Response and regulation of the S6PDH gene in apple leaves under osmotic stress

Sorbitol is the main product of photosynthesis in members of the Rosaceae family, many of which accumulate sorbitol under osmotic stress.To elucidate whether sorbitol-6-phosphate dehydrogenase (S6PDH; the rate-limiting enzyme for sorbitol synthesis) is involved in the response to osmotic stress and to clarify the mechanism by which expression of the S6PDH gene was induced by drought stress, sorbitol contents, the activity of sorbitol-6-phosphate dehydrogenase, and expression levels of the S6PDH gene were assayed in the leaves of apple (Malus × domestica Borkh. cv. Nagano Fuji) subjected to different intensities of osmotic stress induced by PEG-6000. The results showed that the S6PDH gene was induced by osmotic stress, and the more severe the stress the higher the level of expression of the S6PDH gene. Expression of the S6PDH gene almost coincided with the increases in S6PDH enzyme activity and sorbitol accumulation, suggesting that the S6PDH gene played an important role in the response of apple to osmotic stress. The promoter of the S6PDH gene was isolated using chromosome walking. Analysis of the S6PDH promoter revealed three abscisic acid (ABA)-responsive elements (ABRE), four MYB-recognition sites, and three MYC-binding sites. Deletion analysis of the S6PDH promoter was performed in transgenic tobacco plants. β-Glucuronidase (GUS) reporter gene activities driven by different fragments of the S6PDH promoter were detected.This work revealed that the key drought and salt-responsive elements of the S6PDH promoter lay in the region between positions -361 and -221. This region of 141 bp contained two ABA-responsive elements and a putative MYB-recognition sequence, and may regulate transcription of the S6PDH gene under osmotic stress.