Nonphotosynthetic CO2 fixation by root nodules of legumes may provide carbon skeletons for assimilation of symbiotically fixed N2. However, the products formed by this process and their physiological significance in perennial legumes is poorly understood. Therefore, the objectives of this study were to determine if nonphotosynthetic CO2 fixation by root nodules contributes carbon for the assimilation of fixed N2 in alfalfa (Medicago sativa L.) and birdsfoot trefoil (Lotus corniculatus L.) and if assimilation products are partitioned to different plant organs. Effective alfalfa nodules excised from or attached to roots had apparent 14CO2 fixation rates of 50 to 80 μg CO2 kg‐1 s‐1 (dry weight) at 0.0012 to 0.0038 mole fraction CO2. Nodule CO2 fixation rates increased six‐ to seven‐fold as ambient CO2 was raised from 0.0038 to 0.0663 mole fraction. These rates were double to triple those of comparably treated nodules of ineffective alfalfa and those of birdsfoot trefoil. Respiration rates of nodules (3 to 4 mg CO2 kg‐1 s‐1) were 10 to 100‐fold higher than 14CO2 fixation rates of nodules. This finding suggests that attached nodules normally function at saturating CO2 levels and may fix up to 1600 μg CO2 kg‐1 s‐1 when concurrent 14C export to roots and shoots is accounted for. Pulse chase experiments with 14CO2 combined with nodule and xylem sap analysis demonstrated the initial products of root and nodule CO2 fixation were organic acids. However, the export of fixed 14C from effective nodules was primarily in the form of amino acids. In contrast, nodule and/or root fixed 14C in ineffectively nodulated alfalfa and denodulated effective alfalfa and birdsfoot trefoil was transported primarily as organic acids. Aspartate, asparagine, alanine, glutamate, and glutamine were the most heavily labeled compounds in the amino acid fraction of both effective alfalfa and birdsfoot trefoil nodules exposed to 14CO2. By contrast, asparate, asparagine, and glutamine were the predominantly labeled amino acids in xylem sap collected from nodulated effective roots exposed to 14CO2. The occurrence of nodule CO2 fixation in alfalfa and birdsfoot trefoil and the export of fixed carbon as asparagine and aspartate to roots and shoots is consistent with a role for CO2 fixation by nodules in providing carbon skeletons for assimilation and transport of symbiotically fixed N2.
In vivo CO2 fixation activity and in vitro phosphoenolpyruvate carboxylase activty were demonstrated in effective and ineffective nodules of alfalfa (Medicago sativa L.) and in the nodules offour other legume species. Phosphoenolpyruvate carboxylase activity was greatly reduced in nodules from both host and bacterially conditioned ineffective alfalfa nodules as compared to effective alfalfa nodules.Forage harvest and nitrate application reduced both in vivo and in vitro CO2 fixation activty. By day 11, forage harvest resulted in a 42% decline in in vitro nodule phosphoenolpyruvate carboxylase activity while treatment with either 40 or 80 kilograms nitrogen per hectare reduced activity by 65%. In vitro specific activity of phosphoenolpyruvate carboxylase and glutamate synthase were positively correlated with each other and both were positively correlated with acetylene reduction activity.Tlhe distribution of radioactivity in the nodules of control plants (unharvested, 0 kilgrams nitrogen per hectare) averaged 73% into the organic acid and 27% into the amino acid fraction. In nodules from harvested plants treated with nitrate, near equal distribution of radioactivity was observed in the organic acid (52%) and amino acid (48%) fractions by day 8. Recovery to control distribution occurred oniy in those nodules whose in vitro phosphoenolpyruvate carboxylase activity recovered.The results demonstrate that CO2 fixation is correlated with nitrogen fixation in alfalfa nodules. The maximum rate of CO2 fixation for attached and detached alfalfa nodules at low CO2 concentrations (0.13-0.38% C02) were 18.3 and 4.9 nanomoles per hour per milligram dry weight, respectively. Nodule CO2 fixation was estimated to provide 25% of the carbon required for assimilation of symbiotically fixed nitrogen in alfalfa.Current estimates with annual legumes suggest that 30%o of the carbon gained through photosynthesis in the shoot is used for nodule function and maintenance and that approximately 60%o of the carbon partitioned to the nodules is lost as CO2 through respiration (14, 16 fixation via nodule PEP3 carboxylase (EC 4.1.1.3 1) acts as a mechanism for recovery ofsome ofthis respired C02, thus increasing nodule efficiency and providing an added source of carbon for assimilation of fixed N. Studies with lupine and soybean suggest different relationships between N2 fixation and PEP carboxylase. In lupine, manipulations that reduced nitrogen fixation activity caused a concomitant decrease in PEP carboxylase activity (12). Coker and Schubert (7) found that CO2 fixation activity declined in advance of the decrease in N2 fixation activity in soybean nodules.Several investigations on the role of PEP carboxylase in nodule metabolism have involved exposure of either excised or attached nodules to 14CO2. Lawrie and Wheeler (13) demonstrated that high levels ofradioactivity were initially associated with glutamate and aspartate and later with asparagine in excised nodules of Vicia faba after exposure to 14CO2. In excised lupine nodules, as...
Nodule structure, nodule enzymes of ammonia assimilation, nitrogenase-dependent acetylene reduction, and nodule soluble protein were studied during vegetative regrowth of detopped birdsfoot trefoil (Lotus corniculatus L.) seedlings grown in the glasshouse. Nodules senesced rapidly for a period of 14 days following shoot removal, but then pink nodule populations increased as shoot regrowth occurred. The structural sequence of senescence was similar in nodules whether the result of either aging or shoot removal. Membranes surrounding bacteroids showed degenerative changes as bacteroids senesced. Bacteroids aggregated within the nodule cells and ultimately disappeared in senescent cells. Infection threads and bacteria inside infection threads did not disintegrate.Nodule senescence as a result of shoot removal was accompanied by a marked decline in acetylene reduction, nodule soluble protein, nodule host plant glutamine synthetase (GS), and glutamate synthase (GOGAT). Nodule enzyme activity, soluble protein, and acetylene reduction activity recovered to initial values as shoot regrowth occurred and pink nodule populations increased. Nodule host plant glutamate dehydrogenase (GDH) did not change after shoot removal.This study shows that birdsfoot trefoil nodules respond to shoot removal by an increased senescence. Recovery of nodule function is associated with the formation of a new nodule population. The data also indicate that host plant GS and GOGAT function to assimilate fixed N. Spherical nodules with determinant growth may be less efficient than elongate nodules with indeterminant growth in nodule maintenance and function.
A nonphotosynthetic phosphoenolpyruvate carboxylase (EC 4.1.131) was partially purified from the cytosol of root nodules of alfalfa. The enzyme was purified 86-fold by ammonium sulfate fractionation, DEAEcellulose, hydroxylapatite chromatography, and reactive agarose with a final yield of 32%. The enzyme exhibited a pH optimum of 7.5 with apparent K,. values for phosphoenolpyruvate and magnesium of 210 and 100 micromolar, respectively. Two isozymes were resolved by nondenaturing polyacrylamide disc gel electrophoresis. Subsequent electrophoresis of these isozymes in a second dimension by sodium dodecyl sulfate slab gel electrophoresis yielded identical protein patterms for the isozymes with one major protein band at molecular weight 97,000. Malate and AMP were slightly inhibitory (about 20%) to the partially purified enzyme. Phosphoenolpyruvate carboxylase comprised approximately 1 to 2% of the total soluble protein in actively N2-fixing alfalfa nodules.Recent studies have demonstrated that nodules of several legume species actively fix CO2 via the enzyme PEPC2 (EC 4.1.1.31) and that PEPC may increase nodule carbon use efficiency by recycling a portion of the CO2 lost through nodule respiration (1,2,7,20). In alfalfa, nodule CO2 fixation is highly correlated with active N2-fixation and has been shown to contribute up to 25% of the carbon required for the assimilation of symbiotically derived N (20). Both in vitro PEPC activity and in vivo CO2 fixation activity in alfalfa nodules decreased following applications of NO3-(20).Although PEPC has been purified and characterized from nodules of the annual legumes soybean and lupine, regulation of the enzyme activity is not well understood (1, 13). Soybean nodule PEPC activity appears to be regulated by PEP and HCO3 concentrations (13
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.