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

NIH Public Access Author Manuscript In Vitro Cell Dev Biol Plant. Author manuscript; available in PMC 2012 June 1. NIH-PA Author Manuscript Published in final edited form as: In Vitro Cell Dev Biol Plant. 2011 June ; 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 Department of Biology and Biotechnology at Gateway, Worcester Polytechnic Institute, 100 Institute Rd, Worcester, MA 01609, USA Patrick R. Arsenault Department of Biology and Biotechnology at Gateway, Worcester Polytechnic Institute, 100 Institute Rd, Worcester, MA 01609, USA Department of Pathology and Laboratory Medicine, University of Pennsylvania Medical School, Philadelphia, PA 19104, USA NIH-PA Author Manuscript Pamela J. Weathers Department of Biology and Biotechnology at Gateway, Worcester Polytechnic Institute, 100 Institute Rd, Worcester, MA 01609, USA 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 NIH-PA Author Manuscript 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 © The Society for In Vitro Biology 2011 weathers@wpi.edu . Nguyen et al. Page 2 NIH-PA Author Manuscript 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. NIH-PA Author Manuscript 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-4,11diene 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. NIH-PA Author Manuscript 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. In Vitro Cell Dev Biol Plant. Author manuscript; available in PMC 2012 June 1. Nguyen et al. Page 3 Morphological and Developmental Regulation of Artemisinin Trichomes NIH-PA Author Manuscript 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). NIH-PA Author Manuscript 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). NIH-PA Author Manuscript 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. In Vitro Cell Dev Biol Plant. Author manuscript; available in PMC 2012 June 1. Nguyen et al. Page 4 Roots NIH-PA Author Manuscript 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 NIH-PA Author Manuscript 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). NIH-PA Author Manuscript 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 Vitro Cell Dev Biol Plant. Author manuscript; available in PMC 2012 June 1. Nguyen et al. Page 5 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. NIH-PA Author Manuscript 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. NIH-PA Author Manuscript 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 NIH-PA Author Manuscript 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. In Vitro Cell Dev Biol Plant. Author manuscript; available in PMC 2012 June 1. Nguyen et al. Page 6 Phytohormones and Other Signaling Molecules GA, ABA, and BAP effects NIH-PA Author Manuscript 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. NIH-PA Author Manuscript 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 highartemisinin-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-artemisininproducing 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 NIH-PA Author Manuscript 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). In Vitro Cell Dev Biol Plant. Author manuscript; available in PMC 2012 June 1. Nguyen et al. Page 7 Other Regulation of Artemisinin Competing pathways NIH-PA Author Manuscript 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 NIH-PA Author Manuscript 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 NIH-PA Author Manuscript 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 In Vitro Cell Dev Biol Plant. Author manuscript; available in PMC 2012 June 1. Nguyen et al. Page 8 NIH-PA Author Manuscript 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 NIH-PA Author Manuscript 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 3,3diaminobenzidine (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 NIH-PA Author Manuscript 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. 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[PubMed: 18495659] NIH-PA Author Manuscript In Vitro Cell Dev Biol Plant. Author manuscript; available in PMC 2012 June 1. Nguyen et al. Page 13 NIH-PA Author Manuscript Figure 1. 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 1deoxyxylulose 5- phosphate synthase, FPS farnesyl diphosphate synthase, HMGR 3hydroxy-3-methylglutaryl-CoA reductase, HMG-CoA 3-hydroxy-3-methylglutaryl-CoA, MEP nonmevalonate pathway (plastid pathway), RED1 dihydroartemisinic aldehyde reductase 1. NIH-PA Author Manuscript NIH-PA Author Manuscript In Vitro Cell Dev Biol Plant. Author manuscript; available in PMC 2012 June 1. Nguyen et al. Page 14 NIH-PA Author Manuscript Figure 2. 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 1deoxyxylulouse 5-phosphate reductoisomerase, DXS 1-deoxyxylulose 5-phosphate synthase, FPS farnesyl diphosphate synthase. NIH-PA Author Manuscript NIH-PA Author Manuscript In Vitro Cell Dev Biol Plant. Author manuscript; available in PMC 2012 June 1. NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript Table 1 JA SA GA3 BAP ABA DMSO Glucose Fructose AA AN Chilling Drought Water logginga AM 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 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 Nguyen et al. Summary of known regulating factors affecting artemisinin production in A. annua GENES In Vitro Cell Dev Biol Plant. Author manuscript; available in PMC 2012 June 1. METABOLITES 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 Page 15