Organic acid metabolism of Vitis vinifera fruit

Abstract

Tartaric and malic acids are the predominant organic acids of grape berries, and are important for fruit and wine quality. Tartaric acid, despite having been found in several plant species, accumulates to a significant level uniquely in the grape berry. It arises through the metabolism of ascorbic acid (Vitamin C), and while many of the intermediates of the tartaric acid biosynthesis pathway have been determined, the enzymes responsible are still largely unknown. Malic acid is integrated in complex primary metabolism, and displays developmental changes in net accumulation and degradation through transport and metabolism of the acid. The sensitivity of malic acid metabolism to environmental conditions also causes seasonal variation in grape berry acidity. We are investigating four aspects of organic acid metabolism in grapes to advance the current knowledge of organic acid metabolism and fruit acidity. Firstly, to uncover novel enzymes involved in tartaric acid synthesis, we identified a candidate grapevine gene that can be used for recombinant expression and functional characterisation. To determine how the metabolism of tartaric and malic acids are controlled in grape, we screened the concentration of these acids from numerous grapevine populations, to assist the discovery of genes linked to acidity. To identify transporters and transcription factors that regulate malate release from the vacuole during berry ripening, we have cloned candidate genes for functional characterisation. And fourthly, to determine the effect of elevated temperature on organic acid metabolism, we have conducted controlled atmosphere experiments using potted grapevines.