© Springer Nature Singapore Pte Ltd. 2019

M. S. Akhtar, M. K. Swamy (eds.)Natural Bio-active Compoundshttps://doi.org/10.1007/978-981-13-7438-8_10

10. Hairy Root Cultures as an Alternative Source for the Production of High-Value Secondary Metabolites

Arockiam Sagina Rency¹, Subramani Pandian¹, Rakkammal Kasinathan¹, Lakkakula Satish^{1, 2}, Mallappa Kumara Swamy^{3, 4} and Manikandan Ramesh¹  

(1)

Department of Biotechnology, Science Campus, Alagappa University, Karaikudi, Tamil Nadu, India

(2)

Department of Biotechnology Engineering & The Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Beer Sheva, Israel

(3)

Department of Crop Science, Faculty of Agriculture, Universiti Putra Malaysia, Serdang, Selangor, Malaysia

(4)

Department of Biotechnology, East West First Grade College of Science, Bengaluru, Karnataka, India

 

 

Manikandan Ramesh

Email: mrbiotech.alu@gmail.com

10.1 Introduction

10.2 Production of Secondary Metabolites Through Hairy Root Cultures

10.3 Role of Bioreactors in Large-Scale Production of Secondary Metabolites

10.4 Advances in Metabolic Engineering of Hairy Roots

10.5 Enhancement of Secondary Metabolites Through Elicitation

10.6 Biotransformation

10.7 Hairy Root Applications in Environmental Protection (Phytoremediation)

10.8 Germplasm Conservation

10.9 Omics Approaches in Secondary Metabolite Production

10.10 Conclusions and Future Prospects

References

Abstract

Hairy roots are rapidly growing, highly differentiated transformed root cultures induced by Agrobacterium rhizogenes infection usually at the infected site of the representative medicinal plant. Hairy roots have the ability to rapidly multiply in the culture medium devoid of any hormones. Unlike other plant cell cultures, hairy root cultures are genetically and biochemically stable and produce a variety of secondary metabolites. In the past three decades, researchers across the world have successfully initiated and cultured hairy roots in vitro for a large number of medicinal plants. Hairy root technology is becoming a promising source for the production of pharmaceutically and industrially important secondary metabolites. This is due to the characteristics of hairy roots, such as rapid growth, the lack of geotropism, extensive lateral branching, and, more importantly, genetic stability. This chapter explores the applications of secondary metabolites in drug formulation, cosmetic preparation, food processing, and the study of plant metabolic pathways. It also briefs about the recent advancements in the area of hairy root culture involving other biotechnological approaches like metabolic engineering or genetic engineering, elicitation, metabolic trapping, and phytoremediation. This chapter certainly benefits the researchers to further explore on the applications of hairy root culturing technology to produce desired plant secondary metabolites on a large scale.

Keywords

Agrobacterium rhizogenes Genetic engineeringHairy rootsMedicinal plantsSecondary metabolites

10.1 Introduction

Medicinal plants produce a variety of biologically active compounds, i.e., secondary metabolites which play a vital role in plant self-defense mechanisms. Especially, roots play major roles in plants, including anchoring plants to the soil, uptake of minerals and water from the soil, storage of nutrients in perennial plants, and defending themselves from other plants or microbes present in the soil by producing a wide variety of chemical compounds, popularly known as secondary metabolites. These secreted metabolites not only provide protection to plants from biotic and abiotic stresses like pathogens, insects, and other environmental stresses but also useful in improving human’s and other animal’s health (Tian 2015). These compounds are produced in trace amounts during the secondary metabolism, but not essentially necessary for plant growth and development. Plant-based compounds, including alkaloids, flavonoids, saponins, terpenes, anthraquinones, and anthocyanins, are the essential source for the preparation of drugs, food additives, dyes, oils, resins, and agricultural chemicals (Kim et al. 2002; Zhou et al. 2011; Bharati and Bansal 2014). Obtaining the chemical compounds directly from the wild- or field-grown plants is not promising as the yield obtainable is being very low and has limited availability in their habitat. Moreover, it may lead to the destruction of the natural habitat due to over exploitation of these plants. The artificial synthesis of chemical compounds also has several disadvantages including high cost of production, the difficulties in the synthesis, unavailability of the optimized methods for the compound synthesis, and characterization. These problems can be overcome by the using the biotechnological approaches such as plant tissue culture, transgenic medicinal plants, etc. to enhance the synthesis of valuable phytochemicals from medicinal plants (Zhou et al. 2011). In this regard, the hairy root technology is widely preferred by biotechnologists for the large-scale production of diverse secondary metabolites from various medicinal plant resources (Veena and Taylor 2007).

Hairy roots are the by-products from the Agrobacterium rhizogenes (gram negative, soil bacterium)-infected sites, commonly known as hairy root disease or syndrome. This soil bacterium transfers its T-DNA segment from Ri (root-inducing) plasmid into the host plant genome. The T-DNA region contains a set of genes encoding for the specific enzymes, which control the biosynthesis of natural auxins and cytokinins. The new changes, i.e., insertion of new genes, cause hormonal imbalance in the host plant and induce the formation of proliferating roots (hairy roots) from the wounded sites infected with A. rhizogenes (Guillon 2006). Hairy roots are characterized by the abnormal multiplication on the phytohormone free medium by retaining genetic stability. Hairy roots have several unique properties including fast growth rate, able to accumulate vast variety of chemical compounds, no requirement of exogenous hormone in the medium, and genetic and biochemical stability (Giri and Narasu 2000). The schematic representation of hairy root induction and its application is shown in Fig. 10.1. Nowadays, many research groups are paying attention toward in vitro culturing of hairy roots for producing wide varieties of root-oriented plant secondary metabolites. Recent advancements have provided a better understanding about the molecular mechanisms involved in the T-DNA transfer and their integration into the host plant genome. This has paved a new way for producing plant secondary metabolites through employing metabolic engineering strategies. Also, hairy roots have shown the capability of absorbing some of the threatening recalcitrant pollutants and thus can be used to clean the environment (phytoremediation). In this chapter, detailed information about hairy roots and their applications in the production of valuable plant secondary metabolites are discussed. Further, more recent advances in the field of hairy root culture technology are highlighted.

Fig. 10.1

The schematic representation of hairy root induction and its application

10.2 Production of Secondary Metabolites Through Hairy Root Cultures

From several decades to now, worldwide population is still depending on plants and plant-derived products for their daily needs. Even today, around 80% of the human population depends on plants as a traditional medicine to cure several diseases (Ekor 2014; Swamy et al. 2016). Terrestrial plants are the greatest source for several chemical compounds with wide-ranging pharmaceutical applications. As these compounds occur in trace amounts in plants, they generally do not meet the huge demand in the pharmaceutical industry. Hence, this has raised a curiosity among researchers to make use of biotechnological approaches to commercially produce these valuable compounds using plant sources (Verpoorte et al. 1999). In search of this is the hairy root culture technology, an alternative approach which offers the production of secondary metabolites in a large scale. Moreover, hairy roots have the unique characteristics of fast growth, and also levels of secondary metabolites produced are equal to or superior than the parent plants (Roychowdhury et al. 2013). The genetic and biosynthetic stability of hairy roots is another advantage for the production of valuable secondary metabolites. In addition to that, transformed hairy roots can be proficient to regenerate into entire viable plants and also preserve their genetic stability throughout and further successive subculturing and plant regeneration (Giri and Narasu 2000). There are several important secondary metabolites produced through hairy root cultures in many medicinal plant species which are endangered and pharmaceutically important. The list of few important secondary metabolites produced through hairy root cultures from various medicinal plants has been described in Table 10.1. In the recent era, hairy root cultures are not only used for secondary metabolite production but also widely used as model systems for studying plant physiology and metabolism, regulation of metabolic pathways, and identification of key genes for production and regulation of particular metabolite (Shanks and Morgan 1999; Sharma et al. 2013; Tian 2015). For example, the roots of Panax ginseng plants were rich in ginsenosides, saponin which possesses immunomodulatory, adaptogenic, and antiaging properties. The hairy roots of P. ginseng produce twofold increased concentration of ginsenosides than the wild-type roots (Yoshikawa and Furuya 1987). In addition to that, P. quinquefolium is another important Panax species, and its hairy roots produced 0.2 g g⁻¹ dry weight of ginsenoside content within 10 weeks of hairy root culture (Mathur et al. 2010). The hybrid plant was made between P. ginseng and P. quinquefolium which was more dynamic in ginsenoside production than the parental plant. The hairy roots (8-week-old) derived from the hybrid plant containing equivalent amounts of ginsenosides present in the field-grown parental plant roots revealed the biosynthetic potential of hairy roots maintained in the parent plants (Washida et al. 1998; Tian 2015).

Table 10.1

Establishment of hairy root cultures for plant secondary metabolite production

Plant species	Secondary metabolite	Biological properties	References
Artemisia annua	Artemisinin	Antimalarial	Weathers et al. (2005)
Beta vulgaris	Betalains	Antioxidant, colorant	Pavlov and Bley (2006)
Bixa orellana	Stigmasterol	Antimalarial	Zhai et al. (2014)
Chlorophytum borivilianum	Stigmasterol and hecogenin	Antioxidant	Bathoju et al. (2017)
Clitoria ternatea	Taraxerol	Anticancer	Swain et al. (2012)
Datura innoxia	Scopolamine and hyoscyamine	Anticholinergic	Dechaux and Boitel-Conti (2005)
Echinacea sps.	Alkamides	Anti-inflammatory, immune-stimulatory	Romero et al. (2009)
Eschscholzia californica	Benzylisoquinoline	Antimicrobial, anticancer	Vázquez-Flota et al. (2017)
Fragaria x ananassa cv. Reikou	Polyphenols (proanthocyanidins, flavonoids, hydrolyzable tannin)	Antioxidant, anticancer	Motomori et al. (1995)
Gingko biloba	Ginkgolide	Against cardiovascular and aging diseases	Ayadi and Tremouillaux-Guiller (2003)
Hyoscyamus niger	Tropane alkaloids	Anticholinergic	Jaziri et al. (1988)
Isatis tinctoria	Flavonoids	Antioxidant	Gai et al. (2015)
Linum flavum	Aryltetralin lignans Lignans coniferin	Anticancer	Renouard et al. (2018) and Lin et al. (2003)
Linum usitatissimum	Lignan	Anticancer	Gabr et al. (2016)
Nasturtium officinale	Glucosinolates (gluconasturtiin, glucotropaeolin)	Anticancer, antifungal, antibacterial, antinematode, anti-insect	Wielanek et al. (2009)
Ophiorrhiza pumila	Camptothecin	Antitumor	Saito et al. (2001)
Papaver somniferum	Morphine Sanguinarine Codeine	Sedative, analgesic	Le Flem-Bonhomme et al. (2004)
Polygonum multiflorum Thunb	Anthraquinones	Antifungal, anti-inflammatory, antimicrobial	Thiruvengadam et al. (2014)
Rauvolfia micrantha	Ajmalicine Ajmaline	Antihypertensive	Sudha et al. (2003)
Rauwolfia serpentina	Terpenoid indole alkaloids (reserpine, ajmalicine, ajmaline, serpentine, yohimbine)	Hypertension, high blood pressure, mental illness	Mehrotra et al. (2015)
Solanum chrysotrichum	Saponin	Antifungal	Caspeta et al. (2005)
Stevia rebaudiana	Stevioside glycosides	Antioxidant, anti-inflammatory, antihypertensive	Kumari and Chandra (2017)
Taxus brevifolia	Taxol	Anticancer	Huang et al. (1997)
Valeriana wallichii	Iridoids (valepotriates)	Sedative, spasmolytic	Banerjee et al. (1998)
Withania somnifera	Steroidal lactones (withanolide A)	Anticancer	Murthy et al. (2008)

10.3 Role of Bioreactors in Large-Scale Production of Secondary Metabolites

Scaling-up process of commercially important secondary metabolites through bioreactor at the industrial level is the next step after establishing in vitro hairy root cultures (Giri and Narasu 2000; Bourgaud et al. 2001). Bioreactors work as a chemical factory and offer a big hope for the large-scale production of high-quality biologically active compounds from medicinal and aromatic plants cells/tissues. This process is also known as molecular farming (Shanks and Morgan 1999). Large-scale production of secondary metabolites using bioreactor is not an easy process, because designing of the bioreactor and optimization of culture conditions are very difficult. The successful cultivation of hairy roots in bioreactor depends on several requirements, including growth characteristics, morphology, nutrient uptake and availability, oxygen supply, composition of the medium, inoculum concentration, and distribution which can facilitate the growth of inoculum (Giri and Narasu 2000; Roychowdhury et al. 2013; Ho et al. 2017). Also, the productivity in bioreactors depends on several physical and chemical parameters like light, temperature, pH, water, substrate availability, impeller designs, composition of gases, choice of hairy root clone, removal of toxic by-products, reactor operation, etc. (Roychowdhury et al. 2013; Sharma and Shahzad 2013). There are several types of bioreactor designs that have been reported for hairy root culturing. Generally, three major types of bioreactors are used for hairy root cultivation, namely, liquid-phase reactors, gas-phase reactors, and hybrid reactors (a combination of both liquid-phase and gas-phase reactors) (Srivastava and Srivastava 2007). Liquid-phase reactors are commonly known as submerged reactors, in which roots remain submerged in the culture medium and air is passed or bubbled on culture medium to supply oxygen. The best examples for liquid-phase reactors are air lift, stirred tank, bubble column, liquid-impelled loop, and submerged connective flow reactors. In gas-phase bioreactors, hairy roots were occasionally exposed to air, nutrient liquid, and other gaseous mixtures in the bioreactors. In these reactors, nutrients are provided as either in the form of either spraying liquid nutrients onto the roots or roots getting nutrients in the form of droplets, which significantly depends on the varying sizes. Trickle bed, liquid-dispersed, droplet phase, and nutrient mist reactors are some examples for the gas-phase reactors. In hybrid reactors, hairy roots were first exposed to liquid phase and then grown in a gas phase (Roychowdhury et al. 2013). Bioreactor culture systems are mainly used in the industrial application, and they have several advantages, such as requiring very small amount of the inoculum, controlled environmental conditions, increased working volumes, and standardized growth parameters, viz., pH, light, temperature, nutrient media composition, etc. for inducing metabolite production effectively. In addition, easy separation of the target compounds, reproducible yield of the end product, and simpler and quicker harvesting of the cells are some of the other advantages of using bioreactors (Sharma and Shahzad 2013). Some examples for the production of secondary metabolites through the use of bioreactors are mentioned in Table 10.2. For example, artemisinin and its derivatives are high efficient drugs used for the treatment of Plasmodium falciparum (both chloroquine-sensitive and chloroquine-resistant strains) which is the causative agent of cerebral malaria. Traditionally, it is obtained from the plant source Artemisia annua which contains low concentrations of artemisinin. Patra and Srivastava (2016) reported that large-scale artemisinin production by A. annua hairy roots in nutrient mist bioreactor.

Table 10.2

Examples of some important plant secondary metabolites produced through bioreactors

Plant species	Secondary metabolite	Bioreactor type	References
Artemisia annua	Artemisinin	Mist and bubble column reactor; gas- and liquid-phase bioreactors	Kim et al. (2001) and Patra and Srivastava (2016)
Astragalus membranaceus	Astragaloside IV and polysaccharide	Air lift bioreactor	Du et al. (2003)
Artemisia annua	Terpenoids	Mist and bubble column reactor	Souret et al. (2003)
Atropa belladonna	Tropane alkaloids	Stirred bioreactors	Lee et al. (1999)
Atropa belladonna	Tropane alkaloids, atropine	Bubble column bioreactor	Kwok and Doran (1995)
Beta vulgaris	Betalains, peroxidase	Bubble column reactor	Rudrappa et al. (2004, 2005)
Catharanthus roseus	Ajmalicine	Bubble column and rotating drum bioreactor	Thakore et al. (2017)
Datura stramonium	Hyoscyamine	Isolated impeller stirred tank reactor	Hilton and Rhodes (1990)
Eleutherococcus koreanum	Saponins	Air lift bioreactor	Lee et al. (2015a, b)
Genista tinctoria	Phytoestrogens	Prototype basket-bubble bioreactor	Luczkiewicz and Kokotkiewicz (2005)
Hypericum perforatu m	Hypericin	Balloon-type bubble bioreactor	Cui et al. (2010)
Hyoscyamus muticus	Tropane alkaloids	Trickle bed bioreactor	Flores and Curtis (1992)
Nicotiana rustica	Nicotine	Air-sparged vessel stirred tank	Rhodes et al. (1987)
Panax ginseng	Ginsenosides	Air bubble bioreactor	Murthy et al. (2017)
Panax ginseng	Saponins	Air lift bioreactor	Yoshikawa and Furuya (1987)
Panax ginseng	Ginsenosides	Wave bioreactor	Palazon et al. (2003)
Polygonum multiflorum Thunb	Anthraquinones, stilbenes, flavonoids, tannins,	Air lift bioreactor	Lee et al. (2015a, b)
Stizolobium hassjoo	Levodopa	Mesh hindrance mist trickling bioreactor	Sung and Huang (2006)
Trigonella foenumgraceum	Diosgenin	Air lift bioreactor	Rodriguez-Mendiola et al. (1991)

10.4 Advances in Metabolic Engineering of Hairy Roots

A new promising technology known as metabolic engineering or genetic engineering was evolved in the early 1990s (Bourgaud et al. 2001). Metabolic engineering in plants involves the alteration of metabolic pathways to increase the flux toward desired secondary metabolites or to attain better understanding of metabolic pathways and use of cellular pathways for chemical transformation, energy transduction, and supramolecular assembly (Chandra and Chandra 2011; Hussain et al. 2012). In other words, metabolic engineering is the alteration or improvement of the cellular activities involving transport and enzymatic and regulatory functions of the cell by using rDNA technology (Bourgaud et al. 2001; Hussain et al. 2012). It is one of the fastest-growing applications for the production of industrially important bio-active compounds from various plant sources. The main aims of this technique are (1) overproduction of a desired compound which is normally produced in less quantity or increased metabolite production by transferring the pathways to another plant or microorganisms, (2) reducing the production of unwanted compounds, and (3) production of a new compound that is usually produced in nature but not present in the host plant (Verpoorte and Memelink 2002; Capell and Christou 2004; Chandra and Chandra 2011). This can be achieved by conquering the rate-limiting steps or by jamming competitive pathways and blocking of catabolism successfully.

Now, multistep metabolic engineering is possible, which overtakes single-step engineering, and it is the best way to produce secondary metabolites in transgenic plants (Capell and Christou 2004). The main advantage of this method is that it is convenient and cost-effectively produces industrially important secondary metabolites continuously (Hussain et al. 2012). Also, this technique is used as a tool for improving crop plants that are resistant to various diseases, plants producing allelopathic compounds to control the weeds, pest-resistant plants to improve the importance of ornamentals and fruits, and enhanced pollination by modifying scent profiles (Chandra and Chandra 2011). Another advantage is the production of valuable secondary metabolites under controlled environment which is free from climate and soil conditions (Hussain et al. 2012). Engineering or structural design of secondary metabolite pathways is quite difficult in plants, because it requires a detailed knowledge of the whole biosynthetic pathways and a detailed perception of its regulatory mechanisms. But, such information is not explored in many medicinal plants known to have vast variety of bio-active metabolites (Oksman-Caldentey and Inze 2004). Recent advances in metabolic engineering have open a new way for the production of secondary metabolites in higher quantities. However, the success of this approach depends on the metabolic pathway elucidation and metabolite pathway mapping and identifying specific restraining enzyme activities. This process can be further improved by using an appropriate genetic transformation procedure. So far, most of the biosynthetic pathway strategies developed for producing secondary metabolites were through various ways which include isolating and expressing of the respective genes in more efficient organisms, construction of promoters to enhance the expression of a target gene, or antisense and co-suppression techniques for knockdown of particular plants for the desired traits (Bourgaud et al. 2001). For example, engineering of the flavonoid pathway in Saussurea involucrata by a transgenic approach increased the production of apigenin. The gene responsible for apigenin production in S. medusa was found to be chalcone isomerase (chi) gene. A complete cDNA sequence of chi gene construct was prepared under the control of the cauliflower mosaic virus (CaMV) 35S promoter. The chi gene was introduced into the S. involucrata genome by A. rhizogenes-mediated transformation which resulted in the establishment of transgenic hairy root lines. The enzyme chalcone isomerase converts naringenin chalcone into naringenin, which is the precursor of apigenin. After 5 weeks of incubation, C46 hairy root line accumulated 32.1 mg/l of apigenin with total flavonoids at 647.8 mg/l. The accumulation of apigenin and flavonoid content was found to be 12 and 4 times, respectively, which is superior when compared to the wild-type hairy roots. The enhanced enzyme productivity was obtained due to the superior activity of chalcone isomerase (Li et al. 2006). In addition to that, hairy root metabolic engineering has been widely used to enhance the production of pharmaceutically important secondary metabolites and also the production of certain recombinant proteins. For example, solasodine glycoside harmfully controls its own biosynthesis. A recombinant gene construct, i.e., anti-solamargine (As)-scFv gene, contains single-chain fragment variable (scFv) antibody region derived from hybridoma cell lines. Transformed hariy root cultures with anti-solamargine (As)-scFv gene controls and enhances the solasodine glycoside concentration up to 2.3-fold more in the transgenic S. khasianum than wild-type hairy roots (Putalun et al. 2003). Metabolic engineering of the hairy roots is also used to make the de novo synthesis of secondary metabolites by introducing the specific genes that encode related enzymatic process in other organisms. The transfer of three genes from Ralstonia eutropha bacterium into the genome of sugar beet hairy roots directed the accumulation of poly(3-hydroxybutyrate) (Menzel et al. 2003). Recently, Hidalgo et al. (2017) reported the metabolism of tobacco hairy root for the production of stilbenes. In this study, in order to achieve the holistic response in the phenylpropanoid metabolic pathway and also direct the upregulation of multiple metabolic process, transformed tobacco hairy root (HR) cultures carrying the gene stilbene synthase (STS) derived from Vitis vinifera and Arabidopsis thaliana transcription factor (TF) AtMYB12 were established. In addition to that, the normal flux was arrested through the incorporation of an artificial microRNA responsible for chalcone synthase (amiRNA CHS); otherwise there will be a heavy competition with STS enzyme for precursors. The transgenic tobacco hairy roots were capable to synthesize the target compound, stilbenes.

10.5 Enhancement of Secondary Metabolites Through Elicitation

Elicitation is an efficient and promising method for increasing the production of secondary metabolites using an elicitor which is a substance that when introduced into a living cell system in ideal/little concentrations improves the biosynthesis of secondary metabolites. The mechanism involved in this process is that the addition of elicitors (both biotic and abiotic) into the plant system attacks the plant cell wall and triggers the production of plant-defensive secondary metabolites (Namdeo 2007; Bensaddek et al. 2008).

In general, the plant cells recognize the elicitor compounds through various signaling molecules and interact or bind with specific receptors present on the plasma membrane. These interactions later generate signals and activate genes that are responsible for the defense reactions including systemic acquired responses (SAR) and induced systemic resistance (ISR). This stimulates the biosynthesis of pathogenesis-related (PR) proteins or defense secondary metabolites, and these finally lead to the production of secondary metabolites (Zhao et al. 2005). The mechanism involved in the production of secondary metabolites through elicitors was showed in Fig. 10.2. Elicitors are broadly divided into two types, viz., biotic and abiotic; mostly abiotic elicitors are inorganic salts (minerals) and physical and chemical factors such as pH, temperature, UV light, heavy metal salts (Cu and Cd ions), etc., while biotic elicitors are polysaccharides derived from plant cell wall and microorganisms (pectin, cellulose, chitin, and glucans), glycoproteins (G-protein or intracellular proteins), pathogenic fungi and bacteria, plant hormones (methyl jasmonate and salicylic acid), etc. (Donenburg and Knorr 1995; Bourgaud et al. 2001; Namdeo 2007; Ramirez-Estrada et al. 2016). In addition to that, new types of elicitors have been recently introduced and successfully used in few plant cell cultures. These new elicitors include voliticin, caeliferins, and inceptins. These compounds are derived from plants and insects (which are mostly found in oral secretions of insects). Recently, it was found that they act as an elicitor by activating jasmonates and lead to the production of secondary metabolites, mainly the volatile compounds (Ramirez-Estrada et al. 2016). However, improved production of the metabolites from plant cell cultures through elicitation depends on several parameters, such as selection of suitable elicitor, concentration of elicitor, duration of elicitor treatment, age of the explants, cell line, nutrient composition of the media, growth regulation, etc. (Namdeo 2007). Elicitation method for the plant cell culture system has shown a positive result in secondary metabolite production. However, the study about how plant cells or tissues and their metabolic pathways respond to both abiotic and biotic elicitors is a key route to design the new strategies to enhance the industrially important bio-active compounds in a large scale. For example, a few important bio-active compounds produced through elicitation with biotic and abiotic elicitors are Taxol (Veersham et al. 1995), phytoalexins (Kuroyanagi et al. 1998), saponins (Wu and Lin 2002), tropane alkaloids (Lee et al. 1998), etc. Different types of elicitors used for the production of valuable metabolites are listed in Table 10.3. For example, Largia et al. (2016) reported that the transformed hairy roots plants of Bacopa monnieri elicited with 10 mg/L chitosan for 2 weeks enhanced the accumulation of bacoside A (5.83%) content, which is a five- and fourfold increase when compared to wild plants and unelicited transformed plants. Similarly, Shilpha et al. (2016) reported that Solanum trilobatum hairy roots (ST-09 clone) elicited for 2 weeks with 4 μM for methyl jasmonate enhanced the solasodine content, which is 1.9- and 6.5-fold higher than unelicited hairy roots and wild roots.

Fig. 10.2

The mechanism of elicitors in secondary metabolite production

Table 10.3

Production of plant secondary metabolites by using different elicitors

Plant species	Secondary metabolite	Elicitors	References
Ammi majus	Coumarine, furocoumarine	BION® Enterobacter sakazakii	Staniszewska et al. (2003)
Arachis hypogaea	Trans-resveratrol	Sodium acetate	Medina-Bolivar et al. (2007)
Arachis hypogaea	Resveratrol, piceatannol, arachidin-1, and arachidin-3	MeJA and cyclodextrn	Yang et al. (2015)
Astragalus membranaceus	Calycosin and formononetin	Aspergillus niger	Jiao et al. (2017)
Artemisia annua	Artemisinin	Chitosan	Putalun et al. (2007)
Azadirachta indica	Azadirachtin	Salicylic acid, jasmonic acid	Satdive et al. (2007)
Catharanthus roseus	Alkaloids (indole)	Penicillium sp.	Rijhwani and Shanks (1998)
Centella asiatica	Asiaticoside	Methyl jasmonate	Kim et al. (2007)
Datura metel	Atropine	AgNO3, nanosilver, Bacillus cereus, Staphylococcus aureus	Shakeran et al. (2015)
Hyoscyamus muticus	Sesquiterpenes	Rhizoctonia solani	Singh (1995)
Hyoscyamus niger	Polyamines and tropane alkaloids	Methyl jasmonate	Zhang et al. (2007)
Linum album	Lignan	Coniferaldehyde and methylenedioxycinnamic acid	Ahmadian Chashmi et al. (2016)
Oxalis tuberose	Harmaline, harmine	Phytophthora cinnamomi	Bais et al. (2003)
Lotus corniculatus	Isoflavonoids	Glutathione	Robbins et al. (1991)
Papaver orientale	Morphinan alkaloids	MeJA and salicylic acid	Hashemi and Naghavi (2016)
Panax ginseng	Ginseng saponin	Selenium, NiSO4, NaCl	Jeong and Park (2006)
Pharbitis nil	Umbelliferone, scopoletin, skimmin	CuSO4, MeJA	Yaoya et al. (2004)
Salvia miltiorrhiza	Tanshinone	Sorbitol	Shi et al. (2006)
Scopolia parviflora	Scopolamine	Pseudomonas aeruginosa, Bacillus cereus, Staphylococcus aureus	Jung et al. (2003a, b)
Solanum tuberosum	Sesquiterpene, lypooxygenase	Rhizoctonia bataticola, B cyclodextrin, MeJA	Komaraiah et al. (2003)
Tagetes patula	Thiophene	Furasium conglutanis, Aspergillus niger	Mukundan and Hjortso (1990) and Buitelaar et al. (1993)

10.6 Biotransformation

Biotransformation is the process in which a substance is transformed from one chemical to another, and it is catalyzed by the effective enzyme structures of biological systems. Plant cell or organ cultures have the capability to convert exogenously added organic compounds into functional analogs (Banerjee et al. 2012; Roychowdhury et al. 2013). This type of protocols has been done by using plant cell/organ cultures which have generated the libraries of analog compounds with limited structural modifications, and it also ensures the sustainable use of the resource under defined culture conditions free from seasonal variations and pathological constraints. The resulted compounds will have the important characteristic potency of a parent molecule and can also attain a superior selectivity, safety, and physicochemical properties with lower toxicity. This can be more appropriate to be used for newer therapeutic applications. The biotransformation method is very useful for the discovery of novel phytochemicals having therapeutic and commercial advantages. Also, this method is attaining more attention toward the green chemistry, because of the reduced usage of hazardous chemicals in the process of chemical modifications. The major reactions involved in biotransformation methods include oxidation, reduction, glycosylation, esterification, methylation, isomerization, and hydroxylation. Hairy root cultures have various advantages as biocatalysts over cell suspension cultures, because of their genetic and biochemical stability, multi-enzyme biosynthetic potential comparable to the parent plant, and cost-effectiveness. Therefore, hairy root cultures also act as an experimental model system in biotransformation studies (Giri et al. 2001; Banerjee et al. 2012). Biotransformation studies were reported in Ri-transformed root cultures of several plant species for producing valuable secondary metabolites and are briefly described by Banerjee et al. (2012). For example, the biotransformation ability of Atropa belladonna hairy root cultures has been explored by using three carbonyl substrates such as 3,4,5-trimethoxybenzaldehyde, 3,4,5-trimethoxy-acetophenone, and 3,4,5-trimethoxy-benzoic acid. Among the three substrates used, 3,4,5-trimethoxybenzaldehyde and 3,4,5-trimethoxy-acetophenone were biotransformed, but, 3,4,5-trimethoxy-benzoic was not biotransformed. The 3,4,5-trimethoxybenzaldehyde was biotransformed by oxidation and reduction of substrate into 3,4,5-trimethoxy-benzoic acid and 3,4,5-trimethoxy benzyl alcohol, respectively (Srivastava et al. 2012). Overall, the biotransformation using hairy root cultures has got potential to generate new products or to generate already known products very efficiently. The list of reactions involved in biotransformation of hairy roots for metabolites production are shown in Table 10.4.

Table 10.4

Biotransformation of hairy roots for plant secondary metabolite production

Plant species	Types of reaction	Product	References
Anethum graveolens	Acetylation, reduction	Menthyl acetate linalool, α -terpineol, citronellol	Faria et al. (2009)
Anisodus tanguticus	Oxidation	Androst-4-ene-3,17-dione 6 α-hydroxy androst-4-ene-3	Liu et al. (2004)
Astragalus membranaceus	Deglycosylation	Calycosin Formononetin	Jiao et al. (2017)
Atropa belladonna	Reduction	Scopolamine	Subroto et al. (1996)
Brassica napus	Reduction, glycosylation	6-(1(S)-hydroxyethyl)-2,2-dimethyl-2,3-dihydro-4H-chromen-4-one	Orden et al. (2006)
Brugmansia candida	Glucosylation	4-Hydroxyphenyl β-D-glucopyranoside (arbutin)	Casas et al. (1998)
Coleus furskohlii	Glycosylation	Methyl β-D-glucopyranosides, methyl β-D-ribo-hex-3-ulopyranosides	Li et al. (2003)
Cyanotis arachnoidea	Reduction	Deoxyartemisinin	Zhou et al. (1998) and Ligang et al. (1998)
Daucus carota	Reduction	(S)-1-phenyl ethanol)	Caron et al. (2005)
Lobelia sessilifolia	Glucosylation	Protocatechuic acid 3-O-β-D-glucopyranoside	Ishimaru et al. (1996)
Lobelia sessilifolia	Glucosylation	(+)-catechin 7-O-β-D-glucopyranoside	Yamanaka et al. (1995)
		Protocatechuic acid, protocatechuic acid 3-O-β-D-glucopyranoside
		(–)-epicatechin 7-O-β-D-glucopyranoside
		(–)-epiafzelechin 7-O-β-D-glucopyranoside
Levisticum officinale	Isomerization	Linalool, nerol	Nunes et al. (2009)
Panax ginseng	Esterification	Digitoxigenin stearate	Kawaguchi et al. (1990)
		Digitoxigenin palmitate
		Digitoxigenin myristate
		Digitoxigenin laurate
Panax ginseng	Glycosylation	(RS)-2-phenylpropionyl β-D-glucopyranoside	Yoshikawa et al. (1993)
		(2RS)-2-0-(2-phenylpropionyl) D-glucose
		(2RS)-2-phenylpropionyl) 6-0-β-D-xylopyranosyl β-D-glycopyranoside
		Myoinositol ester of (R)-2-phenylpropionic acid
Panax ginseng	Glycosylation	30-O-[β-D-glucopyranosyl (1→2) β-D-glucopyranosyl]	Asada et al. (1993)
		18 β-Glycyrrhetinic acid
		30-O-[β-D-glucopyranosyl] 18 β-glycyrrhetinic acid
		3-O-[β-D-glucopyranosyl -(1→2) β-D- glucopyranosyl] 18 β-glycyrrhetinic acid
		3-0-[β-D-glucopyranosyl-(1→2)-β-D-glucopyranosyl] -30-0-(β-D-glucopyranosyl) 18 β-glycyrrhetinic acid
Panax ginseng	Glycosylation	p-carboxyphenyl β-D-glucopyranoside	Chen et al. (2008)
		p-hydroxybenzoic acid
		β-D-glucopyranosyl ester
		m-carboxyphenyl β-D-glucopyranoside
Pharbatis nil	Glucosylation	Skimmin	Kanho et al. (2004, 2005)
		4-Methylskimmin
		Scopoline
		3,4,8-Tri methylskimmin
		Scopolin, aesculin, eichoriin, vanillin-4-O-β-glucopyranoside
		Vanillyl alcohol-4-O-β-D-glucopyranoside
Physalis ixocarpa	Glucosylation	Arbutin	Bergier et al. (2008)
Plantago lanceolata	Glucosylation	(E)-p-coumaroyl-1-O-β-D-glucopyranoside	Fons et al. (1999)
Polygonum multiflorum	Glycosylation	3-oxo-eremophila 1,7(11)-dien-12,8-olide	Yan et al. (2008)
		3-oxo-8-hydroxy-eremophila 1,7(11)-dien-12,8-olide
Polygonum multiflorum	Glucosylation	4-Hydroxybenzene derivatives: 1-4-benzendiol	Yan et al. (2007)
		4-Hydroxybenzaldehyde
		4-Hydroxybenzyl alcohol
		4-Hydroxybenzoic acid
Polygonum multiflorum	Glucosylation	5-Methyl-2-(1-methylethyl) phenyl-β-D-glucopyranoside	Dong et al. (2009)

10.7 Hairy Root Applications in Environmental Protection (Phytoremediation)

Environmental pollution is a universal problem that adversely affects both the developed and developing countries. The major reason for environmental pollution is due to human activities and natural hazards. Contaminants are usually classified into two types: organic and inorganic. Due to the human activities including oil spills, agriculture wastage, military explosives, fuel production, and wood treatment, organic contaminants are released into the environment. Some of important organic pollutants such as trichloroethylene (TCE), atrazine, trinitrotoluene, polycyclic aromatic hydrocarbons, benzene, toluene, polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons, and methyl tert-butyl ether contaminating the soil and water are a challenge to the world. Generally, inorganic contaminants are originated from either human activities or natural processes. The most dangerous inorganic contaminants include heavy metals such as copper, zinc, manganese, lead, molybdenum, mercury, and nickel which are released into the environment by natural and human activities causing a health threat to humans and livestock (Suza et al. 2008). The removal of these contaminants from the environment is not an easy task, and decontamination is a very expensive process. Phytoremediation, as an emerging alternative technology, is highly appreciated in recent times for its effectiveness in cleaning up of the contaminated environment. Phytoremediation is defined as the ability of plants to uptake contaminants from the polluted environment (soil, water, or air) and convert the toxic chemical molecules to harmless forms enzymatically (Roychowdhury et al. 2013; Guillon et al. 2006). The key advantage of phytoremediation technique is that it is about ten times less expensive than conventional environmental cleanup methods, and it is a safe method. Generally, plants act as natural soil stabilizers, reduce the amount of contaminants, and maintain the surroundings free from pollutants. Phytoremediation is better than bioremediation methods that uses microbes in terms of easy monitoring. This is because, in phytoremediation, the plants’ condition is visible, and the presence of pollutants in plant tissues can be easily tested (Doty 2008). The major phytoremediation strategies involved in the removal of contaminants include phytoextraction, phytostabilization, and rhizofiltration of organic and inorganic pollutants (Gonzalez et al. 2006). In this regard, hairy root technology also plays an important role in the process of phytoremediation. Some of the advantages offered by hairy roots for this purpose include fast growth and high branching of hairy roots allowing increase absorption of contaminants, high biochemical and genetic stability, easy maintenance, scaling-up in bioreactors being easy, and provision of a huge surface area of contact with the contaminants. Moreover, hairy roots contain essential enzymes and metal chelating agents to detoxify the harmful compounds (Gonzalez et al. 2006; Roychowdhury et al. 2013). In recent years, hairy roots are serving as a potential tool to decontaminate the environment and are being highly appreciated by environmental biologists for its effectiveness. A wide variety of environmental pollutants that can be removed by hairy roots derived from different plant species are shown in Table 10.5. However, it is required to completely understand the enzymatic machineries involved in the bioconversion of toxic contaminants to nontoxic complexes and also the mechanisms involved in the hyperaccumulation and metal tolerance (Roychowdhury et al. 2013). In the future, the application of genetic engineering to insert specific detoxifying genes in hairy roots enhances their capacity to effectively clean up the contaminant.

Table 10.5

Phytoremediation of environmental pollutants by hairy root cultures

Plant species	Pollutant	Reference
Solanum nigrum	PCBs (polychlorinated biphenyls) and zinc	Macková et al. (1997a, b) and Subroto et al. (2007)
Thlaspi caerulescens	Cadmium	Nedelkoska and Doran (2000) and Boominathan and Doran (2003)
Alyssum sp.	Nickel	Nedelkoska and Doran (2001) and Suresh et al. (2005)
A. bertolinii, A. tenium, and A. troodi
Catharanthus roseus	RDX (hexahydro-1,3-5-trinitro-1,3-5-triazine) and HMX (oxtahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine)	Bhadra et al. (2001)
Daucus carota	Phenol and chloroderivatives	De Araujo et al. (2002)
A. bertolonii and Thlaspi caerulescens	Nickel, and cadmium	Boominathan and Doran (2002)
Atropa belladonna	TCE (trichloroethylene)	Banerjee et al. (2002)
Brassica napus	2,4-Dichlorophenol, Phenol	Agostini et al. (2003) and Coniglio et al. (2008)
B. juncea and Chenopodium amaranticolor	Uranium	Eapen et al. (2003)
B. juncea and Cichorium intybus	DDT (Dichloro-diphenyl-trichloroethane)	Suresh et al. (2005)
Helianthus annuus	Tetracycline and oxytetracycline	Gujarathi et al. (2005)
Lycopersicon esculentum	Phenols	Wevar-Oller et al. (2005)
Daucus carota, Ipomoea batata, and Solanum aviculare	Guaiacol, catechol, phenol, 2-chlorophenol, and 2,6-dichlorophenol	De Araujo et al. (2004, 2006)
Brassica juncea	Phenol	Singh et al. (2006)
Lycopersicon esculentum	Phenol	Wevar-Oller et al. (2005) and González et al. (2006)
Alyssum murale	Nickel	Vinterhalter et al. (2008)
Solanum lycopersicon	Phenol	Wevar-Oller et al. (2005) and González et al. (2006)
Nicotiana tabacum	Phenol, 2,4-DCP	Alderete et al. (2009) and Talano et al. (2010)
Armoracia rusticana	Uranium	Soudek et al. (2011)

10.8 Germplasm Conservation

Germplasm conservation is one of the prominent techniques to preserve/restore the plant biodiversity, because most of the plants do not produce viable seeds and propagate vegetatively, while some plants produce recalcitrant seeds, and the storage of seeds is affected by pests or other pathogens. So, the conservation of wild, rare, and endangered medicinal plant species for future use has become a big problem, and more efforts are initiated in this direction. Biotechnological tools such as plant tissue culture micropropagation and cryopreservation have certainly benefited in protecting plant germplasms including vegetatively propagated plant species, genetic resources of recalcitrant seeds, rare and endangered plant species, cell lines with special attributes, genetically transformed plant material, and clones obtained from elite genotypes (Engelmann 2011). Based on the storage duration, in vitro conservation methods are classified into three types, namely, short-, medium-, and long-term storage. Among them, cryopreservation is the most efficient technique for long-term conservation of the germplasm of a valuable plant, because of its cost-effectiveness and safety. Three types of cryopreservation methods are highly employed for the biodiversity conservation. They include freeze-induced dehydration, encapsulation-dehydration, and encapsulation-vitrification (Shibli et al. 2006). Hairy root cultures can be used for the germplasm conservation, because hairy root cultures are significantly a good resource for the production of several secondary metabolites and, in recent times, they are obtained in many medicinal plants for commercial applications. Hence, conserving such hairy roots will be more useful for future applications. However, there are only very few reports available on the conservation of hairy roots of medicinal plants. Hairy roots in the form of artificial seeds are a reliable delivery system for the clonal propagation of elite plants with genetic uniformity, high yield, and low production cost. Cryopreservation method for root tips was first developed by Benson and Hamill (1991) from hairy root cultures of Beta vulgaris, and the same technique was implemented in Nicotiana rustica. Yoshimatsu et al. (1996) reported the cryopreservation of Panax ginseng hairy roots. In addition to that, cryopreservation of hairy roots was reported in some more medicinal plants like Artemisia annua (Teoh et al. 1996), Armoracia rusticana (horseradish) (Phunchindawan et al. 1997; Hirata et al. 1998), Atropa belladonna (Touno et al. 2006), Eruca sativa, Astragalus membranaceus and Gentiana macrophylla (Xue et al. 2008), Maesa lanceolata and Medicago truncatula (Lambert et al. 2009), and Rubia akane (nakai) (Kim et al. 2010, 2012; Salma et al. 2014).

10.9 Omics Approaches in Secondary Metabolite Production

The omics approaches, namely, genomics, transcriptomics, proteomics, and metabolomics, have been majorly utilized in hairy root-based secondary metabolite production. As transcriptomic tools the microarrays and expressed sequence tags (EST) were useful in measuring the gene expression studies in large scale. Expression of target genes in a plant cell can be modified through various methods such as precursor feeding, elicitor treatment, overexpression or silencing of transgenes, etc. Generation of cDNA microarrays and EST database provides the information about the changes at mRNA level and also briefs the functions of genes and its regulation in secondary metabolism of hairy root cultures. Transcriptome analysis of hairy root cultures has been done in several plants including P. ginseng (ginsenoside), C. roseus (indole alkaloids), Medicago truncatula (anthocyanin), S. miltiorrhiza (tanshinones), etc. (Jung et al. 2003a, b; Murataa et al. 2006; Pang et al. 2008; Gao et al. 2009; Wang et al. 2010). In studying the tanshinone biosynthesis, S. miltiorrhiza hairy root cultures were used as a model system. The combined analysis of metabolite profiling and cDNA-AFLP identified the candidate genes which are potentially involved in the biosynthetic pathway (Yang et al. 2012). Proteomics is an important, powerful, and under-explored omics technology for the secondary metabolite elucidation in hairy root cultures. Proteomic approach for hairy root cultures has been initiated in P. ginseng and opium poppy (Kim et al. 2003; Zulak et al. 2009). Metabolomics is an emerging approach which is highly useful in secondary metabolite production (Yang et al. 2012). The systems biology approaches with a combination of omics approaches will offer a great opportunity for high-throughput secondary metabolite elucidation in various plant species.

10.10 Conclusions and Future Prospects

In the modern era, humankind is facing the problem of high demand for several potent plant secondary metabolites possessing many bio-pharmacological activities. Previously, in vitro dedifferentiated plant tissue cultures were used for obtaining plant metabolites. As the years passed, cell suspension and adventitious root cultures were widely adopted for the same. However, to elucidate such metabolites, there is a need to develop an efficient and reliable, fast-growing in vitro tissue culture model to overcome the problem of wild plant availability. In this regard, hairy root cultures offer a great value to the continuous production of several precious secondary metabolites, because of their unique characteristics discussed above. Since the emergence of hairy root technology, a lot of improvements have been made day by day especially the use of bioreactors, application of elicitation strategy, and biotransformations. Overall, hairy root technology has shown its wide utility in many medicinal plants. Moreover, the production of plant secondary metabolites in the hairy root culture system has delivered very encouraging findings, for example, illuminating the sites of biosynthesis or rate-regulating stages, precursor’s requirements, role of regulatory genes, transcription factors, and putative metabolite intermediates relating to secondary metabolite biosynthesis. Also, it offers the possibility of recognizing a suitable gene candidate required for metabolic engineering of specific plant traits and to improve their secondary metabolite secretion. However, more efforts are to be encouraged to better understand the biosynthetic pathways and regulatory cascades involved in secondary metabolite synthesis. Therefore, it is crucial to make use of genetic engineering approaches in order to fully realize the biosynthetic prospective of hairy roots. Plant biotechnologists are required to work closely with bioengineers to overcome the challenges faced during the scaling-up of hairy root cultures in bioreactors. In the future, research efforts should be encouraged toward making use of hairy root culture technology for producing high-value secondary metabolites commercially from many unexplored medicinal plant species.

Acknowledgments

The author A. Sagina Rency gratefully acknowledges the Department of Science and Technology for financial support in the form of DST INSPIRE Fellowship (DST/INSPIRE Fellowship/03/2014/004363). Also the authors sincerely thank the Bioinformatics Infrastructure Facility of Department of Biotechnology, Alagappa University (funded by Department of Biotechnology, Government of India: Grant No. BT/BI/25/015/2012), for providing the computational facility.

References

1. Agostini E, Coniglio MS, Milrad S, Tigier H, Giulietti A (2003) Phytoremediation of 2,4-dichlorophenol by Brassica napus hairy roots cultures. Biotechnol Appl Biochem 37:139–144PubMed
2. Ahmadian Chashmi N, Sharifi M, Behmanesh M (2016) Lignan enhancement in hairy root cultures of Linum album using coniferaldehyde and methylenedioxycinnamic acid. Prep Biochem Biotechnol 46:454–460PubMed
3. Alderete LGS, Talano MA, Ibáñez SG, Purro S, Agostini E, Milrad SR, Medina MI (2009) Establishment of transgenic tobacco hairy roots expressing basic peroxidases and its application for phenol removal. J Biotechnol 139:273–279
4. Asada Y, Saito H, Yoshikawa T, Sakamoto K, Furuya T (1993) Biotransformation of 18β-glycyrrhetinic acid by ginseng hairy root culture. Phytochemistry 34:1049–1052PubMed
5. Ayadi R, Tremouillaux-Guiller J (2003) Root formation from transgenic calli of Ginkgo biloba. Tree Physiol 23:713–718PubMed
6. Bais HP, Vepachedu R, Vivanco JM (2003) Root specific elicitation and exudation of fluorescent β-carbolines in transformed root cultures of Oxalis tuberosa. Plant Physiol Biochem 41:345–353
7. Banerjee S, Rahman L, Uniyal GC, Ahuja PS (1998) Enhanced production of valepotriates by Agrobacterium rhizogenes induced hairy root cultures of Valeriana wallichii DC. Plant Sci 131:203–208
8. Banerjee S, Shang TQ, Wilson AM, Moore AL, Strand SE, Gordon MP, Doty SL (2002) Expression of functional mammalian P450 2E1 in hairy root cultures. Biotechnol Bioeng 77:462–466PubMed
9. Banerjee S, Singh S, Rahman LU (2012) Biotransformation studies using hairy root cultures – a review. Biotechnol Adv 30:461–468PubMed
10. Bathoju G, Rao K, Giri A (2017) Production of sapogenins (stigmasterol and hecogenin) from genetically transformed hairy root cultures of Chlorophytum borivilianum (Safed musli). Plant Cell Tissue Organ Cult 131:369–376
11. Bensaddek L, Villarreal ML, Fliniaux MA (2008) Induction and growth of hairy roots for the production of medicinal compounds. Electr J Integr Biosci 3:2–9
12. Benson EE, Hamill JD (1991) Cryopreservation and post freeze molecular and biosynthetic stability in transformed roots of Beta vulgaris and Nicotiana rustica. Plant Cell Tissue Organ Cult 24:163–172
13. Bergier K, Polaszczyk B, Gajewska E, Wielanek M, Krolicka A, Sklodowska M (2008) Glucosylation of hydroquinone to arbutin by hairy roots of Physalis ixocarpa. Zesz Probl Postępów Nauk Rol 2008:524
14. Bhadra R, Wayment DG, Williams RK, Barman SN, Stone MB, Hughes JB, Shanks JV (2001) Studies on plant-mediated fate of the explosives RDX and HMX. Chemosphere 44:1259–1264PubMed
15. Bharati AJ, Bansal YK (2014) In vitro production of flavonoids: a review. World J Pharm Pharm Sci 3:508–533
16. Boominathan R, Doran PM (2002) Ni-induced oxidative stress in roots of the Ni hyperaccumulator, Alyssum bertolonii. New Phytol 156:205–215
17. Boominathan R, Doran PM (2003) Cadmium tolerance and antioxidative defenses in hairy roots of the cadmium hyperaccumulator, Thlaspi caerulescens. Biotechnol Bioeng 83:158–167PubMed
18. Bourgaud F, Gravot A, Milesi S, Gontier E (2001) Production of plant secondary metabolites: a historical perspective. Plant Sci 161:839–851
19. Buitelaar RM, Leenen EJTM, Geurtsen G, Tramper J (1993) Effects of the addition of XAD-7 and of elicitor treatment on growth, thiophene production, and excretion by hairy roots of Tagetes patula. Enzyme Microb Technol 15:670–676
20. Capell T, Christou P (2004) Progress in plant metabolic engineering. Curr Opin Biotechnol 15:148–154PubMed
21. Caron D, Coughlan AP, Simard M, Bernier J, Piché Y, Chênevert R (2005) Stereo selective reduction of ketones by Daucus carota hairy root cultures. Biotechnol Lett 27:713–716PubMed
22. Casas DA, Pitta-Alvarez SI, Giulietti AM (1998) Biotransformation of hydroquinone by hairy roots of Brugmansia candida and effect of sugars and free-radical scavengers. Appl Biochem Biotechnol 69:127–136PubMed
23. Caspeta L, Nieto I, Zamilpa A, Alvarez L, Quintero R, Villarreal ML (2005) Solanum chrysotrichum hairy root cultures: characterization, scale-up and production of five antifungal saponins for human use. Planta Med 71:1084–1087PubMed
24. Chandra S, Chandra R (2011) Engineering secondary metabolite production in hairy roots. Phytochem Rev 10:371–395
25. Chen X, Zhang J, Liu JH, Yu BY (2008) Biotransformation of p-, m-, and o-hydroxybenzoic acids by Panax ginseng hairy root cultures. J Mol Catal B Enzym 54:72–75
26. Coniglio MS, Busto VD, González PS, Medina MI, Milrad S, Agostini E (2008) Application of Brassica napus hairy root cultures for phenol removal from aqueous solutions. Chemosphere 72:1035–1042PubMed
27. Cui XH, Chakrabarty D, Lee EJ, Paek KY (2010) Production of adventitious roots and secondary metabolites by Hypericum perforatum L. in a bioreactor. Bioresour Technol 101:4708–4716PubMed
28. De Araujo BS, Charlwood VB, Pletsch M (2002) Tolerance and metabolism of phenol and chloroderivatives by hairy root cultures of Daucus carota L. Environ Pollut 117:329–335PubMed
29. De Araujo BS, de Oliveira JO, Machado SS, Pletsch M (2004) Comparative studies of peroxidases from hairy roots of Daucus carota, Ipomea batatas and Solanum aviculare. Plant Sci 167:1151–1157
30. De Araujo BS, Dec J, Bollag JM, Pletsch M (2006) Uptake and transformation of phenol and chlorophenols by hairy root cultures of Daucus carota, Ipomoea batatas and Solanum aviculare. Chemosphere 63:642–651PubMed
31. Dechaux C, Boitel-Conti M (2005) A strategy for over accumulation of scopolamine in Datura innoxia hairy root culture. Acta Biol Cracov Ser Bot 47:101–107
32. Donenburg H, Knorr D (1995) Strategies for the improvement of secondary metabolite production in plant cell cultures. Enzym Microb Technol 17:674–684
33. Dong QF, Jia JZ, Zhu JH, Yu RM (2009) Biotransformation of thymol by hairy roots of transgenic Polygonum multiflorum. J Chin Med Mat 32:1495–1499
34. Doty SL (2008) Enhancing phytoremediation through the use of transgenics and endophytes. New Phytol 179:318–333PubMed
35. Du M, Wu XJ, Ding J, Hu ZB, White KN, Branford-White CJ (2003) Astragaloside IV and polysaccharide production by hairy roots of Astragalus membranaceus in bioreactors. Biotechnol Lett 25:1853–1856PubMed
36. Eapen S, Suseelan KN, Tivarekar S, Kotwal SA, Mitra R (2003) Potential for rhizofiltration of uranium using hairy root cultures of Brassica juncea and Chenopodium amaranticolor. Environ Res 91:127–133PubMed
37. Ekor M (2014) The growing use of herbal medicines: issues relating to adverse reactions and challenges in monitoring safety. Front Pharmacol 4:177PubMedPubMedCentral
38. Engelmann F (2011) Use of biotechnologies for the conservation of plant biodiversity. In Vitro Cell Dev Biol Plant 47:5–16
39. Faria JM, Nunes IS, Figueiredo AC, Pedro LG, Trindade H, Barroso JG (2009) Biotransformation of menthol and geraniol by hairy root cultures of Anethum graveolens: effect on growth and volatile components. Biotechnol Lett 31:897–903PubMed
40. Flores HE, Curtis WR (1992) Approaches to understanding and manipulating the biosynthetic potential of plant roots. Ann N Y Acad Sci 665:188–209PubMed
41. Fons F, Tousch D, Rapior S, Gueiffier A, Roussel JL, Gargadennec A, Andary C (1999) Phenolic profiles of untransformed and hairy root cultures of Plantago lanceolata. Plant Physiol Biochem 37:291–296
42. Gabr AM, Mabrok HB, Ghanem KZ, Blaut M, Smetanska I (2016) Lignan accumulation in callus and Agrobacterium rhizogenes-mediated hairy root cultures of flax (Linum usitatissimum). Plant Cell Tissue Organ Cult 126:255–267
43. Gai QY, Jiao J, Luo M, Wei ZF, Zu YG, Ma W, Fu YJ (2015) Establishment of hairy root cultures by Agrobacterium Rhizogenes mediated transformation of Isatis Tinctoria L. for the efficient production of flavonoids and evaluation of antioxidant activities. PLoS One 10:e0119022PubMedPubMedCentral
44. Gao W, Hillwig ML, Huang L, Cui G, Wang X, Kong J, Yang B, Peters RJ (2009) A functional genomics approach to tanshinone biosynthesis provides stereochemical insights. Org Lett 11:5170–5173PubMedPubMedCentral
45. Giri A, Narasu ML (2000) Transgenic hairy roots: recent trends and applications. Biotechnol Adv 18:1–22
46. Giri A, Dhingra V, Giri CC, Singh A, Ward OP, Narasu ML (2001) Biotransformations using plant cells, organ cultures and enzyme systems: current trends and future prospects. Biotechnol Adv 19:175–199PubMed
47. Gonzalez PS, Capozucca CE, Tigierm HA, Milrad SR, Agostini E (2006) Phytoremediation of phenol from wastewater, by peroxidases of tomato hairy root cultures. Enzym Microb Technol 39:647–653
48. Guillon S, Tremouillaux-Guiller J, Pati PK, Rideau M, Gantet P (2006) Hairy root research: recent scenario and exciting prospects. Curr Opin Plant Biol 9:341–346PubMed
49. Gujarathi NP, Haney BJ, Park HJ, Wickramasinghe SR, Linden JC (2005) Hairy roots of Helianthus annuus: a model system to study phytoremediation of tetracycline and oxytetracycline. Biotechnol Prog 21:775–780PubMed
50. Hashemi SM, Naghavi MR (2016) Production and gene expression of morphinan alkaloids in hairy root culture of Papaver orientale (L). using abiotic elicitors. Plant Cell Tissue Organ Cult 125:31–41
51. Hidalgo D, Georgiev M, Marchev A, Bru-Martínez R, Cusido RM, Corchete P, Palazon J (2017) Tailoring tobacco hairy root metabolism for the production of stilbenes. Sci Rep 7:17976PubMedPubMedCentral
52. Hilton MG, Rhodes MJC (1990) Growth and hyoscyamine production of ‘hairy root’cultures of Datura stramonium in a modified stirred tank reactor. Appl Microb Biotechnol 33:132–138
53. Hirata K, Goda S, Phunchindawan M, Du D, Ishio M, Sakai A, Miyamoto K (1998) Cryopreservation of horseradish hairy root cultures by encapsulation-dehydration. J Ferment Bioeng 86:418–420
54. Ho KC, Teow YH, Ang WL, Mohammad AW (2017) An overview of electrically-enhanced membrane bioreactor (embr) for fouling suppression. Int J Eng Sci Rev 10:128–138
55. Huang Z, Mu Y, Zhou Y, Chen W, Xu K, Yu Z, Bian Y, Yang Q (1997) Transformation of Taxus brevifolia by Agrobacterium rhizogenes and taxol production in hairy root culture. Acta Bot Yunnan 19:292–296
56. Hussain S, Fareed S, Ansari S, Rahman A, Ahmad IZ, Saeed M (2012) Current approaches toward production of secondary plant metabolites. J Pharm Bioallied Sci 4:10–20PubMedPubMedCentral
57. Ishimaru K, Yamanaka M, Terahara N, Shimomura K, Okamoto D, Yoshihara K (1996) Biotransformation of phenolics by hairy root cultures of five herbal plants. Jpn J Food Chem Saf 3:38–42
58. Jaziri M, Legros M, Homes J, Vanhaelen M (1988) Tropine alkaloids production by hairy root cultures of Datura stramonium and Hyoscyamus niger. Phytochemistry 27:419–420
59. Jeong GT, Park DH (2006) Enhanced secondary metabolite biosynthesis by elicitation in transformed plant root system. Appl Biochem Biotechnol 130:436–446
60. Jiao J, Gai QY, Niu LL, Wang XQ, Guo N, Zang YP, Fu YJ (2017) Enhanced production of two bioactive Isoflavone aglycones in Astragalus membranaceus hairy root cultures by combining deglycosylation and elicitation of immobilized edible Aspergillus niger. J Agric Food Chem 65:9078–9086PubMed
61. Jung HY, Kang SM, Kang YM, Kang MJ, Yun DJ, Bahk JD, Yang JK, Choi MS (2003a) Enhanced production of scopolamine by bacterial elicitors in adventitious hairy root cultures of Scopolia parviflora. Enzym Microb Technol 33:987–990
62. Jung JD, Park HW, Hahn Y, Hur CG, In DS, Chung HJ, Liu JR, Choi DW (2003b) Discovery of genes for ginsenoside biosynthesis by analysis of ginseng expressed sequence tags. Plant Cell Rep 22:224–230PubMed
63. Kanho H, Yaoya S, Itani T, Nakane T, Kawahara N, Takase Y, Masuda K, Kuroyanagi M (2004) Glucosylation of phenolic compounds by Pharbitis nil hairy roots: I. Glucosylation of coumarin and flavone derivatives. Biosci Biotechnol Biochem 68:2032–2039PubMed
64. Kanho H, Yaoya S, Kawahara N, Nakane T, Takase Y, Masuda K, Kuroyanagi M (2005) Biotransformation of benzaldehyde-type and acetophenone-type derivatives by Pharbitis nil hairy roots. Chem Pharm Bull 53:361–365PubMed
65. Kawaguchi K, Hirotani M, Yoshikawa T, Furuya T (1990) Biotransformation of digitoxigenin by ginseng hairy root cultures. Phytochemistry 29:837–843PubMed
66. Kim Y, Wyslouzil B, Weathers P (2001) A comparative study of mist and bubble column reactors in the in vitro production of artemisinin. Plant Cell Rep 20:451–455
67. Kim Y, Wyslouzil BE, Weathers PJ (2002) Secondary metabolism of hairy root cultures in bioreactors. In Vitro Cell Dev Biol Plant 38:1–10
68. Kim SI, Kim JY, Kim EA, Kwon KH, Kim KW, Cho K, Lee JH, Nam MH, Yang DC, Yoo JS, Park YM (2003) Proteome analysis of hairy root from Panax ginseng CA Meyer using peptide fingerprinting, internal sequencing and expressed sequence tag data. Proteomics 3:2379–2392PubMed
69. Kim OT, Bang KH, Shin YS, Lee MJ, Jung SJ, Hyun DY, Kim YC, Seong NS, Cha SW, Hwang B (2007) Enhanced production of asiaticoside from hairy root cultures of Centella asiatica (L.) urban elicited by methyl jasmonate. Plant Cell Rep 26:1941–1949PubMedPubMedCentral
70. Kim H, Popova E, Yi J, Cho G, Park S, Lee S, Engelmann F (2010) Cryopreservation of hairy roots of Rubia akane (Nakai) using a droplet-vitrification procedure. Cryo Lett 31:473–484
71. Kim H, Popova E, Shin D, Bae C, Baek H, Park S, Engelmann F (2012) Development of a droplet-vitrification protocol for cryopreservation of Rubia akane (nakai) hairy roots using a systematic approach. Cryo Lett 33:506–517
72. Komaraiah P, Reddy GV, Reddy PS, Raghavendra AS, Ramakrishna SV, Reddanna P (2003) Enhanced production of antimicrobial sesquiterpenes and lipoxygenase metabolites in elicitor-treated hairy root cultures of Solanum tuberosum. Biotechnol Lett 25:593–597PubMed
73. Kumari M, Chandra S (2017) Secondary metabolite production in transformed cultures. In: Jha S (ed) Transgenesis and secondary metabolism. Springer International Publishing, Cham, pp 103–121
74. Kuroyanagi M, Arakava T, Mikami Y, Yoshida K, Kawahar N, Hayashi T, Ishimaru H (1998) Phytoalexins from hairy root culture of Hyoscyamus albus treated with methyl jasmonate. J Nat Prod 61:1516–1519PubMed
75. Kwok KH, Doran PM (1995) Kinetic and stoichiometric analysis of hairy roots in a segmented bubble column reactor. Biotechnol Prog 11:429–435
76. Lambert E, Goossens A, Panis B, Labeke MCV, Geelen D (2009) Cryopreservation of hairy root cultures of Maesa lanceolata and Medicago truncatula. Plant Cell Tissue Organ Cult 96:289–296
77. Largia MJV, Satish L, Johnsi R, Shilpha J, Ramesh M (2016) Analysis of propagation of Bacopa monnieri (L.) from hairy roots, elicitation and Bacoside A contents of Ri transformed plants. World J Microbiol Biotechnol 32:131PubMed
78. Le Flem-Bonhomme V, Laurain-Mattar D, Fliniaux MA (2004) Hairy root induction of Papaver somniferum var. album, a difficult-totransform plant by A. rhizogenes LBA 9402. Planta 218:890–893PubMed
79. Lee KT, Yamakawa T, Kodama T, Shimomura K (1998) Effects of chemicals on alkaloid production by transformed roots of Atropa belladona. Phytochemistry 49:2343–2347
80. Lee KT, Suzuki T, Yamakawa T, Kodama T, Igarashi Y, Shimomura K (1999) Production of tropane alkaloids by transformed root cultures of Atropa belladonna in stirred bioreactors with a stainless steel net. Plant Cell Rep 18:567–571
81. Lee EJ, Park SY, Paek KY (2015a) Enhancement strategies of bioactive compound production in adventitious root cultures of Eleutherococcus koreanum Nakai subjected to methyl jasmonate and salicylic acid elicitation through airlift bioreactors. Plant Cell Tissue Organ Cult 120:1–10
82. Lee KJ, Park YK, Kim JY, Jeong TK, Yun KS, Paek KY, Park SY (2015b) Production of biomass and bioactive compounds from adventitious root cultures of Polygonum multiflorum using air-lift bioreactors. Korean J Plant Biotechnol 42:34–42
83. Li W, Koike K, Asada Y, Yoshikawa T, Nikaido T (2003) Biotransformation of low-molecular-weight alcohols by Coleus forskohlii hairy root cultures. Carbohydr Res 338:729–731PubMed
84. Li FX, Jin ZP, Zhao DX, Cheng LQ, Fu CX, Ma F (2006) Overexpression of the Saussurea medusa chalcone isomerase gene in S. involucrata hairy root cultures enhances their biosynthesis of apigenin. Phytochemistry 67:553–560PubMed
85. Ligang Z, Dechun R, Zhengdan H, Hongtao Z, Chongren Y, Junjian W (1998) Biotransformation of artemisinin by hairy roots of Cyanotis arachnoidea. Acta Bot Yunnan 20:229–232
86. Lin HW, Kwok KH, Doran PM (2003) Development of Linum flavum hairy root cultures for production of coniferin. Biotechnol Lett 25:521–525PubMed
87. Liu Y, Cheng KD, Zhu P, Feng WH, Meng C, Zhu HX, He HX, Ma XJ (2004) Biotransformation of dehydroepiandrosterone by hairy root cultures of Anisodus tanguticus. Acta Pharm Sin 39:445–448
88. Luczkiewicz M, Kokotkiewicz A (2005) Co-cultures of shoots and hairy roots of Genista tinctoria L. for synthesis and biotransformation of large amounts of phytoestrogens. Plant Sci 169:862–871
89. Macková M, Macek T, Kučerová P, Burkhard J, Pazlarová J, Demnerová K (1997a) Degradation of polychlorinated biphenyls by hairy root culture of Solanum nigrum. Biotechnol Lett 19:787–790
90. Macková M, Macek T, Ocenaskova J, Burkhard J, Demnerová K, Pazlarová J (1997b) Biodegradation of polychlorinated biphenyls by plant cells. Int Biodeterior Biodegrad 39:317–325
91. Mathur A, Gangwar A, Mathur AK, Verma P, Uniyal GC, Lal RK (2010) Growth kinetics and ginsenosides production in transformed hairy roots of American ginseng-Panax quinquefolium L. Biotechnol Lett 32:457–461PubMed
92. Medina-Bolivar F, Condori J, Rimando AM, Hubstenberger J, Shelton K, O’Keefe SF, Dolan MC (2007) Production and secretion of resveratrol in hairy root cultures of peanut. Phytochemistry 68:1992–2003PubMed
93. Mehrotra S, Goel MK, Srivastava V, Rahman LU (2015) Hairy root biotechnology of Rauwolfia serpentina: a potent approach for the production of pharmaceutically important terpenoid indole alkaloids. Biotechnol Lett 37:253–263PubMed
94. Menzel G, Harloff HJ, Jung C (2003) Expression of bacterial poly (3-hydroxybutyrate) synthesis genes in hairy roots of sugar beet (Beta vulgaris L.). Appl Microbiol Biotechnol 60:571–576PubMed
95. Motomori Y, Shimomura K, Mori K, Kunitake H, Nakashima T, Tanaka M, Miyazaki S, Ishimaru K (1995) Polyphenol production in hairy root cultures of Fragaria × ananassa. Phytochemistry 40:1425–1428
96. Mukundan U, Hjortso MA (1990) Effect of fungal elicitor on thiophene production in hairy root cultures of Tagetes patula. Appl Microbiol Biotechnol 33:145–147
97. Murata J, Bienzle D, Brandle JE, Sensen CW, De Luca V (2006) Expressed sequence tags from Madagascar periwinkle (Catharanthus roseus). FEBS Lett 580:4501–4507PubMed
98. Murthy HN, Dijkstra C, Anthony P, White DA, Davey MR, Power JB, Hahn EJ, Paek KY (2008) Establishment of Withania somnifera hairy root cultures for the production of Withanolide A. J Integr Plant Biol 50:975–981PubMed
99. Murthy HN, Park SY, Paek KY (2017) Production of ginsenosides by hairy root cultures of Panax ginseng. In: Malik S (ed) Production of plant derived natural compounds through hairy root culture. Springer, Cham, pp 203–216
100. Namdeo AG (2007) Plant cell elicitation for production of secondary metabolites: a review. Pharmacogn Rev 1:69–79
101. Nedelkoska TV, Doran PM (2000) Hyperaccumulation of cadmium by hairy roots of Thlaspi caerulescens. Biotechnol Bioeng 67:607–615PubMed
102. Nedelkoska TV, Doran PM (2001) Hyper accumulation of nickel by hairy roots of Alyssum species: comparison with whole regenerated plants. Biotechnol Prog 17:752–759PubMed
103. Nunes IS, Faria JM, Figueiredo AC, Pedro LG, Trindade H, Barroso JG (2009) Menthol and geraniol biotransformation and glycosylation capacity of Levisticum officinale hairy roots. Planta Med 75:387–391PubMed
104. Oksman-Caldentey KM, Inze D (2004) Plant cell factories in the post-genomic era: new ways to produce designer secondary metabolites. Trends Plant Sci 9:433–440PubMed
105. Orden AA, Bisogno FR, Cifuente DA, Giordano OS, Sanz MK (2006) Asymmetric bioreduction of natural xenobiotic diketones by Brassica napus hairy roots. J Mol Catal B Enzym 42:71–77
106. Palazón J, Mallol A, Eibl R, Lettenbauer C, Cusidó RM, Piñol MT (2003) Growth and ginsenoside production in hairy root cultures of Panax ginseng using a novel bioreactor. Planta Med 69:344–349PubMed
107. Pang Y, Peel GJ, Sharma SB, Tang Y, Dixon RA (2008) A transcript profiling approach reveals an epicatechin-specific glucosyltransferase expressed in the seed coat of Medicago truncatula. Proc Natl Acad Sci U S A 105:14210–14215PubMedPubMedCentral
108. Patra N, Srivastava AK (2016) Artemisinin production by plant hairy root cultures in gas-and liquid-phase bioreactors. Plant Cell Rep 35:143–153PubMed
109. Pavlov A, Bley T (2006) Betalains biosynthesis by Beta vulgaris L. hairy root culture in a temporary immersion cultivation system. Process Biochem 41:848–852
110. Phunchindawan M, Hirata K, Sakai A, Miyamoto K (1997) Cryopreservation of encapsulated shoot primordial induced in horseradish (Armoracia rusticana) hairy root cultures. Plant Cell Rep 16:469–473PubMed
111. Putalun W, Taura F, Qing W, Matsushita H, Tanaka H, Shoyama Y (2003) Anti-solasodine glycoside single-chain Fv antibody stimulates biosynthesis of solasodine glycoside in plants. Plant Cell Rep 22:344–349PubMed
112. Putalun W, Luealon W, De-Eknamkul W, Tanaka H, Shoyama Y (2007) Improvement of artemisinin production by chitosan in hairy root cultures of Artemisia annua L. Biotechnol Lett 29:1143–1146PubMed
113. Ramirez-Estrada K, Vidal-Limon H, Hidalgo D, Moyano E, Golenioswki M, Cusidó RM, Palazon J (2016) Elicitation, an effective strategy for the biotechnological production of bioactive high-added value compounds in plant cell factories. Molecules 21:182. https://​doi.​org/​10.​3390/​molecules2102018​2 CrossrefPubMedPubMedCentral
114. Renouard S, Corbin C, Drouet S, Medvedec B, Doussot J, Colas C, Mesnard F (2018) Investigation of Linum flavum (L.) hairy root cultures for the production of anticancer Aryltetralin Lignans. Int J Mol Sci 19:990PubMedCentral
115. Rhodes MJC, Robins RJ, Hamill JD, Parr AJ, Walton NJ (1987) Secondary product formation using Agrobacterium rhizogenes transformed hairy root cultures. TCA Newsl 53:2–15
116. Rijhwani SK, Shanks JV (1998) Effect of elicitor dosage and exposure time on biosynthesis of indole alkaloids by Catharanthus roseus hairy root cultures. Biotechnol Prog 14:442–449PubMed
117. Robbins MP, Hartnoll J, Morris P (1991) Phenylpropanoid defence responses in transgenic Lotus corniculatus 1. Glutathione elicitation of isoflavan phytoalexins in transformed root cultures. Plant Cell Rep 10:59–62PubMed
118. Rodriguez-Mendiola MA, Stafford A, Cresswell R, Aria-Castro C (1991) Bioreactors for growth of plant roots. Enzym Microb Technol 13:697–702
119. Romero F, Delate K, Kraus G, Solco A, Murphy P, Hannapel D (2009) Alkamide production from hairy root cultures of Echinacea. In Vitro Cell Dev Biol Plant 45:599–609
120. Roychowdhury D, Majumder A, Jha S (2013) Agrobacterium rhizogenes-mediated transformation in medicinal plants: prospects and challenges. In: Chandra S, Latha H, Varma A (eds) Biotechnology for medicinal plants. Springer-Verlag, Berlin, pp 29–68
121. Rudrappa T, Bhagyalakshmi N, Ravishankar GA (2004) In situ and ex situ adsorption and recovery of betalains from hairy root cultures of Beta vulgaris. Biotechnol Prog 20:777–785PubMed
122. Rudrappa T, Neelwarne B, Kumar V, Lakshmanan V, Venkataramareddy SR, Aswathanarayana RG (2005) Peroxidase production from hairy root cultures of red beet (Beta vulgaris). Electron J Biotechnol 8:66–78
123. Saito K, Sudo H, Yamazaki M, Koseki-Nakamura M, Kitajima M, Takayama H, Aimi N (2001) Feasible production of camptothecin by hairy root culture of Ophiorrhiza pumila. Plant Cell Rep 20:267–271
124. Salma M, Engelmann-Sylvestre I, Collin M, Escoute J, Lartaud M, Yi J-Y, Kim H, Verdeil J, Engelmann F (2014) Effect of the successive steps of a cryopreservation protocol on the structural integrity of Rubia akane Nakai hairy roots. Protoplasma 251:649–659PubMed
125. Satdive RK, Fulzele DP, Eapen S (2007) Enhanced production of azadirachtin by hairy root cultures of Azadirachta indica A. Juss by elicitation and media optimization. J Biotechnol 128:281–289PubMed
126. Shakeran Z, Keyhanfar M, Asghari G, Ghanadian M (2015) Improvement of atropine production by different biotic and abiotic elicitors in hairy root cultures of Datura metel. Turk J Biol 39:111–118
127. Shanks JV, Morgan J (1999) Plant ‘hairy root’ culture. Curr Opin Biotechnol 10:151–155PubMedPubMedCentral
128. Sharma S, Shahzad A (2013) Bioreactors: a rapid approach for secondary metabolite production. In: Shahid M, Shahzad A, Malik A, Sahai A (eds) Recent trends in biotechnology and therapeutic applications of medicinal plants. Springer, Dordrecht, pp 25–49
129. Sharma P, Padh H, Shrivastava N (2013) Hairy root cultures: a suitable biological system for studying secondary metabolic pathways in plants. Eng Life Sci 13:62–75
130. Shi HP, Qi Y, Zhang Y, Liang S (2006) Induction of cucumber hairy roots and effect of cytokinin 6-BA on its growth and morphology. Chin J Biotechnol 22:514–520
131. Shibli RA, Shatnawi MA, Subaih WS, Ajlouni MM (2006) In vitro conservation and cryopreservation of plant genetic resources: a review. World J Agric Sci 2:372–382
132. Shilpha J, Satish L, Kavikkuil M, Largia MJV, Ramesh M (2016) Methyl jasmonate elicits the solasodine production and anti-oxidant activity in hairy root cultures of Solanum trilobatum L. Ind Crop Prod 71:54–64
133. Singh G (1995) Fungal elicitation of plant root cultures-application to bioreactor dosage. PhD thesis, Pennsylvania State University, USA
134. Singh S, Melo JS, Eapen S, D’Souza SF (2006) Phenol removal using Brassica juncea hairy roots: role of inherent peroxidase and H₂O₂. J Biotechnol 123:43–49PubMed
135. Soudek P, Petrová S, Benesova D, Vanek T (2011) Uranium uptake and stress responses of in vitro cultivated hairy root culture of Armoracia rusticana. Agrochimica 55:15–28
136. Souret FF, Kim Y, Wyslouzil BE, Wobbe KK, Weathers PJ (2003) Scaleup of Artemisia annua L. hairy root cultures produces complex patterns of terpenoid gene expression. Biotechnol Bioeng 83:653–667PubMed
137. Srivastava S, Srivastava AK (2007) Hairy root culture for mass-production of high-value secondary metabolites. Crit Rev Biotechnol 27:29–43PubMed
138. Srivastava V, Negi AS, Ajayakumar PV, Khan SA, Banerjee S (2012) Atropa belladonna hairy roots: orchestration of concurrent oxidation and reduction reactions for biotransformation of carbonyl compounds. Appl Biochem Biotechnol 166:1401–1408PubMed
139. Staniszewska I, Królicka A, Maliński E, Łojkowska E, Szafranek J (2003) Elicitation of secondary metabolites in in vitro cultures of Ammi majus L. Enzym Microb Technol 33:565–568
140. Subroto MA, Kwok KH, Hamill JD, Doran PM (1996) Coculture of genetically transformed roots and shoots for synthesis, translocation, and biotransformation of secondary metabolites. Biotechnol Bioeng 49:481–494PubMed
141. Subroto MA, Priambodo S, Indrasti NS (2007) Accumulation of zinc by hairy root cultures of Solanum nigrum. Biotechnology 6:344–348
142. Sudha CG, Obul Reddy B, Ravishankar GA, Seeni S (2003) Production of ajmalicine and ajmaline in hairy root cultures of Rauvolfia micrantha Hook f., a rare and endemic medicinal plant. Biotechnol Lett 25:631–636PubMed
143. Sung LS, Huang SY (2006) Lateral root bridging as a strategy to enhance L-DOPA production in Stizolobium hassjoo hairy root cultures by using a mesh hindrance mist trickling bioreactor. Biotechnol Bioeng 94:441–447PubMed
144. Suresh B, Sherkhane PD, Kale S, Eapen S, Ravishankar GA (2005) Uptake and degradation of DDT by hairy root cultures of Cichorium intybus and Brassica juncea. Chemosphere 61:1288–1292PubMed
145. Suza W, Harris RS, Lorence A (2008) Hairy roots: from high-value metabolite production to phytoremediation. Electron J Integr Biosci 3:57–65
146. Swain SS, Rout KK, Chand PK (2012) Production of triterpenoid anti-cancer compound Taraxerol in Agrobacterium-transformed root cultures of butterfly pea (Clitoria ternatea L.). Appl Biochem Biotechnol 168:487–503PubMed
147. Swamy MK, Akhtar MS, Sinniah UR (2016) Response of PGPR and AM fungi toward growth and secondary metabolite production in medicinal and aromatic plants. In: Hakeem KR, Akhtar MS (eds) Plant, soil and microbes: mechanism and molecular interactions-volume 2. Springer, Cham, pp 145–168
148. Talano MA, Frontera S, González P, Medina MI, Agostini E (2010) Removal of 2,4-diclorophenol from aqueous solutions using tobacco hairy root cultures. J Hazard Mater 176:784–791PubMed
149. Teoh K, Weathers P, Cheetham R, Walcerz D (1996) Cryopreservation of transformed (hairy) roots of Artemisia annua. Cryobiology 33:106–117PubMed
150. Thakore D, Srivastava AK, Sinha AK (2017) Mass production of Ajmalicine by bioreactor cultivation of hairy roots of Catharanthus roseus. Biochem Eng J 119:84–91
151. Thiruvengadam M, Praveen N, Kim EH, Kim S, Chung IM (2014) Production of anthraquinones, phenolic compounds and biological activities from hairy root cultures of Polygonum multiflorum Thunb. Protoplasma 251:555PubMed
152. Tian L (2015) Using hairy roots for production of valuable plant secondary metabolites. In: Krull R, Bley T (eds) Filaments in bioprocesses. Springer, Cham, pp 275–324
153. Touno K, Yoshimatsu K, Shimomura K (2006) Characteristics of Atropa belladonna hairy roots cryopreserved by vitrification method. Cryo Lett 27:65–72
154. Vázquez-Flota F, de Lourdes Miranda-Ham M, Castro-Concha L, Tamayo-Ordoñez Y (2017) Synthesis of benzylisoquinoline alkaloids and other tyrosine-derived metabolites in hairy root cultures. In: Malik S (ed) Production of plant derived natural compounds through hairy root culture. Springer, Cham, pp 165–182
155. Veena V, Taylor CG (2007) Agrobacterium rhizogenes: recent developments and promising applications. In Vitro Cell Dev Biol Plant 43:383–403
156. Veersham C, Srinivasan V, Shuler ML (1995) Elicitation of Taxus sp. cell cultures for production of taxol. Biotechnol Lett 17:1343–1346
157. Verpoorte R, Memelink J (2002) Engineering secondary metabolite production in plants. Curr Opin Biotechnol 13:181–187PubMed
158. Verpoorte R, Heijden RVD, Hoopen HJGT, Memelink J (1999) Metabolic engineering of plant secondary metabolite pathways for the production of fine chemicals. Biotechnol Lett 21:467–479
159. Vinterhalter B, Savić J, Platiša J, Raspor M, Ninković S, Mitić N, Vinterhalter D (2008) Nickel tolerance and hyperaccumulation in shoot cultures regenerated from hairy root cultures of Alyssum murale Waldst et Kit. Plant Cell Tissue Organ Cult 94:299–303
160. Wang CT, Liu H, Gao XS, Zhang HX (2010) Overexpression of G10H and ORCA3 in the hairy roots of Catharanthus roseus improves catharanthine production. Plant Cell Rep 29:887–894PubMed
161. Washida D, Shimomura K, Nakajima Y, Takido M, Kitanaka S (1998) Ginsenosides in hairy roots of a panax hybrid1. Phytochemistry 49:2331–2335
162. Weathers P, Bunk G, McCoy MC (2005) The effect of phytohormones on growth and artemisinin production in Artemisia annua hairy roots. In Vitro Cell Dev Biol Plant 41:47–53
163. Wevar-Oller AL, Agostini E, Talano MA, Capozucca C, Milrad SR, Tigier HA, Medina MI (2005) Overexpression of a basic peroxidase in transgenic tomato (Lycopersicon esculentum Mill. cv. Pera) hairy roots increases phytoremediation of phenol. Plant Sci 169:1102–1111
164. Wielanek M, Królicka A, Bergier K, Gajewska E, Skłodowska M (2009) Transformation of Nasturtium officinale, Barbarea verna and Arabis caucasica for hairy roots and glucosinolate-myrosinase system production. Biotechnol Lett 31:917–921PubMed
165. Wu J, Lin L (2002) Elicitor-like effects of low-energy ultrasound on plant (Panax ginseng) cells: induction of plant defence responses and secondary metabolite production. Appl Microbiol Biotechnol 59:51–57PubMed
166. Xue SH, Luo XJ, Wu ZH, Zhang HL, Wang XY (2008) Cold storage and cryopreservation of hairy root cultures of medicinal plant Eruca sativa Mill, Astragalus membranaceus and Gentiana macrophylla Pall. Plant Cell Tissue Organ Cult 92:251–260
167. Yamanaka M, Shimomura K, Sasaki K, Yoshihira K, Ishimaru K (1995) Glucosylation of phenolics by hairy root cultures of Lobelia sessilifolia. Phytochemistry 40:1149–1150
168. Yan CY, Yu RM, Zhang Z, Kong LY (2007) Biotransformation of 4 hydroxybenzen derivatives by hairy root cultures of Polygonum multiflorum Thunb. J Integr Plant Biol 49:207–212
169. Yan CY, Ma WL, Yan WW, Yu RM (2008) Biotransformation of furannoligularenone by hairy root cultures of Polygonum multiflorum. J Chin Med Mat 31:633–635
170. Yang D, Ma P, Liang X, Liang Z, Zhang M, Shen S, Liu H, Liu Y (2012) Metabolic profiles and cDNA-AFLP analysis of Salvia miltiorrhiza and Salvia castanea Diel f. tomentosa Stib. PLoS One 7:e29678PubMedPubMedCentral
171. Yang T, Fang L, Nopo-Olazabal C, Condori J, Nopo-Olazabal L, Balmaceda C, Medina-Bolivar F (2015) Enhanced production of resveratrol, piceatannol, arachidin-1, and arachidin-3 in hairy root cultures of peanut co-treated with methyl jasmonate and cyclodextrin. J Agric Food Chem 63:3942–3950PubMed
172. Yaoya S, Kanho H, Mikami Y, Itani T, Umehara K, Kuroyanagi M (2004) Umbelliferone released from hairy root cultures of Pharbitis nil treated with copper sulfate and its subsequent glucosylation. Biosci Biotechnol Biochem 68:1837–1841PubMed
173. Yoshikawa T, Furuya T (1987) Saponin production by cultures of Panax ginseng transformed with Agrobacterium rhizogenes. Plant Cell Rep 6:449–453PubMed
174. Yoshikawa T, Asada Y, Furuya T (1993) Continuous production of glycosides by a bioreactor using ginseng hairy root culture. Appl Microbiol Biotechnol 39:460–464
175. Yoshimatsu K, Yamaguchi H, Shimomura K (1996) Traits of Panax ginseng hairy root after cold storage and cryopreservation. Plant Cell Rep 15:555–560PubMed
176. Zhai B, Clark J, Ling T, Connelly M, Medina-Bolivar F, Rivas F (2014) Antimalarial evaluation of the chemical constituents of hairy root culture of Bixa orellana (L). Molecules 19:756–766PubMedPubMedCentral
177. Zhang L, Yang B, Lu B, Kai G, Wang Z, Xia Y, Tang K (2007) Tropane alkaloids production in transgenic Hyoscyamus niger hairy root cultures over-expressing putrescine N-methyltransferase is methyl jasmonate-dependent. Planta 225:887–896PubMed
178. Zhao J, Davis LC, Verpoorte R (2005) Elicitor signal transduction leading to production of plant secondary metabolites. Biotechnol Adv 23:283–333PubMed
179. Zhou LG, Ruan DC, He ZD, Zhu HT, Yang CR, Wang JJ (1998) Biotransformation of artemisinin by hairy roots of Cyanotis arachnoidea. Acta Bot Yunnanica 20:229–232
180. Zhou ML, Zhu XM, Shao JR, Tang YX, Wu YM (2011) Production and metabolic engineering of bioactive substances in plant hairy root culture. Appl Microbiol Biotechnol 90:1229–1239PubMed
181. Zulak KG, Khan MF, Alcantara J, Schriemer DC, Facchini PJ (2009) Plant defense responses in opium poppy cell cultures revealed by liquid chromatography-tandem mass spectrometry proteomics. Mol Cell Proteomics 8:86–98PubMed