Combined expression of glucokinase and invertase in potato tubers leads to a dramatic reduction in starch accumulation and a stimulation of glycolysis

Abstract

The original aim of this work was to increase starch accumulation in potato tubers by enhancing their capacity to metabolise sucrose.We previously reported that specific expression of a yeast invertase in the cytosol of tubers led to a 95% reduction in sucrose content, but that this was accompanied by a larger accumulation of glucose and a reduction in starch. In the present paper we introduced a bacterial glucokinase from Zymomonas mobilis into an invertase-expressing transgenic line, with the intention of bringing the glucose into metabolism. Transgenic lines were obtained with up to threefold more glucokinase activity than in the parent invertase line and which did not accumulate glucose. Unexpectedly, there was a further dramatic reduction in starch content, down to 35% of wild-type levels. Biochemical analysis of growing tuber tissue revealed large increases in the metabolic intermediates of glycolysis, organic acids and amino acids,two- to threefold increases in the maximum catalytic activities of key enzymes in the respiratory pathways, and three- to fivefold increases in carbon dioxide production.These changes occur in the lines expressing invertase,and are accentuated following introduction of the second transgene, glucokinase. We conclude that the expression of invertase in potato tubers leads to an increased flux through the glycolytic pathway at the expense of starch synthesis and that heterologous overexpression of glucokinase enhances this change in partitioning.