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

antiviral activity against the respiratory syncytial virus (RSV), HSV-1, and HSV-2 were improved by quercetin in dose-dependent ways in cell cultures [208, 209]. Flavans are defined by 2-phenyl-3,4-dihydro-2H-chromene. Flavan-3,4-diol, flavan-4, and flavan-3,3-ol. Epigallocatechin (EGC), catechin, epicatechin gallate, and epigallocatechin gallate (EGCG) are tea flavan-3-ols that have antiviral properties. Since Rawangkan et al. [210] showed that tea catechins, particularly EGCG, may bind to influenza virus haemagglutinin and prevent its adsorption to Madin-Darby canine kidney cells, the influenza virus has drawn the most attention as a potential target. Recent research demonstrates the antiviral potency of quercetin against several influenza virus strains. By interacting with influenza hemagglutinin, it prevents viral-cell fusion [193]. Quercetin inhibits transcription, protein synthesis, and viral endocytosis. Infected mice with airway cholinergic hyperresponsiveness and rhinovirus multiplication are both decreased by quercetin. HCMV is prevented by kaempferol. Coronavirus 3a channel blockers made of kaempferol derivatives with rhamnose residue are available. The influenza A virus is inhibited in vitro by kaempferol 3-O—Lrhamnopyranoside from Zanthoxylum piperitum. HIV-1 reverse transcriptase is inhibited by kaempferol and kaempferol-7-O-glucoside [106, 211–215]. The viral envelope may be damaged by EGCG, which prevents influenza virus and cellular membrane hemifusion [216–219]. Studies have shown that tea catechins are anti- HIV. Due to its ability to combat HIV-1 at all stages of its existence, EGCG is the most potent tea catechin. It inhibits gp120 binding by forming a direct bond with CD4 molecules. CD4's Trp69, Arg59, and Phe43 may interact with EGCG. Viral gp120 interacts with residues [192, 220]. By inhibiting the MEK/ERK1/2 and PI3-K/Akt signaling pathways, EGCG may shorten the EBV lytic cycle. Additionally, HCV cannot adhere to cells or replicate RNA when EGCG is present. The Zika virus (ZIKV) is inhibited by EGCG, according to a recent study: In vitro, the flavanone naringenin inhibits virus multiplication. The absence of anti-Sindbis virus activity in naringenin's glycoside form, naringin, demonstrates that rutinose restricts the antiviral action of this compound. Long-term treatment lowers HCV by 1.4 logs and may inhibit intracellular HCV particle assembly [221–224]. Treatment with naringenin after entrance reduced CHIKV in infected Vero cells. Bovine herpesvirus type 1 and New World

arenavirus Pichinde replication are decreased by the isoflavonoid genistein, which inhibits tyrosine kinase. Resting CD4 T cells and macrophages are protected against HIV infection by genistein. Avian leucosis virus subgroup J, HSV-1, and HSV-2 reproduction were all suppressed by genistein (**Figure 5**) [192, 225].
