**2. Classification of elicitors and secondary metabolite production via in vitro culture of medicinal plants**

Stress is an important factor in determining the chemical composition and therapeutic activity of medicinal plants. Actively stimulating, or eliciting, the plant stress response to induce the desired chemical response is called elicitation, harnessing the connection between plant stress and phytochemistry. "Elicitor may be defined as a substance for stress factors which, when applied in small quantity to a living system, it induces or improves the biosynthesis of specific compound which do have an important role in the adaptations of plants to a stressful condi‐ tions" [18]. Elicitation is the induced or enhanced biosynthesis of metabolites due to addition of trace amounts of elicitors [18]. Several biotechnological strategies have been hypothesized and applied for the productivity enhancement, and elicitation is recognized as the most practically feasible strategy for increasing the production of desirable secondary compounds from cell, organ, and plant systems [19–21].

On the basis of nature, elicitors can be divided into two types abiotic and biotic (Figure 1). Abiotic elicitors comprise of substances that are of nonbiological origin and are grouped in physical, chemical, and hormonal factors. Biotic elicitors are the substances of biological origin that include polysaccharides originated from plant cell walls (e.g. chitin, pectin, and cellulose) and micro–organisms.

**Figure 1.** Elicitors Classification Based on their Nature

### **3. Abiotic elicitors**

As mentioned above, the abiotic elicitors are categorized into physical, chemical, and hormonal elicitors. Abiotic elicitors have wide range of effects on the plants and in the production of secondary metabolites (Table 1).



**Figure 1.** Elicitors Classification Based on their Nature

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

As mentioned above, the abiotic elicitors are categorized into physical, chemical, and hormonal elicitors. Abiotic elicitors have wide range of effects on the plants and in the production of

**Elicitor Plant species Nature of culture Compounds References**

*Hypericum perforatum* Cell suspension Hypericin [164] *Pueraria thomsnii* Cell suspension Puerarin [165]

*Withania somnifera* Hairy root Withanolide A [167] *Withania somnifera* Cell suspension Withanolide A [168]

*Corylus avellana* Cell suspension Paclitaxel [169] *Bacopa monnieri* Shoot Bacoside A [166] *Withania somnifera* Cell suspension Withanolide A [168]

pseudohypericin

[41]

Ozone (O3) *Melissa officinalis* Shoot Rosmarinic acid [163]

pH *Bacopa monnieri* Shoot Bacoside A [166]

Ultraviolet C *Vitis vinifera* Cell suspension Stilbene [34] Proline *Stevia rebaudiana* Callus and suspension Steviol glycoside [40] Polyethylene glycol*Stevia rebaudiana* Callus and suspension Steviol glycoside [40]

Sucrose *Hypericum adenotrichum* Seedling Hypericin and

**3. Abiotic elicitors**

secondary metabolites (Table 1).


**Table 1.** Effect of Different Abiotic Elicitors on the Production of Various Secondary Metabolites in Plants

#### **3.1. Physical elicitors**

Physical elicitors include light, osmotic stress, salinity, drought, and thermal stress.

#### *3.1.1. Light*

The light is a physical factor that can affect the metabolite production. Light can stimulate such secondary metabolites include gingerol and zingiberene production in *Zingiber officinale* callus culture [22]. The effect of light irradiation on anthocyanin production in cell suspension cultures of *Perilla frutescens* was reported [23]. The effect of light and hormones on the digitoxin accumulation in *Digitalis purpurea* L. was reported by Hagimori et al. [24]. Moreover, in hairy root cultures of *Artemisia annua*, the effect of light irradiation influenced the artemisinin biosynthesis [25]. The effect of white light on taxol and baccatin III accumulation in cell cultures of *Taxus cuspidate* was reported by Fett–Neto et al. [26]. Ultraviolet (UV) radiation stimulates secondary metabolite production. Increasing UV–B exposure in field–grown plants not only increased the total essential oil and phenolic content but also decreased the amount of the possibly toxic beta–asarone [27]. These findings are to be expected as phenolics are known UV protectants [28]. *Catharanthus roseus* plants, exposed to UV–B light, show significant increases in the production of vinblastine and vincristine, which have proven effective in the treatment of leukemia and lymphoma [29]. UV–C irradiation promotes the phenylpropanoid pathway and stimulates flavonoid synthesis [30]. UV–C irradiation is an effective method to enhance stilbene production in *Vitis vinifera* berries [31], *V. vinifera* leaves [32], and *V. vinifera* callus of different genotypes [33]. UV–C together with methyl jasmonate (MeJA) or salicylic acid (SA) also used to enhance stilbene production in *V. vinifera* cell cultures [34].
