Flavonoids are C6-C3-C6 carbon framework or more specifically phenylbenzopyran functionality and are a secondary metabolite with various biological activities. Depending upon the position of linkage of the aromatic ring to the benzopyrano (chromano) moiety this natural product can be divided into three classes (Marais et al., 2006):
- Flavonoid (2-phenylbenzopyrans)
- Isoflavonoids (3-benzopyrans)
- Neoflavonoids (4-benzopyrans)
Flavonoid produced by plants perform multifunctional role such as defensive role against pathogens, signaling molecules, protect plant from herbivory and from ultraviolet radiation (Harborne and Williams 2000; Straney et al., 2002; Treutter 2006; Winkel 2006). Antipathogenic properties of flavonoids can be non-specific and result in part from their antioxidative properties (Mierziak et al., 2014). The fungal pathogens of plant specializes on a narrow range of specific plants as host. One of the ramification is, host’s chemical can be used to recognize specific host by specialized pathogen (Straney et al., 2002).
Microbial alteration and attenuation of flavonoid signals may have ecological consequences for rhizosphere plant-microbe and plant-plant interaction (Shaw et al., 2006). Flavonoids exuded from root of leguminous plant act as defensive compounds against pathogenic microorganisms, as plant signals in symbiotic nitrogen fixation and arbuscular mycorrhizal symbiosis and is also a known agent in allelopathic interaction (Aoki et al., 2000; Shaw et al. 2006). Mapope and Dakora (2013) studied the effect of root-nodule bacteria (rhizobia) on the synthesis and release of flavonoid and isoflavonoid signal compound and have investigated the biological significance of phytoalexin production in legume plant nodulation and defense against pathogen and insect pests.
Flavonoids are well known for their antibacterial properties against a wide range of pathogenic microorganisms. The proposed antibacterial mechanisms of flavonoid are inhibition of nucleic acid synthesis, suppression of cytoplasmic membrane function, prohibition of energy metabolism, inhibition of the attachment and biofilm formation, inhibition of the porin on cell membrane, alteration of the membrane permeability and attenuation of the pathogenicity (Xie et al., 2015). Steinkellner and Mammerler (2007) suggest low flavonoid concentration exhibit slight antimicrobial properties against Fusarium oxysporum f. sp. lycopersici.
Major subgroup of flavonoid found in plants are flavonols, flavones, flavanols, flavanones, chalcones, isoflavones, anthocyanins (Tsimogiannis et al., 2007; Brodowska 2017; Vicente and Boscaiu 2018).
- Flavonols: Flavonols (3-hydroxyflavones) are flavonoids with a ketone group. They are building block of proanthocyanins and are present in fruits and vegetables. The efficacy of flavonols as antioxidant agent depends on their chemical structure. Antioxidant activity of flavonols may protect against oxidative damage to cells, lipid or DNA (Brodowska 2017). Quercetin, kaempferol, myricetin, datiscetin, isorhamnetin, rutin, resveratrol, fiestin, galangin, pachypodal and rhamnazin are flavonols (Xu and Lee 2001; Al Aboody and Mickymaray 2020). Flavonols protect plant against fungal pathogens unless the pathogen possess the ability to circumvent the toxicity of these phenolic compounds (Chen et al., 2019). Brassica crop may produce quercetin, kaempferol and isorhamnetin (Cartea et al., 2011), which may serve in antimicrobial defense (Xu and Lee 2001).
- Flavones: Flavones have allelopathic activity against other plants and may protect plant from fungal pathogen (Mathesius 2018). Example of flavones are apigenin, luteolin and tangeretin (Xu and Lee 2001; Al Aboody and Mickymaray 2020). Flavones apigenin is found in onion, chamomile, wheat sprout etc. and possess antibacterial and antioxidant properties (Brodowska 2017). Flavone bioactivity is their structural diversity allowing flavones to interact with different molecules determining their functions in lipid oxidation, DNA and protein binding (Mathesius 2018). The antifungal activity is demonstrated by unsubstituted flavones (Weidenborner et al., 1990). Flavones exhibit antimicrobial activity (Zheng et al., 1996).
- Flavanols: Flavanols are also referred to flavan-3-ols. Catechin is the representative of the group of flavanol and is known as the building block of tannins. These compounds may be found in the seeds and the skin of the fruits which are not fully ripened (Brodowska 2017). Catechins are associated with antimicrobial activity affecting the cell membrane (Gorniak et al., 2019). The major polyphenols in green tea are flavonoids and the four flavonoid in green tea are catechin: epicatechin, epigallocatechin, epicatechin gallate and epigallocatechin gallate. Catechin ruptures bacterial membrane by binding to the lipid bilayer cell membrane (Ikigai et al., 1993; Reygaert 2014). Ullah et al.(2017) indicate that catechin and proanthocyanidins are effective antifungal defences in poplar against foliar rust infection. Catechin plays a crucial role in defense against pathogens of tea (Punyasiri et al., 2004). The existing studies suggest tea polyphenol have potential to control plant pathogen such as fungi, bacteria and viruses (Yang and Zhang 2019).
Identification of flavonoid with possible antifungal/antimicrobial properties may assist in management of plant pathogens.
See Part V (C) for further information ………
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BIOLOGICAL PROCESSES- A NATURE'S GIFT 