*2.3.1.2 Chalcone biosynthesis*

Specific flavonoid synthesis, which starts with chalcone formation, is initiated with the entry of p-coumaroyl-CoA into the flavonoid biosynthesis pathway [30]. One molecule of p-coumaroyl-CoA and three molecules of malonyl-COA are converted into naringenin chalcone (4,2′,4′,6′-tetrahydroxychalcone [THC]) by the action of CHS (produced from acetyl-CoA) [31]. CHS, a polyketide synthase, is the main and first rate-limiting enzyme in the flavonoid biosynthesis pathway [32]. An intermediate of the CHS reaction is subjected to action by the aldo-keto reductase superfamily member chalcone reductase (CHR), which catalyzes its C-6′ dehydroxylation to produce isoliquiritigenin (4,2′,4′-trihydroxychalcone [deoxychalcone]) [33]. In

one of the studies, the amount of anthocyanin decreased, when the *Lotus japonicus* CHR1 gene was overexpressed in petunia [21, 34]. Chalcones are recognized as the first important intermediate metabolite in the production of flavonoids, and are also considered as a crucial yellow pigment in plants [35].

#### *2.3.1.3 Flavanones biosynthesis*

The intramolecular cyclization of chalcones by CHI, which occurs in the cytoplasm to produce flavanones and the heterocyclic ring C, is a step in the flavonoid pathway [30]. According to the substrate used, CHIs in plants may often be split into two classes. Type I CHIs, which are present throughout the entire vascular plant, transform THC into naringenin. Type II CHIs may manufacture naringenin and liquiritigenin utilizing either THC or isoliquiritigenin and are primarily found in leguminous plants [36]. More than these two forms, there are two other variants of CHI (type III and type IV) that retain the catalytic activity of the CHI fold but lack its ability to cycle chalcones [37]. Additionally, flavanones are a frequent substrate for the downstream flavonoid pathway as well as the flavone, isoflavone, and phlobaphene branches [38, 39].

#### *2.3.1.4 Aurone biosynthesis*

Aurones, a family of flavonoids produced from chalcone, are significant yellow pigments in plants. Aurone pigments generate a stronger yellow hue than chalcones and are responsible for the golden coloration of numerous common ornamental plants. Snapdragon, sunflowers, and coreopsis are only a few of the plant species that contain aurones [40, 41]. Aurone production requires THC as a direct substrate [42]. In the cytoplasm of the plant cells, chalcone 4′-O-glucosyltransferase catalyzes the production of THC 4′-O-glucoside from THC. The former is transferred to the vacuole by aureusidin synthase (AS), where it is converted into aureusidin 6-O-glucoside (aurone) [43].

#### *2.3.1.5 Flavone biosynthesis*

In all higher plants, flavone production is an essential branch of the flavonoid pathway. Flavone synthase (FNS) converts flavanones into flavones (FNS) [44, 45]. When present in flavanones, FNSI and FNSII encourage the formation of a double bond between C-2 and C-3 positions of the ring C [46]. FNS is a crucial enzyme in the production of flavones. Both naringenin and eriodictyol can be used as substrates by *Morus notabilis* FNSI to produce flavones [47]. Overexpression of *Pohlia nutans* FNSI causes apigenin accumulation in *A. thaliana* [48]. FNSII expression levels in flower buds of *Lonicera japonica* were shown to be congruent with flavone accumulation patterns [46]. Flavanones can be transformed into C-glycosyl flavones as well [21].

#### *2.3.1.6 Isoflavone biosynthesis*

Leguminous plants serve as the primary source of isoflavones [49]. Isoflavone synthase (IFS) transports flavanone to the isoflavone route [50] and appears to be able to convert liquiritigenin and naringenin into 2,7,4′-trihydroxyisoflavanone and 2-hydroxy-2,3-dihydrogenistein, respectively [51, 52]. Under the action of hydroxyisoflavanone dehydratase (HID), they are further transformed to the isoflavones

#### *Flavonoids: Recent Advances and Applications in Crop Breeding DOI: http://dx.doi.org/10.5772/intechopen.107565*

genistein and daidzein [53]. Additionally, HID, IFS, and isoflavanone O-methyl transferase can catalyze the conversion of liquiditigenin to 6,7,4′-trihydroxyflavanone, which can then be converted to glycitein (an isoflavone) [54]. IFS and HID catalyze two processes that result in the formation of isoflavone: the formation of a double bond between C-2 and C-3 positions of ring C and the transfer of ring B from C-2 position to C-3 position of ring C [55, 56]. The isoflavone production route begins with IFS, a cytochrome P450 hydroxylase. The accumulation of the isoflavone genistein in invitro tissues was caused by Glycine max IFS overexpression in *Allium cepa* [57]. The use of CRISPR/Cas9 to knock off the expression of the *IFS1* gene resulted in a considerable drop in isoflavones like genistein [44].

#### *2.3.1.7 Flavanol biosynthesis*

Flavonols are flavonoid metabolites that have had their ring C-3 hydroxylated [38]. Because their C-3 position is very susceptible to glycosidation, they frequently occur in glycosylated forms in plant cells. Flavonol synthase (FLS) converts the dihydroflavonols like dihydroquercetin (DHQ ), dihydrokaempferol (DHK), and dihydromyricetin (DHM) to the flavonols quercetin, kaempferol, and myricetin, respectively [58]. Through the activity of enzymes such as GTs, methyltransferases, and acyltransferase (AT), quercetin, kaempferol, and myricetin are further changed to numerous flavonol derivatives [59]. A C-2 and C-3 double bonds are formed in ring C via the desaturation of dihydroflavonol, which is catalyzed by FLS, a FeII/2-oxoglutarate-dependent dioxygenase. In the flavonol biosynthesis pathway, FLS is considered the key ratelimiting enzyme [21].
