Abstract
Vitamin C (l-ascorbic acid) is an excellent free radical scavenger, not only for its capability to donate reducing equivalents but also for the relative stability of the derived monodehydroascorbate radical. However, vitamin C is not only an antioxidant, since it is also a cofactor for numerous enzymes involved in plant and human metabolism. In humans, vitamin C takes part in various physiological processes, such as iron absorption, collagen synthesis, immune stimulation, and epigenetic regulation. Due to the functional loss of the gene coding for l-gulonolactone oxidase, humans cannot synthesize vitamin C; thus, they principally utilize plant-based foods for their needs. For this reason, increasing the vitamin C content of crops could have helpful effects on human health. To achieve this objective, exhaustive knowledge of the metabolism and functions of vitamin C in plants is needed. In this review, the multiple roles of vitamin C in plant physiology as well as the regulation of its content, through biosynthetic or recycling pathways, are analyzed. Finally, attention is paid to the strategies that have been used to increase the content of vitamin C in crops, emphasizing not only the improvement of nutritional value of the crops but also the acquisition of plant stress resistance.
Highlights
Vitamin C (l-ascorbic acid) was isolated from the adrenal cortex by Albert Szent-Györgyi in 1928.Szent-Györgyi demonstrated that this compound, which can act as a powerful reducing agent, indicated with the empirical formula of C6 H8 O6, had a molecular mass of 178 ± 3 and was a lactone with an acidic hydrogen atom
Similar positive results have been reported for tomato, where, there was a moderate increase in vitamin C content, an increase in total antioxidants occurred that was linked to redox state regulation [213]; tomato plants overexpressing FaGalUR were found to be more tolerant to abiotic stresses [214]
In the tomato introgression line IL12-4-SL, the genes encoding for pectin methylesterase, polygalacturonase, and UDP-d-glucuronic-acid-4-epimerase, which are involved in pectin degradation, have been identified as candidate genes for a quantitative locus (QTL) associated with high vitamin C content, suggesting that marker-assisted selection could be a good strategy to enhance vitamin C accumulation [215]
Summary
Vitamin C (l-ascorbic acid) was isolated from the adrenal cortex by Albert Szent-Györgyi in 1928. The definitive structure of vitamin C, which is a hexonic acid aldono-1,4-lactone with an enediol group on C2 and C3, was achieved by Norman Haworth in 1933 [5] The evidence that this compound was able to prevent scurvy led to it being renamed from hexuronic acid to ascorbic acid [6]. Studies conducted on humans have not shown significant differences in bioavailability between synthetic and plant-derived vitamin C [28]. Despite the comparable dosage and bioavailability, it has been shown that orange juice and not synthetic vitamin C drink protects leukocytes from oxidative DNA damage [30]. The loss of the capability to synthesize vitamin C in our ancestors would not have been a disadvantage with a diet rich in vegetables and fruits, which could have provided enough vitamin.
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