The genetic basis of acid composition in developing berries of the cultivated grapevine Vitis vinifera

Abstract

Grapevines contain many different organic acids and the two most abundant are tartaric acid and malic acid. Malic acid and tartaric acid both increase in concentration up until veraison then after veraison malic acid is broken down as sugar increases. Malic acid has been studied in a variety of fruits for it is a very common acid. However tartaric acid is an uncommon primary acid in fruits and very little is known about its synthesis in grapevine. However, tartaric acid is important in providing a low pH which is important for the prevention of microbial spoilage during the winemaking process. A high pH of juice means that more tartaric acid will need to be added in the winery increasing the cost to wine makers. By discovering more about the genes involved in the synthesis of malic and tartaric acid and the breakdown of malic acid this knowledge could be used to breed vines with higher acid concentrations. L-Idonate dehydrogenase (L-IDH) is one of only two genes known to participate in the tartaric acid synthesis pathway. Since its initial characterisation two more isoforms have been annotated in the grapevine genome based on sequence similarity. The characterisation of these isoforms was undertaken using a variety of techniques including expression of the proteins in E. coli and in vitro protein activity assays and also in planta expression in the microvine with the creation of transgenic microvines. To try and discover regions of the genome that might be involved in acid metabolism in grapevine berries, malic and tartaric acid concentrations were measured from four progeny populations. The individuals of these populations were then sequenced using a genotyping by sequencing method to find SNPs markers for a Genome Wide Association Study (GWAS). This GWAS was then verified with genetic mapping and QTL analysis. During the process of measuring acid from these progeny populations the question of variability in acid concentration between berries from the same vine arose. A preliminary study into this variability was conducted to determine the variability of malic and tartaric acid in berries both within a bunch and between bunches from the same vine. This data was then used to predict the error in sampling subsets of berries of different sizes. Tartaric acid concentration in tissues other than the berry was also explored. Acid concentration was measured in several tissues including root, shoots and leaves. It was found that tartaric acid was present in these tissues with varying concentrations. Tartaric acid concentration in leaves was then studied further try see if there was a link between the age of the leaf and tartaric acid concentration and also between leaf tartaric acid concentration and berry tartaric acid concentration. There was found to be no link between the two in these preliminary studies.