## 1 Introduction  
Ascorbic acid (AsA), or vitamin C, is a strong antioxidant molecule. AsA is required for multiple biological functions, for instance, the cofactors for enzyme activity, cell division, cross-linking of cell wall protein, photosynthesis, biotic and abiotic stresses and defense response [1, 2, 3, 4]. Plants synthesize AsA mainly in photosynthetic tissues through the Smirnoff-Wheeler pathway via L-galactose [5, 6]. There are several enzymes in the Smirnoff-Wheeler pathway, starting with phosphglucose isomerase (PGI), which converts D-glucose-6-phosphate to D-fructose-6-phosphate, followed by phosphohannose isomerase (PMI), which converts D-fructose-6-phosphate to D-mannose-6-phosphate. Phosphohannose mutase (PMM), GDP-mannose pyrophosphorylase (GMP or VTC1), GDP-mannose-3\({}^{\prime}\),5\({}^{\prime}\)-epimerase (GME), GDP-L-galactose phosphorylase (GGP or VTC2),L-galactose-1-phosphate phosphatase (GPP or VTC4), L-galactose dehydrogenase (GDH) and L-galactono-1,_A_-lactone dehydrogenase (GLDH) are sequentially catalyzed into the final product, L-ascorbic acid [5]. Rice genomes contain one copy of _OsVTC2_, _OsVTC4_ and _OsGDH_ genes, but two copies of _OsGME_ and _OsGDDH_ and three copies of the _OsVTC1_ gene [6]. GDP-D-mannose pyrophosphorylase (GMP or VTC1) is required for the conversion of D-mannose-1-phosphate to GDP-D-mannose [7]. Among three homologs of rice _OsVTC1_, _OsVTC1-1_ is associated with the greatest AsA production in rice leaves, whereas _OsVTC1-3_ plays a role in AsA synthesis in roots. _OsVTC1-8_ may not be involved in AsA synthesis in both leaves and roots [8]. _vtc1_ mutants have been broadly used to study functional roles in plants. In Arabidopsis, _vtc1_ mutant has a smaller cell size than in the wild type [9]. Similarly, _vtc1_ mutant exhibited reduced shoot growth and delayed flowering, supporting the idea that AsA is required for normal plant growth and development [10]. The _VTC1_ gene is also involved in abiotic stress response. _OsVTC1-1_ RNAi rice, which decreases the expression of the _OsVTC1-1_ gene, exhibits reduced salt tolerance at both seedling and reproductive stages [11]. These reports indicate that the _VTC1_ gene plays multiple roles in plants. Two intermediates of the Smirnoff-Wheeler pathway, GDP-D-mannose and GDP-L-galactose, are also precursors of the non-cellulosic components of the plant cell wall. GDP-mannose-3,5'-epimerase (GME) catalyzes the synthesis of GDP-L-galactose, a precursor for both AsA and cell wall polysaccharides synthesis [3]. GME was revealed to be involved in plant growth, leaf senescence and tolerance to acid, drought and salt in Arabidopsis [12; 13]. VTC2 catalyzes the conversion of GDP-L-galactose to L-galactose-1-phosphate. This step is the first rate-determining step in AsA synthesis [14]. Next, VTC4, GDH and GLDH catalyze the formation of AsA via sugar intermediates, including L-galactose and L-galactono-1,4-lactone [5]. The role of these enzymes has been reported in many studies. For example, VTC2 plays a role in salt tolerance in rice [15]. The bacterium _Pseudomonas syringae_ infection is restricted in _vtc2_ mutant of Arabidopsis, indicating that VTC2 involves in defense response [9]. VTC4 is a bifunctional enzyme that affects the synthesis of both AsA and myo-inositol [16]. Moreover, glutathione peroxidalaxies (GPX) is an antioxidant enzyme that reduces hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)) to water against oxidative damage [17]. In this reaction, H\({}_{2}\)O\({}_{2}\) is detoxified by GPX using hydrogens from reduced glutathione (GSH) resulting in water and oxidized glutathione (GSSG) that associates with the ascorbate-glutathione (ASA-GSH) cycle [18]. Due to antioxidant activity, GPX plays an essential role in various abiotic stress responses and defense mechanisms in plants [19]. Plant cell walls are important in their development and for environmental responses [20]. Cell wall polysaccharides, mainly contain of cellulose, hemicellulose and pectin, are built up of sugars by glycosidic linkages to form polymers [21].