Allelic Variation in Paralogs of GDP-l-Galactose Phosphorylase Is a Major Determinant of Vitamin C Concentrations in Apple Fruit      

Abstract

To identify the genetic factors underlying the regulation of fruit vitamin C (L-ascorbic acid [AsA]) concentrations, quantitative trait loci (QTL) studies were carried out in an F1 progeny derived from a cross between the apple (Malus 3 domestica) cultivars Telamon and Braeburn over three years. QTL were identified for AsA, glutathione, total antioxidant activity in both flesh and skin tissues, and various quality traits, including flesh browning. Four regions on chromosomes 10, 11, 16, and 17 contained stable fruit AsA-QTL clusters. Mapping of AsA metabolic genes identified colocations between orthologs of GDP-L-galactose phosphorylase (GGP), dehydroascorbate reductase (DHAR), and nucleobase-ascorbate transporter within these QTL clusters. Of particular interest are the three paralogs of MdGGP, which all colocated within AsA-QTL clusters. Allelic variants of MdGGP1 and MdGGP3 derived from the cultivar Braeburn parent were also consistently associated with higher fruit total AsA concentrations both within the mapping population (up to 10-fold) and across a range of commercial apple germplasm (up to 6-fold). Striking differences in the expression of the cv Braeburn MdGGP1 allele between fruit from high-and low-AsA genotypes clearly indicate a key role for MdGGP1 in the regulation of fruit AsA concentrations, and this MdGGP allele-specific single-nucleotide polymorphism marker represents an excellent candidate for directed breeding for enhanced fruit AsA concentrations. Interestingly, colocations were also found between MdDHAR3-3 and a stable QTL for browning in the cv Telamon parent, highlighting links between the redox status of the AsA pool and susceptibility to flesh browning.

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

“…GGP (VTC2) represents the first committed step of AsA biosynthesis, and has previously been suggested to be a key rate-limiting step in plants [5,9,10,43]. Recent results from our lab indicate that paralogues of apple GGP are key regulators of totAsA-AsA concentrations in fruits [10], and others have shown that ectopic expression of the single kiwifruit GGP orthologue results in enhanced totAsA contents in both transgenic tomato (3- to 6-fold increase) and strawberry (2-fold increase) [5].…”

“…In plants, AsA biosynthesis via the L-galactose pathway (Figure 1) as initially described by Wheeler et al [7] has been proposed to be the main route for AsA accumulation in various species [8-10], and several structural genes from this pathway have been proposed to be key regulators of fruit AsA concentrations in various species. These include GDP-L-galactose phosphorylase ( GGP, VTC2 ) in kiwifruit [9] and apple [10], and L-galactose-1-phosphate-phosphatase ( GPP, VTC4 ) [11] and GDP-mannose-3,5-epimerase ( GME ) [12] in tomato.…”

“…These include GDP-L-galactose phosphorylase ( GGP, VTC2 ) in kiwifruit [9] and apple [10], and L-galactose-1-phosphate-phosphatase ( GPP, VTC4 ) [11] and GDP-mannose-3,5-epimerase ( GME ) [12] in tomato. Possible alternative biosynthetic pathways (Figure 1) involving uronic acids [13,14], L-gulose [15], or myo -inositol [16] have also been demonstrated in several plant species, but to date, there is no convincing evidence that these contribute significantly to the AsA pool under physiological conditions, except possibly in strawberry [14,17].…”

Suppression of tomato SlGGP aggravates methyl viologen-mediated oxidative stress

“…Mutants of the vtc2 gene, which encodes GGP in Arabidopsis, exhibit impaired AsA synthesis and have less abundant AsA (Linster et al 2007). GGP might be a key rate-limiting enzyme for AsA biosynthesis in Arabidopsis (Bulley et al 2009), tobacco (Laing et al 2007), and apple (Mellidou et al 2012). In previous study, SlGGP antisense transgenic tomato lines were found to maintain only 50 -75% of the ascorbate content found in WT and produced more DHA under chilling stress (Wang et al 2013).…”

Using genomics to improve fruit quality

“…In spite of this, QTLs for fl owering and ripening time, fruit quality, tree architecture and resistance to pathogens have been reported (Dirlewanger et al, 1996;Abbott et al, 1998;Dirlewanger et al, 1999;Etienne et al, 2002;Quilot et al 2004;Dirlewanger et al, 2006;Lambert et al, 2007;Ogundiwin et al, 2008;Dirlewanger et al, 2009 (Zini et al, 2005;Dunemann et al, 2009;Dunemann et al, 2012;Khan et al, 2012;Chagne et al, 2012b;Vogt et al, 2013;, texture (King et al, 2000;King et al, 2001;Kenis et al, 2008;Costa et al, 2008;Costa et al, 2010;Longhi et al, 2012;Longhi et al, 2013), ethylene production (Costa et al, 2014), Vitamin C content (Davey et al, 2006;Mellidou et al, 2012;) and fl esh browning (Kenis et al, 2008;Di Guardo et al, 2013). Also, as the cost of map construction was higher, the eff orts were concentrated in a few mapping populations and often they were considered as reference progenies, but these populations only segregated for a few traits.…”