Flavonoid is ubiquitous in plants. Its exudation by roots, control the action of rhizosphere microorganisms. There is a large variation in the composition of flavonoids leading to variation in metabolic profiles (Bovy et al., 2007). Plants often produce pigments around wound, infection sites, insect piercing and feeding sites and physical injuries (Fogelman et al., 2015; Yang et al., 2016; Lu et al., 2017). Flavonoids in general have the capacity to act as antioxidant. Antioxidant activities reduce the damage triggered by reactive oxygen species in plants. Carbohydrates, are a component of osmotic regulation during pathogen infection and may contribute to the accumulation of flavonoid as a defense against infection and disease expansion because of their antioxidant properties (Lu et al., 2017).
- Flavanones: Flavanones are present in all citrus fruits. Citrus flavonoids are antioxidant and anti-inflammatory in action. Example of flavanones are naringenin, hesperidin, naringin, eriodictyol, homoeriodictyol and silybin (Al Aboody and Mickymaray 2020). Naringin and tangeretin are involved in defense mechanism in Citrus aurantium against Penicillium digitatum (Arcas et al., 2000). del Rio et al.(2004) studied the effect of flavanones hesperidin and isonaringin and the polymethoxyflavones sinensetin, nobiletin, tangeretin and heptamethoxyflavone on Phytophthora citrophthora infecting Citrus sinensis fruits. The in vitro studies showed that these compounds acted as antifungal agents, the aglycons (naringenin and hesperetin) being most active followed by polymethoxyflavones and flavanone glycosides. Mixtures containing flavone, flavanone and flavonol caused higher inhibition of fungal growth (Weidenborner and Jha 1993).
- Chalcones: Chalcones are molecule with wide spectrum of biological activities (Diaz -Tielas et al., 2016) such as controls weeds and pests. Chalcones are open–chain flavonoid and are the central precursor of flavonoid. Chalcones are converted into corresponding flavanones by the action of chalcones isomerase or non-enzymatically under alkaline condition (Kaneko et al., 2003). Phloridzin, arbutin, phloretin and chalconaringenin are chalcones. Chalcones possess antifungal activity (Svetaz et al., 2004). Chalchone synthase is a key enzyme of flavonoid/isoflavonoid biosynthesis and is induced in plant against bacterial or fungal infection (Dao et al., 2011). The antifungal, antibacterial activity was observed of chalcones isolated from leaves extract of Myrica serrata (Gafner et al., 1996). Chalcone and dihydrochalcone from Uvaria chamae roots have antimicrobial properties (Koudokpon et al., 2018). Chalcones as other flavonoid and phenolic compounds play a role as signaling molecule in plant-microbe symbioses (Diaz-Tielas et al., 2016).
- Anthocyanins: Anthocyanins are responsible for colours in plants. Cyanidins, delphinidin, malvidin, pelargonidin and peonidin are anthocyanins (Al Aboody and Mickymaray 2020). When one of the phenols is substituted with glycosides, the compound is called an anthocyanin (Barcena et al., 2015). Fruit-rot in grape varieties infected with Botrytis cinerea decreased with increasing anthocyanins content (Schaefer et al., 2008). Himeno et al.(2014) studied anthocyanin accumulation is responsible for purple top symptoms and is associated with reduction of leaf cell death caused by phytoplasma infection. Anthocyanin in plants demonstrate antiviral, antimicrobial activities (Wrolstad 2006). Accumulation of flavonoid in two cultivars (Malus apple) displayed different coloration patterns when their leaves were infected by the rust pathogen (Lu et al., 2017). They observed dark red spots appeared in infected tissues corresponding to increased anthocyanin content and yellow spots with unclear edges appeared because of flavone and flavonol accumulation in the infected tissues. The uninfected and infected tissues of each cultivar presented difference in flavonoid metabolism. Therefore Lu et al.(2017) hypothesized that the flavonoid accumulation-based defense system were different between the two cultivars. Close and Beadle (2003) demonstrate that anthocyanins have strong antifungal activity. Accumulation of anthocyanin reduced susceptibility to Botrytis cinerea (Zhang et al., 2013), it alters the spreading of the reactive oxygen species burst during infection. Anthocyanin have free radical scavenging, antioxidative properties and provides protection against various pathogens (Pervaiz et al., 2017). Skin colour of red potatoes is due to accumulation of anthocyanins in the tuber periderm, a protective tissue that replaces the epidermis at an early stage of tuber development (Fogelman et al., 2015). Anthocyanin function in a diverse way, it attracts pollinators and repel parasites and herbivores (Lev-Yadun and Gould 2008). Coley and Aide (1989) propose that anthocyanins responsible for red coloration of young leaves play a protective role against fungal pathogen. Red cabbage is rich in anthocyanins having antimicrobial action (Hafidh et al., 2011). Production of pigments and defensive compound provides elevated defensive strengths.
See Part V (D) for further information ………
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BIOLOGICAL PROCESSES- A NATURE'S GIFT 