Vitamin C in fleshy fruits: biosynthesis, recycling, genes, and enzymes

Abstract

L-ascorbic acid (vitamin C) is a plant secondary metabolite that has a variety of functions both in plant tissues and in the human body. Plants are the main source of vitamin C in human nutrition, especially citrus, rose hip, tomato, strawberry, pepper, papaya, kiwi, and currant fruits. However, in spite of the biological significance of L-ascorbic acid, the pathways of its biosynthesis in plants were fully understood only in 2007 by the example of a model plant Arabidopsis thaliana. In the present review, the main biosynthetic pathways of vitamin C are described: the L-galactose pathway, L-gulose pathway, galacturonic and myo-inositol pathway. To date, the best studied is the L-galactose pathway (Smyrnoff–Wheeler pathway). Only for this pathway all the enzymes and the entire cascade of reactions have been described. For other pathways, only hypothetical metabolites are proposed and not all the catalyzing enzymes have been identified. The key genes participating in ascorbic acid biosynthesis and accumulation in fleshy fruits are highlighted. Among them are L-galactose pathway proteins (GDP-mannose phosphorylase (GMP, VTC1), GDP-D-mannose epimerase (GME), GDP-L-galactose phosphorylase (GGP, VTC2/VTC5), L-galactose-1-phosphate phosphatase (GPP/VTC4), L-galactose-1-dehydrogenase (GalDH), and L-galactono1,4-lactone dehydrogenase (GalLDH)); D-galacturonic pathway enzymes (NADPH-dependent D-galacturonate reductase (GalUR)); and proteins, controlling the recycling of ascorbic acid (dehydroascorbate reductase (DHAR1) and monodehydroascorbate reductase (MDHAR)). Until now, there is no clear and unequivocal evidence for the existence of one predominant pathway of vitamin C biosynthesis in fleshy fruits. For example, the L-galactose pathway is predominant in peach and kiwi fruits, whereas the D-galacturonic pathway seems to be the most essential in grape and strawberry fruits. However, in some plants, such as citrus and tomato fruits, there is a switch between different pathways during ripening. It is noted that the final ascorbic acid content in fruits depends not only on biosynthesis but also on the rate of its oxidation and recirculation.

Changes in the biochemical components of mandarin fruits after pest control in the Republic of Abkhazia

“…In citrus fruits, three oxocarboxylic acids, i.e. citric, malic, and acetic have the greatest effect on the fruit taste [6,7,15]. The sweetness is determined by low-molecular carbohydrates, i.e.…”

“…Moreover, these compounds make a different contribution to the creation of a feeling of sweetness. If we take the sweetness of sucrose to be 100%, then the sweetness of glucose is 74%, and the sweetness of fructose is 173% [15,16]. Thus, taking into account the content of basic acids and sugars in citrus fruits, it makes it possible to determine the effect of drugs on the main taste properties after treating plants with insecticides.…”

“…The synthesis of the vitamin C in a plant cell is closely related to the synthesis of polysaccharides, and as our study shows, all the insecticides used (primarily, variants with Siliplant, Cytovite and Diatomite) significantly affect the accumulation of sugars in cells. Since the processes of biosynthesis of the vitamin C are triggered in response to a variety of endogenous and exogenous influences, the increase in ascorbic acid in fruits in these variants can be due to the activating effect of both Siliplant and Cytovite, containing chelated forms of trace elements, and diatomite flour, which includes such elements as Fe, Mg and Ca [15].…”

Physiological and metabolomic analysis reveals maturity stage-dependent nitrogen regulation of vitamin C content in pepper fruit

“…Previous studies have indicated that many plants differ in their pathways to synthesize AsA, and may switch pathways during fruit ripening (Badejo et al, 2012;Liao et al, 2021). In an extensive review, Tyapkina et al (2019) conclude that there is no unequivocal evidence for the existence of a predominant pathway of AsA biosynthesis in fleshy fruits. In pepper plants, the L-galactose pathway has been studied by several researchers (Aloś et al, 2013;Goḿez-Garcıá and Ochoa-Alejo, 2016;Rodrıǵuez-Ruiza et al, 2017), but there is very limited understanding about the other AsA biosynthesis pathways.…”

Effect of High Tunnel Coverings on Antioxidants of Breaker and Light Red Tomatoes at Harvest and during Ripening

“…ROS are known to increase with ripening due to increased respiration, which results in increased antioxidant activity resulting from higher AsA and phenolic compounds [47]. In tomato there is a significant increase in AsA synthesis, recycling, and degradation during ripening [48]. It has been suggested that phenolic compounds are involved in the stability of AsA, as both AsA and total phenolic content in pink stage fruit stored at 12 • C increased during a 15-d storage period [47].…”