Expression of a bacterial sucrose phosphorylase in potato tubers results in a glucose-independent induction of glycolysis

Sugars are not only metabolic substrates: they also act as signals that regulate the metabolism of plants. Previously, we found that glycolysis is induced in transgenic tubers expressing a yeast invertase in the cytosol but not in those expressing invertase in the apoplast. This suggests that either the low level of sucrose, the increased formation of cytosolic glucose or the increased levels of metabolites downstream of the sucrose cleavage is responsible for the induction of glycolysis in storage organs. In order to discriminate between these possibilities, we cloned and expressed a bacterial sucrose phosphorylase gene from Pseudomonas saccharophila in potato tubers. Due to the phosphorolytic cleavage of sucrose, formation of glucose was circumvented, thus allowing assessment of the importance of cytosolic glucose - and, by implication, flux through hexokinase - in glycolytic induction. Expression of sucrose phosphorylase led to: (i) a decrease in sucrose content, but no decrease in glucose or fructose; (ii) a decrease in both starch accumulation and tuber yield; (iii) increased levels of glycolytic metabolites; (iv) an induction of the activities of key enzymes of glycolysis; and (v) increased respiratory activity. We conclude that the induction of glycolysis in heterotrophic tissues such as potato tubers occurs via a glucose-independent mechanism.