Regulation of partitioning between sucrose and starch in wheat leaves

Abstract

Manipulation of key steps in sucrose synthesis is one potential approach to altering the partitioning of fixed carbon both within leaves and between organs. In leaves, the regulation of partitioning between sucrose and starch is generally accepted to involve both fructose 2,6-bisphosphate (F26BP), a competitive inhibitor of the cytosolic FBPase, and the enzyme sucrose-phosphate synthase (SPS). However this hypothesis has arisen from studies exclusively with dicot plants which predominantly store starch in their leaves, and details of the regulation in other species, particularly cereals that store predominantly sucrose, are less clear. We have examined the role of F26BP in regulating photosynthetic carbon metabolism in wheat by identifying correlations, or the lack thereof, between amounts of the metabolite and rates of synthesis of sucrose and starch measured by incorporation of label from 14CO2. Rates of synthesis were varied by altering either the external CO2 concentration, the light intensity, or the time of day at which the measurements were made. No correlations were observed between the amounts of F26BP and the ratio of sucrose and starch synthesis, suggesting that a role for F26BP in regulating carbohydrate partitioning in the leaves of plants is not universal. Data on amounts of metabolites known to interact with the regulation by F26BP (e.g. PGA, G6P) and changes in the activation state of SPS in wheat leaves with altered sucrose:starch incorporation ratios will be presented.