Molecular biology of sugar metabolism and its regulation in fruit

In general, plants synthesize sucrose for translocation of photosynthate. However, some horticultural crops of Rosaceae family are known to translocate sorbitol. Both types of crops are important in horticultural science. Although sucrose metabolism in source and sink tissue is well known, regulation of sugar composition has not been investigated yet. Fruit generally contains three major sugars i.e.; sucrose, glucose and fructose. As the sweetness of these three sugars is quite different, the manipulation of sugar composition in fruit could be useful for improving fruit quality. Thus genes encoding fructokinase that might be related to the level of fructose in fruit were cloned and analyzed. In contrast to sucrose, there are fewer studies on sorbitol metabolism in source and sink tissue. Therefore cloning and expression analysis of the gene encoding sorbitol-6-phosphate dehydrogenase (S6PDH) that may play a key role in sorbitol biosynthesis are also reported here. Two fructokinase cDNAs, Frk1 and Frk2, were isolated from tomato and identified using transformed yeast with those genes. The response of both genes to sugar and their expression patterns shown by northern and in situ hybridization suggest that Frk1 responds to carbon availability and plays a primary role in fructose metabolism in plant cells while Frk2 is localized specifically in cells that import and store carbohydrate. On the other hand, S6PDH CDNA was cloned from apple seedlings. The levels of S6PDH activity and protein were high in source tissue such as cotyledons and leaves in apple seedlings. Besides the S6PDH transcripts were induced during germination corresponding to sorbitol synthesis. On peach trees, the levels of S6PDH transcripts were also higher in more developed leaves compared with folded young leaves. The results strongly suggest that the S6PDH gene is expressed for biosynthesis of sorbitol in source tissue. Now some groups are executing mapping and overexpression of S6PDH.