Biochemical and molecular aspects of sorbitol metabolism in rosaceae fruit trees and other plants

Sorbitol is important in translocating photosynthate in fruit trees of the Rosaceae. Sorbitol-6-phosphate dehydrogenase (S6PDH) (or aldose-6-phosphate reductase) catalyzes the NADPH-dependent reduction of glucose-6-phosphate to sorbitol-6-phosphate, and S6PDH likely plays a key role in the biosynthesis of sorbitol in leaves. NAD-dependent sorbitol dehydrogenase (SDH) catalyzes the NAD-dependent oxidation of sorbitol to fructose, and thus SDH is believed to play a key role in the conversion of sorbitol to fructose in sink organs. cDNAs encoding S6PDH and SDH have been cloned for detailed analyses, and molecular genetic approaches for S6PDH have revealed that S6PDH activity regulates the ratio of sorbitol to sucrose in fruit trees of the Rosaceae. In addition, efforts have been made to engineer stress tolerance by introducing S6PDH (and SDH) into plants that do not synthesize sorbitol. Interestingly, S6PDH- and SDH-like sequences are widespread in the plant kingdom; that is, the sequences are found not only in the Rosaceae, but also in other plants that synthesize sucrose instead of sorbitol. In this review, we describe recent progress in the molecular aspects of S6PDH and SDH in plants.