Manipulation of fructose-2,6-bisphosphate levels in transgenic plants.

Introduction This report considers the contribution of recent studies on transgenic plants to our current understanding of the role of fructose-2,6-bisphosphate (Fru-2,6-P,) in the regulation of carbon metabolism. From data obtained using such plants we argue that this metabolite is quantitatively important in the control of carbohydrate metabolism in both photosynthetic and non-photosynthetic tissues. The current consensus is that Fru-2,6-PIS ‘ an important regulator of carbohydrate metabolism in higher plants [ 11. During photosynthesis this metabolite is thought to contribute both to the coordination of sucrose synthesis with the rate of COL fixation, and to the control of partitioning of fixed carbon between the synthesis of sucrose and starch [Z]. This view is based on three lines of evidence. First, Fru-2,6-PL is a potent inhibitor of cytosolic fructoseI ,6-bisphosphatase (FRPase), the first committed step of sucrose synthesis in the cytosol. Second, there is an inverse correlation between the level of Fru-2,6-P2 and the rate of sucrose synthesis when the latter is varied experimentally. Finally, the kinetic properties of the enzymes that metabolize Fru-2,6-P, allow its concentration to vary in response to changes in the levels of other metabolites when the rate of sucrose synthesis is varied. Ilespite the wealth of evidence to support this scheme, it suffers from two important limitations; the evidence is correlative and qualitative. Published data do not allow us to assess the extent to which Fru-2,6-P, regulates carbohydrate metabolism during photosynthesis. This is because the observed changes in F1-u-2,h-P~ are produced indirectly by modifying the levels of other metabolites. These modifications have been achieved by varying the rate of CO, fixation 131, altering the accumulation of sucrose [4], or manipulating the activity of enzymes not directly involved in Fru-ZhP, metabolism [ 5 ) . However, changes in the levels of such metabolites may themselves contribute directly to the regulation of the pathways [2]. Thus,