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 con trast 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 expre ssing the respective cDNA in sense or antisense orientation were const ructed, leading to a range of plant lines with different UDP-Glc epime rase activities. No alterations in morphology were observed and the re lative amounts of different galactose-containing compounds were not af fected if the plants were raised on soil. However, on agar plates in t he presence of galactose, the growth of different lines was increasing ly repressed with decreasing enzyme activity, and an increase in the U DP-Gal content was observed in parallel, whereas the UDP-Glc content w as nearly constant. The amount of galactose in the cell wall was incre ased in plants with low UDP-Glc epimerase activity grown on galactose, whereas the cellulose content in the leaves was not altered. Furtherm ore, starch determined at different times of the day was highly abunda nt in plants with low UDP-Glc epimerase activity in the presence of ga lactose. It is proposed that low endogenous UDP-Glc epimerase activity is responsible for the galactose toxicity of the wild-type. Possible mech anisms by which the starch content might be modulated are discuss ed.