# Responses of Medicinal and Aromatic Plants to Engineered Nanoparticles +
## 4 Effect of Metal-Based NPs on Medicinal and Aromatic Plants
### Silver-Based NPs
#### 4.1.2 Elicitor Effect  
AgNPs also acted as elicitor in HR cultures of _Brassica rapa_, causing a pronounced increase in the contents of glucosinolates and their transcripts, as well as in total phenolic and flavonoid concentrations and transcripts, compared to non-elicited HRs [186]. After treatment of in vitro grown shoots of _Isatis constricta_ with 2 mg/L AgNPs, lasting 5 days, the indigo and tryptanthrin content were higher by 15 and 71% than in the control; however, at prolonged treatment (10 or 15 days), their content showed a decrease. On the other hand, treatments with AgNPs were found to decrease indiribuin production [123]. Foliar treatment of 40 mg/L AgNPs, besides stimulating growth of fenugreek (_Trigonella foenum-graecum_), plants increased the total content of photosynthetic pigments by 25%, while content of indole acetic acid was even doubled compared to the control. Moreover, seeds yielded from treated plants showed pronouncedly higher content of phenolics, flavonoids, and tannins, as well as a higher percentage of proteins and carbohydrates than those of control plants [187]. Diosgenin content in fenugreek seedlings treated with 0.2 mg AgNPs (8-21 nm) per seedling considerably increased compared to the control (214.06 \(\pm\) 17.07 vs. 164.49 \(\pm\) 7.67 ug/mL) [115]. Pronounced enhancement of secondary metabolites, including phenols, tannins, and alkaloids, was found in borago plants sprayed with solutions containing 0.2, 0.4, and 0.6 mM AgNPs [183]. Saluia officinalis plants folilarly treated with AgNPs contained enhanced levels of phenolic compounds; the content of rosmarinic acid (RA) and salvianolic acid A and B was positively correlated with the activity of phenylalanine ammonia-lyase (PAL) and RA synthase, but not with tyrosine aminotransferase. After treatment of plants with 100 mg/L AgNPs, even 8-fold higher RA content was observed than in control plants [128]. _Rosmarinus officinalis_ L. plants sprayed with solution containing 25, 50, 100 ppm AgNPs showed beneficial impact on the contents of secondary metabolites. Application of 200 ppm AgNPs for 12 days resulted in enhanced carnosic acid (CA) content by <11% compared to the control. In AgNP-treated plants, positive correlation was observed between TFC and CA content. Translocation of AgNPs to rosmary roots was confirmed as well [188]. The composition of EO extracted from _Lavandula angustifolia_ propagated on MS medium containing AgNPs (27.5 \(\pm\) 4.8 nm; 10 and 50 mg/L) was considerably altered, showing a decrease in compounds of lower molecular weight, which were replaced by those with higher molecular weight [130]. AgNPs more effectively elicited production of cichoric acid in cell suspension cultures of _Echinacea purpurea_ than AgNO3, which reached even 9.54 mg/g dry weight (d.w.) in cell suspension culture after 48 h of treatment with 2 mg/L AgNPs and 8 mg/g d.w. in leaf suspension culture after 72 h of treatment [122]. Moreover, enhanced production of secondary metabolites, TPC, TFC, and protein content, and improved antioxidant and superoxide dismutase (SOD) activities were observed in _Artemisia absinthium_ grown in vitro on MS medium treated with AgNPs [131]. Due to oxidative stress generated by AgNPs, artemisinin content in HRs increased and its production in 20-day-old HR cultures achieved up to 13.3 mg/L, being 3.9-fold higher than that of the control [164]. By application of 45 mg/L AgNPs to callus culture of _Stevia rebuadiana_, the amount of stevioside was enhanced, up to 32.34 mg/g d.w. of callus, suggesting that by using appropriate concentrations of AgNPs as an elictor enhanced production of this secondary metabolite can be obtained [126]. AgNPs effectively elicited production of both lignans and neolignans in cell suspension culture of _Linum aistatissimum_[125].