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 reg ulate the metabolism of plants. Previously, we found that glycolysis is ind uced in transgenic tubers expressing a yeast invertase in the cytosol but n ot 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 r esponsible for the induction of glycolysis in storage organs. In order to d iscriminate between these possibilities, we cloned and expressed a bacteria l sucrose phosphorylase gene from Pseudomonas saccharophila in potato tuber s. Due to the phosphorolytic cleavage of sucrose, formation of glucose was circumvented, thus allowing assessment of the importance of cytosolic gluco se - and, by implication, flux through hexokinase - in glycolytic induction . Expression of sucrose phosphorylase led to: (i) a decrease in sucrose con tent, but no decrease in glucose or fructose; (ii) a decrease in both starc h accumulation and tuber yield; (iii) increased levels of glycolytic metabo lites; (iv) an induction of the activities of key enzymes of glycolysis; an d (v) increased respiratory activity. We conclude that the induction of gly colysis in heterotrophic tissues such as potato tubers occurs via a glucose -independent mechanism.