## 1 Introduction
Lower AsA content resulted from reduced pectin solubilization in the cell walls of transgenic strawberry fruit, which also had a decreased expression of an endogenous spectate lyase gene [18]. Transforming lettuce and tobacco with cDNA encoding L-gulono-1,4-lactone oxidase increased vitamin C leaf content by 4- to 7-fold [19]. Moreover, the expression of _GalUR_ was shown to be associated with changes in the AsA level of strawberry fruit during the ripening process, as well as with differences in the AsA content of various _Fragaria species_, indicating that the D-galacturonic acid pathway predominantly operates in developing fruits [18]. The 2-3 times higher AsA levels in transgenic _Arabidopsis_ were linked to the overexpression of _FaGalUR_, indicating that it is possible to engineer this gene to produce higher vitamin C levels in plants [18]. Feeding experiments on Arabidopsis, lettuce, potato, and tomato plants demonstrate enhanced AsA levels when pure L-gulose is supplied to plant tissues [20; 21; 22; 23]. In 2003, the L-gulose pathway, also known as the L-gulose shunt, was proposed based on evidence of GDP-D-mannnose conversion to GDP-L-gulose by GDP-D-mannose 3,5-epimerase (GME) and GDP-D-mannnose [15]. In Arabidopsis lines overexpressing _GuILO_, drastically elevated biomass accumulation was seen in both aerial and root tissues compared to wild-type plants and empty vector controls growing under the same conditions [24]. These high-AsA lines also tolerated heat, cold, and salt better. Interestingly, the high-AsA lines did not exhibit the same severity of symptoms at the tested concentrations of protected pyrene (PYR). However, the wild-type plants experienced stunted growth of aerial tissue, fewer root hairs, and hindered leaf expansion [24]. There are multiple isoforms of _L-GuILO_ in the Arabidopsis genome, with unclear activity [20]. AtGuILO5, a purified enzyme, exhibits similar properties to bacterial isozymes in terms of substrate specificity, subcellular localization, electron acceptor use, and activity, distinguishing it from plant and mammalian GuillOs. With the overexpression of L-GuILO genes of Arabidopsis in tobacco  
Figure 1: Overview of pathways leading to L-ascorbic acid (AsA) biosynthesis in plants. From left to right: Myo-inositol pathway, L-gulose pathway, L-galactose pathway, and D-galacturonic acid pathway. cells, higher AsA concentration was observed only when the cultures were supplied with L-gallouno-1,4-lactone, revealing that this pathway is substrate-dependent [25]. The last alternative pathway that contributes to AsA biosynthesis is the Myo-inositol pathway [26]. Molecular and biochemical evidence of the Myo-inositol pathway as a possible biosynthetic route using myoinositol (MI) as the initial substrate was suggested [26]. The enzymatic activity of the MIOX gene was verified in bacterially produced recombinant proteins, and it was discovered on chromosome 4 (MIOX4) of the Arabidopsis ecotype Columbia. The Arabidopsis genome has four MIOX genes, which were given the names _AtMIOX1_, _AtMIOX2_, _AtMIOX4_, and _AtMIOX5_ based on where on the chromosomes they are located [27]. During development, the four MIOX genes were differently regulated, with _MIOX2_ being the most prevalent [27]. The involvement of _MIOX_ in AsA synthesis and the possibility of exploiting this gene for the agronomic and nutritional development of crops were confirmed with a 2- to 3-fold increase in AsA, and the increased level was attributed to the overexpression of _AtMIOX4_[24; 26]. Recently, five MIOX protein-coding genes and _MIOX_ motifs were found in tomato (_S. Lycopersicum_). When compared to controls, transgenic lines created by overexpressing the tomato _SIMOX4_ demonstrated a substantial increase in total AsA in the leaves and red fruits [28].