## 1 Introduction  
L-ascorbic acid (AsA), also known as vitamin C, is an essential component of the human diet, and fruits and vegetables are the main sources of this compound [1]. Moreover, AsA is a powerful antioxidant that may also decrease the incidence of several illnesses such as cancer or cardiovascular diseases [2]. Hence, the elucidation of AsA accumulation and the factors regulating its concentration in horticultural crops has been a topic of interest, and many studies have focused on assessing AsA concentrations in the edible parts of plants [3, 4]. AsA is the most abundant low molecular weight antioxidant present in plant cells and participates in numerous physiological processes, including photosynthesis, growth, and senescence [5]. Due to its notable importance, the enhancement of AsA levels in plants has been carried out by genetic engineering or exogenous application, proving that AsA can mitigate, to some extent, the deleterious effects of environmental stresses in plants [6, 7]. In addition, AsA levels have been associated with improved fruit postharvest properties [8, 9, 10]. In plants, four possible AsA biosynthetic pathways exist: the L-galactose, L-glucose, _myo_-inositol, and D-galacturonic acid pathways, being the first the principal pathway for AsA biosynthesis in plants [11] (Figure 1). In fruits of many species, the L-galactose pathway also seems to prevail, but the predominance of a particular pathway depends on the species and also varies during fruit development [4]. In line with this, in kiwifruit and immature peach fruits, the L-galactose pathway predominates, whereas in strawberry fruit the D-galacturonic acid and the _myo_-inositol pathways seems to prevail, since expression of different _D-galacturonic acid reductase_ isoforms (_GalUR_) and _myo-inositol oxygenase_ (_MIOX_) are positively correlated with AsA contents [3]. In tomato, [PERSON] et al. [12] found that most of the genes involved in the L-galactose pathway followed an opposite expression profile to that of AsA accumulation, except for the expression of _GPP_ that closely correlated with AsA levels, suggesting a regulatory role of this gene. Moreover, in apple fruit, it has been demonstrated that the de novo synthesis of AsA in the peel occurs through the L-galactose and D-galacturonic acid pathways, while in the flesh and seeds, AsA can only be synthetized by the L-galactose pathway, indicating the AsA biosynthetic pathways also seems to be tissue-specific [13]. In addition to the de novo biosynthesis of AsA, other mechanisms such as the rate of degradation and recycling contribute to the regulation of AsA concentrations in plants (Figure 1). AsA can be transformed into monodehydroascorbate (MDHA) by the enzymes ascorbate oxidase (AO) and ascorbate peroxidase (APX) [14], and the MDHA radical can either be recycled back into AsA by monodehydroascorbate reductase (MDHAR) or undergo disproportionation into dehydroroscorbate (DHA) and AsA. Later, DHA can be recycled into AsA by dehydroroscorbate reductase (DHAR) before being irrevocably hydrolyzed (Figure 1) [15]. _Citrus_ fruits are recognized worldwide as one of the main source of vitamin C for human nutrition, and for centuries it has been known that scurvy--a disease caused by vitamin C deficiency--could be alleviated by eating _Citrus_ fruits [16]. Hence, the concentration of AsA in fresh fruits and juices of different species and varieties of _Citrus_ has been widely studied [17; 18]. However, information about the molecular regulation of AsA metabolism in _Citrus_ fruit is still limited. Transcriptional analysis of the genes belonging to the different biosynthetic pathways have been carried out in fruit of orange and mandarin cultivars, which revealed that the L-galactose pathway appears to be the main biosynthetic source in both the flavedo (outer colored part of the peel) and pulp [19; 20].