The regulation of ascorbate in apples (Malus domestica)

Abstract

Ascorbate is an essential biochemical for biological processes in plants and animals. Ascorbate concentrations vary greatly between closely related species of plants, and can even differ across cultivars of the same species. The genetic mechanism which leads to the accumulation of higher levels of ascorbate in apples of some varieties, and not others, remains unknown. What remains unknown is if changes in the primary or alternative biosynthesis pathways, transportation, or recycling lead to variation in ascorbate concentrations observed in apples. Therefore, the aim of this research was to identify significant contributors that underpin this variation in apples. Firstly, transcriptional and translational control were investigated to determine the role of ascorbate related genes in determining the total ascorbate pool in apple flesh in Royal Gala (low ascorbate) and Braeburn (high ascorbate). At the transcriptional level, in contrast to previous research, this research showed no correlation between key regulatory genes MdGGP1, MdGGP2, MdGGP3 or MdDHAR3-3 and ascorbate concentrations in apple flesh. Furthermore, at the translational level; no link between the function of the upstream open reading frame (uORF) of GGP and ascorbate concentrations in apple flesh was identified. To identify genomic regions of significant interest QTL mapping was carried out in a Royal Gala x Braeburn population. Two stable QTLs were identified on LG3 and LG6, the former of which colocated to MdGGP2. The second, does not colocate to the position of any currently known ascorbate synthesis, recycling, or transportation genes. Lastly, to explore and identify potential candidate genes RNA-seq was carried out. This provided a transcriptome of mature apple fruit from the extremes of ascorbate concentrations in the Royal Gala x Braeburn population. A number of differentially expressed genes were identified; of significant interest were two alternative ascorbate biosynthesis pathway genes MI oxygenase 4 (MIOX4) and a putative galacturonic acid reductase (GalUR). Both of the aforementioned genes are upregulated in high ascorbate apple samples, and might be contributing to the differences in phenotype found between Royal Gala and Braeburn. While identifying no single cause, the culmination of this work has shown the complexity of understanding a continuous trait such as ascorbate biosynthesis. The extent of which each of the identified potential contributors influence ascorbate concentrations in apples requires further investigation.