## 2 Biosynthesis and Catabolism of Ascorbic Acid in Plants  
In plants, AsA can be primarily synthesized via the so-called l-galactose pathway, as initially described by [PERSON] et al. [1]. This main biosynthetic pathway consists of eight enzymatic steps (Figure 1), with the first steps prior to GDP-d-mannose involving cell wall polysaccharide precursors and glycoproteins [16]. In this route, d-fructose-6-P derived from d-glucose is initially converted to d-mannose-6-P via phosphomannose isomerase (PMI). In turn, d-mannose-6-P forms d-mannose-1-P via phosphomannomutase (PMM), and d-mannose-1-P forms GDP-d-mannose via GDP-d-mannose pyrophosphorylase (GMP). All the downstream reactions after this step are solely committed to AsA biosynthesis, starting with the conversion of GDP-d-mannose to GDP-l-galactose in a reaction catalyzed by GDP-d-mannose-3,5-epimerase (GME), an enzyme of the extended short-chain dehydase/reductase protein family. This latter gene has been initially considered critical for the regulation of the AsA pool size in plant tissues [17; 18], as it represents the intersection between the l-galactose and cell-wall biosynthesis related pathways [19]. Noteworthy, GDP-l-glucose can also be produced if GME catalyzes 5' epimerization instead of 3',5'epimerization, but as this sugar has no particular function in plants, it has been suggested that it is directly channeled to AsA synthesis [6]. After this key step, GDP-L-galactose is transformed into L-galactose-1-P, L-galactose and L-galactono-1,4-lactone, in the reactions catalyzed by GDP-L-galactose-phosphorylase (GGP) encoded by _VTC2_ and _VTC5_ in _Arabidopsis_[20; 21], by L-galactose-1-phosphate phosphatase (GPP) encoded by _VTC4_ in _Arabidopsis_[22; 23], and by the NAD-dependent L-galactose dehydrogenase (GalDH) [24], respectively. In fact, the reaction catalyzed by GGP serves as the first dedicated step to AsA biosynthesis, and the regulation of the pathway largely resides at this gene [5]. The _Arabidopsis vtc_-deficient mutants enabled the functional characterization of these intermediate steps of the main pathway. Considerable attention has been given to _GMP_ with the identification of the well-characterized _Arabidopsis vtc1_ mutants, which contain 30% less AsA compared to the wild-type (WT) plants [25]. On the other hand, the ROS-sensitive AsA-deficient _vtc2_ mutants have unusually low AsA levels (10-20% of the WT). Despite the fact that _VTC5_ expression is 100-1000 times lower than that of _VTC2_, the double knockout of _vtc2_/_vtc5_ can cause the cessation of seedling growth, damage that  
Figure 1: AsA metabolic pathways in tomato. The main biosynthetic pathway occurring via L-galactose is shown in brown, the D-galacturonate pathway in green, the D-glucuronate pathway in blue, and the AsA recycling pathway in black. Purple symbolizes a reaction taking place in the apoplast. Efficient manipulations of structural genes in either leaves (green) or fruits (red) via transgenic efforts are presented on the left of the reactions with blue arrows, followed by their impact on the AsA pool (increase, decrease, or stable). The fold change >1.5 (overexpression) or <0.5 (silencing) of the AsA contents in transgenic plants compared to wild-type plants was regarded as efficient manipulation. The full list of transgenic approaches is given in Table 1. Regulatory factors affecting positively or negatively the transcription of structural genes are also presented with arrows on the right of the reaction.