**2.2 Evolution of Flavonoid metabolism**

The enzymes, chalcone isomerase (CHI) and isoflavone reductase in Chlamydomonas, dihydrokaempferol-4-reductase and naringenin chalcone synthase (CHS) in *Phaeodactylum*, and CHI and dihydroflavonol reductase in *Ectocarpus* were created as a result of several evolutionary processes in representatives of bryophytes (mosses), liverworts, and hornworts. CHI-like enzymes were discovered in certain proteobacteria and fungi, and they may have been acquired by horizontal gene transfer. Contrarily, the recruitment and gene duplication of polyketide synthases and oxoglutarate-dependent dioxygenases from primary metabolism, respectively, led to the evolution of CHS and F3H. The first three flavonoids, chalcones, flavanols, and flavones, were created as a result of the CHS, CHI, and F3H activities. These metabolites, which have not altered in 500 million years, are essential intermediates in today's irreducibly complicated flavonoid manufacturing pathways in plants [17, 18].

The number of key events that sparked the flavonoid pathway's gradual rise, variety, and evolutionary successes are:

• the recruitment of enzymes from fundamental metabolisms, such as the polyketide, phenylpropanoid, and shikimate pathways

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

**Figure 2.**

*Distribution of flavonoids in plant kingdom and their respective structures.*


• the flexibility of flavonoid pathways and their capacity to shift intermediate molecule fluxes towards the production of complex scaffolds of quite varied chemicals depending on the needs of the local ecosystem.

More than 10,000 of these compounds have been found in over 9000 current plant species as a result of these evolutionary processes, making flavonoids one of the most extensively distributed routes in plants today [16, 19].

#### **2.3 The flavonoid biosynthetic pathways**

From a genetic standpoint, a lot of work has been done to decipher the flavonoids' biosynthesis routes. Flavonoid synthesis mutants have been discovered in a variety of plant species. The first important experimental models in this system were snapdragon (*Antirrhinum majus*), petunia (*Petunia hybrida*), and maize (*Zea mays*), which led to further discovery of several structural and regulatory flavonoid genes. Arabidopsis (*Arabidopsis thaliana*) has recently contributed to the study of flavonoid pathway regulation and subcellular localization [20].

#### *2.3.1 Following are the biosynthetic pathways of some major flavonoids*

The Figure shows the eight branches of flavonoid biosynthetic pathway (showed in the eight different colored boxes) and four important intermediate metabolites (represented by the green boxes) (**Figure 3**). The abbreviated forms of enzyme names and flavonoid compounds mentioned in the figure are as follows: (i) ANR: anthocyanidin reductase; (ii) ACCase: acetyl-CoA carboxylase; (iii) AS: aureusidin synthase; (iv) 4CL: 4-coumarate: CoA ligase; (v) CHS: chalcone synthase; (vi) CHI: chalcone isomerase; (vii) CHR: chalcone reductase; (viii) C4H: cinnamic acid 4-hydroxylase; (ix) CH2′GT: chalcone 2′-glucosyltransferase; (x) CH4′GT: chalcone 4′-O-glucosyltransferase; (xi) ANS: anthocyanidin synthase; (xii) CLL-7: cinnamate–CoA ligase; (xiv) FNS: flavone synthase; (xv) F6H: flavonoid 6-hydroxylase; (xvi) IFS: isoflavone synthase; (xvii) HID: 2-hydroxyisoflavanone dehydratase; (xviii) FNR: flavanone 4-reductase; (xix) F8H: flavonoid 8-hydroxylase; (xx) F3'5'H: flavanone 3′,5′-hydroxylase; (xxi) F3H: flavanone 3-hydroxylase; (xxii) DHK: dihydrokaempferol; (xxiii) DHM: dihydromyricetin; (xxiv) DFR: dihydroflavonol-4-reductase; (xxv) DHQ: dihydroquercetin; (xxvi) FLS: flavonol synthase; (xxvii) OMT: O-methyl transferases; (xxviii) PAL: phenylalanine ammonia lyase; (xxix) UFGT: UDP-glucose flavonoid 3-Oglucosyltransferase; (xxx) LAR: leucoanthocyanidin reductase [21].

#### *2.3.1.1 Phenylpropanoid pathway*

The phenylpropanoid route produces flavonoids from phenylalanine, whereas the shikimate pathway produces phenylalanine [22, 23]. The general phenylpropanoid route refers to the first three steps of the phenylpropanoid pathway [24]. The aromatic amino acid phenylalanine is transformed to p-coumaroyl-CoA in this route. The typical phenylpropanoid route begins with the deamination of phenylalanine to trans-cinnamic acid, which is catalyzed by the enzyme phenylalanine ammonia lyase (PAL) [25]. In plants, PAL also has a significant role in controlling the transfer of carbon from primary to secondary metabolism [26]. The second step in the general phenylpropanoid route is catalyzed by the activity of C4H, a cytochrome P450

**Figure 3.**

monooxygenase found in plants that hydroxylates trans-cinnamic acid to produce p-coumaric acid [27]. The quantity of lignin, a crucial phenylpropanoid metabolite, in *Populus trichocarpa* and *Arabidopsis thaliana*, is connected to the degree of C4H expression [24, 28]. In the third step, 4-coumarate (4CL) catalyzes the production of p-coumararoyl-CoA by incorporating a coenzyme A (CoA) unit into p-coumaric acid [29].
