## 2 Biosynthesis and Catabolism of Ascorbic Acid in Plants
PGI: Phosphoglucose isomerase; PMI: Mannose-6-phosphate isomerase; PMM: Phosphomannoutase; GMP: GDP-d-mannose pyrophosphylase; GME: GDP-D-mannose 3′5′5’: epimerase; GGP: GDP-L-galactose-phosphorylylase; GPP: L-galactose-1-P phosphatase; GalDH: L-galactose dehydrogenase; GLDH: L-galactono-1,4-lactone dehydrogenase; GalUR: _D_-galacturonate reductase; MIOX: _my_-inositol oxygenase; GuL: _D_-gallono-1,4-lactone dehydrogenase; AO: ascorbate oxidase; APX: ascorbate peroxidase; MDHAR: monodehydro-ascorbate reductase; DHAR: dehydrohydro-ascorbate reductase; MDHA: monodehydro-phosphate; DHA: dehydroresource; GSH: glutathione; GSSG: oxidized glutathione. can be reversed when plants are fed with AsA itself or l-galactose [20; 21], suggesting the requirement of both genes for AsA biosynthesis. As both _VTC2_ and _VTC5_ transcript levels are not induced in response to H\({}_{2}\)O\({}_{2}\) or other oxidative stress, indicating that the signal is not ROS-related, _VTC2_ is suggested to regulate the pathway by feedback inhibition via reduced translation [5]. Indeed, _VTC2_ can be regulated by a non-canonical upstream open reading frame (uORF), which encodes for a peptide that acts as an inhibitor of translation when AsA is highly abundant, or a stimulator of GGP translation when AsA is at low levels [26]. Recent studies on a cis-acting uORF in GGP1 highlighted its role in regulating redox homeostasis, normal plant development, and pollen fertility in tomato [27]. On the other hand, the partial AsA deficiency in _Arabidopsis vtc4_ mutants, in combination with the fact that these mutants also contained low levels of _myo_-inositol reinforces the notion that this enzyme has probably a double activity on both L-galactose-1-P and d-myo-inositol 3-P [23]. As the last enzyme of the l-galactose pathway, L-galactono-1,4 lactone dehydrogenase (GLDH), is placed and active in the mitochondrion, L-galactono-1,4-lactone must be transferred from cytosol to the inner mitochondrion membrane, to produce AsA (Figure 1). An interesting note here is that this final reaction in the AsA biosynthetic pathway is catalyzed by a dehydrogenase via cytochrome c, and not by an oxidase, as in the animal pathway. Consequently, no H\({}_{2}\)O\({}_{2}\) is released, and no side effects over the cell redox state occur [28], highlighting the significance of the interaction between AsA synthesis and energy metabolism via the respiratory electron chain [29], which is yet to be entirely clear. Several alternative AsA biosynthetic pathways have been proposed in different plant species, including those related to d-galacturonic acid or _myo_-inositol (Figure 1), but the overall contribution of these routes on AsA accumulation is highly debatable and seems to be predominantly species- and stage-dependent. For example, the d-galacturonic acid pathway via d-galacturonate reductase (GalUR) may supplement the AsA pool size in certain species such as strawberry [30], orange [31], apple [32], grape [33], and rose [34], or at specific developmental stages, for example, ripe tomato fruit [35]. On the other hand, the alternative route derived from _myo_-inositol, which is predominantly involved in hexose, starch, and pectin metabolism, rather than in AsA biosynthesis, seems to be less clear, and unlikely to significantly fine-tune AsA accumulation [14; 36; 37]. Notwithstanding, as the AsA pathway through this way is considerably shorter than through the l-galactose pathway, it may complement the predominant biosynthetic route particularly in fruit tissues under stress conditions [33]. Beyond its biosynthesis, AsA can undergo regeneration through the so-called ascorbate-glutathione cycle (Figure 1), as initially proposed by [PERSON] and [PERSON] [38], to preserve redox cellular homeostasis. Being a strong antioxidant molecule, AsA can receive electrons from several free radicals.