# Vitamin C in Plants: From Functions to Biofortification
## 3 Multiple Roles of Vitamin C in Plants  
A significant part of accessible glucose (about 1%) is used for vitamin C production, which is present at high concentration in plants [60]. Vitamin C was found in all cell compartments, including the apoplast (the cell wall and extracellular space), reaching a concentration of 20 mM in chloroplasts [61]. However, the vitamin C content significantly differs among plant species and in the same species between diverse cultivars [62]. Moreover, the vitamin C content varies among different tissues and organs, usually being high in leaves, meristematic tissues, flowers, or young fruits and low in non-photosynthetic organs such as stems and roots [54; 62]. Only seeds that reach maturity in a stage of strong dehydration (orthodox seeds) contain little vitamin C, which is essentially in the oxidized form [63; 64]. In the same organ or tissue, vitamin C content is influenced by the plant developmental stage and environmental changes [65; 66; 67; 68]. Light is one of the most significant environmental signals involved in the regulation of vitamin C levels [69; 70]. As in humans, vitamin C favors iron uptake in plants. The ASC efflux in the apoplast contributes to the reduction of Fe\({}^{3+}\), catalyzed by the ferric chelate reductase plasma membrane enzyme. _Arabidopsis_ mutants having low vitamin C content (_vtc_ mutants) show a decrease in Fe\({}^{3+}\) reducing capability and a consequent reduction of iron accumulation in the seeds [71]. Vitamin C, having different functions in chloroplasts, is essential for the correct functionality of photosynthesis. Firstly, ASC has a key role in the direct scavenging of ROS and in the removal of H\({}_{2}\)O\({}_{2}\) through the water-water cycle [72; 73]. ASC also participates in the xanthophyll cycle, which is needed to protect photosystem II (PSII) from photoinhibition. In this cycle, ASC is the cofactor of violaxanthin de-epoxidase, which converts violaxanthin in zeaxanthin, the xanthophyll responsible for dissipating excess excitation energy in the light harvesting complexes of PSII [74]. Finally, ASC can donate electrons to both photosystems, especially when they are damaged by stress conditions [75; 76]. Changes in vitamin C content significantly modify the expression of genes linked to photosynthesis [77]. The lowering of the ASC content, through the suppression of DHAR expression, leads to the loss of chlorophyll a, the reduction of the RUBISCO large subunit, and a decrease in CO\({}_{2}\) assimilation [78]. Consistently, vitamin C-deficient _Arabidopsis_ mutants enter senescence earlier than wild-type [79]. Thus, vitamin C, by preserving photosynthetic functioning and limiting ROS-mediated damage, slows down leaf senescence [78; 79; 80; 81]. Vitamin C is involved in the synthesis of the plant hormone ethylene, acting as a cofactor of 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase, the enzyme that catalyzes the last biosynthetic step. Indeed, ASC contributes to the ring opening of ACC by supplying the electron to the active site of the enzyme [62; 82]. Being a cofactor of dioxygenases, vitamin C could also be involved in the synthesis of abscisic acid and gibberellins, as well as in the catabolism of auxins [62]. A complex interplay between vitamin C and hormone signaling intervenes in different phases of plant growth and development, as well as in plant response to the environment and pathogens [83; 84]. In particular, vitamin C involvement in the defense response against pathogens is strictly dependent on pathogen lifestyles [84]. It is known that defense against biotrophic pathogens is mediated by salicylic acid signaling, whereas defense against heterotrophic pathogens is mediated by jasmonic acid and ethylene signaling [85]. _Arabidopsis_ mutants with low vitamin C levels show an increase in salicylic acid, pathogenesis-related proteins, and camalexin and are more resistant to _Pseudomonas syringae_ and _Peronospora parasitica_[79; 86; 87; 88].