemisinin cultivar were already replete with AN.

ABA plays an important role in a plant's response to different biotic and abiotic stresses; also, it is elevated often in response to drought. When soil-grown A. annua plants were sprayed with 1–100 μM ABA, there was a significant increase in AN content compared to controls (Jing et al. 2009). There also was an increase in HMGR, FPS, and CYP transcripts in ABA-treated plants after 4, 12, and 8 h, respectively. This is consistent with the observations of Yang et al. (2010) who reported increased ADS expression in plants when roots were subjected to drought.

JA and SA effects

Like many secondary metabolites, AN is thought to play a part in plant defense, including against oxidative stress. JA is a secondary messenger known to activate a plant's defense system against oxidative stress and to induce production of secondary metabolites (van der Fits and Memelink 2000). Wang et al. (2009) sprayed soil-grown plants with 300 μM methyl jasmonate and 6 d after the treatment saw a significant increase in AN compared to controls. Because trichomes are the site of AN biosynthesis and sequestration, Liu et al. (2009) also measured trichome density 14 d after JA treatment. Both trichome density and AN levels increased after JA treatment compared to controls. Later Maes et al. (2010) sprayed and soil-drenched two A. annua cultivars with 100 μM JA or water and observed a response similar to that of GA3 treatment: enhanced trichome density, larger trichomes, and more AN. JA stimulated AN and DHAA production concurrent with increases in FPS and DBR2 expression in a high-artemisinin cultivar. JA also stimulated AB and AA production concurrent with increases in ALDH1 in the low-artemisinin cultivar (Maes et al. 2010). In addition, JA increased expression of ADS, CYP, and CPR in both cultivars.

Salicylic acid (SA) is another plant signal involved in defense (Durrant and Dong 2004). When SA was sprayed onto A. annua plants, AN, AA, and DHAA all increased (Pu et al. 2009). Only ADS and HMGR showed a measurable increase in transcripts. The ROS molecules, H2O2 and O2− both showed significant increases about 4 h after treatment, and this timing coincided with a significant increase in AN. This study was later confirmed by Guo et al. (2010).

Other Regulation of Artemisinin

Competing pathways

Inhibition of squalene synthase (SQS) in other plants has been shown to be coordinately regulated with sesquiterpene cyclases. For example, Vögeli and Chappell (1988) showed that in tobacco when sterols were inhibited, sesquiterpene production increased; conversely, inhibition of sesquiterpene synthesis resulted in enhanced sterol synthesis. In A. annua, when SQS was inhibited with miconazole, AN yield increased (Kudakasseril et al. 1987; Towler and Weathers 2007). Moreover, studies using either an antisense or interference strategy with A. annua (Yang et al. 2008; Feng et al. 2009), or promoter replacement in Saccharomyces cerevisiae (Paradise et al. 2008), showed that when SQS was inhibited, AN metabolites and transcripts of genes in the AN biosynthetic pathway both increased. Feng et al. (2009) further showed that downregulation of SQS results in preferential upregulation of ADS instead of the other competing sesquiterpene cyclases.

Transcription factors

As mentioned previously, a transcription factor, AaWRKY1, was found in A. annua and affects AN biosynthesis. AaWRKY1 binds to the W-box cis-acting elements of the ADS promoter and likely regulates ADS gene expression (Ma et al. 2009). WRKY transcription factors are plant specific transcription factors that bind to the W-box of the promoter of defense-related genes (Ülker and Somssich 2004). These transcription factors are known to contain a conserved amino acid sequence (WRKYGQK) and a zinc finger-like motif. Using the plant cis-acting regulatory DNA element database, Ma et al. (2009) found that the ADS promoter contains two reverse-oriented TTGACC W-boxes. Subsequently, AaWRkY1 was isolated by constructing a cDNA library using mRNA isolated from GSTs and shown to contain the consensus WRKY sequence and a zinc finger-like motif. Ma et al. (2009) showed through an electrophoretic mobility shift assay that AaWRKY1 was able to bind to the ADS promoter but unable to bind to a mutated ADS promoter. They also showed that an increase in AaWRKY1 resulted in an increase in ADS in transformed A. annua. JA also upregulates AaWRKY1, which correlates well with the ADS results observed by Maes et al. (2010).

Sugars

Sugars are known to stimulate production of many secondary metabolites, including AN (Wang and Weathers 2007). When A. annua seedlings were inoculated into Gamborg's B5 medium containing 3% (w/v) sucrose, glucose, or fructose for 14 d, glucose stimulated AN production while fructose inhibited production. Increasing the ratio of glucose to fructose showed a direct correlation with increasing AN level (Wang and Weathers 2007).

In a subsequent study, Arsenault et al. (2010b) reported that expression of the genes HMGR, FPS, DXS, DXR, ADS, and CYP in the AN biosynthetic pathway as well as production of AN metabolites, AN, AB, DHAA, and AA were also affected. AN was the highest 2 d after inoculation of seedlings into B5 medium containing glucose, and as AN increased from day 1 to day 2, DHAA began to decrease (Arsenault et al. 2010b). In medium containing fructose, both AN and DHAA decreased, further supporting the earlier observation by Wang and Weathers (2007) that fructose inhibits AN biosynthesis (Arsenault et al. 2010b). On the other hand, AA and AB continued to decline after inoculation into medium containing glucose. In medium containing fructose, however, the AA concentration was always higher than AB. It appeared that AA does not show the same precursor kinetic patterns as was seen with DHAA and AN. Although after 1 d glucose- and fructose-fed plants increased HMGR transcripts, 3.5- and 2.5-fold, respectively, compared to seedlings fed sucrose, FPS increased 8-fold but only in the glucose-fed seedling (Arsenault et al. 2010b). Glucose also increased DXS and DXR expression in seedlings 1 d after inoculation. After a week's incubation, and compared to fructose-fed seedlings, glucose-fed seedlings showed a 3-fold increase in ADS expression relative to sucrose-fed seedlings. CYP expression, on the other hand, showed a bimodal increase 2 and 7 d after inoculation of seedlings into glucose. Only glucose induced expression of all six genes involved in AN biosynthesis. While not practical from the standpoint of production, these studies show that in planta regulation of AN production, at least in young seedlings, is subject to fluctuations in native sugars that are in constant flux in plants.

DMSO

Although AN is known to be phytotoxic and is thought to be involved in plant defense, it has also been suggested that DHAA eliminates reactive oxygen species (ROS) by reacting with ROS to form AN as a by-product (Mannan et al. 2010). Staining with 3,3-diaminobenzidine (DAB) for hydrogen peroxide, a common ROS in plants, Mannan et al. (2010) reported an increase in hydrogen peroxide in leaves of 14-d rooted shoots that were exposed to DMSO. The highest amounts of AN were detected in rooted shoots exposed to 0.25% (v/v)and 2% (v/v) DMSO after 7 d. It was also found that AN levels of rooted shoots exposed to 0.25% (v/v) DMSO produced the highest AN level, which continued to increase after 7 d. Interestingly, analysis of DHAA and AA, the precursors of AN and AB, respectively, in the same experiment described above showed that DMSO was able to increase production of DHAA but not AA. The highest level of CYP transcripts were detected in cultures exposed to 0.5% (v/v) DMSO after 24 h, but at 0.5% (v/v) DMSO, AN level was the lowest compared to cultures exposed to 0.1%, 0.25%, 1%, and 2% (v/v) DMSO. There were no significant changes to ADS transcripts level in any of the DMSO concentrations.

Ascorbic acid (vitamin C, AsA) is a ROS scavenger, and Mannan et al. (2010) hypothesized that the hydrogen peroxide induced by DMSO should decrease in the presence of AsA. When DMSO-treated plants were incubated with either 10 or 20 mM AsA, levels of peroxide decreased and production of AN significantly decreased by about 80%. This study showed that although DMSO is not part of the natural biochemistry of the plant, it can be a useful tool in helping to elucidate the role of ROS in AN biosynthesis. Indeed, ROS, possibly as hydrogen peroxide, is playing a role in AN biosynthesis and DHAA is likely acting as a ROS sink with concomitant production of AN.

Conclusions

Overall, a number of different factors are now known to affect the production of AN in A. annua, and these are summarized in Table 1. Until recently, the one broad rule had been: trichomes + roots + ROS = artemisinin. However, it is now clear that, not surprisingly, phytohormones, in particular jasmonic acid, are playing an important role in regulating production of this terpene. As finer points of control become elucidated, our understanding of terpene production in plants will hopefully enable us to better harness the plant for production of this important therapeutic.

Table 1.

Summary of known regulating factors affecting artemisinin production in A. annua

JA SA GA3 BAP ABA DMSO Glucose Fructose AA AN Chilling Drought Water
logginga 		AM
GENES
  HMGR nm + nm nm + nm + + ns ns + ns ns nm
  DXS nm nm nm nm nm nm + ns nm nm + + ns nm
  DXR nm nm nm nm nm nm + ns nm nm + ns ns nm
  FPS + ns nm nm + nm + ns ns ns + + ns nm
  ADS + + nm nm ns ns + ns − ns + + ns nm
  CYP + ns nm nm + + + ns − − + + ns nm
  DBR2 + nm nm nm nm nm nm nm nm nm ns ns + nm
  ALDH1 + nm nm nm nm nm nm nm nm nm nm nm nm nm
METABOLITES
 AA + + ns ns nm ns − − nm nm nm nm nm nm
 AB + nm ns ns nm ns − − nm nm nm nm nm nm
 DHAA + + ns ns nm + − − nm nm nm nm nm nm
 AN + + ns ns + + + − nm − + + nm +
 Glandular trichome # + nm ns + nm nm nm nm nm nm nm nm nm nm
 Shoot response to root-perceived signal? Yes nm Yes Yes nm Yes Yes Yes nm nm nm nm Yes Yes
Open in a new tab
a

Yang et al. (2010) suggested that water logging induced hypoxia. +

nm not measured, ns not significant, + upregulated or increased; − downregulated or decreased, AA artemisinic acid; AB arteannuin B; ABA abscisic acid; AM arbuscular mycorrhizae; AN artemisinin; DMSO dimethyl sulfoxide; DHAA dihydroartemisinic acid; JA jasmonic acid; SA salicylic acid; GA3 gibberellin 3; BAP benzylaminopurine

Acknowledgments

The authors thank Dr. Patrick Covello, Plant Biotechnology Institute, Saskatoon, SK, Canada for providing a critical review of the manuscript. This work was supported in part by the National Institutes of Health grant number 2R15GM069562–03.

References

  1. Aftab T, Khan MMA, Idrees M, Naeem M, Ram M. Boron induced oxidative stress, antioxidant defence response and changes in artemisinin content in Artemisia annua L. J. Agron. Crop Sci. 2010 doi:10.1111/j.1439-037X.2010.00427.x. [Google Scholar]
  2. Arsenault PR, Vail D, Wobbe KK, Erickson K, Weathers PJ. Reproductive development modulates gene expression and metabolite levels with possible feedback inhibition of artemisinin in Artemisia annua L. Plant Physiol. 2010a;154:958–968. doi: 10.1104/pp.110.162552. [DOI] [PMC free article] [PubMed] [Google Scholar]
  3. Arsenault PR, Vail DR, Wobbe KK, Weathers PJ. Effect of sugars on artemisinin production in Artemisia annua L.: transcription and metabolite measurements. Molecules. 2010b;15:2302–2318. doi: 10.3390/molecules15042302. [DOI] [PMC free article] [PubMed] [Google Scholar]
  4. Arsenault PR, Wobbe KK, Weathers PJ. Recent advances in artemisinin production through heterologous expression. Curr. Med. Chem. 2008;15:2886–2896. doi: 10.2174/092986708786242813. [DOI] [PMC free article] [PubMed] [Google Scholar]
  5. Banyai W, Kirdmanee C, Mii M, Supaibulwatana K. Overexpression of farnesyl pyrophosphate synthase (FPS) gene affected artemisinin content and growth of Artemisia annua L. Plant Cell Tissue Organ. 2010;103:255–265. [Google Scholar]
  6. Brown GD, Sy LK. In vivo transformations of artemisinic acid in Artemisia annua plants. Tetrahedron. 2007;63:9548–9566. [Google Scholar]
  7. Chang YJ, Song SH, Park SH, Kim SU. Amorpha-4,11-diene synthase of Artemisia annua: cDNA isolation and bacterial expression of a terpene synthase involved in artemisinin biosynthesis. Arch. Biochem. Biophys. 2000;383:178–184. doi: 10.1006/abbi.2000.2061. [DOI] [PubMed] [Google Scholar]
  8. Chen D, Ye H, Li G. Expression of a chimeric farnesyl diphosphate synthase gene in Artemisia annua L. transgenic plants via Agrobacterium tumefaciens-mediated transformation. Plant Sci. 2000;155:179–185. doi: 10.1016/s0168-9452(00)00217-x. [DOI] [PubMed] [Google Scholar]
  9. Covello PS. Making artemisinin. Phytochemistry. 2008;69:2881–2885. doi: 10.1016/j.phytochem.2008.10.001. [DOI] [PubMed] [Google Scholar]
  10. Covello PS, Teoh KH, Polichuk DR, Reed DW, Nowak G. Functional genomics and the biosynthesis of artemisinin. Phytochemistry. 2007;68:1864–1871. doi: 10.1016/j.phytochem.2007.02.016. [DOI] [PubMed] [Google Scholar]
  11. Delabays N, Simonnet X, Gaudin M. the genetics of artemisinin content in Artemisia annua L. and the breeding of high yield cultivars. Curr. Med. Chem. 2001;8:1795–1801. doi: 10.2174/0929867013371635. [DOI] [PubMed] [Google Scholar]
  12. Dhingra V, Narasu ML. Purification and characterization of an enzyme involved in biochemical transformation of arteannuin B to artemisinin from Artemisia annua. Biochem. Biophys. Res. Commun. 2001;281:558–561. doi: 10.1006/bbrc.2000.4197. [DOI] [PubMed] [Google Scholar]
  13. Duke MG, Paul RN, Elsohly HN, Sturtz G, Duke SO. Localization of artemisinin and artemisitene in foliar tissues of glanded and glandless biotypes of Artemisia annua L. Int. J. Plant Sci. 1994;155:365–372. [Google Scholar]
  14. Durrant WE, Dong X. Systemic acquired resistance. Annu. Rev. Phytopathol. 2004;42:185–209. doi: 10.1146/annurev.phyto.42.040803.140421. [DOI] [PubMed] [Google Scholar]
  15. Efferth T. Herbal drugs : ethnomedicine to modern medicine. Springer-Verlag; Berlin Heidelberg, Germany: 2009. Artemisinin: a versatile weapon from traditional Chinese medicine; pp. 173–194. [Google Scholar]
  16. Feng LL, Yang RY, Yang XQ, Zeng XM, Lu WJ, Zeng QP. Synergistic re-channeling of mevalonate pathway for enhanced artemisinin production in transgenic Artemisia annua. Plant Sci. 2009;177:57–67. [Google Scholar]
  17. Ferreira JFS, Gonzalez JM. Chemical and biological stability of artemisinin in bovine rumen fluid and its kinetics in goats (Capra hircus) Rev. Bras. Parasitol. Vet. 2008;17(Supp. 1):103–109. [PubMed] [Google Scholar]
  18. Ferreira JFS, Janick J. Progress in new crops. ASHS Press; Arlington, Virginia, USA: 1996a. Distribution of artemisinin in Artemisia annua; pp. 579–584. [Google Scholar]
  19. Ferreira JFS, Janick J. Roots as an enhancing factor for the production of artemisinin in shoot cultures of Artemisia annua. Plant Cell Tissue Organ. 1996b;44:211–217. [Google Scholar]
  20. Ferreira JFS, Simon JE, Janick J. Developmental studies of Artemisia annua: flowering and artemisinin production under greenhouse and field conditions. Planta Med. 1995;61:167–170. doi: 10.1055/s-2006-958040. [DOI] [PubMed] [Google Scholar]
  21. Graham IA, Besser K, Blumer S, Branigan CA, Czechowski T, Elias L, Guterman I, Harvey D, Isaac PG, Khan AM, Larson TR, Li Y, Pawson T, Penfield T, Rae AM, Rathbone DA, Reid S, Ross J, Smallwood MF, Segura V, Townsend T, Vyas D, Winzer T, Bowles D. The genetic map of Artemisia annua L. identifies loci affecting yield of the antimalarial drug artemisinin. Science. 2010;327:328–331. doi: 10.1126/science.1182612. [DOI] [PubMed] [Google Scholar]
  22. Guo XX, Yang XQ, Yang RY, Zeng QP. Salicylic acid and methyl jasmonate but not Rose Bengal enhance artemisinin production through involving burst of endogenous singlet oxygen. Plant Sci. 2010;178:390–397. [Google Scholar]
  23. Han JL, Liu BY, Ye HC, Wang H, Li ZQ, Li GF. Effects of overexpression of the endogenous farnesyl diphosphate synthase on the artemisinin content in Artemisia annua L. J. Integr. Plant Biol. 2006;48:482–487. [Google Scholar]
  24. Jing F, Zhang L, Li M, Tang Y, Wang Y, Wang Y, Wang Q, Pan Q, Wang G, Tang K. Abscisic acid (ABA) treatment increases artemisinin content in Artemisia annua by enhancing the expression of genes in artemisinin biosynthesis pathway. Biologia. 2009;64:319–323. [Google Scholar]
  25. Kapoor RR, Chaudhary V, Bhatnagar AK. Effects of arbuscular mycorrhiza and phosphorus application on artemisinin concentration in Artemisia annua L. Mycorrhiza. 2007;17:581–587. doi: 10.1007/s00572-007-0135-4. [DOI] [PubMed] [Google Scholar]
  26. Kim SH, Chang YJ, Kim SU. Tissue specificity and developmental pattern of amorpha-4, 11-diene synthase (ADS) proved by promoter-driven GUS expression in the heterologous plant Arabidposis thaliana. Planta Med. 2008;74:188–193. doi: 10.1055/s-2008-1034276. [DOI] [PubMed] [Google Scholar]
  27. Kindermans JM, Pilloy J, Olliaro P, Gomes M. Ensuring sustained ACT production and reliable artemisinin supply. Malaria J. 2007;6:125. doi: 10.1186/1475-2875-6-125. doi:10.1186/1475-2875-6-125. [DOI] [PMC free article] [PubMed] [Google Scholar]
  28. Kudakasseril GJ, Lam L, Staba EJ. Effect of sterol inhibitors on the incorporation of 14 C-isopentenyl pyrophosphate into artemisinin by a cell-free system from Artemisia annua tissue cultures and plants. Planta Med. 1987;53:280–284. doi: 10.1055/s-2006-962706. [DOI] [PubMed] [Google Scholar]
  29. Liu S, Tian N, Li J, Huang J, Liu Z. Isolation and identification of novel genes involved in artemisinin production from flowers of Artemisia annua using suppression subtractive hybridization and metabolite analysis. Planta Med. 2009;74:1542–1547. doi: 10.1055/s-0029-1185809. [DOI] [PubMed] [Google Scholar]
  30. Lommen WJM, Elzing S, Verstappen FWA, Bouwmeester HJ. Artemisinin and sesquiterpene precursors in dead and green leaves of Artemisia annua L. crops. Planta Med. 2007;73:1133–1139. doi: 10.1055/s-2007-981567. [DOI] [PubMed] [Google Scholar]
  31. Lommen WJM, Schenk E, Bouwmeester HJ, Verstappen FWA. Trichome dynamics and artemisinin accumulation during development and senescence of leaves of Artemisia annua leaves. Planta Med. 2006;72:336–345. doi: 10.1055/s-2005-916202. [DOI] [PubMed] [Google Scholar]
  32. Lulu Y, Chang Z, Ying H, Ruiyi Y, Qingping Z. Abiotic stress-induced expression of artemisinin biosynthesis genes in Artemisia annua L. Chin. J Appl. Environ. Biol. 2008;14:001–005. [Google Scholar]
  33. Ma C, Wang H, Lu X, Xu G, Liu B. Metabolic fingerprinting investigation of Artemisia annua L. in different stages of development by gas chromatography and gas chromatography-mass spectrometry. J. Chromatogr. 2008;1186:412–419. doi: 10.1016/j.chroma.2007.09.023. [DOI] [PubMed] [Google Scholar]
  34. Ma DD, Pu G, Lei C, Ma L, Wang H, Guo Y, Chen J, Du Z, Wang H, Li G, Ye H, Liu B. Isolation and characterization of AaWRKY1, an Artemisia annua transcription factor that regulates the amorpha-4, 11-diene synthase gene, a key gene of artemisinin biosynthesis. Plant Cell Physiol. 2009;50:2146–2161. doi: 10.1093/pcp/pcp149. [DOI] [PubMed] [Google Scholar]
  35. Maes L, Inze D, Goossens A. Functional specialization of the TRANSPARENT TESTA GLABRA1network allows differential hormonal control of laminal and marginal trichome initiation in Arabidopsis rosette leaves. Plant Physiol. 2008;148:1453–1564. doi: 10.1104/pp.108.125385. [DOI] [PMC free article] [PubMed] [Google Scholar]
  36. Maes L, Van Nieuwerburgh FCW, Zhang Y, Reed DW, Pollier J, Vande Casteele S, RF ID, Covello PS, Deforce DLD, Goossens A. Dissection of the phytohormonal regulation of trichome formation and biosynthesis of the antimalarial compound artemisinin in Artemisia annua plants. New Phytol. 2010 doi: 10.1111/j.1469-8137.2010.03466.x. doi:10.1111/j.1469-8137.2010.03466.x. [DOI] [PubMed] [Google Scholar]
  37. Mannan AA, Liu C, Arsenault PR, Towler MJ, Vail DR, Lorence A, Weathers PJ. DMSO triggers the generation of ROS leading to an increase in artemisinin and dihydroartemisinic acid in Artemisia annua shoot cultures. Plant Cell Rep. 2010;29:143–152. doi: 10.1007/s00299-009-0807-y. [DOI] [PMC free article] [PubMed] [Google Scholar]
  38. Marchese JA, Ferreira JF, Rehder V, Rodrigues O. Water deficit on the accumulation of biomass and artemisinin in annual wormwood (Artemisia annua L., Asteraceae) Congr. Braz. Soc. Plant Pathol. 2010;22:1–9. [Google Scholar]
  39. Mercke P, Bengtsson M, Bouwmeester HJ, Posthumus MA, Brodelius PE. Molecular cloning, expression, and characterization of amorpha-4, 11-diene synthase, a key enzyme of artemisinin biosynthesis in Artemisia annua L. Arch. Biochem. Biophys. 2000;381:173–180. doi: 10.1006/abbi.2000.1962. [DOI] [PubMed] [Google Scholar]
  40. Olsson ME, Olofsson LM, Lindahl AL, Lundgren A, Brodelius M, Brodelius PE. Localization of enzymes of artemisinin biosynthesis to the apical cells of glandular secretory trichomes of Artemisia annua L. Phytochemistry. 2009;70:1123–1128. doi: 10.1016/j.phytochem.2009.07.009. [DOI] [PubMed] [Google Scholar]
  41. Paradise EM, Kirby J, Chan R, Keasling JD. Redirection of flux through the FPP branch-point in Saccharomyces cerevisiae by down-regulating squalene synthase. Biotechnol. Bioeng. 2008;100:371–378. doi: 10.1002/bit.21766. [DOI] [PubMed] [Google Scholar]
  42. Pu GB, Ma DM, Chen JL, Ma LQ, Wang H, Li GF, Ye HC, Liu BY. Salicylic acid activates artemisinin biosynthesis in Artemisia annua L. Plant Cell Rep. 2009;7:1127–1135. doi: 10.1007/s00299-009-0713-3. [DOI] [PubMed] [Google Scholar]
  43. Ro DK, Paradise EM, Ouellet M, Fisher KJ, Newman KL, Ndungu JM, Ho KA, Eachus RA, Ham TS, Kirby J, Chang MCY, Withers ST, Shiba Y, Sarpong R, Keasling JD. Production of the antimalarial drug precursor artemisinic acid in engineered yeast. Nature. 2006;440:940–943. doi: 10.1038/nature04640. [DOI] [PubMed] [Google Scholar]
  44. Rydén AM, Ruyter-Spira C, Quax WJ, Osada H, Muranaka T, Kayser O, Bouwmeester H. The molecular cloning of dihydroartemisinic aldehyde reducatse and its implication in artemisinin biosynthesis in Artemisia annua. Planta Med. 2010;76:1778–1783. doi: 10.1055/s-0030-1249930. [DOI] [PubMed] [Google Scholar]
  45. Sa G, Ma M, Ye HC, Liu BY, Li GF, Chong K. Effects of ipt gene expression on the physiological and chemical characteristics of Artemisia annua L. Plant Sci. 2001;160:691–698. doi: 10.1016/s0168-9452(00)00453-2. [DOI] [PubMed] [Google Scholar]
  46. Schramek N, Wang H, Römisch-Margl W, Keil B, Radykewicz T, Winzenhörlein B, Beerhues L, Bacher A, Rohdich F, Gershenzon J, Liu B, Eisenreich W. Artemisinin biosynthesis in growing plants of Artemisia annua. A 13CO2 study. Phytochemistry. 2010;71:179–187. doi: 10.1016/j.phytochem.2009.10.015. [DOI] [PubMed] [Google Scholar]
  47. Teoh KH, Polichuk DR, Reed DW, Covello PS. Molecular cloning of an aldehyde dehydrogenase implicated in artemisinin biosynthesis in Artemisia annua. Botany. 2009;87:635–642. [Google Scholar]
  48. Teoh KH, Polichuk DR, Reed DW, Nowak G, Covello PS. Artemisia annua L. (asteraceae) trichome-specific cDNAs reveal CYP71AV1, a cytochrome P450 with a key role in the biosynthesis of the antimalarial sesquiterpene lactone artemisinin. FEBS Lett. 2006;580:1411–1416. doi: 10.1016/j.febslet.2006.01.065. [DOI] [PubMed] [Google Scholar]
  49. Towler MJ, Weathers PJ. Evidence of artemisinin production from IPP stemming from both the mevalonate and the nonmevalonate pathways. Plant Cell Rep. 2007;26:2129–2136. doi: 10.1007/s00299-007-0420-x. [DOI] [PubMed] [Google Scholar]
  50. Ülker B, Somssich IE. WRKY transcription factors: from DNA binding towards biological function. Curr. Opin. Plant Biol. 2004;7:491–498. doi: 10.1016/j.pbi.2004.07.012. [DOI] [PubMed] [Google Scholar]
  51. Van der Fits L, Memelink J. ORCA3, a jasmonate-responsive transcriptional regulator of plant primary and secondary metabolism. Science. 2000;289:295–297. doi: 10.1126/science.289.5477.295. [DOI] [PubMed] [Google Scholar]
  52. Vögeli U, Chappell J. Induction of sesquiterpene cyclase and suppression of squalene synthetase activities in plant cell cultures treated with fungal elicitor. Plant Physiol. 1988;88:1291–1296. doi: 10.1104/pp.88.4.1291. [DOI] [PMC free article] [PubMed] [Google Scholar]
  53. Wallaart TE, Bouwmeester HJ, Hille J, Poppinga L, Maijers NCA. Amorpha-4, 11-diene synthase: cloning and functional expression of a key enzyme in the biosynthetic pathway of the novel antimalarial drug artemisinin. Planta. 2001;212:460–465. doi: 10.1007/s004250000428. [DOI] [PubMed] [Google Scholar]
  54. Wallaart TE, Pras N, Beekman AC, Quax WJ. Seasonal variation of artemisinin and its biosynthetic precursors in plants of Artemisia annua of different geographical origin: proof for the existence of chemotypes. Planta Med. 2000;66:57–62. doi: 10.1055/s-2000-11115. [DOI] [PubMed] [Google Scholar]
  55. WHO . World malaria report 2009. WHO Press; Switzerland: 2009. http://www.who.int/topics/malaria/en/ [Google Scholar]
  56. Wang H, Ge L, Ye HC, Chong K, Liu BY, Li GF. Studies on the effects of fpf1 gene on Artemisia annua flowering time and on the linkage between flowering and artemisinin biosynthesis. Planta Med. 2004;70:347–352. doi: 10.1055/s-2004-818947. [DOI] [PubMed] [Google Scholar]
  57. Wang H, Liu Y, Chong K, Liu BY, Ye HC, Li ZQ, Yan F, Li GF. Earlier flowering induced by over-expression of CO gene does not accompany increase of artemisinin biosynthesis in Artemisia annua. Plant Biol. 2007;9:442–446. doi: 10.1055/s-2006-924634. [DOI] [PubMed] [Google Scholar]
  58. Wang H, Ma C, Li Z, Ma L, Wang H, Ye H, Xu G, Liu B. Effects of exogenous methyl jasmonate on artemisinin biosynthesis and secondary metabolites in Artemisia annua L. Ind. Crops Prod. 2009;31:214–218. [Google Scholar]
  59. Wang YY, Weathers PJ. Sugars proportionately affect artemisinin production. Plant Cell Rep. 2007;26:1073–1081. doi: 10.1007/s00299-006-0295-2. [DOI] [PubMed] [Google Scholar]
  60. Weathers PJ, Elkholy S, Wobbe KK. Artemisinin: the biosynthetic pathway and its regulation in Artemisia annua, a terpenoid-rich species. In Vitro Cell Dev-Pl. 2006;42:309–317. [Google Scholar]
  61. Yang RY, Feng LL, Yang XQ, Yin LL, Xu XL, Zeng QP. Quantitative transcript profiling reveals down-regulation of a sterol pathway relevant gene and overexpression of artemisinin biogenetic genes in transgenic Artemisia annua plants. Planta Med. 2008;74:1510–1516. doi: 10.1055/s-2008-1081333. [DOI] [PubMed] [Google Scholar]
  62. Yang RY, Zeng XM, Lu YY, Lu WJ, Feng LL, Yang XQ, Zeng QP. Senescent leaves of Artemisia annua are one of the most active organs for overexpression of artemisinin biosynthesis responsible genes upon burst of singlet oxygen. Planta Med. 2010;76:734–742. doi: 10.1055/s-0029-1240620. [DOI] [PubMed] [Google Scholar]
  63. Zhang Y, Nowak G, Reed DW, Covello PS. The production of artemisinin precursors in tobacco. Plant Biotechnol. J. 2010 doi: 10.1111/j.1467-7652.2010.00556.x. doi:10.1111/j.1467-7652.2010.00556.x. [DOI] [PubMed] [Google Scholar]
  64. Zhang YY, Teoh KH, Reed DW, Maes L, Goossens A, Olson DJH, Ross ARS, Covello PS. The molecular cloning of artemisinic aldehyde 11(13) reductase and its role in glandular trichome-dependent biosynthesis of artemisinin in Artemisia annua. J. Biol. Chem. 2008;283:21501–21508. doi: 10.1074/jbc.M803090200. [DOI] [PubMed] [Google Scholar]

RESOURCES