## 1 Introduction  
L-Ascorbic acid (AsA, vitamin C) is a highly abundant and essential organic acid for plants and animals. As an antioxidant and enzyme cofactor, AsA plays a crucial role in various plant physiological processes, including removing reactive oxygen species (ROS) [1; 2], enhancing oxidative stress tolerance [3], premature senescence and programmed cell death (PCD) [4; 5], and cell elongation and division in plants [6]. It has been shown that 30% to 40% of AsA in plant cells is located in the chloroplast, which plants use to produce ROS under light respiration and photosynthesis, suggesting AsA plays a vital role in protecting against side effects of photosynthesis in plant [7; 8]. In humans, AsA can scavenge free radicals, thereby preventing and suppressing the occurrence of cancer, reducing blood cholesterol, and enhancing the immune system [9; 10]. Unfortunately, humans have lost the ability to synthesize vitamin C due to the functional loss of L-gulono-gamma-lactone oxidase [11]. Thus, fresh vegetables and fruits including tomato are considered as the primary sources of vitamin C in the human diet. Four pathways for biosynthesis of AsA have been identified (Appendix A), including the D-mannose/L-galactose (D-Man/L-Gal) pathway, the D-gululose pathway, the D-galacturonate pathway, and the myo-inositol pathway [12; 13; 14; 15]. The D-Man/L-Gal pathway is recognized as the most critical biosynthesis pathway in plants, and all structural genes in this pathway have been identified, including those encoding GDP-D-Man pyrophosphorylase (GMP) [16], GDP-Man-3\({}^{\prime}\),5\({}^{\prime}\)-epimerase (GME) [13], GDP-L-Gal phosphorylase (GGP) [17], L-Gal-1-P phosphatase (GPP) [18], L-Gal dehydrogenase (GalDH) [19], and L-galactono-1,4-lactone dehydrogenase (GLDH) [20]. GMP is a rate-limiting enzyme in the D-Man/L-Gal pathway and is involved in the reversible conversion of D-mannose-1-phosphate to GDP-D-mannose. Several molecular studies have proved that _GMP_ has an important role in the D-Man/L-Gal pathway. The AsA content in a _GMP_ mutant named _vtc1_ in Arabidopsis was 75% decreased compared to the wild-type. When _vtc1_ plants were transformed with the wild-type VTC1 gene, foliar AsA concentration was restored to the wild-type level [21]. In the tomato genome, there are four members of _GMP_ gene family (_GMP1-GMP4_). Previously, loss- and gain-of-function of _GMP3_ in tomato resulted in a decreased and increased accumulation of AsA, respectively, confirming the fact that _GMP_ plays a vital role in the biosynthesis of AsA in tomato [22; 23]. GME, which is located downstream of GMP and converts GDP-D-mannose to GDP-L-galactose, is considered to be the central enzyme in AsA biosynthesis. Transgenic tomato plants with _GMEs_ (_SIGME1_ and _SIGME2_) overexpression or suppression exhibit increased or decreased AsA concentration, respectively [24; 25]. The effects of inhibiting _SIGME1_ and _SIGME2_ on ascorbate content showed a consistent function, but only _SIGME1_ but not _SIGME2_ affect the pollen development and pollination process [26]. The enzymatic reaction of GGP is the first step in the particular trail of D-Man/L-Gal pathway different from the D-glucose pathway and is involved in the conversion of GDP-L-galactose into L-galactose-1-phosphate. Three _GGP_ defective mutants, _vtc2-1_, _vtc2-2_, and _vtc2-3_ were screened in ethyl methanesulfonate (EMS)-mutagenized _Arabidopsis_ seedlings by nitroblue tetrazolium (NBT)-based assay [16]. Additionally, _vtc5_ mutant (a close homolog of _VTC2_) was isolated and resulted in a decreased accumulation (80% of the wild type) of AsA _Arabidopsis_[17]. Moreover, the double mutant (_vtc2/vtc5_) showed even less ascorbate accumulation, resulting in immediate growth arrest upon germination and cotyledon bleaching, while normal growth was restored by supplementation with ascorbate or L-Gal [17]. _GPP_, named as _VTC4_ in _Arabidopsis_, converts L-galactose-1-phosphate to L-galactose.