Sugar metabolism and partitioning in cytosol and bacteroid fractions of chickpea nodules

Abstract

The contents of free sugars in nodules of chickpea ( Cicer arietinum) were maximum around flowering. In stem and root tissues, the relative incorporation of 14C from [ 14C]-labelled sucrose or glucose into extracted sucrose was over 70 %. In the former tissue, the relative incorporation of 14C from glutamate into sucrose was about 50 % at 50 d after sowing (DAS) but the same decreased to about 25 % at 80 DAS. However, from glutamate, 63–68 % of 14C from extracted sugars of root tissue appeared in invert sugars. Feeding via stem [ 14C]-glutamate to intact nodules led to intense labelling of sucrose and invert sugars in nodule cytosol. Upon injecting labelled sugars or glutamate into isolated nodules, maximum 14C appeared in glucose of this nodule fraction. In bacteroids, incorporation of 14C from glutamate was much higher in amino acids. In the cytosol of younger (50 DAS) nodules, sucrose was cleaved largely by soluble alkaline invertase (EC 3.2.1.26). However, sucrose cleavage in this fraction of older (80 DAS) nodules was catalysed by this enzyme as well as sucrose synthase (reversal, EC 2.4.1.13) and such nodules also contained higher activity of nitrogenase. The bacteroid fraction, which contained 10–17 % of nodule sugars, lacked the activities of sucrose-cleaving enzymes. The activities of ATP-dependent phosphofructokinase (EC 2.7.1.11), glyceraldehyde-3-phosphate dehydrogenase (EC 1.1.1.12), NADP +-dependent isocitrate dehydrogenase (EC 1.1.1.41) and malate dehydrogenase (EC 1.1.1.37) were higher in cytosol than bacteroids. However, the reverse was true for glucose-6-phosphate dehydrogenase (EC 1.1.1.49) and 6-phosphogluconate dehydrogenase (EC 1.1.1.44). The results suggest that in chickpea nodules sugar metabolism occurs largely via the glycolytic pathway in cytosol and the pentose phosphate pathway in bacteroids and there is some transport of glutamate from cytosol to bacteroids.