Regulation of carbon partitioning between sucrose and nitrogen assimilation in cotyledons of germinating Ricinus communis L. seedlings.

The interactions between carbon and nitrogen metabolism in cotyledons of germinating Ricinus communis L. seedlings were investigated. The endosperm was removed for 6-d-old seedlings and their cotyledons were supplied with 50 mM glucose and mM potassium phosphate without a nitrogen source, or supplemented with 10 mM glutamine or 5 mM NH4Cl. [U(14)C] Glucose labelling patterns were used to investigate the effect of fluxes. Addition of glutamine or NH4Cl led to a 3.5- to 5-fold increase of labelling in amino acids (most of which were exported) and increased (14)CO2 release. Glutamine also led to a stimulation of glucose uptake, sucrose synthesis and export. Measurements of metabolites showed that glutamine or NH4Cl led to a decrease of a-ketoglutarate, pyruvate, phosphoenolpyruvate, glycerate-2-phosphate and glycerate-3-phosphate, a small increase of triose-phosphate and fructose-1,6-bisphosphate, a small decrease of hexose-phosphate (in the case of glutamine), and an increase of UDP glucose. In both treatments, fructose-2,6-bisphosphate doubled, and inorganic pyrophosphate decreased slightly. Similar results were obtained in detached cotyledons, except that glutamine did not alter the rate of glucose uptake or sucrose synthesis. The increased rate of sucrose synthesis after supplying glutamine to intact seedlings is ascribed to an increased rate of sucrose export from the cotyledons due to enhanced water flow in the phloem, brought about by loading of glutamine. The doubling of the rate of glycolysis after adding glutamine or NH4Cl in intact seedlings or excised cotyledons is ascribed to activation of the terminal enzymes in glycolysis, pyruvate kinase and phosphoenolpyruvate carboxylase. The resulting decrease of phosphoenolpyruvate and glycerate-3-phosphate leads to activation of phosphofructokinase. It also relieves inhibition of fructose-6-phosphate,2-kinase, leading to increased fructose-2,6-bisphosphate and activation of pyrophosphate; fructose-6-phosphate phosphotransferase.