## 3 Nano-Elicitation of Flavonoids
### AgNPs as a Novel Elicitor of Flavonoids  
AgNPs find their applications in medicine, biotechnology and agriculture owing to their desirable physico-chemical properties [34]. Amongst different NPs, AgNPs are widely reported as a novel elicitor of flavonoids in in vitro culture (Table 1). _Momordia charantia_, commonly known as bitter gourd, is a highly nutritive vegetable containing bioactive compounds such as phenolics and flavonoids [35]. The cell suspension culture of _M. charantia_ was elicited with _Bacillus licheniformis_ biosynthesized AgNPs (50 nm) at 5 mg/L concentration. The elicited culture showed enhanced concentration of different flavonoids including quercetin, kaempfer, catechin, myricetin, rutin, naringenin and biochanin A [36]. Flavonols are the most ubiquitous flavonoids found in food, also called 3-hydroxylavones. They are closely related to flavones in structure but differ in having a hydroxyl group at 3-position on the C-ring, while flavones contain a ketone group with an unsaturated carbon-carbon bond [37]. Quercetin, being the most abundant flavonoid in plants, possesses anti-inflammatory, anti-oxidative and anticancer activities, and it is therefore a widely studied flavonoid. As quercetin has a special ability to scavape highly reactive species such as hydroxyl radical, so it has importance as a nutraceuticaltial [38]. The hairy root culture of _Cucumis anguria_ when elicited with AgNPs showed an increase in the accumulation of phenolic compounds as well as flavonoid compounds. AgNP elicited culture showed a higher amount of flavonoids (3136.20 \(\upmu\)g/g) as compared to non-elicited culture [39]. Similarly, biogenic AgNPs effectively elicited the hairy root culture of _Brassica rapa_ at concentrations 50, 100, and 150 mg/L. Hairy roots treated with 100 mg/L AgNP solution showed a gradual increase in the concentration of total phenolic content (TPC) and total flavonoid content (TFC), while treatment with 150 mg/L decreased TPC and TFC. Moreover, nine different types of flavonoids (myricetin, naringenin, catechin, quercetin, rutin, kaempfer, biochanin A, formononetin, and hesperidin) were accumulated in a higher quantity in AgNP elicited culture [40]. The influence of biologically synthesized polydispersed AgNPs from a pod extract of _Cola nitida_ on the phytochemical profile of _Amaranthus caudatus_ plant was investigated. There was 32.58, 35.80, and 7.20% improvement in flavonoid content for 25, 50 and 100 ppm concentration of AgNPs, respectively. Moreover, kaempfer and quercetin were the most abundant flavonoids produced at 50 ppm AgNP concentration [41]. The callus culture of _Caralluma tuberculata_ when elicited with 90 ug/L AgNPs (40 nm) led to 1.8 mg TFCproduction [42]. _Caralluma tuberculata_ contains valuable bioactive compounds mostly used to treat diabetes, asthma, paralysis, inflammation, obesity and fever [43]. Anthocyanins are another member of the flavonoids family which have angiocardiopathy, high antioxidant and cancer prevention properties [44]. Anthocyanins are water soluble colored pigments which have been used as food pigments, nutraceuticals and traditional medicine [45]. When the seedlings of _Arabidopsis thaliana_ were elicited with 50 uM spherical shaped AgNPs, a 56% increase in anthocyanins was observed. Moreover, anthocyanin accumulation was correlated with the increase in the expression of antioxidant enzyme Cu/Zn superoxide dismutase 2 [46]. When the cell suspension culture of _Linum usitatissimum_ was elicited with AgNPs during the early phase, enhanced flavonoid production was observed, which declined during the later phase, and the highest TFC (11.85 mg/g dry weight (DW)) was recorded when cultures were elicited on day 10 [47]. A possible reprogramming of the phenylpropanol pathway through this AgNPs elicitation was suggested since the accumulation of other phenylpropanoids such as lignans and neolignans was also affected [47]. The addition of different NPs in combination is also effective in inducing stress in plants.