Plant antioxidants and their non-traditional sources (review)

The viability of eukaryotes largely depends on a biochemical defense system that protects the body from damage. Antioxidants that neutralize free radicals are significant components of biochemical protective system (M.G. Uzbekov, 2014). Oxidative stress underlies many diseases, e.g., oncological, rheumatoid, bronchopulmonary, cardiovascular, and premature aging (S. Miwa et al., 2016; J.G. Geisler, 2019). There are more than 5,000 antioxidants which differ in chemical composition, antiradical and antiproliferative activity. Many studies show the synergism or additive effect of antioxidants (V. Polonsky et al., 2018). That is, to effectively protect the body, the range of antioxidants consumed must be quite broad. In this regard, it becomes urgent to search for new sources of biologically active substances and increase their content in already cultivated species. This work provides a classification of antioxidants. Among exogenous ones, carotenoids, polyphenols (flavonoids), and trace elements are considered in more detail. The various antioxidant activities of these substances are considered. Flavonoids are the most significant antioxidants. The antiradical activity of flavonoids can be 50 times higher than that of many plant substances, vitamins E and C are notably inferior to them (Y. Yao et al., 2010). Black grain rice varieties are rich sources of flavonoids (U.K.S. Kushwaha, 2016). Carotenoids are another effective antioxidants, the distinctive feature of which is interaction with other substances of this nature which increases the biological activity of the compounds (W. Stahl et al., 2004; С. Hu et al., 2020). Sources with high antioxidant potential and significant accumulation of carotenoids can be red grain varieties of rice, momordica, amaranth (Yu. Fotev et al., 2018; D. Shafigullin et al., 2018). The intraspecific diversity observed at the phenotypic level in terms of color characteristics is associated with both regulatory and structural genes (E.K. Khlestkina et al., 2014). The increased content of proanthocyanidins in the seed coat determines resistance to germination on the root, and the presence of anthocyanins contributes to better preservation of seeds after long-term storage and increased plant resistance to stress (T.L. Korotenko, 2018). Antioxidants increase plant resistance to biotic and abiotic stresses. However, this aspect has not been sufficiently studied in rice varieties with colored pericarp. The study of genetic mechanisms that control plant color traits is relevant in connection with the antioxidant and antimicrobial properties of pigments and their colorless precursors (Y. Qin et al., 2018). These compounds provide the prevention of cancer, reduce the risk of cardiovascular diseases, atherosclerosis, type 2 diabetes, increase immunity, improve the synthesis of visual pigments, activate metabolic processes, and slow down aging (C. Xu et al., 2017). Color variations and grain quality traits in rice samples is controled by 41 loci. The Ra ( Prp-b for varieties with purple pericarp) and Rc (brown pericarp and aleurone layer) genes mainly contribute to the phenotypic effect on rice grain color and nutritional quality (Y. Shao et al., 2011). These genes are located on chromosomes 9, 10 and 8 in the regions of the markers RM228 (amplification product size 90-154 bp), RM339 (166-148 bp), and RM316 (160-210 bp) location (T. Furukawa et al., 2007). Molecular characterization of key genes involved in the biosynthesis of the above compounds will allow breeders to control and accelerate selection for color traits, important for improving the nutritional value of functional products.