Change in sugar levels and related gene expression during bilberry fruit development and ripening

Abstract

Vaccinium myrtillus L., commonly known as European blueberry or bilberry, has been used as food and medicine since ancient times. Bilberries are highly valued in European and Asian markets. Sugar accumulation is responsible for the sweet taste of fruit, which is one of the main determinants of fruit quality. However, sugar metabolism in bilberry has not been studied before. This study was aimed to measure sugar content and to examine the expression of genes encoding the invertase enzymes; which are one of the key enzymes involved in the metabolism of sugars. All the analyses were done from four ripening stages of bilberry fruit (small unripe, large unripe, ripening, and fully ripe). Fructose was found to be predominant sugar in bilberry, followed by glucose. Sucrose was present in lower amount than fructose and glucose. The total sugar content was highest in fully ripe berries. Altogether 32 bilberry specific sugar metabolism genes were identified by bioinformatics tools, and expression of invertases, cell wall invertases (CwINVs), vacuolar invertases (VINVs), and neutral invertases (NINVs) were studied in detail. The different isoforms of invertases were differentially expressed, suggesting that they have specialized functions in the sugar metabolism pathway. VmCwINV2 and VmNINV5 were highly expressed in unripe berries while the levels of VmVINV1 and VmVINV2 transcripts were high in ripe berries. The findings of this study provide a basis for the further sugar metabolism research in bilberry fruit and deepen understanding of the role of sugar metabolism pathway to the fruit quality.