## 3 Flavonoids  
### Biosynthesis of Flavonoids  
Flavonoids are a class of water-soluble polyphenic secondary metabolites containing a 15-carbon phenylpropanol core, which is modified by rearrangement, alkylation, oxidation, and glycosylation. In the structure of flavonoids, benzene rings A and B, along with heterocycle C, include not only carbon atoms but also oxygen (Figure 3). Flavonoids represent the most numerous classes of natural phenolic compounds. The classification of flavonoids into 12 groups is based on the oxidation state of heterocycle C and the number of hydroxyl or methyl groups on the benzene ring. The four key classes of flavonoids--chalcones, flavanones, dihydroflavonols, and leucoanthocyanidins--also act as intermediate metabolites. They contribute to the synthesis of other flavonoid forms such as flavones, isoflavones, flavonols, flobaphenes, proanthocyanidins, anthocyanins, stilbenes, and aurones. By functioning as signaling molecules, allebotathic compounds, phytoalexins, detoxifying agents, and antimicrobial protective compounds, flavonoids protect plants from various biotic and abiotic stresses as well as nutritional deficiencies [174]. In addition, flavonoids in plants serve as natural UV filters [175]. Biosynthesis of flavonoids (Figure 4) occurs at the cytosolic side of the endoplasmic reticulum, leading to the accumulation of flavonoids in the central vacuole. As with isoprenoids, flavonoid biosynthesis begins with phenylalanine, one of the products of the shikimate pathway (Figure 2). The first three steps of flavonoid biosynthesis from phenylalanine are referred to as the general phenylpropanolid pathway resulting in the formation of 4-counaroyl-CoA (4-C-CoA) from p-counaric acid (Figure 4). The initial step, the deamination of phenylalanine to trans-cinnamic acid, is catalyzed by phenylalanine ammonia lyase (PAL) [176]. The information on the genes, which encode the main enzymes for flavonoid biosynthesis in _A. thaliana_, is given in Table 3. PAL exhibits strong inducibility under stress conditions, enhancing the synthesis of all phenolic compounds, including phytoalexins, thereby serving as a primary adaptive response to a number of biotic and  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Proteins** & **Species** & **Target Genes** & **Anti-Oxidants** & **Editing Type** & **Result** \\ \hline \multirow{4}{*}{Phosphoryylase, GGP} & \multirow{4}{*}{_Lactuca sativa_} & uORF\({}_{\text{AIVTC2}}\)\(L\)\(S\  
abiotic stressors [177]. Subsequent steps of this pathway are catalyzed by cinnamic acid 4-hydroxylase (C4H) and 4-cumarate CoA ligase (4 CL). Figure 4: The pathway of flavonoid biosynthesis in plants. Each colored frame represents a different class of flavonoids. The enzyme names are abbreviated as follows: ACCase, acetyl-CoA, carboxylase; ANS, anthocyanin synthase; ANR, anthocyanin reductase; C4H, cinnamic acid 4-hydroxylase; CHI, chalclone isomerase; CH4’GT, chalclone 4’-O-glucosyltransferase; 4 CL, 4-cumarate CoA ligase; CHS, chalclone synthase; CHR, chalclone reductase; CLL-7, cinnate-CoA ligase; DFR, dihydroflavonol 4-reductase; FGT (AGT), flavonoid glycosyltransferases; FNS, flavone synthase; FNR, flavanone 4-reductase; F3H, flavanone 3-hydroxyylase; F3’H, flavanone 3’-hydroxyylase; FLS, flavonol synthase; IFS, isoflavone synthase; LAR, leucanthocyanidin reductase; PAL, phenylalanine ammonia lyase; STS, stilbene synthase.