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Metabolic regulation and engineering of artemisinin biosynthesis in A. annua

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  • Artemisinin is a potent anti-malarial sesquiterpene lactone that is naturally biosynthesized in glandular trichomes of a Chinese herbal plant, Artemisia annua. Although semi-synthesis of artemisinin in yeast has been feasible, at present the A. annua plant is still the main commercial source of artemisinin. The content of artemisinin however is low in A. annua, limiting its supply to malarial victims. So it's crucial to elevate artemisinin production in A. annua. Up to date artemisinin biosynthetic pathway has been completely elucidated. And many regulatory factors, mediating diverse plant hormone or environmental signaling routes, have been identified to get involved in the regulation of artemisinin biosynthesis and glandular trichome formation. Understanding the regulatory mechanisms of artemisinin biosynthesis and glandular trichome formation will be conducive to our practice for improving artemisinin production by metabolic engineering. In this review, the metabolic regulatory network with regard to artemisinin biosynthesis and glandular trichome formation is summarized, and the advance on metabolic engineering to increase artemisinin content in A. annua is also discussed.
  • Bletilla Rchb. f. is one of the most economically valuable groups of orchids in the world. Due to its ornamental significance, the genus Bletilla occupies an important place in the worldwide horticultural market. Furthermore, in China, Japan, South Korea, and other Asian countries, it is highly valued for its medicinal use[1].

    There are eight species in the genus Bletilla, including Bletilla chartacea (King & Pantl.) Tang & F.T. Wang, Bletilla cotoensis Schltr., Bletilla foliosa (King & Pantl.) Tang & F.T. Wang, Bletilla formosana Schltr., Bletilla guizhouensis J. Huang & G.Z. Chen, Bletilla morrisonensis Schltr., Bletilla ochracea Schltr., and Bletilla striata Rchb.f.[2,3]. The distribution area spans from northern Myanmar in Asia to Japan via China[4]. Five species are native to China, namely, B. foliosa, B. formosana, B. guizhouensis, B. ochracea, and B. striata. In China, people have assigned various names to Bletilla based on its morphology and efficacy, such as baiji (白及/白芨), baigen (白根), baige (白给), baijier (白鸡儿), baijiwa (白鸡娃), diluosi (地螺丝), gangen (甘根), junkouyao (皲口药), lianjicao (连及草), and yangjiaoqi (羊角七)[5]. These diverse appellations highlight the importance of this genus in Chinese folk biological culture.

    The medicinal material known as 'baiji' in traditional Chinese medicine (TCM) is usually the dried tuber of B. striata, which is also the authentic product included in the Chinese Pharmacopoeia[6]. According to the Chinese Pharmacopoeia (2020), TCM baiji is sliced, dried, and crushed into a powder that can be used topically or internally, with a recommended dosage of 3–6 g at a time, offering astringent, hemostatic, detumescence, and myogenic effects. It is often used for conditions such as hemoptysis, hematemesis, traumatic bleeding, sores, and skin chaps[7]. Although only B. striata is the authentic product of TCM baiji, the other four Bletilla species native to China are also used as substitutes, and this practice is widespread[8].

    Modern research indicates that Bletilla contains a variety of chemical components, including benzol, dihydrophenanthrene, phenanthrene, and quinone derivatives. These components confer pharmacological effects on Bletilla, such as hemostasis, anti-tumor activity, and promotion of cell growth[9]. Due to its outstanding medicinal value, Bletilla can be found in nearly every corner of the traditional medicine market (Fig. 1). However, habitat destruction and uncontrolled mining have led to a significant reduction in the native populations of Bletilla, making its protection an urgent priority. Therefore, this paper provides a comprehensive review of relevant research up to August 2023, covering botanical characteristics, resource distribution, ethnobotanical uses, chemical components, pharmacological effects, clinical applications, and safety evaluations of Bletilla. The aim is to raise awareness and promote the protection and sustainable use of this genus.

    Figure 1.  Varieties of Bletilla at the traditional March Medicinal Market in Dali, Yunnan, China.

    The morphology of different Bletilla species is highly similar. The primary taxonomic feature distinguishing each species is the characteristics of the flower, particularly the lip of the flower, including its size, shape, and the number and shape of longitudinal ridges on the lip plate (Table 1, Fig. 2)[1014].

    Table 1.  The morphological differences among five species of Bletilla plants native to China.
    Morphological featureBletilla striataBletilla formosanaBletilla ochraceaBletilla foliosaBletilla guizhouensis
    Plant height (cm)18−6015−8025−5515−2045−60
    Rhizome shapeCompressedCompressedSomewhat compressedSubgloboseCompressed
    Rhizome diameter (cm)1−31−2About 21−1.53−4
    Stem characteristicsStoutEnclosed by sheathsStoutStout, shortThin
    Leaf shapeNarrowly oblongLinear-lanceolateOblong-lanceolateElliptic-lanceolateNarrowly lanceolate
    Leaf size (cm)8−29 × 1.5−46−40 × 0.5−4.58−35 × 1.5−2.85−12 × 0.8−325−45 × 1.2−4.5
    Flower colorPurplish red or pinkPale purple or pinkYellowPale purpleDeep purple
    Flower sizeLargeMediumMediumSmall to mediumLarge
    Inflorescence structureBranched or simpleBranched or simpleSimpleSimpleBranched
    Pedicel and ovary length (mm)10−248−12About 187−913−17
    Sepal shapeNarrowly oblongLanceolateLanceolateLinear-lanceolateOblong-elliptic
    Petal shapeSlightly larger than sepalsSlightly narrower than sepalsObliqueLanceolateOblong-elliptic
    Lip shapeObovate-ellipticBroadly ellipticNarrowly rhombic-obovateNarrowly oblongNarrowly oblong
    Lip colorWhite with purplish veinsWhitish to pale yellow with small dark purple spotsWhitish to pale yellow with small dark purple spotsWhite with purplish spots and purple edgeWhite with deep purple edge
    Number of lip Lamellae5 lamellae5 undulate lamellae5 longitudinal lamellae3 fimbriate lamellae7 longitudinal lamellae
    Column characteristicsSubterete, dilated towards apexSubterete, dilated towards apexSlender, dilated towards apexCylindric, dilated towards apexSuberect, with narrow wings
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    Figure 2.  (a)−(d) Bletilla striata (Thunb. ex Murray) Rchb. f. (e)−(h) Bletilla formosana (Hayata) Schltr. (i)−(l) Bletilla ochracea Schltr. (m), (n) Bletilla sinensis (Rolf) Schltr. (o), (p) Bletilla guizhouensis Jie Huang & G.Z. Chen (Photographed by Wang Meina, Zhu Xinxin, and He Songhua).

    The flowers of B. striata are large and purplish-red or pink, with narrowly oblong sepals and petals measuring 25−30 mm in length and 6−8 mm in width. They have acute apices, nearly as long as the sepals and petals. The lip is obovate or elliptic, predominantly white with purplish-red coloration and purple veins, measuring 23−28 mm in length, slightly shorter than the sepals and petals. The lip disc exhibits five longitudinal folds extending from the base to near the apex of the middle lobe, with waviness occurring only above the middle lobe[11]. In China, B. striata is found in regions such as Anhui, Fujian, Guangdong, Guangxi, Gansu, Guizhou, Hubei, Hunan, Jiangsu, Jiangxi, Shaanxi, Sichuan, and Zhejiang. It also occurs in the Korean Peninsula and Japan, thriving in evergreen broad-leaved forests, coniferous forests, roadside grassy areas, or rock crevices, at altitudes ranging from 100−3,200 m[12].

    B. ochracea's flowers are medium to large, featuring yellow or yellow-green exteriors on the sepals and petals, while the insides are yellow-white, occasionally nearly white. The sepals and petals are nearly equal in length, oblong, measuring 18−23 mm long and 5−7 mm wide, with obtuse or slightly pointed apices, often adorned with fine purple spots on the reverse side. The lip is elliptic, typically white or light yellow, measuring 15−20 mm in length and 8−12 mm in width, with three lobes above the middle. The lip disc is characterized by five longitudinally ridged pleats, with undulations primarily occurring above the middle lobe[13]. B. ochracea is native to southeastern Gansu, southern Shaanxi, Henan, Hubei, Hunan, Guangxi, Guizhou, Sichuan, and Yunnan, thriving in evergreen broad-leaved forests, coniferous forests, or beneath shrubs, in grassy areas or alongside ditches at altitudes ranging from 300−2,350 m[14].

    B. formosana's flowers come in shades of lavender or pink, occasionally white, and are relatively small. The sepals and petals are narrowly oblong, measuring 15−21 mm in length and 4−6.5 mm in width, and are nearly equal in size. The sepals have subacute apices, while the petal apices are slightly obtuse. The lip is elliptic, measuring 15−18 mm in length and 8−9 mm in width, with three lobes above the middle. The lip disc exhibits five longitudinal ridge-like pleats, which are wavy from the base to the top of the middle lobe[15]. B. formosana is indigenous to southern Shaanxi, southeastern Gansu, Jiangxi, Taiwan, Guangxi, Sichuan, Guizhou, central to northwest Yunnan, southeast Tibet (Chayu), and Japan. It is typically found in evergreen broad-leaved forests, coniferous forests, road verges, valley grasslands, grassy slopes, and rock crevices, at altitudes ranging from 600−3,100 m[16].

    The flowers of B. foliosa are small and lavender, with white sepals and petals featuring purple apices. The sepals are linear-lanceolate, measuring 11−13 mm in length and 3 mm in width, with subacute apices. The petals are lanceolate, also measuring 11−13 mm in length and 3 mm in width, with acute apices. The lip is white, oblong, adorned with fine spots, and features a purple apex. It measures 11−13 mm in length and 5−6 mm in width, tapering near the base and forming a scaphoid shape. The lip is anteriorly attenuated, unlobed, or abruptly narrowing with inconspicuous three lobes and exhibits fringe-like fine serrations along the edge. Three longitudinal ridge-like pleats are present on the upper lip disc[17]. B. foliosa typically grows on hillside forests, with its type specimen collected from Mengzi City, Honghe Hani and Yi Autonomous Prefecture, Yunnan Province, China[17].

    B. guizhouensis is a recently discovered species in Guizhou, China. In terms of shape, B. guizhouensis closely resembles B. striata, but it can be distinguished by its ovate-oblong buds, oblong dorsal sepals, obovate lips, and middle lobes of the lips, which are oval in shape. The disc of B. guizhouensis features seven distinct longitudinal lamellae, setting it apart from other known Bletilla species and establishing it as a distinct species[2]. Presently, B. guizhouensis has only been found in Guizhou, China, primarily thriving in evergreen broad-leaved forests at altitudes ranging from 900−1,200 m[3].

    Understanding the morphology, habitat, and distribution of Bletilla species is crucial for the conservation and propagation of these resources. To effectively implement plant conservation and breeding programs, a comprehensive understanding of the specific morphological characteristics, growth environments, and native habitats of these plants is essential, as without this knowledge, effective results cannot be achieved.

    The ethnobotanical uses of Bletilla worldwide primarily fall into two categories: ornamental and medicinal purposes. Bletilla orchids, renowned for their striking and distinct flowers, are commonly cultivated for ornamental purposes across many countries[18]. Valued for their aesthetic appeal, these orchids are frequently grown in gardens and utilized as potted plants. Among the various cultivars, B. striata stands out as the most favored choice for ornamental horticulture due to its ease of cultivation and adaptability to diverse climates[19,20].

    Contrastingly, in select Asian countries, Bletilla assumes a crucial role as a medicinal plant. For instance, influenced by TCM, the tuber of Bletilla also serves as a crude drug for hemostatic and anti-swelling purposes in Japan[21]. Likewise, traditional Korean medicine, deeply rooted in TCM principles, extensively documents the versatile use of Bletilla in addressing issues such as alimentary canal mucosal damage, ulcers, bleeding, bruises, and burns[22]. In Vietnam, Bletilla has been used as a medicinal herb for treating tumors and skin fissures, aligning with practices observed in the ethnic communities of southwest China[23].

    In China, Bletilla boasts a longstanding medicinal history, with numerous classical ancient Chinese medicine books containing detailed records of its medicinal applications[2432]. Even in contemporary society, many ethnic groups residing in mountainous areas in China continue to uphold the traditional medical practice of using Bletilla medicinally[31].

    In ancient Chinese medical literature, detailed records of Bletilla's morphology can be traced back to the late Han Dynasty, around 200 AD[24]. The Mingyi Bielu, a historical source, documented, 'Bletilla grows in the valley, with leaves resembling those of Veratrum nigrum L., and its root is white and interconnected. The ideal time for harvesting is September'. As awareness of the medicinal significance of Bletilla grew, successive dynastic-era Chinese medical texts consistently included descriptions of Bletilla's morphology (Table 2). In the Ming Dynasty, Li Shizhen compiled these earlier accounts of Bletilla's plant characteristics in his work, the Compendium of Materia Medica. He even provided an illustrative depiction of this plant genus (Fig. 3)[25].

    Table 2.  Morphological description of the plants belonging to Bletilla in the ancientChinese medicinal books.
    Dynasty (Year)TitleAuthorOriginal ChineseEnglish translation
    Late Han
    (184−220 AD)
    Mingyi Bielu/白给生山谷, 叶如藜芦,
    根白相连, 九月采
    Bletilla grows in the valley, with leaves like Veratrum nigrum L., root is white and connected. September is the time for harvesting.
    Wei-Jin period
    (220−420 AD)
    WuPu BencaoWu Pu白根, 茎叶如生姜, 藜芦,
    十月花, 直上, 紫赤色,
    根白连, 二月, 八月, 九月采
    Bletilla, stems and leaves like Zingiber officinale Roscoe and V. nigrum. It blooms in October and is purple and red, the inflorescence is vertical and upward. The roots are white and connected. It can be dug in February, August, and September.
    the Northern and Southern
    (420−589 AD)
    Bencao JizhuTao Hongjing近道处处有之, 叶似杜若,
    根形似菱米, 节间有毛
    It is everywhere near the road. The leaves are like Pollia japonica Thunb. The roots are like the fruit of Trapa natans L., and internode are many fibrous roots.
    Tang
    (618−907 AD)
    Su Jing, Zhangsun Wuji, etcTang materia medica生山谷, 如藜芦, 根白连, 九月采Born in the valley, with leaves like V. nigrum, root is white and connected. September is the time for harvesting.
    Song
    (960−1279 AD)
    Su SongCommentaries on the Illustrations白芨, 生石山上。春生苗,
    长一尺许, 似栟榈及藜芦,
    茎端生一台, 叶两指大, 青色,
    夏开花紫, 七月结实, 至熟黄黑色。
    至冬叶凋。根似菱米, 有三角白色, 角端生芽。二月, 七月采根
    Bletilla grow on the stone hill. It sprouts in spring and grows about a foot long. The seedlings are like Trachycarpus fortunei (Hook.) H. Wendl. and V. nigrum. The leaves are two finger-size. In summer, it blooms purple flowers and bears fruit in July. The ripe fruit is yellow-black. The leaves wither in winter. The root is like the fruit of T. natans, with three corners, white, and sprouting at the corners. The roots are dug in February and July.
    Ming
    (1368−1644 AD)
    Li ShizhenCompendium of Materia Medica一棵只抽一茎, 开花长寸许,
    红紫色, 中心如舌, 其根如菱米,
    有脐, 如凫茈之脐,
    又如扁扁螺旋纹, 性难干
    Only one stem per herb. The flower is more than one inch long, red and purple, and the center resembles a tongue. Its root is similar to the fruit of T. natans, possessing an umbilicus akin to that of Eleocharis dulcis (N. L. Burman) Trinius ex Henschel. It has spiral veins and is challenging to dry.
    −, Anonymous.
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    Figure 3.  Bletilla in Compendium of Materia Medica.

    Generally, ancient Chinese medical texts did not make clear distinctions between different Bletilla species. They collectively referred to plants with similar morphological traits as 'baige', 'baigen', 'baiji', 'gangen', 'lianjicao', or 'ruolan'. However, through textual analysis, it has been established that the descriptions of Bletilla in ancient texts before the Ming Dynasty largely align with Bletilla striata in terms of plant height, pseudo-bulb shape, leaf morphology, flower and fruit colors, and other characteristics. While the Bletilla portrayed in attached images may not precisely match B. striata in terms of morphology, considering the textual descriptions, it generally corresponds with B. striata. In writings from the Ming Dynasty and later periods, more specific descriptions of Bletilla emerged, encompassing details about its vascular arrangement, inflorescence, and flower structure, which consistently align with B. striata. Consequently, researchers have corroborated that the original plant of Bletilla described in ancient texts is Bletilla striata[24,33].

    According to the ancient Chinese medicinal books, Bletilla was used to treat a wide variety of conditions, including coughing, bruising, and bleeding, but their most mentioned use in ancient Chinese texts is for skin whitening and freckle removal[25]. Since ancient times, Bletilla species have been used consistently for skin care and whitening, and there are many well-known skincare products related to Bletilla. These Chinese formulae with Bletilla are similar to modern facial masks, face creams, facial cleanser, hand creams and other skin care products[26].

    For example, a prescription for 'facial fat (面脂)' in Medical Secrets from the Royal Library (752 AD) is made by boiling Bletilla with other traditional ingredients, and is applied to the face, resulting in skin whitening, freckle and wrinkle removal[27]. The 'Angelica dahurica cream (白芷膏)' in the General Medical Collection of Royal Benevolence (1111−1125 AD) is reputed to whiten facial skin through a seven-day treatment regiment, and contains Bletilla as the main botanical ingredient along with Angelica dahurica[28]. Jingyue Quanshu (1563-1640 AD) also contains a prescription called 'Yurong powder (玉容散)' for facial skin care. 'Yurong powder' is made of a fine powder of Bletilla, Nardostachys jatamansi (D. Don) DC., Anthoxanthum nitens (Weber) Y. Schouten & Veldkamp and other herbs[29]. Washing the face with Yurong powder in the morning and evening every day is said to make a person's face white without blemishes (Fig. 4)[29].

    Figure 4.  Yurong powder made of Bletilla and other traditional Chinese medicines in Jingyue Quanshu.

    In addition, in ancient Chinese medicine texts, Bletilla is also a well-known medicine for treating hematemesis, hemoptysis and bruises[23]. According to Shennong's Classic of Materia Medica (25−220), grinding the white fungus into fine powder and taking it after mixing with rice soup can be effective for treating lung damage and hematemesis[30]. Among the Prescriptions for Universal Relief (1406), 18 traditional Chinese medicines, such as Bletilla, are used to make 'snake with raw meat cream', which is said to be useful to treat carbuncles and incised wounds[31]. There is also a record of Bletilla powder treating lung heat and hematemesis in the Collected Statements on the Herbal Foundation (1624)[32].

    In ancient Chinese medicinal texts, most Bletilla are said to be useful for lung injury and hemoptysis, epistaxis, metal-inflicted wounds, carbuncles, burns, chapped hands and feet, whitening and especially for skin care. In the ancient medicinal texts, Bletilla is used alone or mixed with other traditional Chinese medicines. It is usually used in the form of a powder. The various medicinal effects of Bletilla described in these ancient texts suggest the great potential of this genus in clinical application, especially in the market of skin care products and cosmetics.

    As a skin care herb praised by ancient medical classics, 11 ethnic minorities in China, such as Bai, Dai, De'ang, Jingpo, Lisu, Miao, Mongolian, Mulao, Tu, Wa, and Yi still retain the traditional habit of using Bletilla for skin care in their daily life (Table 3). In addition to B. striata, B. formosana and B. ochracea are also used as substitutes. Although Chinese ethnic groups have different names for Bletilla spp., the skin care methods are basically the same. Dry Bletilla tubers are ground into a powder and applied to the skin[34], and this usage is also confirmed by the records in ancient medical texts[23, 24]. The various local names of Bletilla by different ethnic groups also indirectly suggests which ethnic groups play an important role in the traditional use. For example, Bai people called B. striata baijier (白鸡儿), goubaiyou (狗白尤), and yangjiaoqi (羊角七) (Table 3).

    Table 3.  The traditional medicinal knowledge of Bletilla in ethnic communities, China.
    Ethnic groupLatin nameLocal nameUsed partUse methodMedicinal effect
    AchangBletilla striata (Thunb. ex Murray) Rchb. F.BaijiTuberAfter the roots are dried, chew them orally or grind them into powder for external applicationTuberculosis, hemoptysis, bleeding from gastric ulcer, burns and scalds
    BaiBaijier, Goubaiyou, YangjiaoqiTuberTreatment of tuberculosis hemoptysis, bronchiectasis hemoptysis, gastric ulcer hemoptysis, hematochezia, skin cracking
    DaiYahejieTuberUsed for tuberculosis, tracheitis, traumatic injury, and detumescence
    De'angBageraoTuberTuberculosis, hemoptysis, bleeding from gastric ulcer, burns and scalds
    DongShaque, SanjueTuberTreat hematemesis and hemoptysis
    JingpoLahoiban, PusehzuotuberFor tuberculosis, bronchiectasis, hemoptysis, gastric ulcer, hematemesis, hematuria, hematochezia, traumatic bleeding, burns, impotence
    MengMoheeryichagan, NixingTuberFor tuberculosis hemoptysis, ulcer bleeding, traumatic bleeding, chapped hands, and feet
    MiaoBigou, Wujiu, SigouTuberUsed for hemoptysis of tuberculosis, bleeding of ulcer disease, traumatic bleeding, chapped hands, and feet
    MolaoDajiebaTuberTreat internal and external injuries caused by falls
    TibetanSanchabaijiTuberFresh chopped and soaked with honey; Powdered after sun-dried, then taken with honey and waterMainly used to treat cough, asthma, bronchitis, lung disease and a few gynecological diseases
    TuRuokeyeTuberAfter the roots are dried, chew them orally or grind them into powder for external applicationTreatment of tuberculosis, hemoptysis, bloody stool, chapped skin
    WaBaijiTuberAfter the roots are dried, chew them orally or grind them into powder for external applicationFor tuberculosis, hemoptysis, gastrointestinal bleeding, scald and burn
    YaoBiegeidaiTuberTreat gastric ulcer, pulmonary tuberculosis, cough, hemoptysis, and hematemesis
    YiDaibaij, Tanimobbaili, Niesunuoqi, AtuluoboTuberTreatment of tuberculosis, hemoptysis, golden wound bleeding, burns, chapped hands and feet
    ZhuangManggounuTuberTreat stomachache and hemoptysis
    BaiBletilla formosana (Hayata) Schltr.Baijier, YangjiaoqiTuberAfter the roots are dried, chew them orally or grind them into powder for external applicationIt is used for emesis, hemoptysis due to tuberculosis, and hemoptysis due to gastric ulcer. External application for treatment of incised wound
    MiaoLianwuTuberThe effect is the same as that of B. striata
    LisuHaibiqiuTuberIt can treat tuberculosis, hemoptysis, epistaxis, golden sore bleeding, carbuncle and swelling poison, scald by soup fire, chapped hands and feet
    YiNiesunuoqi, Yeruomaoranruo, Atuluobo, Ribumama, Atuxixi, Abaheiji, Binyue, ZiyouTuberIt is used for tuberculosis, hemoptysis, traumatic injury, treatment of frostbite, burn, scald, bed-wetting of children and other diseases
    BaiBletilla ochracea Schltr.Baijier, YangjiaoqiTuberAfter the roots are dried, chew them orally or grind them into powder for external applicationFor hematemesis, epistaxis, hemoptysis due to tuberculosis, hemoptysis due to gastric ulcer; External application of golden sore and carbuncle
    MengMoheeryichagan, NixingtuberThe effect is the same as that of B. striata
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    The formation of traditional medical knowledge among Chinese people is often directly related to the specific living environment and cultural background[34]. For example, the Bai, Dai, De'ang, Jingpo, Lisu Yi, Wa and other ethnic minorities live in mountainous areas. The cold weather in winter and year-round outdoor manual work makes it difficult to maintain their skin[35, 36]. In the face of this situation, the ethnic people who are concerned about their physical appearance have long ago chosen local Bletilla species for skin care, and have handed down this tradition for many generations[34]. This important traditional skin care tradition is worthy of further in-depth study.

    The six main classes of Bletilla chemical components, phenanthrene derivatives, phenolic acids, bibenzyls, flavonoids, triterpenoids, and steroids, have been described previously. Almost three hundred compounds have been isolated from Bletilla, including 116 phenanthrene derivatives, 58 phenolic acids, 70 bibenzyls, 8 flavonoids, 24 triterpenoids and steroid and 13 other compounds (Figs 514). Chemical structures of the isolates of Bletilla species most are phenanthrene derivatives, which have been demonstrated to possess various pharmacological activities.

    The prominent opioids oxycodone, hydrocodone, naloxone, and naltrexone are all phenanthrene derivatives[37]. Currently, phenanthrene derivatives (Fig. 5, 1 to 66) were isolated from B. formosana, B. ochracea, and B. striata. In 2022, 17 phenanthrene derivatives (117) were isolated from the ethyl acetate (EtOAc) extracts of B. striata tubers[38]. Then, other phenanthrene derivatives were isolated from Bletilla, such as dihydrophenanthrenes (1841), phenanthrenes (4266), biphenanthrenes (Fig. 6, 6789), dihydro/phenanthrenes with uniquestructures (90112) and phenanthraquinones (Fig. 7, 113116). Thus far, this genus has been documented to include these compounds, which have been shown to exhibit pharmacological actions[3945].

    Figure 5.  Phenanthrene derivatives from Bletilla species (1−66)[3841,4345,47,49,58,7072,7479].
    Figure 6.  Phenanthrene derivatives from Bletilla species (67−105)[41,43,49,5961,70,76,7986].
    Figure 7.  Phenanthrene derivatives from Bletilla species (106−116)[43,49,70,75,84,85,87].

    Phenolic acids are carboxylic acids created from the skeletons of either benzoic or cinnamic acids[4648]. Fifty-eight phenolic acids (Figs 810, 117 to 174) were isolated from B. formosana, B. ochracea, and B. striata.

    Figure 8.  Phenolic acids from Bletilla species (117−134)[1,5,36,4752,54,67,88,90].
    Figure 9.  Phenolic acids from Bletilla species (135−169)[39,45,4854,56,61,68,69,73,76,82,83,8994].
    Figure 10.  Phenolic acids from Bletilla species (170−174)[20,95].

    For example, compounds 121, 126, 139, 141, 148, 149, 154, 155 and 157 were isolated from the rhizomes of B. formosana[1,49,5052]. The structures of these compounds were determined, mostly from their NMR spectroscopy data. Additionally, protocatechuic (136) and vanillin (137) also have been isolated from B. striata[53]. Moreover, some bioactive components such as 2-hydroxysuccinic acid (164) and palmitic acid (165) have been discovered and identified from B. striata[20,5456].

    The bibenzyls were small-molecular substances with a wide range of sources, which were steroidal ethane derivatives and resembling the structural moiety of bioactive iso-quinoline alkaloids[57].

    For example, depending on their structural characteristics, 70 bibenzyl compounds (Fig. 11, 175 to 244) can be grouped into three groups, simple bibenzyls (175186, 233238), complex bibenzyls (189225) and chiral bibenzyls (226-232, 239-244)[5860].

    Figure 11.  Bibenzyls from Bletilla species (175-244)[1,4042,47,49,50,5860,70,73,76,9699].

    Flavonoids are among the most common plant pigments. Eight bibenzyls (Fig. 12, 245 to 252) have been isolated from B. formosana, B. ochracea, and B. striata. Apigenin (245) and 8-C-p-hydoxybenzylkaempferol (249) were isolated from the whole plant of B. formosana[45]. Bletillanol A (250), bletillanol B (251) and tupichinol A (252) were isolated from B. striata[61]. The names and chemical structures of the flavonoids reported from Bletilla are shown below (Fig. 12).

    Figure 12.  Flavonoids from Bletilla species (245–252)[45,61].

    Twenty-four triterpenoids and steroids (Fig. 13, 253 to 276) have been reported from Bletilla (Fig. 13), such as, tetracyclic triterpenes (253259) and pentacyclic triterpenes (189225) and chiral bibenzyls (260)[6264]. Steroids (261276) isolated from the Bletilla and have shown some bioactivity. For example, bletilnoside A (272) was isolated from Bletilla species and displayed anti-tumor activity[65,66].

    Figure 13.  Triterpenoids and steroid compounds from Bletilla species (253-276)[56,6265].

    Thirteen other compounds (Fig. 14, 277 to 289) were isolated from B. formosana, B. ochracea, and B. striata. These compounds included amino acids, indoles and anthraquinones[67,68]. For example, syringaresinol (285) and pinoresinol (286) have been described in the methanol extract of the tubers of B. striata[61].

    Figure 14.  Others compounds from Bletilla species (277−289)[50,54,61,62,67,68,94,97].

    Based on the information about the chemical constituents of Bletilla species, it appears that there is a substantial body of research on these compounds. However, there are some areas that may warrant further investigation and research. At first, it would be valuable to investigate potential synergistic effects and interactions between the different classes of compounds within Bletilla species, as some of the compounds may work together. Besides, it is worth considering the improvement of compound yield. Optimizing extraction methods and finding the most efficient and environmentally friendly techniques are vital for both research purposes and potential commercial applications. It is also important to take into account the variability in chemical composition among different Bletilla species and even within the same species from different cultivars.

    The rich and varied chemical components make the plants of Bletilla have a wide variety of pharmacological activities (Table 4). Many studies have shown that the plants of this genus have anti-inflammatory, antineoplastic, antiviral, antioxidant, hemostatic, antibacterial, and other biological activities, which help to support the traditional medicinal practice of Bletilla in folk medicine.

    Table 4.  Summary of the pharmacological activities of Bletilla species.
    Pharmacological activityTested substance/partTested system/organ/cellTested dose/dosing methodResultsRefs.
    Anti-inflammatoryEthanol extract of Bletilla striataRAW264.7 cells RAW264.7 cells were pre-treated with ethanol extract of B. striata for 1 h and then stimulated with LPS (200 ng/mL) for 12 h, 0.05% DMSO was applied as the parallel solvent control. The culture supernatant was collected for IL-6 and TNF-α detection.Ethanol extract of B striata significantly inhibited LPS-induced interleukin-1β (IL-1β), interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α) expression at 2.5 µg/mL.[41]
    The ethyl acetate-soluble (EtOAc) extract of tubers of B. striataH2O2-induced PC12 cell injury modelPC12 cells were seeded in 96-well multiplates at a density of 1.5 × 105 cells/mL. After overnight incubation at 37 °C with 5% CO2, 10 μM test samples and H2O2 (final concentration of 450 μM) were added into the wells and incubated for another 12 h.It protected the cells with the cell viabilities of 57.86 ± 2.08%, 64.82 ± 2.84%, and
    64.11 ± 2.52%.
    [98]
    Ethanol extract of tubers of B. striataRAW264.7 cellsCells were treated with ethanol extracts (25 μM) dissolved in DMSO, in the presence of
    1 μg/mL lipopolysacchride
    (LPS) for 18 h
    The anti-inflammatory activity with IC50 of 2.86 ± 0.17 μM.[54]
    PE extract of the tubers of B. striataLPS-stimulated BV2 cellsCells treated with extract
    (0, 1, 10, 30, 100 μg/mL) and dihydropinosylvi (0, 1, 10, 30, 100 μM) in presence of LPS
    (1 μg/mL)
    The anti-inflammatory activity with IC50 values of 96.0 μM.[96]
    Ethanol extract of the roots of B. striataCox-1 and Cox-2Treated with the ethanol extracts at various concentrations
    (0, 1, 10, 100 μM)
    The compounds with sugar moieties displayed selective inhibition of Cox-2 (N90%).[38]
    B. striata polysaccharide (BSPb)Human mesangial cells (HMCs)HMCs were pre-treated with BSPb (5, 10, 20 μg/mL)BSPb efficiently mediated expression of NOX4 and TLR2, to attenuate generation of ROS and inflammatory cytokines.[12]
    Compounds extracted from the rhizomes of Bletilla ochraceaRAW264.7 cellsAfter 24 h preincubation,
    cells were treated with serial dilutions in the presence of
    1 μg/mL LPS for 18 h. Each compound was dissolved in DMSO and further diluted in medium to produce different concentrations. NO production in the supernatant was assessed by adding 100 μL of Griess regents.
    It showed the inhibitory effects with IC50 values in the range of 15.29–24.02 μM.[76]
    Compounds extracted from the rhizomes of B. ochraceaMurine monocytic RAW264.7 cellsAfter 24 h preinubation, RAW 264.7 cells were treated with compounds (25 μM) dissolved in DMSO, in the prenence of
    1 μg/mL LPS for 18 h. NO production in each well was assessed by adding 100 μL of Giress regent
    It showed the inhibitory effects with IC50 2.86 ± 0.17 μM.[86]
    Compounds extracted from the rhizomes of Bletilla formosanaElastase Release AssaysNeutrophils (6 × 105 cells/mL) were equilibrated in MeO-Suc-Ala-Ala-Pro-Val-p-nitroanilide (100 μM) at 37 °C for 2 min and then incubated with a test compound or an equal volume of vehicle (0.1% DMSO, negative control) for 5 min.Most of the isolated compounds were evaluated for their anti-inflammatory activities. The results showed that IC50 values for the inhibition of superoxide anion generation and elastase release ranged from 0.2 to 6.5 μM and 0.3 to 5.7 μM, respectively.[49]
    Anti-tumorTwo compounds from Bletilla striataA549 cellsCompounds were tested for their ability to induce ROS generation in A549 cells at concentrations of 20 two compounds for 24 h, the cells were harvested to evaluate the ROS production.The two compounds exhibited antiproliferative effects using the MTT test; these effects may be due to cell cycle arrest and inducing ROS generation.[87]
    Stilbenoids from B. striataBCRP-transduced K562 (K562/BCRP) cellsIt showed antimitotic activity and inhibited the polymerization of tubulin at IC50 10 μM.[78]
    Compounds extracted from the rhizomes of B. ochraceaThe human tumor cell lines HL-60 (acute leukemia), SMMC-7721 (hepatic cancer), A-549 (lung cancer), MCF-7 (breast cancer), and SW480 (colon cancer)100 μL of adherent cells were seeded into each well of 96-well cell culture plates. After 12 h of incubation at 37 °C, the test compound was added. After incubated for 48 h, cells were subjected to the MTS assay.All isolated metabolites except 7 were evaluated for cytotoxic activity against five human cancer cell lines (HL-60, SMMC7721, A-549, MCF-7 and SW480).[76]
    AntiviralThe tuber of B. striataMadin-Darby canine kidney model and embryonated eggs modelAs simultaneous treatment with 50% inhibition concentration (IC50) ranging from 14.6 ± 2.4 to 43.3 ± 5.3 μM.Phenanthrenes from B. striata had strong anti-influenza viral activity in both embryonated eggs and MDCK models.[107]
    The 95% ethanol
    Extract of B. striata
    BALB/C miceIt has significant anti-influenza
    virus effect in mice, which may be related to the increase of IL-2, INFα, INF-β and thus enhance the immune function of mice.
    [12]
    AntioxidantCompounds extracted from the rhizomes of B. formosanaDPPH radical-scavenging assaySolutions containing 160 μL of various concentrations of sample extract, 160 μL of various concentrations of BHA, 160 μL of various concentrations of ascorbic acid, and the control (160 μL of 75% methanol) were mixed separately with 40 μL of 0.8 mM DPPH dissolved in 75% methanol. Each mixture was shaken vigorously and left to stand for 30 min at room temperature in the dark.Tthe seedlings grown by tissue culture of B. formosan collected in Yilan County had the best antioxidant capacity. In addition, B. formosana collected in Taitung County had the best scavenging capacity in the tubers, leaves and roots.[93]
    Fibrous roots of B. striataDPPH model and superoxide anion systemThe ABTS+ solution was prepared by reacting 7 Mm ABTS with 2.45 mM potassium persulfate (final concentrations both dissolved in phosphate buffer, 0.2 M, pH 7.4) at room temperature for 12–16 h in the dark.It removed free radicals and inhibit tyrosinase activity.[33]
    B. striata extracts (BM60)The murine macrophage cells NR8383, male SD mice (180~200 g)NR8383 were pretreated with extracts (1, 10 and 100 g/mL) for 4 h and then 65 stimulated with 1 g/mL of LPS for 24 h. Acute lung injury was induced in mice by nonhexposure intratracheal instillation of LPS (3.0 mg/kg). Administration of the BM60 extract of 35, 70, and 140 mg/kg (L, M, H) was performed by oral gavages.The BM60 treatment reduced the production of NO in NR8383 macrophages. Treatments with BM60 at the doses of 35, 70, 140 mg/kg significantly reduced macrophages and
    neutrophils in the bronchoalveolar lavage fluid (BALF).
    [12]
    The crude
    polysaccharides obtained from B. striata
    DPPH free radical scavenging activityConcentration
    range of 2.5–5.0 mg/mL
    The IC50 of BSPs-H was 6.532 mg/mL.[35]
    HemostasisB. striata polysaccharide (BSP)Diabetes mellitus (DM) mouse models were induced by high fat-diet feeding combined with low-dose streptozocin injectionDM mouse models were induced by high fat-diet feeding combined with low-dose streptozocin injection. The BSP solutions were applied on the surface of each wound at a volume of 50 μl. RD mice were assigned as normal controls and received saline treatment (n = 6). All mice were treated with vehicle or BSP once daily from the day of wounding (d0) until 12 days later (d12).BSP administration accelerated diabetic wound healing, suppressed macrophage infiltration, promoted angiogenesis, suppressed NLRP3 inflammasome activation, decreased IL-1β secretion, and improved insulin sensitivity in wound tissues in DM mice.[112]
    B. striata Micron Particles (BSMPs)Tail amputation model and healthy male Sprague-Dawley (SD) rats
    (250 ± 20 g, 7 weeks of
    age)
    Rats were divided into six groups of five treated with cotton gauze and BSMPs (350–250, 250–180, 180–125, 125–75, and < 75 μm), respectively.Compared to other BSMPs of different size ranges, BSMPs of 350–250 μm are most efficient in hemostasis. As powder sizes decrease, the degree of aggregation between particles and hemostatic BSMP effects declines.[109]
    Rhizoma Bletillae polysaccharide (RBp)Adult male SD rats weighing 220 ± 20 gAfter incubation for 1 min at 37 °C, 300 μL of PRP was dealt with different concentrations of RBp (50, 100, 150, and 200 mg/L) under continuous stirring, and the vehicle was used as the blank control.RBp significantly enhanced the platelet aggregations at concentrations of 50−200 mg/L in a concentration-dependent manner.[113]
    AntibacterialBibenzyl derivatives from the tubers of Bletilla striataS. aureus ATCC 43300, Bacillus subtilis ATCC 6051, S. aureus ATCC 6538 and Escherichia coli ATCC 11775Using a microbroth dilution method, bacteria were seeded at
    1 × 106 cells per well (200 μL) in a
    96-well plate containing Mueller-Hinton broth with different concentrations (from 1 to 420 μg/mL, 300 μg/mL and so on;
    2-fold increments) of each test compound.
    It showed inhibitory activities with MIC of (3–28 μg/mL) against S. aureus ATCC6538[116]
    The crude extract of B. striataS. album, A. capillaris, C. cassiaThey were seeded at 1 × 106 cells per well (200 μL) in a 96-well plate containing Mueller−Hinton broth (meat extracts 0.2%, acid digest of casein 1.75%, starch 0.15%) with different concentrations (from 1 to 128 μg/mL; 2-fold increments) of each test compound.It showed S. album (0.10%), A. capillaris (0.10%), and C. cassia (0.10%) to have the strongest antibacterial properties.[118]
    The ethyl acetate-soluble (EtOAc) extract of tubers of B. striataS. aureus ATCC 43300, S. aureus ATCC 6538, and Bacillus subtilis ATCC 6051) and Escherichia coli ATCC 11775)Bacteria were seeded at 1 × 106 cells per well (200 μL) in a 96-well plate containing Mueller Hinton broth with different concentrations (from 1 to 420 μg/ml; 2-fold increments) of each test compound.The extract was effective against three Gram-positive bacteria with minimum inhibitory concentrations (MICs) of 52–105 μg/ml.[98]
    The phenanthrene fraction (EF60) from the ethanol extract of fibrous roots of Bletilla striata pseudobulbsS. aureus ATCC 25923, S. aureus ATCC 29213, S. aureus ATCC 43300, E. coli ATCC 35218, and P. aeruginosa ATCC 27853, Bacillus subtilis 168EF60 was active against all tested strains of Staphylococcus aureus, including clinical isolates and methicillin-resistant S. aureus (MRSA). The minimum inhibitory concentration (MIC) values of EF60 against these pathogens ranged from 8 to 64 μg/mL.EF60 could completely kill S. aureus ATCC 29213 at 2× the MIC within 3 h but could kill less than two logarithmic units of ATCC 43300, even at 4× the MIC within 24 h. The postantibiotic effects (PAE) of EF60 (4× MIC) against strains 29213 and 43300 were 2.0 and 0.38 h, respectively.[117]
    Anti-adhesiveBletilla striata extraction solutionPPA was induced by cecal wall abrasion, and Bletilla striata was injected to observe its efect on adhesion in ratsThe rats in the sham operation group was not treated; the other rats of the three experimental groups were intraperitoneally injected with 8 ml of phosphate-buffered saline (Control group), 15% Bletilla striata extraction solution (BS group), and 0.2% hyaluronic acid solution (HA group), respectively.Bletilla striata decreased the development of abdominal adhesion in abrasion-induced model of rats and reduced the expression of the important substance which increased in PPAs.[120]
    ImmunomodulatoryB. striata polysaccharide (BSPF2)Mouse spleen cellsTo observe the immune activity of BSPF2, mouse spleen cells were stimulated with BSPF2 at 10–100 g/mL for 72 h.Immunological assay results demonstrated that BSPF2 significantly induced the spleen cell proliferation in a dose-dependent manner.[121]
    Anti-pulmonary fibrosisB. striata polysaccharideClean grade male SD ratsSD rats were randomly divided into 5 groups, sham operation group (equal volume of normal saline), model group (equal volume of normal saline), tetrandrine positive control (24 mg/kg) group and white and Polysaccharide low
    (100 mg/kg) and high (400 mg/kg) dose groups.
    The Bletilla striata polysaccharide has remarkable regulation effect on anti-oxidation system and immune system, but cannot effectively prevent lung fibrosis.[127]
    Small molecule components of Bletilla striataClean grade male SD ratsSD rats were randomly divided into 5 groups, sham operation group (0.5 mL normal saline), model group (0.5 mL normal saline), and positive control group (tetrandrine 24 mg/kg) and low (20 mg/kg) and high (40 mg/kg) dosage groups of the small molecule pharmacological components of Bletilla, which were administered by gavage once a day for 2 consecutive months.The small molecule components of Bletilla striata can effectively prevent lung fibrosis though regulating the anti-oxidation system,immune system and cytokine level; SMCBS is one of the active components of Bletilla striata on silicosis therapy[124]
    —, not given.
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    Many phytochemicals have been well characterized to lessen swelling or inflammation[89]. A series of phenolic acid and polysaccharide compounds isolated from Bletilla demonstrated anti-inflammatory bioactivity against BV-2 microglial, RAW 264.7, and PC12 cells[96,100102]. For example, phochinenin K (106) exhibited growth inhibitory effects with an IC50 of 1.9 μM, and it is a possible candidate for development as neuroinflammation inhibitory agent[43]. Using the H2O2-induced PC12 cell injury model, (7S)-bletstrin E (242), (7R)-bletstrin F (243) and (7S)-bletstrin F (244) could clearly protect the cells with the cell viabilities of 57.86% ± 2.08%, 64.82% ± 2.84%, and 64.11% ± 2.52%, respectively[98]. With an IC50 of 2.86 ± 0.17 μM, 2,7-dihydroxy-4-methoxyphenanthrene (53) showed potential action against NO generation in RAW 264.7 macrophages[54]. The use of Bletilla in traditional skin care, it is said to function as an astringent, hemostatic and wound healing[33]. Modern medical pharmacology research has validated that this plant has antibacterial effects, which may may help to explain, in part, its traditional use in skin care[24].

    Though it's mentioned that some of these compounds from Bletilla have demonstrated anti-inflammatory action, more extensive studies are needed to fully understand their mechanisms of action, potential therapeutic applications, and safety profiles. Conducting in vivo studies and clinical trials can provide more concrete evidence of their effectiveness.

    There are important antineoplastic agents that have originated from plant natural products[103]. In recent years, several bibenzyl and flavonoid compounds have been discovered from Bletilla that have antineoplastic activity against A549 cells and other cells. For example, 7-hydroxy-2-methoxy-phenanthrene-3,4-dione (160) and 3′,7′,7-trihydroxy-2,2′,4′-trimethoxy-[1,8′-biphenanthrene]-3,4-dione (163) have shown strong antiproliferative effects and induced ROS production after 24 h in A549 cells[87]. The doxorubicin (Dox)/FA (folate)-BSP-SA (stearic acid) modified Bletilla striata polysaccharide micelles boosted the drug enrichment in tumors and improved the in vivo anticancer effects[104,105]. Micelles, nanoparticles, microspheres, and microneedles are examples of B. striata polysaccharide-based drug delivery systems that exhibit both drug delivery and anti-cancer functionality. These experiments confirmed that some of the compounds isolated from the Bletilla have potential activity for the treatment of cancer.

    However, most of the evidence presented in the previous studies is based on in vitro experiments or cell culture studies. It is highly necessary to use animal models to study the in vivo anti-tumor effects of Bletilla extracts or compounds. These studies can help evaluate the safety and effectiveness of treatments based on Bletilla. Additionally, through such methods, researchers can further investigate the mechanisms of Bletilla's anti-tumor activities, exploring how Bletilla compounds interact with cancer cells, immune responses, and signaling pathways involved in tumor growth and metastasis.

    Antiviral medications are essential for preventing the spread of illness, and are especially important nowadays with pandemics and drug-resistant viral strains[5, 6]. Therefore, it is vitally necessary to find novel, safe, and effective antiviral medications to treat or prevent viral infections[106]. B. striata plant contains compounds that have been recorded in ancient texts to cure cough, pneumonia, and skin rashes, and these may be related to potential antiviral constituents[23]. Some constituents of B. striata have antiviral activity, for example, phenanthrenes and diphenanthrenes from B. striata displayed potent anti-influenza viral in a Madin-Darby canine kidney model and embryonated eggs model, diphenanthrenes with parentally higher inhibitory activity than monophenanthrenes[107]. But more research is needed to further determine the antiviral activity of Bletilla, understand how Bletilla compounds interact with viral proteins or the host immune response, and conduct safety and toxicity studies, which are crucial for the development of related materials.

    Free radicals have the potential to exacerbate lipid peroxidation and harm cell membranes, which can lead to several prevalent human diseases, including cancer, cataracts, and coronary heart disease[108]. Research has shown that extracts from Bletilla possess strong antioxidant activity. However, this antioxidant activity can vary depending on the different growing environments of the plant. Additionally, the antioxidant capabilities of extracts from different parts of the Bletilla plant also vary[93]. Clinical studies have shown that traditional Chinese medicine formulas containing Bletilla can inhibit tyrosinase activity and possess antioxidant properties, thus resulting in skin-whitening effects[108]. Furthermore, some research reveals that the polysaccharides in the plant exhibit significant antioxidant activity, effectively scavenging free radicals and inhibiting tyrosinase activity[33]. This highlights the skin-whitening potential of the fibrous root of Bletilla striata, indicating promising prospects for the comprehensive utilization of the B. striata plant[33]. However, most studies on the pharmacological activities of Bletilla have focused solely on B. striata, neglecting other species within the genus. Different species may possess varying phytochemical compositions and antioxidant properties, which can lead to an incomplete understanding of the genus as a whole.

    Available hemostatic agents are expensive or raise safety concerns, and B. striata may serve as an inexpensive, natural, and promising alternative[109]. Polysaccharides of B. striata displayed hemostatic activity through inhibition of the NLRP3 inflammasome[110112]. The ADP receptor signaling pathways of P2Y1, P2Y12, and PKC receptors may be activated as part of the hemostasis[113]. Alkaloids from Bletilla have hemostatic activities through platelet deformation, aggregation, and secretion. In addition, polysaccharides of Bletilla striata have potential wound-healing medicinal value[110]. Currently, Bletilla plants have been used in various traditional systems, such as traditional Chinese medicine and Ayurveda, to control bleeding.

    Previous studies revealed that Bletilla displayed antibacterial effects[114]. For example, bletistrin F, showed inhibitory activities with MIC of (3–28 μg/mL) against S. aureus ATCC 6538[115,116]. Antimicrobial screening of Bletilla showed S. album (0.10%), A. capillaris (0.10%), and C. cassia (0.10%) to have the strongest antibacterial properties[117,118]. In addition, phenanthrenes are worthy of further investigation as a potential phytotherapeutic agent for treating infections caused by S. aureus and MRSA[119]. However, further in vivo studies on the antibacterial activity of Bletilla are lacking, which is needed for clinical application. For example, the specific mechanism of antibacterial activity of Bletilla still needs to be elucidated. While research on the antibacterial activity of Bletilla plants is promising, it faces several shortcomings and challenges that need to be addressed for a more comprehensive understanding of their potential therapeutic applications. Further studies with standardized methodologies, mechanistic insights, clinical trials, and consideration of ecological and safety concerns are essential to advance this field.

    There are other pharmacological activities of Bletilla, like anti-fibrosis activity, anti-adhesive activity, and immunomodulatory activity. For example, B. striata has been studied as a new and cheaper antiadhesive substance which decreased the development of abdominal adhesion abrasion-induced model in rats[120]. However, the natural resources of Bletilla are also getting scarcer. To preserve the sustainable development of Bletilla species, proper farming practices are required, along with the protection and economical use of these resources. The immunomodulatory activity of the Bletilla species was assessed using the 3H-thymidine incorporation method test, and BSP-2 increased the pinocytic capacity and NO generation, which improved the immunomodulatory function[121,122].

    B. striata extract was shown to have anti-pulmonary fibrosis effect[123]. B. striata polysaccharide can successfully prevent lung fibrosis through established by invasive intratracheal instillation method and evaluated by lung indexes[123,124]. Moreover, Bletilla species need further investigations to evaluate their long-term in vivo and in vitro activity before proceeding to the development of pharmaceutical formulation.

    While there is currently a deep understanding of the pharmacological activity of plants in the Bletilla genus, there are still many gaps that need to be addressed. To overcome these shortcomings, future research on the pharmacological activity of Bletilla species should emphasize comprehensive, well-designed studies with a focus on species-specific effects, mechanistic insights, and rigorous clinical trials. Additionally, collaboration among researchers, standardization of methods, and transparent reporting of results can help advance our understanding of the therapeutic potential of Bletilla plants. Researchers should also consider safety aspects and explore potential herb-drug interactions to ensure the responsible use of Bletilla-based therapies.

    There are several common clinical applications of Bletilla striata in TCM. The gum of B. striata has unique viscosity characteristics and can be used as thickener, lubricant, emulsifier and moisturizer in the petroleum, food, medicine, and cosmetics industries[125130]. B. striata is used as a coupling agent, plasma substitute, preparation adjuvant, food preservative and daily chemical raw material[131133]. In clinical practice, B. striata glue has also been proven to control the infections and is beneficial to the healing of burns and wounds[133135].

    In ethnic communities in Southwest China, the locals chew fresh Bletilla tubers directly or take them orally after soaking in honey to treat cough, pneumonia and other diseases[33, 34]. This traditional use is common in local communities in Southwest China, and suggests at the safety of Bletilla. However, current research shows it is still necessary to control the dosage when using Bletilla[136].

    Zebrafish embryos and larvae respond to most drugs in a manner similar to humans[137]. Militarine, the main active ingredient of Bletilla, was tested in a zebrafish embryo development assay at concentrations of 0.025 g/L and 0.05 g/L, and with the increased concentration, the heart rate of zebrafish embryos is slowed. Mortality and malformation rates of zebrafish embryos gradually increased with time and militarine concentration[138]. Although Bletilla species are safe at therapeutic dose ranges, further research on their safety is required[136]. More in-depth studies should be carried out on Bletilla to extract effective ingredients and make better preparations for clinical use[139].

    According to the traditional medicinal knowledge in ancient Chinese texts, Bletilla has been an important ingredient for skin care since ancient times. Many ethnic minority groups in China still retain the practice of using Bletilla for skin care, and the plant parts and preparation methods of use are consistent with the records in ancient texts. Almost 300 phytochemicals have been identified from Bletilla, and some of them possess important pharmacological activities, which support its traditional uses and suggest the important medicinal development potential of this genus. This review has demonstrated that Bletilla, as an important medicinal plant of Orchidaceae, still requires further research to fathom its medicinal potential.

    For instance, it is necessary to enhance the quality control procedures based on the chemical components and pharmacological activity of Bletilla. The chemical composition and pharmacological properties of Bletilla are critical areas of current research. According to previous studies, the main bioactive components of Bletilla can vary greatly according to its origin, harvest time, distribution, storage, and adulteration. However, variation in bioactivities caused by the differences in Bletilla constituentshave not been explored extensively yet. To develop clinical applications of Bletilla, it is crucial to further explore the mechanism of action between its chemical composition variation and its pharmacological actions.

    In addition, although the tuber has historically been the main medicinal part of Bletilla, research has shown that the chemical composition in other parts of Bletilla, such as stems, leaves, and flowers, also give these parts a variety of pharmacological activities. Further in-depth analysis of the chemical components and pharmacological activities of different parts of this genus is worthwhile, to explore the specific chemical basis of its pharmacological activities, develop related drugs, and promote clinical applications. For example, Bletilla polysaccharide has good hemostasis and astringent wound effects[110], so it may have the potential to be developed into a drug or related medical materials to stop bleeding and heal wounds.

    Finally, as a cautionary note, many unrestrained collections and the destruction of habitats have made the resources of wild Bletilla rarer. In addition to protecting the wild populations of Bletilla, appropriate breeding techniques should be adopted to meet the commercial needs of this economically important genus, thereby allowing its sustainable use in commerce.

    The authors confirm contribution to the paper as follows: study conception and design, funding acquirement: Long C; data analysis, draft manuscript preparation, literature review: Fan Y, Zhao J; manuscript revise and language editing: Wang M, Kennelly EJ, Long C. All authors reviewed the results and approved the final version of the manuscript.

    The raw data supporting the conclusion of this article will be made available by the authors, without undue reservation, to any qualified researcher. Requests to access these datasets should be directed to Yanxiao Fan (fanyanxiao0510@163.com).

    This research was funded by the Yunnan Provincial Science and Technology Talent and Platform Plan (202305AF150121), Assessment of Edible & Medicinal Plant Diversity and Associated Traditional Knowledge in Gaoligong Mountains (GBP-2022-01), the National Natural Science Foundation of China (32370407, 31761143001 & 31870316), China Scholarship Council (202206390021), and the Minzu University of China (2020MDJC03, 2022ZDPY10 & 2023GJAQ09).

  • The authors declare that they have no conflict of interest.

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  • Cite this article

    Tang Y, Xiang L, Zhang F, Tang K, Liao Z. 2023. Metabolic regulation and engineering of artemisinin biosynthesis in A. annua. Medicinal Plant Biology 2:4 doi: 10.48130/MPB-2023-0004
    Tang Y, Xiang L, Zhang F, Tang K, Liao Z. 2023. Metabolic regulation and engineering of artemisinin biosynthesis in A. annua. Medicinal Plant Biology 2:4 doi: 10.48130/MPB-2023-0004

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Metabolic regulation and engineering of artemisinin biosynthesis in A. annua

Medicinal Plant Biology  2 Article number: 4  (2023)  |  Cite this article

Abstract: Artemisinin is a potent anti-malarial sesquiterpene lactone that is naturally biosynthesized in glandular trichomes of a Chinese herbal plant, Artemisia annua. Although semi-synthesis of artemisinin in yeast has been feasible, at present the A. annua plant is still the main commercial source of artemisinin. The content of artemisinin however is low in A. annua, limiting its supply to malarial victims. So it's crucial to elevate artemisinin production in A. annua. Up to date artemisinin biosynthetic pathway has been completely elucidated. And many regulatory factors, mediating diverse plant hormone or environmental signaling routes, have been identified to get involved in the regulation of artemisinin biosynthesis and glandular trichome formation. Understanding the regulatory mechanisms of artemisinin biosynthesis and glandular trichome formation will be conducive to our practice for improving artemisinin production by metabolic engineering. In this review, the metabolic regulatory network with regard to artemisinin biosynthesis and glandular trichome formation is summarized, and the advance on metabolic engineering to increase artemisinin content in A. annua is also discussed.

    • Malaria is a life-threatening disease that is particularly epidemic in sub-Saharan Africa and South-East Asia. According to the World Malaria Report, approximately 2.2 billion people were at the risk of malaria; of them, an estimated 219 million cases occurred worldwide, and consequently 435,000 people, mostly children, were killed by malaria (World Malaria Report 2018, WHO). Fortunately, artemisinin, a kind of sesquiterpene lactone, which was first found by Chinese scientists from a traditional Chinese herbal plant Artemisia annua, can effectively cure malaria[1]. To date, artemisinin-based combination therapies (ACTs) have been recommended as the first-choice treatment method for drug-resistant malaria by the World Health Organization (WHO)[2, 3]. Besides the anti-malarial effect, artemisinin also exhibits pharmaceutical activities in the treatment of diabetes[4], inflammatory response[5] and tumor[6].

      Different strategies were applied for increasing artemisinin supply such as chemical synthesis, semi-synthesis from microbially sourced artemisinic acid, and metabolic engineering in A. annua. Chemical synthesis of artemisinin is difficult and costly. Although a concise synthesis of artemisinin from inexpensive cyclohexanone was reported, which reached the gram scale, there is still quite a lot to do before large-scale production can be achieved[7]. Semi-synthesis of artemisinin has achieved success. Artemisinic acid, the precursor for chemically synthesizing artemisinin, can be biosynthesized through successive enzymatic reactions in engineered yeast cells at 25 g/L, ensuring artemisinin production can reach an industrial scale[810]. But the semi-synthesis by yeast is still relatively high in cost and low in production, limiting its extensive application. Therefore, at present, Artemisia annua plants remain the primary source for obtaining artemisinin.

      Artemisinin is mainly extracted from A. annua leaves, but its content in wild A. annua is low (0.1%–1% dry weight)[11, 12], unable to meet global demand. It is therefore of great importance to elevate artemisinin production in A. annua and develop new A. annua lines with high yield artemisinin by metabolic engineering and genetic breeding.

      Over a decade ago, the artemisinin biosynthetic pathway has been completely elucidated at the molecular level. The precursor for artemisinin biosynthesis is farnesyl diphosphate (FDP) containing three isoprenyl 5-carbon (C5) units, which is formed by the condensation of three isopentenyl diphosphates (IPPs) through the catalysis of farnesyl diphosphate synthase (FPS). Isopentenyl diphosphate (IPP) and its isomer dimethylally diphosphate (DMAPP) are the general 5-carbon precursors for all terpenoids, including monoterpenes, sesquiterpenes, diterpenes and so on. IPP can be synthesized through two distinct pathways in the plant: the cytosolic mevalonate pathway (MVA) and the plastidial methylerythritol phosphate (MEP) pathway[13, 14]. In the MVA pathway, HMG-CoA reductase (HMGR) is the rate-limiting enzyme[15], while in the MEP pathway, DXR is the rate-limiting enzyme[16]. The two enzymes are the preferred target enzymes for metabolically engineering artemisinin biosynthesis in the upstream pathway of artemisinin biosynthesis.

      After the formation of FDP, amorpha-4,11-diene synthase (ADS), which is believed to be the first committed step in artemisinin biosynthesis, catalyzes the cyclization of FDP into cyclic amorpha-4,11-diene as the unique precursor for artemisinin[17]. Then a multiple-function enzyme CYP71AV1 converts amorpha-4,11-diene into artemisinic alcohol, artemisinic aldehyde and artemisinic acid sequentially[18]. Subsequently, artemisinic aldehyde is reduced to dihydroartemisinic aldehyde under the catalysis of artemisinic aldehyde Δ11 (13) reductase (DBR2)[19]. Then dihydroartemisinic aldehyde is converted into dihydroartemisinic acid by aldehyde dehydrogenase (ALDH1)[20]. Dihydroartemisinic acid is the direct precursor for artemisinin, which is transformed into artemisinin by photo oxidative reaction without enzymes[21, 22]. Meanwhile, artemisinic acid can be converted into arteannuin B via a photo-oxidative non-enzymatic reaction[21, 22]. The four genes, including ADS, CYP71AV1, DBR2 and ALDH1, which are artemisinin-specific biosynthetic genes, are specifically expressed in glandular trichomes of A. annua[23, 24]. The complete elucidation of the artemisinin biosynthetic pathway provides the biosynthetic genes necessary for engineering artemisinin biosynthesis, and facilitates the development of new A. annua varieties with a high-yield of artemisinin.

      Besides the elucidation of the artemisinin biosynthetic pathway, huge progress has been made in dissecting the complicated regulatory networks of artemisinin biosynthesis at the transcriptional and post-translational levels. Many transcription factors, which mediate different signaling pathways from plant hormones and other environmental factors like light, coldness etc, are involved in the regulation of artemisinin biosynthesis. To date, a variety of regulatory genes, such as those encoding transcription factors and kinases, have been characterized to play important parts in regulating artemisinin biosynthesis. These identified regulatory genes are valuable in developing A. annua plant lines with high-yield artemisinin and certain beneficial horticultural traits. In a recently published review, Zheng et al. made a comprehensive description of transcriptional regulatory network of artemisinin biosynthesis induced by diverse phytohormones and environmental factors, which would be helpful for better understanding the regulatory mechanism of artemisinin biosynthesis[25].

      Moreover, as artemisinin is exclusively synthesized and stored in glandular trichomes (GT) of A. annua[23, 24], the density and number of GTs has a large effect on the content of artemisinin in A. annua plants. Therefore, it is also necessary to dissect the regulatory mechanism of glandular trichome formation and identify the regulatory factors involved.

      In this review, we summarize the accomplishments in the study of the molecular regulation of artemisinin biosynthetic pathway and glandular trichome initiation, and also discuss biotechnological approaches toward higher artemisinin production in A. annua plants.

    • Plant hormones, like jasmonate (JA), abscisic acid (ABA), salicylic acid (SA) etc, and other environmental signals regulate artemisinin biosynthesis via the mediation of transcription factors (TF), and they form a cascade signaling network. Among the hormones, jasmonate (JA) signaling is most extensively studied. A number of transcription factors, which are responsive to these signals, have been identified to be involved in regulating artemisinin biosynthesis. These transcription factors belong to different TF families respectively, such as APETALA2/ethylene-response factors (AP2/ERF), WRKY, basic helix-loop-helix (bHLH), NAC and so on (Fig. 1).

      Figure 1. 

      Regulatory network of artemisinin biosynthetic pathway. The arrows represent direct upregulation of downstream targets. Arrows started from the common upstream regulators or directed toward the common downstream targets are in one colour. Transcription factors of the same family are marked with the same colour.

    • The APETALA2/Ethylene-Responsive Factors (AP2/ERFs) are one of the most important TF families that participate in stress response, developmental process and the regulation of secondary metabolism[26]. Some of them have been reported to regulate the biosynthesis of secondary metabolites, such as ORCA3 regulating the biosynthesis of monoterpenoid indole alkaloids (MIAs) in Catharanthus roseus[27] and NtERF32 regulating nicotine biosynthesis in tobacco[28]. In Artemisia annua, four AP2/ERF TFs (AaERF1, AaERF2, AaORA and AaTAR1) have been sequentially reported to be involved in regulating artemisinin biosynthesis[2931]. Three of them, including AaERF1, AaERF2 and AaORA, are markedly induced by JA. AaERF1, AaERF2 and AaTAR1 are capable to bind to the CBF2 and RAA motifs in ADS and CYP71AV1 promoters, and activate the two genes’ expression[29, 31]. Besides, the trichome morphology/shape and cuticle composition are also regulated by AaTAR1 in A. annua[31]. AaORA is specifically expressed in the glandular and non-glandular T-shaped trichomes of A. annua, and positively regulates artemisinin biosynthesis and resistance to the fungal pathogen Botrytis cinerea[30]. However, AaORA was found not to directly bind to the promoters of the four artemisinin biosynthetic genes; rather, it interacts with AaTCP14 or AaTCP15, two Teosinte branched1/Cycloidea/Proliferating Cell Factors, to form a complex and enhance the transactivation activity of the two TCPs on their target genes such as DBR2 or ALDH1[32, 33], thereby promoting artemisinin biosynthesis.

    • WRKY transcription factors (TFs) make up a large family specific to plant species. WRKY TFs, with a conserved WRKY domain, regulate target genes’ expression through directly binding to their cis-elements called W-box[34]. AaWRKY1 is the first reported TF controlling artemisinin biosynthesis in A. annua[35]. AaWRKY1, highly expressed in GTs and induced by JA, could bind to the W-box of ADS and CYP71AV1 promoters and activate their expression[35, 36]. Then another WRKY TF, namely GLANDULAR TRICHOME-SPECIFIC WRKY 1 (AaGSW1), was identified to activate the expression of CYP71AV1 and AaORA by binding to their promoters, thus upregulating artemisinin biosynthesis[37]. AaGSW1 is responsive to both JA and ABA signals, and can be transactivated by AaMYC2 (a JA-responsive bHLH TF) and AabZIP1 (an ABA-responsive bZIP TF). Therefore, AaGSW1 integrates JA and ABA signaling pathways to regulate artemisinin biosynthesis. Furthermore, a recently reported AaWRKY9, which integrates JA and light signals, positively regulates artemisinin biosynthesis by activating the expression of AaGSW1 and DBR2[38]. Hence WRKY TFs are widely involved in mediating the regulation of artemisinin biosynthesis by diverse signals.

    • The basic helix-loop-helix (bHLH) TFs play an important part in the regulation of growth and development, secondary metabolism, stress response in plants[3941]. So far many bHLH TFs have been functionally identified. AabHLH1, identified from A. annua, is responsive to JA and transactivates ADS and CYP71AV1 through binding to the E-box cis-elements in their promoters[42]. MYC2, another member of the bHLH family, plays a crucial role in regulating terpene biosynthesis in plants[43]. In A. annua, AaMYC2 is induced by JA and upregulates the expression of CYP71AV1, DBR2 and ALDH1 by binding to the G-box in their promoters[44, 45]. AaMYC2 could also directly activate AaGSW1 expression. At the post-translational level, the JASMONATE ZIM-DOMAIN (JAZ) proteins in JA signaling pathway and DELLA proteins in gibberellin signaling pathway could interact with AaMYC2, probably suppressing its transactivating function. Transgenic results indicated that AaMYC2 upregulated the biosynthesis of artemisinin and anthocyanins in A. annua plants[44]. Besides, AabHLH112, highly expressed in glandular trichomes of A. annua, is induced by coldness and upregulates the expression of AaERF1 by binding to its promoter, thus promoting artemisinin biosynthesis[46].

    • The NAC transcription factors are of a large plant-specific family that regulate stress response and cell development[4749]. In A. annua, a NAC TF, AaNAC1, was identified to promote artemisinin production by enhancing ADS expression, as well as to enhance drought tolerance and resistance to Botrytis cinerea[50]. AaNAC1 can be induced by SA, JA, dehydration and coldness respectively, suggesting that this transcription factor may integrate diverse signaling pathways[50]. Moreover, AabZIP1, a transcription factor of basic leucine zipper (bZIP) family, is responsive to ABA signals and upregulates artemisinin biosynthesis through directly activating the expression of ADS, CYP71AV1 and AaMYC2[45, 51]. In addition, ETHYLENE-INSENSITIVE 3 (EIN3), a key TF involved in ethylene signaling, was found to negatively regulate the expression of artemisinin biosynthetic genes. Therefore, AaEIN3 is a negative regulator of artemisinin biosynthesis[52]. HY5, a key TF in light signaling, was found to transactivate AaWRKY9 and AaGSW1, thereby upregulating artemisinin biosynthesis[53]. These findings indicate that artemisinin biosynthesis can be regulated by diverse transcription factors that mediate diverse signaling pathways.

    • Artemisinin is specifically synthesized and stored in glandular secretory trichomes (GT) composed of 10 cells, due to GT-specific expression of artemisinin biosynthetic genes[23, 24]. The amount of GT has a determinative effect on the content of artemisinin in A. annua plants. So it is important to reveal the regulatory mechanism of glandular trichome formation, which would provide candidate regulatory factors valuable for increasing GT formation and elevating artemisinin production. So far, the identified regulatory factors involved in regulating GT formation in A. annua are mainly homeodomain-leucine zipper (HD-ZIP) IV and R2R3-MYB TFs (Fig. 2).

      Figure 2. 

      Identified regulation of glandular trichome (GT) formation in A. annua. The arrows represent upregulation of downstream targets, while short lines represent inhibition or downregulation of downstream targets. Transcription factors of the same family are marked with the same colour.

    • Homeodomain-leucine zipper (HD-ZIP) TFs, especially the IV (HD-ZIP IV) subfamily, were reported to regulate trichome development in plants[54, 55]. Two HD-ZIP IV transcription factors, AaHD1 and AaHD8, were sequentially identified to be positive regulators of trichome formation in A. annua. The JA-responsive AaHD1, specifically expressed in the basal cells of glandular and non-glandular trichomes of young A. annua leaves, positively regulates trichome formation through activating the expression of AaGSW2, a GT-specific WRKY TF that promotes GT initiation[56, 57]. Biochemical assays demonstrated that AaJAZ8 physically interacted with AaHD1 and suppressed its transactivating function[56]. AaHD8, highly expressed in trichomes, could bind to the L1-box of AaHD1 promoter and activate its expression; besides, it can activate the expression of cuticle biosynthesis genes[58]. AaMIXTA1 (a R2R3 MYB TF) interacted with AaHD8 and enhanced the function of AaHD8 in transactivating AaHD1 and cuticle biosynthesis genes[59]. These findings reflect the complicated associations between cuticle biosynthesis and trichome formation.

    • As the largest TF family in plants, the myeloblastosis (MYB) proteins regulate trichome development, root hair density, secondary metabolism, and stress response[6063]. Among the MYB family, the R2R3 MYB TFs are of the largest subfamily, which contain two MYB domain repeats[64, 65]. Some R2R3 MYB TFs, including AaMYB1, AaMIXTA1, AaTAR2, AaMYB17, were sequentially identified to positively regulate glandular trichome formation, thereby elevating artemisinin content in A. annua[59, 63, 66, 67]. Overexpression of AaMYB1 led to an increase in the trichome density, probably by affecting gibberellin metabolism[63]. AaMIXTA1, mainly expressed in the GT basal cells of A. annua, positively regulated trichome initiation and cuticles biosynthesis through interacting with AaHD8 to strengthen its transactivation function[59]. AaTAR2 is mainly expressed in glandular and non-glandular trichome cells. Both AaHD1 and AaHD8 were found to be able to promote the expression of AaTAR2 by binding to its promoter[66]. Besides, two R2R3 MYB factors, AaMYB5 and AaMYB16, were reported to competitively interact with AaHD1 and affect its transactivation function on AaGSW2 promoter, thereby regulating GT initiation in A. annua[68]. AaMYB16 enhanced the transactivation function of AaHD1, while AaMYB5 weakened this function. Therefore, AaMYB16 is a positive regulator of GT initiation, while AaMYB5 is a negative regulator of GT formation[68]. These results show that the MYB proteins get involved in regulating GT initiation not only at the transcriptional level, but also at the post-translational level.

    • The elucidation of the artemisinin biosynthetic pathway as well as its regulatory network provided theoretical basis and potentially useful genes for engineering artemisinin production in A. annua. Based on these discoveries, several strategies are designed for promoting artemisinin production in A. annua, including: (1) overexpressing key enzymes in biosynthesis pathway, (2) repressing the competitive metabolic branch pathway, (3) making use of transcription factors (to regulate the biosynthetic pathway), and (4) increasing the glandular trichome density.

    • Generally, the common strategy for enhancing artemisinin production in A. annua is to overexpress rate-limiting enzymes in the biosynthetic pathway to break through the committed steps, in order for more metabolic flux to flow toward artemisinin biosynthesis. The cytosolic MVA pathway and the plastidial MEP pathway provide 5-carbon precursors (IPP and DMAPP) for artemisinin biosynthesis. HMGR is the rate-limiting enzyme in MVA pathway. When HMGR gene from Catharanthus roseus was overexpressed in A. annua, the transgenic lines displayed an increase of 22.5%–38.9% in artemisinin content, compared with wild-type plants[69, 70]. DXR is the rate-limiting enzyme in the MEP pathway, and overexpression of DXR caused a maximum of 1.3 fold increase of artemisinin content in A. annua[71]. Besides, overexpression of FPS, which catalyzes the condensation of IPPs to form FDP, led to a maximum of 1.5 fold increase in artemisinin level in A. annua[72, 73].

      ADS is the first key enzyme in the artemisinin specific biosynthetic pathway. The content of artemisinin was increased by about 82% in ADS-overexpressing transgenic A. annua lines, compared to that in wild type plants[74]. Likewise, the genes of CYP71AV1 and its redox partner CPR were co-overexpressed in A. annua, and artemisinin content in transgenic lines was about 38% higher than that in the controls[71, 75].

      Overexpressing a single pathway gene can enhance artemisinin accumulation only to a limited extent. Co-overexpression of two or more key enzyme genes in artemisinin biosynthetic pathway would be more effective to elevate artemisinin production. For example, co-overexpression of HMGR and ADS led to a maximum of 7.65 fold higher artemisinin content in transgenic A. annua lines than in the control lines[76]. Transgenic A. annua plants overexpressing the combination of FPS, CYP71AV1 and CPR had the artemisinin level 2.6 fold higher than that of the control plants[77]. In addition, co-overexpressing ADS, CYP71AV1 and CPR genes caused a maximum of 2.4 fold increase in artemisinin content in transgenic A. annua, compared to the control plants[78]. Meanwhile, co-overexpression of four genes including ADS, CYP71AV1, CPR and ALDH1 in A. annua caused an increase of artemisinin content by 2.4 fold at the most[79]. Exceptionally, co-overexpression of HMGR and FPS, which increased artemisinin level merely by 80% relative to the control[80], did not bring about a more significant elevation in artemisinin content compared with the single-gene (HMGR or FPS) overexpression in A. annua plants (Table 1). More combinations of target genes could be tried in the future for metabolic engineering of artemisinin production, and these combinations should be well evaluated and compared to identify the optimal one that can most effectively enhance artemisinin biosynthesis.

      Table 1.  Summary of genes used for metabolic engineering of artemisinin in A. annua.

      Strategies for metabolic engineering of artemisininTarget genes usedArtemisinin increment relative to controlReferences
      Overexpressing key enzymes in artemisinin biosynthesisHMGR22.5%−38.9%[69, 70]
      DXR1.3 fold[71]
      FPS1.5 fold[72, 73]
      ADS82%[74]
      CYP71AV1/CPR38%[71, 75]
      HMGR + FPS80%[80]
      HMGR + ADS7.65 fold[76]
      ADS + CYP71AV1/CPR2.4 fold[78]
      FPS + CYP71AV1/CPR2.6 fold[77]
      ADS + CYP71AV1/CPR+ ALDH12.4 fold[79]
      Repressing competitive pathwaysSQS71%[82]
      CPS77%[82]
      BFS77%[82]
      GAS1.03 fold[82]
      Overexpressing transcription factorsAaERF168%[29]
      AaERF250%[29]
      AaORA53%[30]
      AaTAR138%[31]
      AaTCP14Nearly 1 fold[32]
      AaTCP15Nearly 1 fold[33]
      AaTCP14+AaORA1.8 fold[32]
      AaWRKY11.3-2 fold[36]
      AaGSW10.5-1 fold[37]
      AaWRKY90.6-1.2 fold[38]
      AaMYC223%−55%[44]
      AabHLH11248%−70%[46]
      AaNAC146%−79%[50]
      AaEIN3(repression)About 35%[52]
      AabZIP10.7-1.5 fold[51]
      AaHY5Nearly 1 fold[53]
      Increasing glandular trichome densityAaHD150%[56]
      AaHD835%[58]
      AaMIXTA11 fold[59]
      AaMYB11 fold[63]
      AaTAR250%[66]
      AaMYB1787%[67]
      AaMYB5(knockdown)45%−84%[68]
      AaMYB1643%−56%[68]
      AaGSW21 fold[57]
    • Farnesyl diphosphate (FDP) acts as a common precursor for the biosynthesis of artemisinin and other sesquiterpenes like β-farnesene, β-caryophyllene, squalene and so on[14, 81]. These diverse metabolic pathways are in competition for the same precursor. Repressing other sesquiterpenes' biosynthetic pathways would conduce to more metabolic flux into artemisinin biosynthetic pathway. For example, squalene synthase (SQS) is the key enzyme converting FDP into squalene, an intermediate in sterol biosynthesis[14]. Suppressing SQS expression by antisense technology in A. annua increased artemisinin biosynthesis by 71%. Repressing the expression of β-caryophyllene synthase (CPS), an enzyme converting FDP into β-caryophyllene, led to a 77% increase of artemisinin content in A. annua[82]. Likewise, repression of β-farnesene synthase (BFS), which converts FDP into β-farnesene, caused the artemisinin content to increase by 77% in A. annua. And repression of germacrene A synthase (GAS), an enzyme converting FDP into germacrene A, caused the artemisinin content to increase by 103%[82] (Table 1).

    • Transcription factors can usually regulate the expression of multiple genes in a certain pathway, and overexpression of these factors has been proposed as a promising way for efficiently upregulating a target metabolic pathway. At present, many transcription factors of different families have been identified to positively regulate artemisinin biosynthesis, which are potentially useful for engineering artemisinin production. For example, overexpression of AaORA in A. annua led to a maximum of 53% increase in artemisinin level, compared to wild type plants[30]; overexpressing AaMYC2 in A. annua increased artemisinin content by 23%–55% compared to the wild type[44]. The artemisinin level in AabHLH112-overexpression lines was 48%–70% higher than that in the control lines[46]. The artemisinin content in AabZIP1-overexpressing A. annua lines was increased by 0.7–1.5 fold compared with the wild-type[51]. In addition, transgenic lines overexpressing AaWRKY9 produced 0.6–1.2 fold more artemisinin than the control[38]. More examples of transcription factors overexpressed for elevating artemisinin yield are listed in Table 1.

    • Since artemisinin is exclusively synthesized and stored in glandular trichomes of A. annua, enhancing the formation of glandular trichomes would conduce to elevating artemisinin content in A. annua plants. Many regulatory factors related to glandular trichome formation have been identified, which are potentially useful for increasing glandular trichome density. For example, overexpression of AaHD1 in A. annua led to about 50% increase in glandular trichome density on mature leaves, with a concomitant increase of 50% in artemisinin content, compared to the control[56]. Overexpressing AaMYB17 in A. annua made the number of GTs on the adaxial leaf side increase by 30%–60% with a concomitant increase of artemisinin content, compared to the control[67]. The GT number on the leaf adaxial side in AaMIXTA1-overexpessing lines increased by approximately 50%, with the artemisinin content also rising by 1 fold, compared with that of the control[59]. Besides, overexpression of AaGSW2 in A. annua caused the GT density on the leaves to be double that of the wild type, with the artemisinin content also 2-fold that of wild-type controls[57]. More examples of regulatory factors used for increasing GT density are listed in Table 1.

    • Although artemisinin production by chemical synthesis or semi-synthesis in yeast has become successful, the A. annua plant remains the main commercial source of artemisinin. So it is of great importance to elevate artemisinin content in A. annua and develop new A. annua lines with high yield artemisinin by metabolic engineering and genetic breeding. To date, the artemisinin biosynthetic pathway has been completely elucidated, and many regulatory factors regulating artemisinin biosynthesis and glandular trichome formation have been identified. Many of these genes involved in artemisinin biosynthesis or its regulation have been used in the study for metabolic engineering of artemisinin production, and exhibited good potential for developing A. annua lines with high-yield artemisinin.

      However, the complicated regulatory mechanism of artemisinin biosynthesis and GT initiation is still far from clear and requires further exploration. The majority of TFs identified to regulate artemisinin biosynthesis are related to JA signaling pathway. Those TFs involved in signaling pathways from other hormones like SA, ABA, ethylene, or environmental factors such as temperature, light, etc, have not been thoroughly characterized. Moreover, the presently identified regulators that regulate GT formation mostly lie upstream of the regulatory cascade pathways, and their targets downstream directly participating in the process of GT formation remain unknown. For example, what are the target genes downstream of AaGSW2? How do these genes function in the GT initiation process? The lack of such knowledge limits our comprehensive grasp of regulatory network of artemisinin biosynthesis and GT development. Elucidation of the above issues would provide more solid theoretical foundation for genetic engineering of artemisinin.

      According to the present data, it appears that the single strategy adopted above for metabolic engineering have a relatively limited effect on the elevation of artemisinin content in A. annua. To further increase artemisinin production, the combination of multiple strategies could be tried, such as overexpressing multiple key enzyme genes in artemisinin biosynthetic pathway, combined with repressing competitive pathways and increasing the trichome number by genetic manipulation. Besides, it is also important to find out the optimal environmental parameters (such as light, temperature and humidity conditions) that are most suitable for A. annua growth. To conclude, more in-depth study is required to ultimately solve the shortage of artemisinin.

      • Zhihua Liao is the Editorial Board member of journal Medicinal Plant Biology. He was blinded from reviewing or making decisions on the manuscript. The article was subject to the journal's standard procedures, with peer-review handled independently of this Editorial Board member and his research groups.

      • Copyright: © 2023 by the author(s). Published by Maximum Academic Press, Fayetteville, GA. This article is an open access article distributed under Creative Commons Attribution License (CC BY 4.0), visit https://creativecommons.org/licenses/by/4.0/.
    Figure (2)  Table (1) References (82)
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    Tang Y, Xiang L, Zhang F, Tang K, Liao Z. 2023. Metabolic regulation and engineering of artemisinin biosynthesis in A. annua. Medicinal Plant Biology 2:4 doi: 10.48130/MPB-2023-0004
    Tang Y, Xiang L, Zhang F, Tang K, Liao Z. 2023. Metabolic regulation and engineering of artemisinin biosynthesis in A. annua. Medicinal Plant Biology 2:4 doi: 10.48130/MPB-2023-0004

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