Regulation of fruit ascorbic acid concentrations during ripening in high and low vitamin C tomato cultivars

Abstract

BackgroundTo gain insight into the regulation of fruit ascorbic acid (AsA) pool in tomatoes, a combination of metabolite analyses, non-labelled and radiolabelled substrate feeding experiments, enzyme activity measurements and gene expression studies were carried out in fruits of the ‘low-’ and ‘high-AsA’ tomato cultivars ‘Ailsa Craig’ and ‘Santorini’ respectively.ResultsThe two cultivars exhibited different profiles of total AsA (totAsA, AsA + dehydroascorbate) and AsA accumulation during ripening, but both displayed a characteristic peak in concentrations at the breaker stage. Substrate feeding experiments demonstrated that the L-galactose pathway is the main AsA biosynthetic route in tomato fruits, but that substrates from alternative pathways can increase the AsA pool at specific developmental stages. In addition, we show that young fruits display a higher AsA biosynthetic capacity than mature ones, but this does not lead to higher AsA concentrations due to either enhanced rates of AsA breakdown (‘Ailsa Craig’) or decreased rates of AsA recycling (‘Santorini’), depending on the cultivar. In the later stages of ripening, differences in fruit totAsA-AsA concentrations of the two cultivars can be explained by differences in the rate of AsA recycling activities. Analysis of the expression of AsA metabolic genes showed that only the expression of one orthologue of GDP-L-galactose phosphorylase (SlGGP1), and of two monodehydroascorbate reductases (SlMDHAR1 and SlMDHAR3) correlated with the changes in fruit totAsA-AsA concentrations during fruit ripening in ‘Ailsa Craig’, and that only the expression of SlGGP1 was linked to the high AsA concentrations found in red ripe ‘Santorini’ fruits.ConclusionsResults indicate that ‘Ailsa Craig’ and ‘Santorini’ use complementary mechanisms to maintain the fruit AsA pool. In the low-AsA cultivar (‘Ailsa Craig’), alternative routes of AsA biosynthesis may supplement biosynthesis via L-galactose, while in the high-AsA cultivar (‘Santorini’), enhanced AsA recycling activities appear to be responsible for AsA accumulation in the later stages of ripening. Gene expression studies indicate that expression of SlGGP1 and two orthologues of SlMDHAR are closely correlated with totAsA-AsA concentrations during ripening and are potentially good candidates for marker development for breeding and selection.

Integrated bioinformatics to decipher the ascorbic acid metabolic network in tomato

“…It is thus conceivable that the genes of the main biosynthetic pathway in the clusters 7 and 8 are functionally related to AsA accumulation only in the first stages of fruit development, when transcript abundance is still relevant. This inverse trend was well known and observed by different authors (Ioannidi et al 2009;Badejo et al 2012;Mellidou et al 2012). However, these observations support the idea that the biosynthesis and the maintenance of AsA pool rely on different mechanisms, including not only the main biosynthetic pathway (galactose) but also the alternative ones, the recycling and translocation processes.…”

“…These genes could cooperate to favor the maintenance of AsA in later stages of ripening, while the contribution of the main biosynthetic pathway decreases. Indeed the presence of a MDHAR could play a fundamental role in the recycling of the ascorbate in later stages of ripening (Mellidou et al 2012). Concerning the three genes associated with the translocation, although previous feeding experiments revealed that translocation of AsA in fruit at red ripe stage is not relevant (Badejo et al 2012), the high expression detected for two of these three genes could underlie others mechanisms involved in the regulation/homeostasis of AsA pool in the mature fruit.…”

“…The correlation of AsA content with the expression of genes contributing to its metabolism across fruit stages was already investigated (Ioannidi et al 2009;Mellidou et al 2012). Nevertheless, this is the first time that an entire collection of genes associated with the AsA network was explored through RNA-Seq data.…”

“…A phosphodiesterase (EC 3.1.4.1), a sugar-phosphatase (EC 3.1.3.23), a D-threo-aldose 1-dehydrogenase (EC 1.1.1.122) and an L-gulonolactone oxidase (EC 1.1.3.8) could be the putative enzymes for the conversion of GDPgulose in L-ascorbate (Wolucka and Van Montagu 2007;Cruz-Rus et al 2011;Mellidou et al 2012). In our study, four putative loci were surveyed for EC 3.1.3.23 and three for EC 1.1.1.222.…”

Gene expression and enzyme activities of the D-mannose/L-galactose pathway influence L-ascorbic acid content in Myrciaria dubia

“…Likewise, Yang et al (2011) showed that a higher AsA accumulation in fruit pulp of Citrus sinensis may be related to a higher expression of GME, GPP, GDH, and GLDH. However, Mellidou et al (2012) showed that only the expression of one ortologue of GGP correlates with a high AsA content found in a tomato cultivar characterized by fruits with a high AsA content.…”