Consequences of the expression of a bacterial glucokinase in potato tubers, both in combination with and independently of a yeast-derived invertase

The aim of this work was to further define the metabolic factors that regulate carbohydrate metabolism in potato (Solanum tuberosum L. cv. Desiree) tubers. We previously found that glycolysis is induced (and starch accumulation reduced) in transgenic tubers in which a yeast invertase and a glucokinase from Zymomonas mobilis were expressed in the cytosol, whereas potato tuber size is dramatically increased when invertase expression is targeted to the apoplast. In this study we describe the further characterisation of potato tubers expressing a yeast invertase in the apoplast. We also report the generation of two novel transgenic plants in which the Z. mobilis glucokinase gene is expressed tuber-specifically (either in the wild type or apoplastic invertase-expressing back- ground). We evaluated the influence that increasing the glucokinase activity, independent of invertase activity, had on the shift in carbon partitioning, and assessed if the hexoses produced by the apoplastic cleavage of sucrose could be brought into metabolism. We found that expression of glucokinase either in the wild type or in the apoplastic invertase-expressing background led to changes in the levels of glucose and glucose 6-phosphate. However, these changes had little effect on carbon partitioning or tuber size with respect to the parent line. We conclude that neither the accumulation nor the phosphorylation of glucose play a pivotal role in the regulation of metabolism or morphology in the potato tuber.