Sucrose is metabolised by sucrose synthase and glycolysis within the phloem complex of Ricinus communis L. seedlings

Metabolites and enzyme activities were measured in the phloem sap exuding from a cut hypocotyl of germinating castor-bean (Ricinus communis L.) seedlings. The sap contained considerable quantities of adenine nucleotides, uridine nucleotides, uridine diphosphoglucose (UDPGlc), all the major phosphorylated metabolites required for glycolysis, fructose-2,6-bisphosphate and pyrophosphate. Supplying 200 mM glucose instead of sucrose to the cotyledons resulted in high concentrations of glucose in the sap, but did not modify the metabolite levels. In contrast, when 200 mM fructose was supplied we found only a low level of fructose but a raised sucrose concentration in the sap, which was accompanied by a three-to fourfold decrease of UDPGlc, and an increase of pyrophosphate, UDP and UTP. The measured levels of metabolites are used to estimate the molar mass action ratios and in-vivo free-energy change associated with the various reactions of sucrose breakdown and glycolysis in the phloem. It is concluded that the reactions catalysed by ATP-dependent phosphofructokinase and pyruvate kinase are removed from equilibrium in the phloem, whereas the reactions catalysed by sucrose synthase, UDPGlc-pyrophosphorylase, phosphoglucose mutase, phosphoglucose isomerase, aldolase, triose-phosphate isomerase, phosphoglycerate mutase and enolase are close to equilibrium within the conducting elements of the phloem. Since the exuded sap contained negligible or undetectable activities of the enzymes, it is concluded, that the responsible proteins are bound, or sequesterd behind plasmodesmata, possibly in the companion cells. It is argued that sucrose mobilisation via a reversible reaction catalysed by sucrose synthase is particularily well suited to allow the rate of sucrose breakdown in the phloem to respond to changes in the metabolic requirement for ATP, and for UDPGlc during callose production. It is also calculated that the transport of nucleotides in the phloem sap implies that there must be a very considerable uptake or de-novo biosynthesis of these cofactors in the phloem.