# Metabolism and Regulation of Ascorbic Acid in Fruits
## 3 Transcriptional Regulation of Ascorbic Acid Metabolic Genes in Fruit  
Ascorbic acid is enriched in fruit and is closely related to fruit development and resistance. Changing the content of ascorbic acid in fruit is of great significance for improving fruit quality. Tomato, strawberry, kiwifruit, citrus, and other fruits are rich in ascorbic acid, with complete genome information and a mature genetic transformation system, which are the main objects of research related to the regulation of ascorbic acid content. For example, there were significant differences in the expression levels of AsA synthesis and metabolism-related genes in citrus fruits with different ascorbic acid contents [83; 84]. It is of great significance to improve the ascorbic acid content in fruit by changing the expression of genes related to AsA synthesis and metabolism. Substrate feeding experiments have shown that AsA synthesis in tomato leaves mainly depends on the L-galactose pathway, while the L-galactose pathway, the D-galacturonate pathway, and the myo-inositol pathway are all involved in AsA synthesis in tomato fruit [35]. Many genes encoding key enzymes in the AsA metabolism pathway have been identified. The content of AsA is significantly changed in fruits when the expression of these genes is altered through genetic engineering (Table 2). GMP is a key rate-limiting enzyme in the L-galactose pathway. There are four members of the GMP family in tomato (_SIGMP1-SIGMP4_). Upregulation or downregulation of _SIGMP3_ expression in tomato fruit can significantly increase or decrease the content of AsA [85]. Overexpression of tomato _SIGME1_ and _SIGME2_ increase AsA content to 1.60- and 1.24- fold in ripe red fruit, respectively [86]. However, silencing _SIGME1_ and _SIGME2_ alone does not affect AsA content in tomato fruit [87]. Conversely, simultaneous silencing of both _SIGMEs_ genes significantly reduces AsA content in tomato fruit, suggesting a functional redundancy between _SIGME1_ and _SIGME2_[38]. Overexpression of _GGP_ significantly increases AsA content in tomato leaves but does not affect AsA content in fruit [35]. However, studies have also shown that the AsA content in leaves, flowers, and fruits is significantly reduced in tomato _SIGGP1_ mutants [88; 89]. Heterologous expression of the _GGP_ gene of kiwifruit in tomato and strawberry results in a twofold increase in AsA content in the fruits [90]. In addition, overexpression of _AcGGP3_ can significantly increase the AsA content in leaves and fruits of kiwifruit, indicating that the role of GGP in the AsA synthesis pathway is tissue-specific in different plants [91; 92]. GPP catalyzes the dephosphorylation of L-galactose-1-phosphate to L-galactose, an essential part of the L-galactose pathway. However, no significant changes in AsA content are found in the fruits of the _GGP_ overexpression line and the _GGP_\(\times\)_GPP_ pyramiding lines [35]. A recent study by [PERSON] et al. indicates that overexpression of _SIMP3_, a gene encoding a bifunctional enzyme with GPP and IMP activities, significantly increased AsA content in tomato leaves, stems, and fruits [93]. Studies on the functions of GalDH and GLDH in AsA synthesis are limited. The only research shows that GLDH has no significant effect on AsA accumulation in tomato fruits [17]. L-gulono-\(\gamma\)-lactone oxidase (_GLOase_) is the homologous gene of _GLDH_ in animals. A 1.5-fold increase in AsA levels is found in tomato fruit heterologously expressing rat GLOase [94]. In the myo-inositol pathway, MIOX is involved in the production of D-glucuronate. Heterologous expression of _AtMIOX_ increases the AsA content of tomato fruit by 1.4-fold [95]. [PERSON] et al. identifies five MIOX (_MIOX1\(\sim\)MIOX5_) in tomatoes, and overexpression of _MIOX4_ significantly increases the AsA content in leaves and red fruits [96]. _GalUR_ is downstream of _MIOX_, which catalyzes the conversion of D-glucuronate to L-gulonate.