# Vitamin C in Plants: From Functions to Biofortification
## 3 Multiple Roles of Vitamin C in Plants
On the contrary, the same mutants are more susceptible to the necrotic ascomycete _Alternaria brassicola_[89]. Nevertheless, exogenous addition of ASC acts as an inducer of disease resistance in different plant-pathogen interactions [89; 90; 91; 92]. In plants, vitamin C can control the division, elongation, and differentiation of cells, as well as programmed cell death (PCD). Vitamin C plays a significant role in the control of cell division. This metabolite in the meristematic cells of root meristems can shorten the G1 phase and stimulate entry into the S phase [93; 94]. In the quiescent center of the root meristem, where cells are not dividing, low levels of ASC, linked to a significant increase of the ASC-consuming enzyme ASC oxidase (AOX), are responsible for the arrest of the cell cycle in the G1 phase [95]. In tobacco BY-2 cells, a peak in ASC, as well as l-galactono-1,4-lactone dehydrogenase (GLDH), activity overlaps with the peak in the mitotic index [96; 97]. Moreover, cells enriched with ASC show stimulation of cell division, whereas enrichment with DHA leads to a reduction in cell division, suggesting that the ASC redox state is fundamental to cell cycle progression [96]. Low ASC levels and an altered redox state negatively affect cell cycle progression in the root meristem of _Arabidopsis_, with a consequent decrease in the number of cells in the proliferation zone [98]. An ASC increase has also been shown during cell divisions in developing embryos [99]. The stimulation of division in the apical meristem by ASC seems to be principally due to the inhibition of peroxidase involved in the crosslink of cell wall components. Inhibition of vitamin C biosynthesis leads to abortion of the meristem [100]. The stimulation of cell elongation is due to the expression of AOX, the activity of which determines an increase in oxidized forms of vitamin C in the apoplast [101; 102]. Indeed, apoplastic MDHA participates in transmembrane electron transfer, accepting electrons from cytochrome b. This process induces plasma membrane hyperpolarization and activation of H+-ATPase, with an acidification of the apoplast that favors cell wall relaxing [103]. The parallel oxidation of NADH acidifies the cytoplasm and activates vacuolar H+-ATPase, increasing vacuolization and cell expansion [103; 104; 105]. In the apoplast, ASC in the presence of Cu2+ can exert its pro-oxidant role, producing H2O2, which induces degradation of polysaccharides [106; 107]. Moreover, by reducing lignin precursors utilized by peroxididases, ASC delays cell wall lignification [108]. With the transition from meristematic to differentiated cells, ASC levels significantly decrease, permitting the activity of secretory peroxididases and, consequently, cell wall stiffening and lignification, occurring during differentiation [109]. Vitamin C is also involved in the control of PCD. _A. thaliana_ mutants, with low vitamin C content, spontaneously trigger localized cell death like that which occurs during hypersensitive response, a plant-defense mechanism activated to block pathogen invasion [87]. An ASC decrease is necessary for PCD induced by H2O2 and heat shock (HS) in tobacco BY-2 cells [110; 111; 112; 113]. The decrease in ASC during HS-induced PCD is due to inactivation of the last enzyme of the vitamin C biosynthetic pathway [114]. Low levels of ASC during PCD are parallel to a decrease in the level of transcript, protein, and activity of APX [111; 112; 115]. The impairment in ASC and APX is needed to increase ROS, which are essential for PCD induction [116; 117]. Interestingly, the increase in vitamin C biosynthesis by the supply of l-galactono-1,4-lactone delays PCD occurring during kernel maturation in durum wheat, with a consequent postponement of dehydration and improvement in kernel filling [118]. Vitamin C, regulating the abovementioned processes at molecular and cellular levels, is therefore involved in different phases of plant growth and development, such as seed maturation and germination, flowering, fruit ripening, and senescence [119].