To clarify the role of sorbitol-6-phosphate dehydrogenase (S6PDH) in sugar accumulation in apple (Malus domestica Borkh. cv. 'Orin'), transgenic apple plants expressing a S6PDH cDNA under the control of a CaMV 35S promoter were obtained and studied. The sorbitol content in leaves changed markedly in several transformed lines. In five transgenic lines, the levels of mRNA, protein and activity of S6PDH decreased markedly, but in three transgenic lines they increased. The five lines that had less activity contained only a small levels of sorbitol, but showed six- to seven-fold increase in sucrose compared with the non-transgenic control. However, two lines with increased S6PDH activity had increased sorbitol and sucrose. In contrast, a third line having the highest S6PDH activity did not increase the sorbitol content. This study showed that S6PDH is a key enzyme regulating partitioning between sorbitol and sucrose in apple leaves. (C) 2004 Elsevier Ireland Ltd. All rights reserved.