**3.2 Hairy root cultures**

Off late, the cultivation of hairy roots has been seen as a sustainable strategy for the production of medicinally important metabolites of plants not only due to the reason that harvesting roots has been destructive for the plants in nature but also


#### **Table 1.**

*Bioactive secondary metabolites produced through shoot/callus cultures/suspension cultures.*

due to the ease of growing hairy roots in mass cultures in the absence of external hormones, absence of geotropism and high branching, etc. Furthermore, hairy roots produce secondary metabolites for larger periods of time, unlike natural roots which are not only in limited supply but are available at specific times in a year. For these reasons, switching from culturing natural plant-organs to hairy roots is considered as an attractive alternative for the production of many valuable natural secondary metabolites [15].

For establishing hairy root cultures, the plants are infected by *Agrobacterium rhizogenes* which induces hairy roots by the transfer of T-DNA from Ri plasmid into the plant genome. This ability of *A. rhizogenes* has led to studies on it as a source of root-derived pharmaceuticals [16]. Important metabolites produced through hairy roots are serpentine production from *Catharanthus roseus*, ajmalicine from *Rauvolfia micrantha* [17] and ginkgolides from hairy roots of *Gingko biloba* [18]. Large scale production of ginsenoside from *Panax ginseng* hairy roots has been achieved by optimizing organic nutrients in bioreactor for enhancing their production. Recent developments have indicated that hairy root culture technology has moved from small laboratory scale to a large scale industrial production. For example, the German Co. RooTec has been carrying out production of camptothecin and podophyllotoxin through hairy root cultures. In a cross-species co-culture system, hairy roots of *Linum flavum* have been found to increase the production of podophyllotoxin by 240% in the cell suspensions of *Podophyllum hexandrum*. It has been reported that secondary metabolites accumulating in aerial plant have also been accumulated in the hairy roots such as artemisinin which was thought to accumulate only in the aerial parts of *Artemisia annua* also accumulated in the hairy roots. Higher production of forskolin in transformed roots of *Coleus forskolli* was achieved by using various concentrations of auxins and auxin conjugates, cytokinins and GA3 [19]. The enhanced production of picroside-1 has been reported through hairy root cultures of *P. kurroa* [20].

### **3.3 Elicitation of phytochemicals production in callus/cell/hairy root cultures**

The lower yield of phytochemicals in plant cell cultures prompted researchers to look into various other means of enhancing their production. The recognition that certain specific secondary metabolites such as phytoalexins are produced by plants in response to microorganisms has led to the concept of using such stimulators (known as elicitors) for in vitro cultures. The substances used as elicitors can be of biotic or abiotic origin [21]. The plants also elicit the same response when challenged by compounds of pathogenic origin [22]. The elicitation of cell suspension cultures or hairy root cultures with biotic or abiotic elicitors has been found to enhance the rate of production as well as the yields of plant secondary metabolites [23].

The biotic elicitors are substances of biological origin, which include fungal homogenate, chitosan, microorganisms (*Pseudomonas aeruginosa*, *Bacillus cereus*), glycoprotein or intracellular proteins whose function are coupled to receptors and act by activating or inactivating a number of enzymes or ion channels [24]. Abiotic elicitors include physical and chemical stresses such as UV radiations, temperature, antibiotics, salts of heavy metals, etc. [22].

Various fungal elicitors including cell wall fragments, polysaccharides, glycoproteins and oligosaccharides have been used for the production of secondary metabolites in many plant spp. and their cell cultures. The cell extracts and filtrates of four species of fungi were used for the production of taxol from elicited cell cultures of *Taxus* sp. [25]. The cell wall fractions of *Aspergillus niger* have been used as an

#### *Production of Medicinal Compounds from Endangered and Commercially Important Medicinal… DOI: http://dx.doi.org/10.5772/intechopen.90742*

elicitor in cell suspension cultures of *Taxus chinensis* thereby resulting in more than two fold increase in taxol yield and about six fold increase in total secretion.

Jasmonic acid (JA) and its methyl esters, methyl jasmonate (MJ) have been reported as key signaling compounds in the process of elicitation leading to the accumulation of various secondary metabolites. Lu et al. (2001) reported 28 fold higher saponin production in the elicited cultures of *Panax ginseng* by using yeast extract and methyl jasmonate as elicitors. Production of many valuable secondary metabolites using various elicitors have been reported successfully in various other plant species [26–29]. Enhanced production of podophyllotoxin in suspension cultures of *Linum album* was reported by using biotic (yeast extract) and abiotic (Ag+ , Pb2+ and Cd2+) elicitors.

Methyl jasmonate, vanadyl sulphate and chitosan were used for enhancing the production of ginsenoside from hairy root cultures of *P. ginseng* [23].

Pitta-Alvarez and Giulietti et al. (2000) used jasmonic acid and aluminium chloride as elicitors for enhancing the production of scopolamine and hyoscyamine in hairy root cultures of *Brugmansia candida*. Bacterial elicitors like *Bacillus cereus*, *Staphylococcus aureus*, etc. have been used for enhancing scopolamine production from the adventitious hairy roots of *Scopolia parviflora* (**Table 2**).


#### **Table 2.**

*Elicitors used for the production of secondary metabolites by cell cultures of medicinal plants.*
