**4. Role of biotic and abiotic elicitors for enhancing its therapeutic potential**

In plants, increase synthesis and accumulation of secondary metabolites occur upon their exposure to adverse climatic conditions which not only strengthen their growth but also revamp their innate immune response [107]. Several studies have indicated that distinct physical, chemical and microbial factors could act as abiotic/biotic elicitors for stimulating genes of metabolic pathways which will in turn result in the increase production important/specialised metabolites [108]. Now a day's elicitation is extensively used as a biotechnological tool to induce the biosynthesis of medicinally important metabolites in various tissues and organs of tissue cultured plants. The functional mechanism behind elicitor's elicitation involves signal perception by the receptors designed specifically to initiate signal transduction of the genes/transcription factor involved in the biosynthetic pathway (**Figure 2**) which in turn enhance the production and accumulation of different metabolites [107]. This section briefly describes different biotic/abiotic elicitors that can be employed for enhancing the production of secondary metabolites in medicinal as well as crop plants. A list of different biotic/abiotic elicitors involved in the regulation of bioactive metabolites in legumes are presented in **Table 2**.

*Unlocking Pharmacological and Therapeutic Potential of Hyacinth Bean (*Lablab purpureus *L.)… DOI: http://dx.doi.org/10.5772/intechopen.99345*

#### **Figure 2.**

*Schematic representation of mechanisms by which biotic/abiotic elicitors signalling pathways involved in the biosynthesis of important bioactive metabolites in plants. The elicitors are perceived by the receptors which then interacts with various components of signal atransduction pathway. This interaction activates certain transcription factors which in turn regulate the expression of various biosynthetic genes/proteins thus stimulating enhanced production of important therapeutic metabolites. SA: Salicylic acid; NO: Nitric oxide; MeJA: Methyl jasmonate; ROS: Reactive oxygen species; CDPKs: Calcium dependent protein kinases; MAPKs: Mitogen activated protein kinases; IP3: Inositol triphosphate; DAG: Diacyglycerol; TFs: Transcription factors.*

#### **4.1 Biotic elicitors**

Biological materials such as proteins, carbohydrates, inactivated enzymes, and polysaccharides etc. whether of plant, fungi or bacterial origin either in crude or purified form is used to induce the synthesis of secondary metabolites are termed as biotic elicitors [120]. Researchers have well indicated that proteins/enzymes are being explicitly used to stimulate the defence system of plants by increasing the synthesis of secondary metabolites involved in the regulation of stress responsive genes [121]. In tissue culture generated plants, several glycoprotein elicitors have been shown to elicit the production of phytoalexin, lectins and agglutinins that tremendously ameliorate the stress-induced oxidative damage [122]. Similarly, various fungal elicitor proteins such as PebC and PevD1 from *Botrytis cinerea* from and *Ss*Cut from *Sclerotinia sclerotiorum* elicited multiple defence response tomato and cotton plants in response to various biotic and abiotic stresses [123]. Furthermore, these elicitors have also been shown to activate G-proteins which in turn can also act as elicitor in stimulating secondary metabolite synthesis in plants such as stimulation of flavonoids and isoflavanones in soybean, benzophenanthridine alkaloids in bloodroot and β-thujaplicin in *Cupressus lusitanica* [122]. Ca2+ signalling also play pivotal role in the activation of various protein kinases such as calcium dependent protein kinase (CDPKs) and mitogen activated protein kinases (MAPKs) that have stimulated the sesquiterpenes biosynthesis in tobacco and French bean plants [120].

Polysaccharides such as xyloglucans, oligogalacturonides, hemicellulose and pectin derived from plant, bacterial or fungal cell wall could also be exploited as an



*List of different abiotic/biotic elicitors used for eliciting secondary metabolites production in legume crops.*

#### *Unlocking Pharmacological and Therapeutic Potential of Hyacinth Bean (*Lablab purpureus *L.)… DOI: http://dx.doi.org/10.5772/intechopen.99345*

**209**

elicitor to stimulate secondary metabolite synthesis in plants [124]. For instance, a polysaccharide derived from *Trichoderma atroviride* D16 was successfully regulated the genes involved in the production of tanshinone diterpene in *Salvia miltiorrhiza* and also increased the production of hairy roots up to 60% compared to control plants [125]. Likewise, oligosaccharide derived from *Fusarium oxysporum* efficiently stimulated the production of artemisinin in *Artemisia annua* and flavonoids in *Fagopyrum tataricum* plants suggesting definitive role of biotic elicitors in the stimulation of therapeutically important metabolites [126]. In another study, researchers also successfully exploited oligogalacturonides as biotic elicitor to stimulate the synthesis of phytoalexins in soybean plants [125]. The oligogalacturonides was also further utilised efficiently to stimulate stress defence response in *Nicotiana tabacum* plants by stimulating the biosynthesis of nutraceuticals [126]. Furthermore, the oligogalacturonides based elicitation of phytoalexins was also confirmed by Ferrari et al. [127] in soybean cell cultures.

Various phytohormones/signalling molecules such as salicylic acid (SA), nitric oxide (NO) jasmonic acid (JA), ethylene (ET) and abscisic acid (ABA) which can serve as an elicitor to elicit secondary metabolites production and stress-induced defence response in various plant species [127]. Among all, the role of SA, NO and JA have been extensively investigated for the elicitation of secondary metabolites synthesis and imparting resistance against biotic/abiotic stress induced oxidative damage in plants [128]. Methyl-jasmonate a derivative of jasmomic acid precisely activated the production of indole glucosinolate, β-thujaplicin and terpenes indole alkaloids in *Arabidopsis*, *C. roseus* and *C. lusitanica* plants [129, 130]. Similarly, studies have also indicated that both methyl jasmonate and salicylic acid either alone or in combination significantly enhanced the therapeutic attributes of *Hemidesmus indicus* by stimulating the synthesis of 2-hydroxy 4-methoxy benzaldehyde [131]. Moreover, Gai et al. [132] also observed SA and methyl jasmonate based elicitation of pharmacologically active alkaloids in the hairy root cultures of *Isatis tinctoria* L. Apart from plants, researchers have also widely used SA and methyl jasmonate to elicit the pharmaceutical alkaloids biosynthesis in microalgae *Arthrospira platensis* suggesting their robust application in prokaryotic system [133].

Various plant growth promoting rhizobacteria (PGPR) and arbuscular mycorrhizal fungal inoculum in conjunction with methyl jasmonate have been shown to enhance production of various secondary metabolites in *Rauvolfia serpentine,* fenugreek and *Solanum khasianum* [118, 134]. Bacterial and fungal based elicitation of therapeutically important compounds is being commonly used by various researchers because apart from increasing secondary metabolites production they also significantly improve growth and developments of plant exposed to various biotic and abiotic stress conditions [118]. In a study, Gorelick and Bernstein [135] observed differential effect of fungal elicitation in *Cannabis sativa* plants where fungal elicitation increases the production of cannabinoid and 3-deoxyanthocyanidin but causes significant reduction in anthocyanin content. Furthermore, researchers have also used arbuscular mycorrhizal fungal inoculum along with foliar application of chitosan that significantly boosted the biosynthesis of menthol and essential oils in *Mentha × Piperita* L. [136].

#### **4.2 Abiotic elicitors**

Elicitation of secondary metabolite synthesis by using substance of non-biological such as inorganic salts of heavy metals (VOSO4, NiSO4, CdCl2, AgNO3, CuCl2), UV-radiation, heat, light etc. is known as abiotic elicitation and the substance

#### *Unlocking Pharmacological and Therapeutic Potential of Hyacinth Bean (*Lablab purpureus *L.)… DOI: http://dx.doi.org/10.5772/intechopen.99345*

used are known as abiotic elicitors. Abiotic elicitors such as high temperature, salt, drought, light and heavy metals etc. have also been successfully used as physical and chemical stimuli to elicit the biosynthesis of medicinally important metabolites in various plants [122]. These abiotic elicitors have been successfully used either independently or in combination either by foliar spray, irrigation or as hydroponics under both open field or controlled conditions for secondary metabolite production in medicinally important plants [122]. Present section deciphers the functional mechanism by which these different abiotic based elicitors elicit the production of therapeutically important compounds.

Drought is one of the most prevalent abiotic stress that alter plant growth and productivity around the globe [108]. Researchers have also indicated that in order to cope up with drought induced oxidative stress, plants synthesise certain metabolites such as glycine betaine and proline as mean to strengthen their defence system [122]. Based on this notion, researchers are using mannitol, calcium chloride and polyvinyl pyrrolidone (chemical which are used to induce drought stress) as a physical elicitor to induce the production of terpene indole alkaloids up to 2-fold in treated *C. roseus* plants compared to non-treated control [107]. Likewise, researchers have also observed increased synthesis of anti-inflammatory metabolite saikosaponins in *Bupleurum chinense* plants exposed to mild drought stress [137]. Furthermore, exposure of drought stress also significantly enhanced the production of rosmarinic ursolic and oleanolic acid in *Prunella vulgaris* [122]. Similarly, researchers have also observed sharp increase in the glycyrrhizic acid and betulinic acid content in *Glycyrrhiza uralensis* and *Hypericum brasiliense* plants upon exposure to drought stress conditions [137].

Salinity is also known to affect wide array of physiological and biochemical properties in plants thus affecting their growth and development [137]. Prolong exposure to salinity stress causes cellular dehydration and generation of oxidative stress in plants thus limiting their ion/osmotic homeostasis [122]. However, in order to withstand to salinity stress, plants synthesised various secondary metabolites like phenols, alkaloids and terpenes as an ameliorative mechanism to overcome oxidative damage. For instance, researchers observed significant increase in the biosynthesis of terpene indole alkaloids (TIAs) in *C. roseus* plants exposed to mild salt stress as compared to control plants. Similarly, another group of researchers also enhanced production of vincristine alkaloids and anthocyanin in *C. roseus* and *Grevillea ilicifolia* plants exposed to salt stress [137]. Salinity induced elicitation of secondary metabolites are also reported in *Datura innoxia, Oryza sativa, Triticum aestivum* and *Trifolium repens* where researchers have observed enhance synthesis of various alkaloids, polyamine and glycine betaine using NaCl as an elicitor [138].

High light intensity and temperature are also able to alter the course of secondary metabolites production in plants [138]. Prolong exposure of both high light and temperature can induce oxidative stress in plants that can have adverse effect growth, ontology and development. High temperature can also lead to the induction of premature leaf senescence, stomatal closure and can stimulate transpiration rate to a greater extent [137]. Nonetheless, despite affecting plant's growth these physical factors have also been reported to stimulate the biosynthesis of important secondary metabolites in the root of *Panax quinquefolius* [138]. Likewise, researchers have stimulated the production of gingerol and zingiberene metabolites by culturing *Zingiber officinale* explants under high light conditions. Moreover, exposing plants to short-term UV-B radiations have also been reported to stimulate the secondary metabolite synthesis. For example., Klein et al. [139] observed increase biosynthesis of betacyanin and betaxanthin metabolites in *Alternanthera sessilis* and *Alternanthera brasiliana* by exposing them to 10–40 J cm−2 of UV-B radiation.

Similarly, UV-B radiation (up to 30–90 min) and low temperature treatment significantly improved hypericin biosynthesis in *Hypericum perforatum* adventitious roots and enhances the synthesis of total hydroxycinnamic acids (HCAs) and some sesquiterpenes in *Crepidiastrum denticulatum* [140]. Furthermore, exposure of both high/low temperature have also been shown to improve biosynthesis of ginsenoside, hypericin and hyperforin metabolites in *Panax ginseng* and *Hypericum perforatum* plants [137].

Increasing bioaccumulation of heavy metals such as As, Cd, Cu, Ni, Co and Ag have significantly impacted the agricultural lands and productivity. These heavy metals when presence in excess amount adversely affects plant growth and development [137]. However, at low levels these heavy metals act as co-enzymes/ co-factors in various cellular and metabolic pathways thus stimulating secondary metabolite production in plants [122]. Several researchers have well documented the role of heavy metals in stimulating oil content, shikonin/digitalin levels in *Brassica juncea* and production of betalains in *Beta vulgaris* [107]. Likewise, stimulatory effect of Cu2+, Co2+, CdCl2 and AgNO3 in stimulating lepidine in cultures of *Lepidium sativum*, betacyanins in callus of *Amaranthus caudatus*, tanshinone in root culture of *Perovskia abrotanoides* and various sesquiterpenoids in *Datura stramonium* [122]. In the recent years, researchers have synthesised various nanoparticles using various metals that have induce compelling impact on the plant, secondary metabolite production. For example., CdO nanoparticles not only induced the biosynthesis of phenolic compounds but also significantly improve growth and productivity of barley plants [141]. Likewise, Tripathi et al. [142, 143] also reported development of silver nanoparticles in *Withania coagulans* possess strong antibacterial, cytotoxic and antioxidative properties and was also able to enhance the production of withanolides. A large body of literature have well characterised the functional mechanism by which the nanoparticles can induce secondary metabolite synthesis is by stimulating ROS bursts [122]. These ROS act as signalling molecules at lower levels but can impose severe repercussion on the growth and development of plants when their level reaches beyond their antioxidative defence system. Thus, these nanoparticles presented themselves as most efficient mean to strengthen secondary metabolite production that will not only improve plant performance under stress conditions but can also improve the therapeutic/pharmacological potential of plants.
