Fructose 2,6-bisphosphate activates pyrophosphate: fructose-6-phosphate 1-phosphotransferase and increases triose phosphate to hexose phosphate cycling in heterotrophic cells

The aim of this work was to establish the influence of fructose 2,6-bisphos phate (Fru-2,6-P-2) on non-photosynthetic carbohydrate metabolism in plants . Heterotrophic callus lines exhibiting elevated levels of Fru-2,6-P-2 were generated from transgenic tobacco (Nicotiana tabacum L.) plants expressing a modified rat liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase. Lines containing increased amounts of Fru-2,6-P-2 had lower levels of hexo se phosphates and higher levels of 3-phosphoglycerate than the untransforme d control cultures. There was also a greater redistribution of label into t he C6 position of sucrose and fructose, following incubation with [1-C-13]g lucose, in the lines possessing the highest amounts of Fru-2,6-P-2, indicat ing a greater re-synthesis of hexose phosphates from triose phosphates in t hese lines. Despite these changes, there were no marked differences between lines in the metabolism of C-14-substrates, the rate of oxygen uptake, car bohydrate accumulation or nucleotide pool sizes. These data provide direct evidence that physiologically relevant changes in the level of Fru-2,6-P-2 can affect pyrophosphate: fructose-6-phosphate 1-phosphotransferase (PFP) a ctivity in vivo, and are consistent with PFP operating in a net glycolytic direction in the heterotrophic culture. However, the results also show that activating PFP has Little direct effect on heterotrophic carbohydrate meta bolism beyond increasing the rate of cy hexose phosphates and triose phosph ates. of cycling between hexose phosphates and triose phosphates.