## 3. Asxorbic Acid Accumulation and Metabolism in Tomato
\begin{table}
\begin{tabular}{c c c c} \hline
**Species** & **Fruit** & **AsA Content (mg/100 g FW)** & **Reference** \\ \hline _Malpighia marginata_ & Acerola & 1190–2187 & [52] \\ _Actinidia deliciosa_ & Kiwifruit & 29–120 & [2; 4] \\ _Fragaria \(\times\) ananassa Duch._ & Strawberry & 10–80 & [2; 4; 6] \\ _Solanum lycopersicum_ & Tomato (cultivated varieties) & 6–23 & [14; 47] \\ _S. pennellii_, _S. pimpinellifolium_ & Tomato (wild species) & 6–50 & [47] \\ _Cucumis melo_ L. & Melon & 10–35 & [2; 6; 7] \\ _Malus domestica Borkh._ & Apple (cultivated varieties) & 1–13 & [53; 54] \\ _Malus_ spp. & Apple (wild varieties) & 2–28 & [53] \\ \hline \end{tabular}
\end{table} 
Table 1. The AsA content in commonly consumed fruits. silencing of \(AO\) caused a significant increase in AsA content in ripe melon [44]. Several genes involved in pectin degradation such as polygalacturonase (_PG_), pectinethylesterase (_PME_), and UDP-D-glucuronic-acid-4-epimerase were also found to enhance the AsA pool via the alternative D-galacturonate pathway [60]. Recently, a group of genes related to sugar and hormone pathways were mapped within, and out of, the introressed region of IL 12-4, and thus considered candidate genes regulating the AsA pool in the fruit of the sub-line [61]. Recombinant inbred lines have been also employed to identify key genes governing AsA levels in tomato fruits, evidencing the co-regulation of AsA and hormone metabolism [62]. Nonetheless, apart from the profound genetic differences in the AsA pool between cultivars, remarkable differences can be also attributed to different environmental (presumably light and temperature) and growing conditions, as well as ripening stage at harvest and postharvest handling [2; 63]. Among them, the ripening stage at harvest is probably the key regulatory factor in climacteric species such as tomato, as ripening is thought to be an oxidative process influencing both antioxidant machinery and senescence-related processes such as cell-wall loosening [64]. Despite the inconsistency of findings related to AsA accumulation patterns in tomatoes, the general consensus is that young fruits at early developmental stages usually exert higher AsA biosynthetic capacities than mature ones, to support cell division and expansion. By contrast, as ripening progresses, AsA accumulation can be either due to increased recycling and/or decreased breakdown [14; 47], or due to enhanced biosynthesis via _GGP_ and _GPP_, and also correlated with increased respiration rates [13]. Using weighted gene correlation network analysis (WGCNA), others reported a lack of correlation between AsA biosynthetic genes and AsA accumulation rate in ripening tomato fruits [65], indicating the cultivar-dependent manner of AsA regulation under different growing conditions. At the gene level, the changes in AsA accumulation during tomato ripening have been correlated with transcript levels of key genes of the l-galactose pathway such as _GGP_, _GPP_[13], or _GGP_[14], and of the recycling pathway, that is, _MDHAR_, but not with the expression of genes from the alternative biosynthetic pathways such as _myo_-inositol oxygenase (_MIOX_), suggesting that it may serve as a supplementary pathway to support biosynthesis via the main route. By contrast, other genes from the main biosynthetic pathway were found to be related only in early fruit development [13] and in co-expression analyses using RNA-Seq data [58]. The same study confirmed a putative role for the D-galacturonate reductase gene during the last stages of fruit ripening, in line with previous findings based on feeding experiments [35]. As for AsA degradation, \(AO\) transcript levels were high early in tomato fruit ripening and then declined [13]. Nevertheless, a WGCNA study between tomato genotypes with contrasting antioxidant levels revealed an \(AO\) ortholog (Solyc07g052230), which favors the accumulation of the reduced form of AsA during ripening by regulating AsA redox state in the apoplast [66].