Overexpression of two different potato UDP-Glc-4-epimerases can increase the galactose content of potato tuber cell walls

UDP-glucose 4-epimerases (UGE) catalyze the reversible conversion of UDP-glucose to UDP-galactose. Two potato UGE genes (StUGE45 and StUGE51) were isolated from a potato cDNA library using the Arabidopsis thaliana UGE1 cDNA as a probe. The cDNA clones cluster differently in a phylogenetic tree of plant epimerases. StUGE51 showed higher expression than StUGE45, particularly, in older tubers, flowers, stems and in vitro plants. Transgenic potato plants were generated to examine the effect of modified UGE expression on the amount of cell wall galactose. Sense expression of the two StUGE clones in potato gave plants with no overt developmental phenotype. In contrast to WT plants, a large number of transformants had the ability to overcome the toxic effect of galactose in the culture medium during in vitro growth. The UGE45 transformants showed a clear correlation of increased UGE RNA expression levels with this tolerance to galactose, whereas the UGE51 transformants did not. The elevated UGE expression in the transformants resulted in an increased galactose content in potato tuber cell walls. This effect was more profound for the UGE45 transformants than for the UGE51 ones. Our results suggest that the two potato UGEs have different biochemical properties, and that they have a different function in plant development. (C) 2004 Elsevier Ireland Ltd. All rights reserved.