Transcriptomic Analysis and the Effect of Maturity Stage on Fruit Quality Reveal the Importance of the L-Galactose Pathway in the Ascorbate Biosynthesis of Hardy Kiwifruit (Actinidia arguta)

Hardy kiwifruit (Actinidia arguta) has recently become popular in fresh markets due to its edible skin and rich nutritional value. In the present study, different harvest stages of two A. arguta cultivars, ‘Issai’ and ‘Ananasnaya’ (“Ana”), were chosen for investigating the effects of maturity on the quality of the fruit. Interestingly, Issai contained 3.34 folds higher ascorbic acid (AsA) content than Ana. The HPLC method was used to determine the AsA content of the two varieties and revealed that Issai had the higher content of AsA and DHA. Moreover, RNA sequencing (RNAseq) of the transcriptome-based expression analysis showed that 30 differential genes for ascorbate metabolic pathways were screened in Issai compared to Ana, which had 16 genes down-regulated and 14 genes up-regulated, while compared to the up-regulation of 8 transcripts encoding the key enzymes involved in the L-galactose biosynthesis pathway. Our results suggested that AsA was synthesized mainly through the L-galactose pathway in hardy kiwifruit.