Flavonoid with two aromatic rings joined by three carbon chain (heterocyclic pyran ring) (C6-C3-C6) are structurally diverse secondary metabolites in plant with different functions. The structural diversity of flavonoids is derived by complex substitution of these carbon skeletons through hydroxylation, glycosylation, methylation, phenylation etc. (Naoumkina et al., 2010). Different modifications of flavonoid skeleton affect their antimicrobial activity. The flavonoid glycoside analogues exhibited antifungal activity against different Fusarium oxysporum f. sp. dianthi pathotype (Galeotti et al., 2008). The prenylated flavonoids showed antibacterial and antifungal activity (Sohn et al., 2004). Flavonoid with variable phenolic structures are found in fruits, vegetables and certain beverages. They function as signal molecule, phytoalexins, detoxifying agents and antimicrobial defensive compounds (Panche et al., 2016). Flavonoid class include chalcones, aurones, isoflavones, flavones, flavonols, anthocyanins, proanthocyanidins, flavanones and flavanols (Nix et al., 2017). Specialized cells of plant synthesize phenolics and store them in their vacuoles. Such phenolic-storing cells are distributed within most tissues. Beckman (2000) proposed that these cells by decompartmentation, rapid oxidation of their phenolic content followed by lignification and suberization of cells and cell death, seal off infection or injuries at the site of cellular penetration. If this defense fails and the infection persist, then the same processes promote build-up of indole acetic acid (IAA) and ethylene that causes metabolic cascade in cells that includes secondary metabolism and growth response to produce a peridermal defense. Flavonoid compounds are transported to the site of infection and induce the hypersensitivity reaction, a defense mechanism of plant in response to the infection resulting in programmed cell death (Mierziak et al., 2014).
Flavonoid inhibit fungal growth either by disrupting plasma membrane or by induction of mitochondrial dysfunction. The inhibition mechanisms may also include cell wall formation, cell division, ribonucleic acid (RNA) and protein synthesis (Al Aboody and Mickymaray 2020). Increased resistance in older seedlings of cotton to Rhizoctonia solani infection was due to increase in catechin (Hunter 1974), catechin is flavan-3-ol monomer is an effective antifungal defense against rust infection (Ullah et al., 2017). Flavonoid can kill or inhibit bacterial cell by causing membrane disruption, inhibition of nucleic acid synthesis, quorum sensing which impairs their ability to form biofilms, the antimicrobial action can also inhibit cell envelop synthesis (Gorniak et al., 2019). Sophoraflavanone G isolated from Sophora exigua exerts antibacterial effect by reducing the fluidity of cellular membrane (Tsuchiya and Iinuma 2000). Many flavonoid have evolved as bioactive compounds that interfere with nucleic acid or proteins and show antimicrobial or insecticidal properties (Panche et al., 2016). Flavonoid can modulate the activity of auxin-transporting P-glycoproteins and appears to modulate the activity of regulatory proteins such as phosphatases and kinases (Peer and Murphy 2007). Rutin a flavonoid can function as an activator to improve plant disease resistance (Yang et al., 2016). Polyphenol-rich plant extract may have antibacterial activity (Taguri et al., 2006). Rutin classified as a polyphenolic substance exhibits bactericidal and fungicidal activity in vitro assay. Polyphenolic substances disrupt the cell wall and the cell membrane integrity of microbial cells thereby releases intracellular components (Yang et al., 2016), inhibits nucleotide synthesis and ATP synthesis resulting in inhibition of microorganism (Ahmad et al., 2012). Kaempferol a polyphenol can bind to DNA helicase and inhibits its ATPase activity (Adamczak et al., 2020). Likic et al. (2014) propose modulation of IAA transport through the action of kaempferol has a regulatory role in plant defense responses against virus infection. They also suggest kaempferol to be part of auxin dependent defense response which limits virus infection and that this defense response is activated prior to salicylic acid dependent defense response.
Flavonoid and nitrogenous metabolites such as alkaloids, terpenoids, peptides and amino acids are components of plant seeds. Conjugated form of these compounds are soluble in water and are released in soil. These metabolites in soil may serve as eco-sensing signals for rhizobia and arbuscular mycorrhiza for establishment of symbioses. They may also serve as defense molecules against pathogens and insect pests and can control the plant pest (Striga) of cereal crops (Ndakidemi and Dakora 2003).
Formation of reactive oxygen species (ROS), a by-product of oxidation/reduction (redox) reaction can attack biomolecule causing DNA mutation, protein denaturation and membrane lipid peroxidation they disturb normal cellular metabolism and cause molecular damage or if severe can result in cell death (Racchi 2013). Flavonoid play a role as antioxidant agent and scavenge ROS which are generated on pathogen attack. Flavonoid reduce the production of ROS either by suppression of singlet oxygen or by inhibition of ROS-generating enzymes (cyclooxygenase, lipoxygenase, monooxygenase and xanthine oxidase) or by chelation of transition metal ions which may catalyse ROS production (Mierziak et al., 2014). Flavonoid comprises of phenolic compound with range of biological function. Many biological roles of flavonoid is attributed to cytotoxicity and antioxidant abilities (Pourcel et al., 2007). The variability in content and composition of phenolic compounds provides opportunity to develop resistant cultivar to pathogen attack.
See Part V (B) for further information ……..
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