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 ………
Al Aboody, M. S. and Mickymaray, S. 2020 Anti-Fungal Efficacy and Mechanisms of Flavonoids. Antibiotics (Basel) 9(2): 45
Aoki, T., Akashi, T. and Ayabe, S-I. 2000 Flavonoids of Leguminous Plants: Structure, Biological Activity and Biosynthesis. J. Plant Res. 113(4): 475 – 488
Brodowska, K. M. 2017 Natural Flavonoids: Classification, Potential Role and Application of Flavonoid Analogues. Eur. J. Biol. Res. 7(2):108 – 123
Cartea, M. E., Francisco, M., Soengas, P. and Velasco, P. 2011 Phenolic Compounds in Brassica Vegetables. Molecules 16: 251 – 280
Chen, J., Ullah, C., Reichelt, M., Gershenzon, J. and Hammerbacher, A. 2019 Sclerotinia sclerotiorum Circumvents Flavonoid Defense by Catabolizing Flavonols Glycosides and Aglycones. Plant Physiol. 180: 1975 – 1987
Gorniak, I., Bartoszewski, R. and Kroliczewski, J. 2019 Comprehensive Review of Antimicrobial Activities of Flavonoids. Phytochem. Rev. 18: 241 – 272
Harborne, J. B. and Williams, C. A. 2000 Advances in Flavonoid Research since 1992. Phytochem. 55(6): 481 – 504
Ikigai, H., Nakae, T., Hara, Y. and Shimamura, T. 1993 Bactericidal Catechins Damage the Lipid Bilayer. Biochimica et Biophysica Acta 1147(1):132 – 136
Mapope, N. and Dakora, F. D. 2013 Role of Flavonoid and Isoflavonoid Molecules in Symbiotic Functioning and Host-Plant Defence in the Leguminosae. Chemistry for Sustainable Development in Africa 33 – 48
Marais, J. P. J., Deavours, B., Dixon, R. A. and Ferreira, D. 2006 The Stereochemistry of Flavonoids In: “Science of Flavonoids”. Grotewold, E. (ed.). Springer, New York, NY. Chapter 1: 1 -46
Mathesius, U. 2018 Flavonoid Functions in Plants and their Interaction with Other Organisms. Plant (Basel) 7(2): 30
Mierziak, J., Kostyn, K. and Kulma, A. 2014 Flavonoids as Important Molecules of Plant Interactions with the Environment. Molecules 19(10): 16240 – 16265
Punyasiri, P. A. N., Abeysinghe, I. S. B., Kumar, V., Treutter, D., Duy, D., Gosch, C., Martens, S., Forkmann, G. and Fischer, T. C. 2004 Flavonoid Biosynthesis in the Tea Plant Camellia sinensis: Properties of Enzymes of the Prominent Epicatechin and Catechin Pathways. Arch. Biochem. Biophys. 431(1): 22 – 30
Reygaert, W. 2014 The Antimicrobial Possibilities of Green Tea. Frontiers in Microbiol. 5(434): 1 – 8
Shaw, L. J., Morris, P. and Hooker, J. E. 2006 Perception and Modification of Plant Flavonoid Signals by Rhizosphere Microorganisms. Environ. Microbiol. 8(11): 1867 – 1880
Steinkellner, S. and Mammerler, R. 2007 Effect of Flavonoids on the Development of Fusarium oxysporum f. sp. lycopersici. J. Plant Interactions 2(1): 17 – 23
Straney, D., Khan, R., Tan, R. and Bagga, S. 2002 Host Recognition by Pathogenic Fungi through Plant Flavonoids. In: “Flavonoids in Cell Function” Buslig, B. S. and Manthey, J. A. (eds.). Advances in Experimental Medicine and Biology Springer Boston, MA. 505: 09 – 22
Treutter, D. 2006 Significance of Flavonoids in Plant Resistance: A Review. Environmental Chemistry Letters 4(3): 147 – 157
Tsimogiannis, D., Samiotaki, M., Panayotou, G. and Oreopulou, V. 2007 Characterization of Flavonoid Subgroups and Hydroxy Substitution by HPLC-MS/MS. Molecules 12(3): 593 – 606
Ullah, C., Unsicker, S. B., Fellenberg, C., Constabel, C. P., Schmidt, A., Gershenzon, J. and Hammerbacher, A. 2017 Flavan-3-ols are an Effective Chemical Defense against Rust Infection. Plant Physiol. 175: 1560 – 1578
Vicente, O. and Boscaiu, M. 2018 Flavonoids: Antioxidant Compounds for Plant Defence and for Healthy Human Diet. Not. Bot. Horti. Agrobo. 46(1): 14 – 21
Weidenborner, M., Hindorf, H., Jha, H. M. and Tsotsonos, P. 1990 Antifungal Activity of Flavonoids against Storage Fungi of the Genus Aspergillus. Phytochem. 29(4): 1103 – 1105
Winkel, B. S. J. 2006 The Biosynthesis of Flavonoids In: “The Science of Flavonoids”. Grotewold, E. (eds.). Springer New York, NY. Chapter 3: 71 – 95
Xie, Y., Yang, W., Tang, F., Chen, X. and Ren, L. 2015 Antibacterial Activities of Flavonoids: Structure Activity Relationship and Mechanism. Curr. Med. Chem. 22(1): 132 – 149
Xu, H. X. and Lee, S. F. 2001 Activity of Plant Flavonoids against Antibiotic Resistant Bacteria. Phytotherapy Res. 15(1): 39 – 43
Yang, Y. and Zhang, T. 2019 Antimicrobial Activities of Tea Polyphenol on Phyopathogens: A Review. Molecules 24(4): 816
Zheng, W. F., Tan, R. X., Yang, L. and Liu, Z. L. 1996 Two Flavones from Artemisia giraldii and their Antimicrobial Activity. Planta Med. 62(2): 160 – 162