The role of UDP-glucose epimerase in carbohydrate metabolism of Arabidopsis.

Uridine 5'-diphospho-glucose-4-epimerase (UDP-Glc epimerase) catalyses the reversible epimerization of UDP-galactose and UDP-glucose. In contrast to bacteria and yeast, expression of the UDP-Glc epimerase gene in Arabidopsis was found not to be induced by galactose. To elucidate the metabolic role of this enzyme, transgenic Arabidopsis plants expressing the respective cDNA in sense or antisense orientation were constructed, leading to a range of plant lines with different UDP-Glc epimerase activities. No alterations in morphology were observed and the relative amounts of different galactose-containing compounds were not affected if the plants were raised on soil. However, on agar plates in the presence of galactose, the growth of different lines was increasingly repressed with decreasing enzyme activity, and an increase in the UDP-Gal content was observed in parallel, whereas the UDP-Glc content was nearly constant. The amount of galactose in the cell wall was increased in plants with low UDP-Glc epimerase activity grown on galactose, whereas the cellulose content in the leaves was not altered. Furthermore, starch determined at different times of the day was highly abundant in plants with low UDP-Glc epimerase activity in the presence of galactose. It is proposed that low endogenous UDP-Glc epimerase activity is responsible for the galactose toxicity of the wild-type. Possible mechanisms by which the starch content might be modulated are discussed.