A Transcriptional Analysis of the Genes Involved in the Ascorbic Acid Pathways Based on a Comparison of the Juice and Leaves of Navel and Anthocyanin-Rich Sweet Orange Varieties

Abstract

Sweet oranges are an important source of ascorbic acid (AsA). In this study, the content of AsA in the juice and leaves of four orange clonal selections, different in terms of maturity time and the presence/absence of anthocyanins, was correlated with the transcription levels of the main genes involved in the biosynthesis, recycling, and degradation pathways. Within each variety, differences in the above pathways and the AsA amount were found between the analysed tissues. Variations were also observed at different stages of fruit development and maturation. At the beginning of fruit development, AsA accumulation was attributable to the synergic action of l-galactose and Myo-inositol, while the l-gulose pathway was predominant between the end of fruit development and the beginning of ripening. In leaves, the l-galactose pathway appeared to play a major role in AsA accumulation, even though higher GalUr isoform expression suggests a synergistic contribution of both pathways in this tissue. In juice, the trend of the AsA content may be related to the decrease in the transcription levels of the GME, GDH, MyoOx, and GalUr12 genes. Newhall was the genotype that accumulated the most AsA. The difference between Newhall and the other varieties seems to be attributable to the GLDH, GalUr12, APX2, and DHAR3 genes.

Ascorbic Acid Content and Transcriptional Profiling of Genes Involved in Its Metabolism during Development of Petals, Leaves, and Fruits of Orange (Citrus sinensis cv. Valencia Late)

“…Moreover, the absence of light markedly repressed the expression of GalUR8, and also GalUR12 in some species, concomitantly with the reduction of AsA concentrations in the flavedo [23]. These findings support the notion that the D-galacturonic acid plays an important role in AsA accumulation in Citrus fruits [19,21]. Information of AsA metabolism in Citrus organs other than fruits is very limited and mainly restricted to the analysis of enzymatic activities of AsA degrading and recycling enzymes under stress conditions [24][25][26][27][28].…”

“…Nonetheless, Alós et al [19] suggested the importance of the D-galacturonic acid and the myo-inositol pathways in the regulation of AsA accumulation in Citrus fruit tissues, with the genes D-galacturonic acid reductase12 (GalUR12) and myo-inositol oxygenase (MIOX) playing a key role. More recently, Caruso et al [21] associated the decrease in AsA content in the juice of blood oranges during maturation to the down-regulation of MIOX and GalUR12, together with GDP-mannose-3 ,5 -epimerase (GME) and L-Galactose dehydrogenase (GalDH) of the L-galactose pathway. Interestingly, these authors also proposed that the contribution of the different biosynthetic pathways to the AsA pool varied during fruit development, with the L-galactose and myo-inositol being important at the early stages of fruit development and the L-gulose pathway at the end of ripening [21].…”

“…More recently, Caruso et al [21] associated the decrease in AsA content in the juice of blood oranges during maturation to the down-regulation of MIOX and GalUR12, together with GDP-mannose-3 ,5 -epimerase (GME) and L-Galactose dehydrogenase (GalDH) of the L-galactose pathway. Interestingly, these authors also proposed that the contribution of the different biosynthetic pathways to the AsA pool varied during fruit development, with the L-galactose and myo-inositol being important at the early stages of fruit development and the L-gulose pathway at the end of ripening [21]. Moreover, AsA accumulation in Citrus fruit seems to be regulated by complex mechanisms affected also by changes in the rate of degradation and recycling [19][20][21].…”

“…Interestingly, these authors also proposed that the contribution of the different biosynthetic pathways to the AsA pool varied during fruit development, with the L-galactose and myo-inositol being important at the early stages of fruit development and the L-gulose pathway at the end of ripening [21]. Moreover, AsA accumulation in Citrus fruit seems to be regulated by complex mechanisms affected also by changes in the rate of degradation and recycling [19][20][21]. In particular, differences in APX enzymatic activity have been reported in fruits with contrasting AsA concentrations [20,21], and a marked induction of the APX2 isoform was observed during development and ripening, suggesting that this enzyme could be crucial for the degradation of AsA in the pulp [19].…”

“…Moreover, AsA accumulation in Citrus fruit seems to be regulated by complex mechanisms affected also by changes in the rate of degradation and recycling [19][20][21]. In particular, differences in APX enzymatic activity have been reported in fruits with contrasting AsA concentrations [20,21], and a marked induction of the APX2 isoform was observed during development and ripening, suggesting that this enzyme could be crucial for the degradation of AsA in the pulp [19]. In addition, the expression of genes involved in AsA recycling have been correlated with changes in AsA concentrations in the flavedo and pulp during fruit ripening, and also with differences in AsA contents among species or cultivars [19,21].…”