Control of Photosynthetic Sucrose Synthesis by Fructose 2,6-Bisphosphate

In this article, we propose a model describing how the spinach leaf cytosolic fructose 1,6-bisphosphatase is regulated in vivo by an interaction between fructose 2,6-bisphosphate and metabolic intermediates during photosynthesis. Previously published results are reanalyzed to provide a description of the way in which fluxes and metabolites vary in spinach leaves, depending on the relation between the supply of photosynthate and the demand for sucrose. The activity of the spinach leaf cytosolic fructose 1,6-bisphosphatase has been assayed in conditions simulating those expected in leaves as the rate of sucrose synthesis increases in response to rising rates of photosynthesis, and as sucrose synthesis declines in response to accumulation of sucrose in the leaf so that more starch is synthesized. The results show that regulation of the cytosolic fructose 1,6-bisphosphatase by alterations of fructose 2,6-bisphosphate, dihydroxyacetone phosphate, adenosine monophosphate, and phosphate can account for the alterations of flux found in vivo. The properties of a regulatory network, which allows the distribution of triose P between the Calvin cycle, sucrose synthesis, and starch synthesis to be balanced and adjusted, are described.