Vitamin C Content in Fruits: Biosynthesis and Regulation

Abstract

Throughout evolution, a number of animals including humans have lost the ability to synthesize ascorbic acid (ascorbate, vitamin C), an essential molecule in the physiology of animals and plants. In addition to its main role as an antioxidant and cofactor in redox reactions, recent reports have shown an important role of ascorbate in the activation of epigenetic mechanisms controlling cell differentiation, dysregulation of which can lead to the development of certain types of cancer. Although fruits and vegetables constitute the main source of ascorbate in the human diet, rising its content has not been a major breeding goal, despite the large inter- and intraspecific variation in ascorbate content in fruit crops. Nowadays, there is an increasing interest to boost ascorbate content, not only to improve fruit quality but also to generate crops with elevated stress tolerance. Several attempts to increase ascorbate in fruits have achieved fairly good results but, in some cases, detrimental effects in fruit development also occur, likely due to the interaction between the biosynthesis of ascorbate and components of the cell wall. Plants synthesize ascorbate de novo mainly through the Smirnoff-Wheeler pathway, the dominant pathway in photosynthetic tissues. Two intermediates of the Smirnoff-Wheeler pathway, GDP-D-mannose and GDP-L-galactose, are also precursors of the non-cellulosic components of the plant cell wall. Therefore, a better understanding of ascorbate biosynthesis and regulation is essential for generation of improved fruits without developmental side effects. This is likely to involve a yet unknown tight regulation enabling plant growth and development, without impairing the cell redox state modulated by ascorbate pool. In certain fruits and developmental conditions, an alternative pathway from D-galacturonate might be also relevant. We here review the regulation of ascorbate synthesis, its close connection with the cell wall, as well as different strategies to increase its content in plants, with a special focus on fruits.

Impact of Silicon Nanoparticles on the Antioxidant Compounds of Tomato Fruits Stressed by Arsenic

“…Vitamin C has a main function as an antioxidant and cofactor in redox reactions, in addition to being involved in the activation of epigenetic mechanisms that control cell differentiation [70]. However, vitamin C is highly unstable, and its levels may vary if subjected to stress factors [71].…”

Ascorbate and Thiamin: Metabolic Modulators in Plant Acclimation Responses

“…Besides GGP, GLDH has been also suggested as an important controlling point for light regulation of ascorbate biosynthesis at the level of the enzyme activity [67,68]. Arabidopsis plants, grown under high light after supplementation with L-galactone-1,4-lactone (L-Gal; the precursor of ascorbate), accumulated up to twofold ascorbate levels and had twice as high GLDH activities of the low-light grown plants, assumed as higher respiration rates [69].…”

“…GLDH is located in the inner membrane of the mitochondria, which carries a redox-sensitive thiol residue (Cys-340), critical for the conversion of L-Gal into ascorbate [70]. This residue has been validated to be irreversibly oxidized by H 2 O 2 , inactivating GLDH [70], and has been suggested to be responsible for regulation of GLDH activity during the early stages of heat stress produced programmed cell death [67,71]. Moreover, Arabidopsis GLDH overexpressing lines accumulated higher ascorbate levels and demonstrated higher chlorophyll fluorescence parameters after exposure to high light for 14 days, which led them to have lower sensitivity to light stress [72].…”

Accumulation of Ascorbic Acid in Tomato Cell Culture: Influence of the Genotype, Source Explant and Time of In Vitro Cultivation

“…Fruits and vegetables are the main dietary source of antioxidants [5]. After oranges, tomatoes (Solanum lycopersicum L.) are the most important source of vitamin C [6], also because they are often eaten raw and in relatively large quantity compared to other spices, fruits and aromatic herbs. AsA accumulation in tomato and in other plant species is a complex process that involve different metabolic 2 of 14 pathways [7].…”

“…Antioxidants usually occur at low concentrations in plants [6]. Moreover, AsA content decreases with the storage time of the fresh vegetable [25,26].…”