**2. Flavonoids: classification and biosynthesis network in plant**

Flavonoids are the most diverse group of natural products; they are found in plants in over 10,000 different compounds [28]. In contrast to stilbenes (a class of *Chemistry and Role of Flavonoids in Agriculture: A Recent Update DOI: http://dx.doi.org/10.5772/intechopen.106571*

flavonoids) which has a C6-C2-C6 structure (**Figure 1**), flavonoids have a C6-C3-C6 basic structure composed of three phenolic rings, A (6 carbons) and B (6 carbons), linked by a 3-carbon heterocyclic ring (ring C). This structure, in turn, can give rise to several derivatives and sub-classes of compounds with distinct substituents [11, 29]. According to the degree of oxidation of the heterocyclic ring and the number of hydroxyl or methyl groups on the benzene ring, flavonoids can be divided into 12 subgroups: anthocyanins, aurones, chalcones, dihydroflavonols, flavanones, flavones, flavanols, isoflavones, leucoanthocyanidins, phlobaphenes, proanthocyanidins and stilbenes (**Figure 1**) [9, 30, 31]. However, in terms of attachment of the B ring to the C ring, flavonoids are into three main groups: Flavonoids (2-phenylbenzopyrans): the B ring is attached at the 2-position of the C ring, Isoflavonoids (3-phenylbenzopyrans): the B ring is attached at the 3-position of the C ring, and Neoflavonoids (4-phenylbenzopyrans): the B ring is attached at position 4 of the ring C [11, 32].

The phenylpropanoid pathway produces flavonoids from phenylalanine, whereas the shikimate pathway produces phenylalanine [33]. It is generally recognized that the first three steps of the phenylpropanoid pathway are known as the general phenylpropanoid pathway [28]. Using this pathway, aromatic amino acid phenylalanine is transformed to top-coumaroyl-CoA via phenylalanine ammonia lyase (PAL), cinnamic acid 4-hydroxylase (C4H), and 4-coumarate: CoA ligase (4CL). A primary catalytic function of PAL is to catalyze deamination of phenylalanine to trans-cinnamic acid, the first in a general phenylpropanoid pathway [34]. Further, PAL is essential for the regulation of carbon flux from primary to secondary metabolism in plants [35]. StlA, which encodes PAL in *Photorhabdus luminescens*, has been

shown to play a role in generating a stilbene antibiotic [34]. PAL activity has also been linked to anthocyanins and other phenolic compounds in strawberry fruit [36]. In the general phenylpropanoid pathway, the second step involves C4H, a monooxygenase found in cytochrome P450 and responsible for hydroxylating trans-cinnamic acid to generate p-coumaric acid. The flavonoid synthesis pathway involves this first oxidation reaction as well [37]. It has been found that the expression of C4H in *Populus trichocarpa* and *Arabidopsis thaliana* can be correlated with lignin content, an important phenylpropanoid metabolite [28]. The enzyme 4CL catalyzes the synthesis of p-coumaroyl-CoA by coupling with a co-enzyme A (CoA) unit to *p*-coumaric acid at the third step of the general phenylpropanoid pathway. A chalcone synthesizing enzyme, chalcone synthase (CHS), contributes to the biosynthesis of specific flavonoid-based compounds by combining one molecule of 4-coumaroyl-CoA (6-carbon) with three molecules of malonyl-CoA. As a result of two different pathways of cell metabolism, ring A and ring B are generated via the acetate pathway and shikimate

**Figure 2.** *Biosynthesis network of flavonoids in plant.*

#### *Chemistry and Role of Flavonoids in Agriculture: A Recent Update DOI: http://dx.doi.org/10.5772/intechopen.106571*

pathway, respectively, with chain linkages delivering ring C. During the acetate pathway, malonyl-CoA is converted to ring A by carboxylation of acetyl-CoA, whereas ring B and the linking chain (ring C) are generated via the shikimate pathway (**Figure 2**) from coumaroyl-CoA. In the phenylpropanoid pathway, coumaryl-CoA is directly generated by three enzymatic reactions from phenylalanine [29]. Following the condensation of these aromatic rings, these pathways lead to the synthesis of chalcone, which will then undergo isomerase-catalyzed cyclization to form flavanone (**Figure 2**). In addition to hydroxylation, glycosylation, and methylation, the latter compounds undergo additional modifications, resulting in an enormous variety of colors (**Figure 2**).
