## 3 Flavonoids
### Genetic Approaches for Boosting Flavonoid Production in Plants  
Mutagenesis of plants for enhancement of flavonoid production is also a rapidly developing area. There are two main approaches to increase the level of various flavonoids in plants: the regulation of the activity of individual enzymes involved in flavonoid metabolism or the regulation of the activity of TFs, which in vivo are involved in the activation of the entire flavonoid biosynthesis pathway. .3.1 Regulation of the Expression of Individual Genes Encoding Key Enzymes in Flavonoid Biosynthesis  
Amplification of the expression level of the genes encoding the enzymes of the initial stages of flavonoid biosynthesis sometimes gives ambiguous results. For instance, overexpression of the _CHI_ gene (Figure 4, Table 3) in tomato fruits led to a remarkable 78-fold increase in flavonoid content [239]. Overexpression of the _CHI_ gene from _Petunia hybrida_ in potato plants resulted in a slight but significant increase in flavonoid content [240]. CHS is responsible for catalyzing the formation of chalcones--a distinct class of flavonoids and a precursor to other flavonoid classes. When the _CHS_ gene is overexpressed in potato plants, it frequently results in unexpected outcomes. It often leads to reduction in CHS levels in the plants [240]. Apparently, this effect was observed due to suppression of _CHS_ gene expression by its extra copies. However, when the _CHS_ gene from barley was overexpressed in potato plants, this effect was not observed, although no significant increase in anthocyanin accumulation was detected either [240]. Flavan-3-ols formation is directly catalyzed by FLS (Figure 4). The effect of the intensification of _FLS_ gene expression on flavonoid accumulation and the resistance of resulting plants to various stresses was studied using different plant species. For instance, overexpression of the _FLS_ gene from _Euphoria kansui_ Liou in Arabidopsis resulted in an approximately 75% increase in flavonoid content compared to the wild-type plants [241]. The transgenic Arabidopsis plants generated in this study demonstrated enhanced resistance to salinity (200 mM NaCl) and drought induced by 20% PEG. These plants accumulated lower levels of ROS compared to control plants. This was attributed to the antioxidant activity of flavonoids and the increased activity of superoxide dismutase and peroxidase in these plants [241]. Transgenic tobacco plants overexpressing FLS from _Apocynum veneetum_ also contained a significantly higher amount of flavonoids, approximately 2.8 times more than the wild-type plants [242]. These plants accumulated lower levels of ROS and exhibited increased resistance to salinity (150 mM NaCl) [242]. Arabidopsis plants with overexpression of FLS1 from _Triticum aestivum_ were also more resistant to salt stress [243]. However, there are data in the literature that overexpression of FLS does not always lead to an increase in the flavonoid content in plants. For example, in Arabidopsis plants with an overexpressed _FLS1_ gene, the flavonoid content did not significantly differ from that in the wild-type plants [244]. It appears that different _FLS_ genes perform distinct roles in flavonoid biosynthesis, and selecting the right gene variant is essential. An increase in the content of flavonoids in the transgenic plants of tea and tobacco was found after overexpression of the _DFR_ gene [245], encoding the enzyme, catalyzing the production of leucoanthocyanidins, an alternative class of flavonoids (Figure 4, Table 3). The extract from these plants demonstrated the elevated ability to scavenge stable diphenylpicryl hydrazyl free radicals (70-185%), indicating increased antioxidant activity compared to wild-type plants. Furthermore, these tobacco plants exhibited an increased resistance to biotic stress, particularly against _Spodoptera litura_ infestation [245].