Sorbitol- and other sugar-induced expressions of the NAD+-dependent sorbitol dehydrogenase gene in Japanese pear fruit

The regulation of NAD-dependent sorbitol dehydrogenase (NAD-SDH, EC 1.1.1.14) by sugar was investigated by using sliced tissues of japanese pear (Pyrus serotina Nakai ev. Kousui) fruit in order to determine its role in the mechanism of sugar accumulation in fruit tissue. The results of the activities and steady-state levels of the protein and mRNA indicate that NAD-SDH in japanese pear fruit is among the sugar-inducible genes. By preincubating the sliced tissues for 16 hours in a medium without sugar, NAD-SDH activity declined and reached a stable level that was maintained for tip to 40 hours. The washing procedure also reduced the sugar concentration in the apoplast and cytosol of the sliced tissues to low concentrations and enabled them to be manipulated by exogenous applications of carbohydrate solutions. Incubation of tissues in 50 or 100 mm sorbitol for 8 hours led to enhanced expression of the NAD-SDH gene as determined by increased mRNA and protein levels and enhanced enzyme activity. The presence of 100 mm glucose, sucrose, or mannitol also gave significant stimulation on the levels of activity, protein, and mRNA of NAD-SDH compared with those of control tissues bathed in media in which the osmotic potential had been adjusted to that of the sugar solutions by adding polyethylene glycol. However, fructose was ineffective in stimulating NAD-SDH activities and the level of the protein was not enhanced but the level of mRNA was increased. Therefore, it is suggested that NAD-SDH gene transcription is enhanced by each sugar investigated, and fructose appears to be unique as it also influences NAD-SDH at a post-transcriptional level.