Insufficient levels of malate and lack of sourness in commercial grape cultivars (Vitis vinifera) deteriorates the quality of fruit grown in warm climates, and is further exacerbated due to climate change. Conversely, excessive levels of malate in the fruit of wild Vitis species forms a major limiting factor for the introgression of disease resistance alleles to future cultivars by breeding. These contrasting interspecific phenotypes were harnessed, for the first time, to deepen our understanding of malate regulation, with the overarching goal of controlling grape sourness through breeding.
Our aim was to highlight metabolic pathways associated with the interspecific differences in fruit malate levels, and identify associated genetic loci and regulatory genes. We integrated metabolomics, transcriptomics, and physiological analyses of the fruit of wild and domesticated genotypes, and complemented these with QTL mapping, using novel genus-wide genetic markers, on several interspecific mapping families.
Three polymorphic loci were associated with over two-fold differences in malate and 40.6% of the phenotypic variation. Two of these loci were stable in all and in three out of five years, respectively. Elevated malate levels were associated with higher fruit respiration rates, higher levels of amino acids, TCA and fermentation metabolites, and higher expression of their corresponding genes. In addition, transcripts of a cytosolic malate dehydrogenase and malate/dicarboxylate transporters (ALMT/TDT), associated with fruit malate levels, were identified.
These results advance current knowledge regarding the regulation of malate at the mechanistic and metabolic levels, and provide novel genetic markers for improving fruit quality and disease-resistance of future cultivars.