Regulation of sucrose and starch synthesis in wheat (Triticum aestivum L.) leaves: role of fructose 2,6-bisphosphate.

Fructose 2,6-bisphosphate (F26BP) is a competitive inhibitor of the cytosolic fructose 1,6-bisphosphatase (cytFBPase, EC 3.1.3.11). In spinach (Spinacia oleracea L.) leaves it is a significant component of the complex regulatory network that co-ordinates rates of photosynthesis, sucrose synthesis and starch synthesis. However the role of F26BP has only been studied in plants that predominantly store starch in their leaves and its role in other species is not clear. This paper examines the significance of F26BP in the regulation of photosynthetic carbon metabolism in the intact leaves of wheat (Triticum aestivum L.), a plant that accumulates predominantly sucrose. The approach taken was to vary rates of photosynthesis and then correlate measurements of F26BP and a range of other metabolites with rates of carbohydrate synthesis obtained from (14)CO(2)-feeding experiments performed under physiological conditions. It was found that: (i) Amounts of 3-phosphoglycerate and fructose-6-phosphate are correlated with the amount of F26BP. (ii) F26BP is involved in inhibiting cytFBPase at low light and low CO(2), but other factors, for example triose-phosphate, must also be involved. (iii) Amounts of both F26BP and substrate are involved in co-ordinating rates of photosynthesis and sucrose synthesis, but the relative importance of these depends on the conditions. (iv) Amounts of F26BP do not correlate with the partitioning of fixed carbon between sucrose and starch. Together these data suggest that the amount of F26BP in wheat is regulated by mechanisms similar to those in spinach, and that the metabolite is one of the factors involved in co-ordinating sucrose synthesis and photosynthesis. However F26BP does not appear to be involved in regulating the partitioning of fixed carbon between sucrose and starch in wheat under the experimental conditions examined.