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

Abstract

The role of fructose 2,6-bisphosphate in partitioning of photosynthate between sucrose and starch was studied in spinach (Spinacia oleracea U.S.A. hybrid 424). Spinach leaf material was pretreated to alter the sucrose content, so that the rate of starch synthesis could be varied. The level of fructose 2,6-bisphosphate and other metabolites was then related to the accumulation of sucrose and the rate of starch synthesis. Fructose 2,6-bisphosphate is apparently involved in a sequence of events which provide a fine control of sucrose synthesis so that more photosynthate is diverted into starch under conditions when sucrose accumulates to high levels in the leaf tissue. As sucrose levels in the leaf rise, there is an accumulation of triose phosphates and hexose phosphates, implying an inhibition of sucrose phosphate synthase and cytosolic fructose 1,6-bisphosphatase. Under these conditions, fructose 2,6-bisphosphate increases. The increased fructose 2,6-bisphosphate can be accounted for by the increased fructose 6-phosphate in the leaf. Fructose 2,6-bisphosphate inhibits the cytosolic fructose 1,6-bisphosphatase so more photosynthate is retained in the chloroplast, and converted to starch.