## 1 Introduction
These changes activate various signaling pathways, such as the mitogen-activated protein kinase (MAPK) pathway, and lead to the activation of NADPH oxidase and the accumulation of reactive oxygen and nitrogen species (ROS and RNS). Next, the expression of late defense response genes and the production of jasmonate occur. Finally, the accumulation of secondary metabolites takes place [11; 12]. Figure 1: _Cont._  
Figure 1: Chemical structures of three ginseng saponin types: (**a**) protopanaxadiol type (PPD); (**b**) protopanaxatriol type (PPT); and (**c**) oleanolic acid type [7]. Figure 2: Scheme of the general mechanism action of elicitors [11]. PL—phospholipase; ROS—reactive oxygen species; RNS—reactive nitrogen species; MAPK—mitogen-activated protein kinases. The application of elicitors is one of most effective methods to increase the production of secondary metabolites in hairy root cultures. These cultures are able to accumulate very important pharmaceutical compounds: tropane alkaloids (scopolamine and hyosocyanine), indole alkaloids, cardenolide glycosides, polyacetylenes, thiophenes, anthraquinones, taxol, and others. Moreover, the profiles of these metabolites are the same as in mother plants, and their level can be higher than that of the parent plant or of other types of in vitro plant tissue cultures [13; 14; 15; 16; 17]. Biotic and abiotic elicitors were also applied to enhance ginseng production in _Panax ginseng_ hairy root cultures. According to scientific data, the external application of jasmonic acid and its methyl derivative significantly improved ginseng production in hairy root cultures of _P. ginseng_ growing both in flasks and a bioreactor [18; 19; 20; 21]. Similar effects were noticed after peptone treatment [20]. Other elicitors such as vanadyl sulphate, phenol derivatives (caffeic acid and catechine), fucoidan, chitosan, and gum karaya decreased the ginseng saponin content [19; 20; 21]. [PERSON] et al. [22] examined the influence of different types of light on ginseng accumulation. Fluorescent light stimulated ginseng production, whereas metal halide, blue, red, and blue plus red light reduced ginseng saponin content in hairy root cultures of _P. ginseng_ growing in a flask bubble bioreactor. [PERSON] et al. [23] reported that the level of total ginseng was enhanced under Tween treatment, and most of these metabolites were released into the medium. Moreover, it was stated that the level of dammarenediol synthase, being one of the enzymes involved in ginseng biosynthesis, was up-regulated at both the gene expression and enzyme activity levels. Tween elicitation also slightly contributed to the up-regulation of the expression of genes _CYP716A47_ and _CYP716A53v2_, encoding Cyt P450 enzymes and catalyzing the formation of protopanaxadiol from dammarenediol and protopanaxatriol from protopanaxadiol, respectively. Other authors demonstrated that the treatment of _P. ginseng_ hairy root cultures with methyl jasmonate (MJ) also resulted in the increased expression of genes and coding enzymes involved in the ginseng pathway. In consequence, the increased activity of farnesyl diphosphate synthase (FPS), squalene synthase (SS), and squalene epoxidase (SQE1) and enhanced ginseng acumulation were observed [24; 25; 26; 27]. Transformed root cultures were also obtained from another species of ginseng--_P. quinquefolium_--and several elicitors have, so far, been used to promote gin-sensoide biosynthesis in these cultures [28]. The properties of well-known elicitors such as yeast extract, abscisic acid, and methyl jasmonate were studied [29; 30; 31]. Furthermore, our team attempted to find new stressors that would increase the accumulation of ginensoides in the studied cultures. We turned our interest to essential oils. Their high, multi-directional biological activity [32] gave rise to the idea that they may also affect plant metabolism.