Activities of the Pentose Phosphate Pathway and Enzymes of Proline Metabolism in Legume Root Nodules

Abstract

Based on localization and high activities of pyrroline-5-carboxylate reductase and proline dehydrogenase activities in soybean nodules, we previously suggested two major roles for pyrroline-5-carboxylate reductase in addition to the production of the considerable quantity of proline needed for biosynthesis; namely, transfer of energy to the location of biological N2 fixation, and production of NADP+ to drive the pentose phosphate pathway. The latter produces ribose-5-phosphate which can be used in de novo purine synthesis required for synthesis of ureides, the major form in which biologically fixed N2 is transported from soybean root nodules to the plant shoot. In this paper, we report rapid induction (in soybean nodules) and exceptionally high activities (in nodules of eight species of N2-fixing plants) of pentose phosphate pathway and pyrroline-5-carboxylate reductase. There was a marked increase in proline dehydrogenase activity during soybean (Glycine max) ontogeny. The magnitude of proline dehydrogenase activity in bacteroids of soybean nodules was sufficiently high during most of the time course to supply a significant fraction of the energy requirement for N2 fixation. Proline dehydrogenase activity in bacteroids from nodules of other species was also high. These observations support the above hypothesis. However, comparison of pentose phosphate pathway and pyrroline-5-carboxylate reductase activities of ureide versus amide-exporting nodules offers no support. The hypothesis predicts that pyrroline-5-carboxylate and pentose phosphate pathway activities should be higher in ureide-exporting nodules than in amideexporting nodules. This predicted distinction was not observed in the results of in vitro assays of these activities.P5CR2 catalyzes the NAD(P)H-dependent reduction of P5C to form proline. The activity of P5CR is substantially higher in soybean nodules than that reported for other plant and animal tissues (1 1 addition to the production of the considerable quantity of proline needed for biosynthesis, particularly of cell wall glycoproteins. These possible roles are: the production ofreduced carbon skeletons (proline) whose subsequent oxidation contributes to fueling energy-intensive biological N2 fixation; and the production of NADP+ which makes possible, in a manner described below, the assimilation of biologically fixed N2 into ureides, the major form in which biologically fixed N2 is transported from soybean root nodules to the plant shoot (19,21). Increasing the availability of NADP+, as a consequence of the reduction of P5C to proline, should increase flux through the PPP, since this flux is tightly regulated by the intracellular concentration of NADP+ (12). P5C has been shown to stimulate the activity of the PPP in human fibroblasts, Chinese hamster ovary cells, rabbit kidney cells (18), human erythrocytes (27), and rat and mice lens (24). Based on analogy with these results in animal systems, Kohl et al. (I 1) proposed that the high P5CR activity in soybean nodules might function to i...

Evidence for Channeling of Intermediates in the Oxidative Pentose Phosphate Pathway by Soybean and Pea Nodule Extracts, Yeast Extracts, and Purified Yeast Enzymes

“…High rates of activity of the enzymes of the OPPP in the plant fraction of soybean nodules have been reported by Hong and Copeland (1990). High rates of NADP '-dependent evolution of I4CO, from [l-'4C]glucose in the 12000 g supernatant fluid of soybean nodule extracts have been observed in our laboratory (Kohl et al, 1990). Given the NADP' specificity, the source of the evolved CO , i s certainly the OPPP.…”

“…Soybean plants (Glyciize max L. Merr., cv Williams 82) were grown under greenhouse conditions as described earlier (Kohl et al, 1990). During growth, plants were fertilised with N-free nutrients (Fishbeck et al, 1973).…”

Effect of Hydrogen Cyanamide on Amino Acid Profiles in Kiwifruit Buds during Budbreak

“…To keep the pentose-phosphate pathway functioning, NADPH would need to be consumed, and this could be done by reducing A'-pyrroline-5-carboxylate to Pro (8). This type of interaction between Pro and the pentose-phosphate pathway has been documented in legume root nodules (11,12). Elevated levels of Pro in HC-treated plants could be associated with a greater stimulation of the pentose-phosphate pathway, resulting in a greater percentage of budbreak (9,14) and increased bud fruitfulness (14, E.F. Walton, P.J.…”

Proline Metabolism and Its Implications for Plant-Environment Interaction

“…Proline metabolism in nodules may be coupled to the pentose phosphate pathway, similar to the mechanism described above for leaf tissue (Section IV.-A). In bacteroids, proline synthesis could produce NADP + which would allow continued operation of the pentose phosphate pathway to generate precursors for ureide synthesis (Kohl et al, 1988;Kohl et al, 1990).…”