Genomic Structure, Sub-Cellular Localization, and Promoter Analysis of the Gene Encoding Sorbitol-6-Phosphate Dehydrogenase from Apple

Sorbitol is the primary photosynthetic product and the translocatable and storage carbohydrate in apple (Malus domestica) and other fruit tree species within the Rosaceae family. Sorbitol-6-phosphate dehydrogenase (S6PDH, EC 1.1.1.200) is the key enzyme in the biosynthesis of sorbitol. In this study, we isolated two full-length genomic sequences for S6PDH from "Gala" apple. The two sequences have same six exons and first two introns, but the sizes of their last three introns are different. The two sequences were mapped to the same loci on chromosome 10. Immunogold electron microscopy analysis demonstrates that the S6PDH is localized mainly in leaf cytosol and chloroplasts. We also isolated and analyzed the promoter region of S6PDH and constructed a series of promoter deletion derivatives with beta-glucuronidase (GUS) gene to identify the upstream region of the S6PDH gene required for promoter activity. The GUS activity in Agrobacterium-mediated transient transformation of tobacco leaves reveals that the -1719 region is more important for gene expression contrasting with other regions in the S6PDH promoter. The promoter region can be induced by cold, dark, and abscisic acid treatment.