**1. Introduction**

Flavonoids with more than 6000 individuals are divided into six main categories: chalcones, flavones, flavonols, flavandiols, anthocyanins, and proanthocyanidins are present in all plants. The aurones group is present in several species [1]. Legumes and a few non-legume plants produce isoflavonoids, but few plants produce

3-deoxyanthocyanins and phlobaphenes. Stabileneds resemble chalcones and are made from grape and peanuts [2]. Flavonoids have several functions, such as protecting plants from UV radiation and phytopathogens, regulating signals, promoting male fertility, transporting auxin, and giving flowers their color to draw pollinators [3]. Flavonoids may increase nutrient recovery during senescence by shielding leaf cells from photooxidative damage. The oldest and most prevalent flavonoids are flavonols, which have potent physiological effects [4]. The phenylpropanoid pathway, which turns phenylalanine into 4-coumaroyl-CoA, produces flavonoids. Chalcone scaffolds are produced by the first flavonoid-specific enzyme, chalcone synthase. Although the principal method for producing flavonoids in plants is consistent, the different flavonoid subclasses are produced depending on the species via isomerases, reductases, hydroxylases, and various Fe2+/2-oxoglutarate-dependent dioxygenases [4]. Transferases alter the solubility, reactivity, and interaction of flavonoid molecules with biological targets by adding sugars, methyl groups, and acyl moieties to the flavonoid backbone [5]. Plants can produce specific organic molecules and prevent metabolic interference thanks to metabolic channeling. P450s-related metabolons have been discovered in several biosynthetic pathways, including phenylpropanoid, flavonoid, cyanogenic glucoside, and others [6]. More proof of intermediate channeling is provided by transgenic tobacco plants that produce two phenylalanine ammonia-lyase isoforms (PAL1 and PAL2) and cinnamate-4-hydroxylase [7]. For example, Yeast-two hybrid assays indicate that rice contains an anthocyanin multienzyme complex [8]. For flavonoids from marine environments, for example, unknown and uncommon marine flavonoids precursors of algal flavonoids are biosynthesized using comparable metabolic mechanisms to those seen in plants. Flavonoids are created by several metabolic pathways [9]. There are a variety of structures in algal flavonoids. Specify flavones, isoflavones, flavanols, flavanones, and favonols. The C6/ C3 unit of t-cinnamic acid and the unit of malonyl-C3 CoA make up the backbone. Pcoumaric acid is present in *Anabaena doliolum, Spongiochloris spongiosa, Porphyra tenera,* and *Undaria pinnatifida* [10]. Phenylalanine Ammonia Lyase is responsible for producing t-cinnamic acid. While p-coumaroyl-CoA may be converted into a chalcone derivative by Claisen condensation, Michael addition, and chalcone synthasecatalyzed enolization, the poly-b-keto ester can be made using the phenylpropanoid pathway. Chalcone is created from the poly-b-keto ester [11] Chalcone is transformed into flavonoid structures via several reductases, isomerases, hydroxylases, acyltransferases, and glycosyltransferases. Within each category, structural heterogeneity is brought on by variations in the number of linked hydroxyl groups, position, degree, type of alkylation, and glycosylation [12].

Further evidence is required for the algal flavonoid synthesis route. For Algal flavonoid composition, the 15-carbon skeleton of flavonoids comprises two phenyl rings (A and B) connected by a 3-carbon unit to form a heterocyclic ring. Their structural categorization is based on where the benzenoid substituent is located [13]: 2-Phenylchromans (flavonoids), which include anthocyanidins, flavanones, flavonols, flavones, and flavan-3-ols. 2. Pterocarpans, isoflavones, and 3-phenylchroman isoflavones. Glycosides represent the majority of flavonoids in marine [14]. Phlorotannins are more often used polyphenols in algae than flavonoids. There is not much literature on these chemicals. Flavanols. The most varied flavonoids found in algae are flavanols. The double bond between carbons 2 and 3 and the carbonyl group in carbon 4 of ring C is absent from flavanols. Occupational and health advantages Polyphenols have a variety of bioactivities in addition to being antioxidants that protect against UV rays and are poisonous to predators [15]. The bioactivity of

*Flavonoids Biosynthesis in Plants as a Defense Mechanism: Role and Function Concerning… DOI: http://dx.doi.org/10.5772/intechopen.108637*

phlorotannins and flavonoids is regulated by the pattern of -OH group substitutions, double bonds, and the site of their conjugation [16]. Also, Phytoflavonoids bioactivity of the flavonoids found in algae is unclear, but there are several applications for marine flavonoids in medical and cosmetic applications. Acanthophorin A and B, isolated from *A. spicifera*, shield the rat liver from lipid peroxidation and stop the malondialdehyde generation [17]. Because OH groups combine with H radicals to generate persistent semiquinone radicals, flavonoids have antioxidant properties. Flavonoids scavenge hydroxyl, other functional groups, and unsaturated and conjugated pi bonds [18]. Flavonoids are vital components of the human diet and potent antioxidants that can lower oxidative stress and several diseases in people [19]. Due to their antioxidant, anti-inflammatory, antibacterial, and affinity/inhibitory properties toward inflammatory enzymes, plant extract rich in flavonoids are employed in dermatology and cosmetics. As dietary components, algal flavonoids may protect against several human diseases (**Figure 1** and **Table 1**) [25].

#### **2. Flavonoids as a defense mechanism**

Flavonoids are necessary for plant development and plaque resistance. Flavonoids are responsible for many of the hues of angiosperm flowers. They can be found throughout the plant, not just in the blooms [26]. Plant-based foods and drinks like fruits, vegetables, tea, chocolate, and wine are rich in flavonoids. Plants, animals, and even microorganisms contain flavonoids. Flavonoids responsible for The color and aroma of flowers, fruit dispersal, the germination of seeds and spores, and the growth and development of seedlings in plants are all influenced by flavonoids, which are produced in particular locations [27]. In addition to acting as UV filters [28], signal molecules, allopathic chemicals, phytoalexins, detoxifying agents, and antimicrobials, flavonoids shield plants against biotic and abiotic stresses. Plants' ability to adapt to heat and tolerate freezing may be influenced by flavonoids [29]. Early advances in

**Figure 1.** *The metabolic pathways of flavonoids.*



*Flavonoids Biosynthesis in Plants as a Defense Mechanism: Role and Function Concerning… DOI: http://dx.doi.org/10.5772/intechopen.108637*

> **Table 1.**

*The different classes of flavonoids with their activities and examples of*

 *each class.* floral genetics were made possible by mutation techniques that altered flower colors generated from flavonoids, and functional gene silencing in plants was associated with flavonoid synthesis. Today, flavonoids treat illnesses and prevent cancer, and more than 6000 flavonoids color fruits, herbs, vegetables, and medicinal plants. Flavones are a subclass of flavonoids. Positions 2 and 3 of the C ring have double bonds, and position 4 is a ketone [30], Most flavones found in fruits and vegetables have a hydroxyl group at position 5 of the A ring, but other positions—particularly position 7 of the A ring and 3<sup>0</sup> and 4<sup>0</sup> of the B ring—can vary depending on the taxonomic group. Keto-flavonoids are flavanols. Flavanols can act as antioxidants and reduce the risk of vascular disease [31]. The third hydroxyl group on the C ring of flavonols can be glycosylated. Compared to flavones, flavonols exhibit distinct methylation, hydroxylation, and glycosylation patterns. The most varied group of bioactive polyphenols is flavonoids [32]. A phenyl ring (A ring) joined with heterocyclic benzo-c-pyrone (C ring), which connects to another phenyl ring (B ring) via a carbon-carbon bond, makes up the three rings that make up the diphenyl propane skeleton of flavonoids (C6C3C6). These chemicals contain hydroxyls. The gymnosperms, angiosperms, ferns, and bryophytes contain more than 4000 flavonoids [2]. The first plant to possess flavonoids is green algae [33]. According to Bonfante [34], a symbiotic relationship between algae and a tip-growing fungus is the reason for the plants' biphyletic origin. When plants transitioned from marine to terrestrial habitats, flavonoids developed primarily to protect against rising UV exposure [35].

Research on plant flavonoid production is an important area of research. The synthesis of flavonoids in algae may differ from higher plants due to algae development. Microalgae contained flavonoids and flavonoid intermediates by Goiris et al. [36]. Phloretin and dihydrochalcone might be intermediates in the production of flavonoids. The findings suggest that flavonoid biosynthesis enzymes may be present in microalgae. Diverse flavonoids compatible with better plant flavonoid synthesis are present in certain algae [36]. Plant-plant interactions may be impacted by flavonoids. Negative relations are mainly based on the inhibition of seedling development and germination. Flavonoids are frequently released into the soil by roots, where they prevent seed germination. They may also be found in leaves and pollen, which prevents the germination of other plants [3, 37]. Barley flavones lessen weed seed germination, while *Centaurea maculosa* catechins limit *Centaurea diffusa* and *Arabidopsis thaliana* germination and growth [38]. The precise allelopathic mechanism of flavonoids is unknown. Allelopathy can be affected by preventing cell division, ATP production, and auxin activity [39]. The Ca2+ signal cascade and root system death are stimulated by flavanols. Due to its ability to inhibit weed development, allelopathy is becoming increasingly important in agriculture [40].

Plants may fight against bacteria and fungi with the assistance of flavonoids. The general antipathogenic properties of flavonoids are largely attributed to their antioxidant properties. They suppress ROS produced by both pathogens and plants [41]. The B ring of flavonoids can intercalate or form hydrogen bonds with nucleic acid bases, limiting bacterial DNA and RNA synthesis and influencing DNA gyrase activity [42]. They can bind to viral nucleic acids or capsid proteins and inhibit viral polymerases [43]. The antipathogenic activity of flavonoids depends on their structure. Flavones and flavanones without substitution have strong antifungal properties. The antifungal activities of these compounds are reduced by hydroxyl and methyl groups, but methylated flavonoids have a greater effect. Isoflavones, flavanes, and flavanones are powerful antibacterial compounds, whereas flavonoids inhibit root infections, particularly fungus (**Figure 2**) [44, 45].

*Flavonoids Biosynthesis in Plants as a Defense Mechanism: Role and Function Concerning… DOI: http://dx.doi.org/10.5772/intechopen.108637*

**Figure 2.** *The different role of flavonoids and their mechanism of action as a defense mechanism in human Vis plant.*
