**5. Hormonal elicitors**

Various plant hormones have been extensively used in elicitation studies. The most studied, because of their key roles in the plant defense response, are jasmonic acid (JA) and SA and its derivatives.

#### **5.1. Jasmonates**

*3.1.4. Drought stress*

*3.1.5. Thermal stress*

*perforatum* [66].

**4. Chemical elicitors**

One of the most important abiotic stress is drought, which affect plant growth and their developmental process [56]. The available water in the soil is reduced to such critical levels, and atmospheric conditions add to the continuous loss of water; the situation is called drought stress. The high temperature in the environment and solar radiations add up the water deficit in the soil, which leads to drought stress. Drought stress tolerance is observed in all types of plants, but its extent varies from species to species [56]. Drought stress, which can also greatly reduce plant growth, can increase secondary metabolite content. Mild water stress significantly increased the content of the anti–inflammatory saikosaponins in *Bupleurum chinense* [57]. Moderate water stress increased the content of salvianolic acid in roots of *Salvia miltiorrhiza*, although the content of other bioactives, including tanshinone, was lowered [58]. Moderate drought stress also increased the production of rosmarinic, ursolic, and oleanolic acid in *Prunella vulgaris* [59]. A weak water deficit greatly increased the glycyrrhizic acid content in roots of *Glycyrrhiza uralensis* [60]. In *Hypericum brasiliense*, the amounts of various phenols and

Although thermal stress can greatly reduce plant growth and induce senescence, elevated temperatures (heat stress) or low temperatures (cold stress) have also been shown to increase secondary metabolite production. Temperature strongly influences metabolic activity and plant ontology, and high temperatures can induce premature leaf senescence [62]. Elevated temperatures increase leaf senescence and root secondary metabolite concentrations in the herb *Panax quinquefolius* [63]. A 5°C increase in temperature significantly increased the ginsenoside content in roots of *P. quinquefolius* [63]. A temperature variation has multiple effects on the metabolic regulation, permeability, and rate of intracellular reactions in plant cell cultures [62]. Temperature range of 17–25°C is normally used for the induction of callus tissues and growth of cultured cells [16]. The temperature and light quality influences on the production of ginsenoside in hairy root culture of *Panax ginseng* [64]. The *Melastoma malaba‐ thricum* cell cultures incubated at a lower temperature range (20 ± 2°C) grew better and had higher anthocyanin production than those grown at 26 ± 2°C and 29 ± 2°C [65]. Fifteen days at 35°C significantly increased the hypericin and hyperforin content in shoots of *Hypericum*

Heavy metals have become one of the main abiotic stress agents for living organisms because of their increasing use in the developing fields of industry and agrotechnics and high bioaccumulation and toxicity [67]. Although a lot of information is available concern‐ ing the effects of heavy metals on plant growth and physiology, much less is known regarding their effects on the production of secondary metabolites. Heavy metal–induced

betulinic acid were drastically increased under drought stress [61].

254 Abiotic and Biotic Stress in Plants - Recent Advances and Future Perspectives

Jasmonates, including JA and MeJA, are a family of cyclopentanone compounds that modulate a wide range of plant responses [80,81] and act as effective elicitors to enhance secondary metabolites in in vitro cultures. They constitute an important class of elicitors for many plant secondary metabolic pathways, which are typically manifested by the elicitation of secondary metabolite biosynthesis when plants face particular environmental stresses [82]. JA is an important signal molecule of plant in response to wound and pathogen attack [83]. JA and its more active derivative MeJA can induce the production of a wide range of plant secondary metabolites such as rosmarinic acid, terpenoid indole alkaloid, and plumbagin in various cell cultures [84–86]. JA elicitation are reported to induce the production of rosmarinic acid in *Mentha piperita* [84], anthocyanin in *V. vinifera* [87], and plumbagin in hairy roots of *Plumbago indica* [88]. JA and MeJA have been used as elicitors for stilbene biosynthesis in *V. vinifera* foliar cuttings [89], *V. vinifera* cell cultures [90,91], and *Vitis rotundifolia* hairy root cultures [92]. The addition of MeJA to *V. vinifera* cell cultures also promoted anthocyanin accumulation [93]. MeJA with transgenic technology highly enhanced the production of tanshinones in *Salvia miltiorrhiza* hairy roots [94]. In the hairy root culture of *Withania somnifera*, MeJA elicited the production of withanolide A, withanone, and withaferin A [95]. The MeJA enhanced the production of bacoside A, a valuable triterpenoid saponin having nootropic therapeutic activity in in vitro shoot cultures of *Bacopa monnieri* [96]. In *Andrographis paniculata* cell culture, the MeJA induced the production of andrographolide content [97]. In *Glycyrrhiza glabra*, methyl jasmonate induced the production of the saponin soyasaponin [98]. It enhanced the production of paclitaxel in *Tacus canadensis* and *T. cuspidate* [99], and in *Rubus idaeus*, it stimulated the production of the raspberry ketone benzalacetone [100].

#### **5.2. Salicylic acid**

Salicylic acid, well known for the systemic acquired resistance it induces in the plant response to many pathogens, can also elicit the production of secondary metabolites in plants [101,102]. SA with transgenic technology highly enhanced the production of tanshinones in *S. miltior‐ rhiza* hairy roots [94]. The higher production of withanolide A, withanone, and withaferin A was reported in the elicited–hairy roots of *W. somnifera* [95]. SA induced the stilbene production in the cell suspension of *V. vinifera* [34]. It stimulated the production of alkaloids such as vincristine and vinblastine in periwinkle [103], the tropane alkaloid scopolamine in hairy root cultures of *Brugmansia candida* [77], and pilocarpine in jaborandi leaves [104]. Anthraquinone production was greatly increased in *Rubia cordifolia* after a SA treatment [105]. SA also affects terpenoid secondary metabolism in plants. It induced accumulation of the triterpenoids ginsenosides in ginseng and glycyrrhizin in licorice [106,107]. Recent evidence demonstrated that suitable concentrations of SA can also promote monoterpene production [108].

#### **5.3. Gibberellic acid**

Gibberellin (GA), a phytohormone, is also well known as an effective elicitor for the production of secondary metabolites [109].
