COMBINED EXPRESSION OF GLUCOKINASE AND INVERTASE IN POTATO-TUBERS LEADS TO A DRAMATIC REDUCTION IN STARCH ACCUMULATION AND A STIMULATION OFGLYCOLYSIS

The original aim of this work was to increase starch accumulation in p otato tubers by enhancing their capacity to metabolise sucrose. We pre viously 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 reducti on in starch. In the present paper we introduced a bacterial glucokina se from Zymomonas mobilis into an invertase-expressing transgenic line , with the intention of bringing the glucose into metabolism. Transgen ic 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 conte nt, down to 35% of wild-type levels. Biochemical analysis of growing t uber tissue revealed large increases in the metabolic intermediates of glycolysis, organic acids and amino acids, two- to threefold increase s in the maximum catalytic activities of key enzymes in the respirator y pathways, and three- to fivefold increases in carbon dioxide product ion. These changes occur in the lines expressing invertase, and are ac centuated following introduction of the second transgene, glucokinase. We conclude that the expression of invertase in potato tubers leads t o an increased flux through the glycolytic pathway at the expense of s tarch synthesis and that heterologous overexpression of glucokinase en hances this change in partitioning.