## 3 Flavonoids
### Genetic Approaches for Boosting Flavonoid Production in Plants
#### 3.3.2 Regulation of Transcription Factor Activity to Enhance Flavonoid Biosynthesis
\"Indigo Rose\" led to a decrease in the content of flavonoids only in the vegetative parts of plants and was accompanied by a number of morphological changes (reduction in plant height, decrease in fruit size). In the fruits of the edited plants, the flavonoid content was the same as in the unedited plants [161]. In carrots, the knockout of the _DcMYB113-like_ gene in the purple variety resulted in depignement regenerants [54]. The R2R3-MYB transcription factor PtrMYB57 is a repressor of anthocyanin and proanthocyanidin biosynthesis. In poplar, the CRISPR/Cas9 mutant _PtrMYB57_ was characterized by high levels of anthocyanin and proanthocyanidin in leaves compared to wild-type plants [162]. The R2R3-MYB transcription factor PtrMYB45 suppresses flavonoid biosynthesis in Tartary buckwheat (_Fagorynrun tataricum_). Knockout of this TF resulted in increased content of rutin, catechin, and other flavonoids in hairy root mutants [163]. In the basic helix-loop-helix (bHLH) group of TFs, the _Transparent Testa_ gene plays an important role in the accumulation of flavonoids. CRISPR/Cas9 BnTT8 mutants in _Brassic napus_ possessed yellow seed color associated with a block in tissue-specific proanthocyanidin deposition in the seed coat, as well as increased protein and lipid content. Transcriptome analysis showed that targeted mutations resulted in suppressed expression of phenylpropanoloid/flavonoids biosynthetic genes [167]. Targeted mutagenesis of homologous _TT8_ genes in tobacco (_NtAnt1a_ and _NtAn1b_) also resulted in seeds with a yellow seed coat phenotype and increased lipid and protein accumulation [168]. In the MYB-bHLH-WD40 (MBW) complex, the repeat protein WD40 is involved in transcriptional regulation of the flavonoid metabolic pathway in many plant species. The _Transparent Testa Glabra1_ (_TTG_) locus related to WD40 was knocked out using CRISPR/Cas9 gene editing technology in _A. thaliana_ plants with different ploidy levels [165] and in rice plants [166]. Mutations in the gene resulted in a decrease in the synthesis of flavonoids in plants that led to light coloration of seeds and disturbances in the formation of trichomes. The basic region/leucine zipper (bZIP) TF gene family also plays a key role in the regulation of flavonoid biosynthesis in many plant species. Knockout of the _VvbZIP36_ gene in grapevine (_Vitis vinifera_) using CRISPR/Cas9 led to the accumulation of flavonoids and a number of related metabolites (naringenin chalcone, naringenin, dihydroflavonols, and cyanidin-3-O-glucose), which was accompanied by the appearance of red pigmentation on the leaves. Editing revealed that VvbZIP36 is a negative regulator of flavonoid biosynthesis [164]. Thus, obtaining targeted mutations in various transcription factor genes makes it possible to identify their functions in regulating the expression of flavonoid biosynthesis genes (activation or repression) and clarify their role in other processes related to plant development. Other TFs, such as PAP1, MYB1, MYB2, MYB10, DcMYB6, and the Lateral Organ Boundary Domain (LBD) TF family are known to increase the accumulation of flavonoids. These TFs are potential candidates for genome editing [251; 252]. An example is _FaMYB10-2_, one of three MYB10 homologues responsible for fruit color in strawberries. It is known that the insertion of a transposon into the promoter region of this gene can alter the biosynthesis of flavonoids. Depending on the location, this insertion can either enhance the biosynthesis of flavonoids in fruits or lead to its inhibition and the appearance of white fruits [253]. Using genome editing, it becomes possible to obtain various types of mutations with desired change in the phenotype of cultivar strawberry fruits. Uridine diphosphate-dependent glucosyltransferases (UGTs) are responsible for the transfer of monosaccharide residues to their acceptor molecules in plants. Two enzymes, encoded by the _UGT79B2_ and _UGT79B3_ genes, were identified in Arabidopsis.