Mining and comparison of the genes encoding the key enzymes involved in sugar biosynthesis in apple, grape, and sweet orange

Abstract Fruit taste depends on the contents and types of soluble sugars and organic acids. The large-scale identification and comparison of genes involved in sugar biosynthesis in apple, grape, and sweet orange based on the genome and expressed sequence tag information have not been reported. In this study, we found that sorbitol-6-phosphate dehydrogenase ( S6PDH ) was one of the most important genes in sorbitol biosynthesis in apple leaf, and was decomposed by NAD + -dependent sorbitol dehydrogenase ( NAD + SDH ) in apple fruit. The high expression level of soluble acidic invertase ( AIV ) gene possibly caused the high soluble sugar content (i.e., glucose and fructose) in grape. The expression level of sucrose synthase ( SUS ) in grape and sweet orange was also significantly higher than that in apple. The results show that SUS was involved in the resynthesis of fructose and UDP-glucose into sucrose in grape and sweet orange. By contrast, sucrose was transported into a vacuole and decomposed into glucose and fructose by AIV . Thus, ADP-glucose pyrophosphorylase ( AGP ), granule-bound starch synthase ( GBSS ), isoamylase 1 ( ISA1 ), and isoamylase 2 ( ISA2 ) could be the key genes in starch biosynthesis, whereas α-amylase ( AMY3 ), isoamylse 3 ( ISA3 ), and pullulanase ( PUL ) could be the key genes in starch degradation in apple. This study provides new insight into the isolation and comparison of candidate genes involved in the sugar biosynthetic pathway among multiple plants.