# Vitamin C in Plants: From Functions to Biofortification
## 1 Introduction  
Vitamin C (\(\iota\)-ascorbic acid) was isolated from the adrenal cortex by [PERSON] in 1928. [PERSON] demonstrated that this compound, which can act as a powerful reducing agent, indicated with the empirical formula of C\({}_{6}\)H\({}_{8}\)O\({}_{6}\), had a molecular mass of \(178\pm 3\) and was a lactone with an acidic hydrogen atom. Due to its similarity to simple sugars and its acidic properties, [PERSON] called this compound \"hexuronic acid\" [1]. In 1932, [PERSON] isolated an antiscorbitule compound from lemon juice that was recognized as the hexuronic acid found by [PERSON] [2, 3]. At the same time, [PERSON] showed that 1 mg/day of hexuronic acid provided ample protection against scurvy [4]. 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 [PERSON] 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]. Vitamin C is the most abundant water-soluble compound working in one-electron reactions, and it is an essential micronutrient and a key element for the metabolism of almost all living organisms. In humans, vitamin C has numerous functions, mainly acting as an antioxidant and a cofactor for mono-oxygenases and dioxygenases [7]. The roles of vitamin C as an antioxidant in humans has been established based on a large body of scientific evidence. Vitamin C, by scavenging free radicals, protects DNA, proteins, and lipids fromoxidative damages [8]. Vitamin C is used as an antioxidant throughout the body but may have specific roles in some organs. For instance, vitamin C is required in the eyes at a millimolar concentration to guarantee protection from oxidative damage due to solar radiation [9]. Vitamin C inhibits the synthesis of the carcinogenic nitrosamines, which can be synthesized in the intestine or absorbed with food [10], and reduces tetrahydrorobiopterin, the cofactor of nitric oxide synthase, which catalyzes the synthesis of nitric oxide [11]. Vitamin C is also important for iron bioavailability, reducing non-heme iron from the ferric (Fe\({}^{3+}\)) to the ferrous (Fe\({}^{2+}\)) form, which is more easily absorbed in the intestine; for this reason, this vitamin is indirectly needed to protect against anemia [12]. Vitamin C also influences iron metabolism through the stimulation of ferritin synthesis and the inhibition of ferritin degradation [13]. Vitamin C, as a cofactor of peptidyl-glycine alpha-amidating monooxygenase, is involved in the biosynthesis of many signaling peptides, such as oxytocin, vasopressin, cholecystinin, and calcitonin [14; 15; 16]. Vitamin C functions as a cofactor for many dioxygenases, reducing the iron in the active site of these enzymes to Fe\({}^{2+}\). Vitamin C contributes to the correct formation of collagen through post-translational modifications of procollagen. In particular, this vitamin acts as a cofactor for the reaction catalyzed by prolyl 3-hydroxylase, prolyl 4-hydroxylase, and lysyl hydroxylase, which are involved in the hydroxylation of lysine and proline and permit the formation of the stable structure of collagen [17; 18; 19]. Vitamin C, as a cofactor of hydroxylase, is used for the synthesis of norepinephrine and carnitine [20; 21; 22]. Vitamin C intervenes in many cytochrome-P450-dependent hydroxylation reactions, such as the transformation of cholesterol into bile acids, the degradation of exogenous substances such as pollutants and drugs, and the synthesis of steroid hormones [23]. Recently, vitamin C has been identified as a cofactor for the methylcytosine dioxygenases ten-eleven translocation (TET), which is involved in DNA demethylation and JmjC-domain-containing proteins, which catalyze the demethylation of histones [24].