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

The contents of free sugars in nodules of chickpea (Cicer arietinum) were m aximum around flowering. In stem and root tissues, the relative incorporati on of C-14 from [C-14]-labelled sucrose or glucose into extracted sucrose w as over 70 %. In the former tissue, the relative incorporation of C-14 from glutamate into sucrose was about 50 % at 50 d after sowing (DAS) but the s ame decreased to about 25 % at 80 DAS. However, from glutamate, 63-68 % of C-14 from extracted sugars of root tissue appeared in invert sugars. Feedin g via stem [C-14]-glutamate to intact nodules led to intense labelling of s ucrose and invert sugars in nodule cytosol. Upon injecting labelled sugars or glutamate into isolated nodules, maximum C-14 appeared in glucose of thi s nodule fraction. In bacteroids, incorporation of C-14 from glutamate was much higher in amino acids. In the cytosol of younger (50 DAS) nodules, suc rose was cleaved largely by soluble alkaline invertase (EC 3.2.1.26). Howev er, sucrose cleavage in this fraction of older (80 DAS) nodules was catalys ed by this enzyme as well as sucrose synthase (reversal, EC 2.4.1.13) and s uch nodules also contained higher activity of nitrogenase. The bacteroid fr action, which contained 10-17 % of nodule sugars, lacked the activities of sucrose-cleaving enzymes. The activities of ATP-dependent phosphofructokina se (EC 2.7.1.11), glyceraldehyde-3-phosphate dehydrogenase (EC 1.1.1.12), N ADP(+)-dependent isocitrate dehydrogenase (EC 1.1.1.41) and malate dehydrog enase (EC 1.1.1.37) were higher in cytosol than bacteroids. However, the re verse was true for glucose-6-phosphate dehydrogenase (EC 1.1.1.49) and 6-ph osphogluconate dehydrogenase (EC 1.1.1.44). The results suggest that in chi ckpea 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. (C) 1999 Editions scient ifiques ct medicales Elsevier SAS.