Effects of low temperature and abscisic acid on the expression of the sorbitol-6-phosphate dehydrogenase gene in apple leaves

Sorbitol-6-phosphate dehydrogenase (S6PDH) is known to be a key enzyme in the biosynthesis of sorbitol, an early product of photosynthesis, common to fruit trees of the Rosaceae family. Effects of low temperature and abscisic acid (ABA) on the expression of the S6PDH gene were investigated in apple leaves. In leaf-disc experiments, the expression of S6PDH was enhanced by an ABA treatment, as well as by low temperature and high-salinity stresses. The level of S6PDH mRNA increased 8 h after the addition of ABA; the highest level of S6PDH mRNA resulted on exposure to 10 μM ABA. The level of S6PDH mRNA in leaves of apple trees growing in an orchard increased with a decrease in temperature in the fall while ABA content increased. This induction may partly be a stress-response to low temperature, a prerequisite for freezing tolerance during the coming winter. Southern blot analysis revealed that S6PDH is a single-copy gene in the apple genome, indicating that it is a unique, multifunctional one, for sorbitol biosynthesis under various stress responses, as well as for the translocation of photosynthates.