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1.
Pyrroline-5-Carboxylate Reductase in Soybean Nodules : Comparison of the Enzymes in Host Cytosol, Bradyrhizobium japonicum Bacteroids, and Cultures
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Characteristics of pyrroline-5-carboxylate reductase (P5CR) from Bradyrhizobium japonicum bacteroids and cultured rhizobia were compared with those of the enzyme in soybean nodule host cytosol. Reductase from host cytosol differed from that in bacteroids in: (a) the effect of pH on enzymic activity, (b) the capacity to catalyze both reduction of pyrroline-5-carboxylic acid and NAD+-dependent proline oxidation, (c) apparent affinities for pyrroline-5-carboxylic acid, and (d) sensitivities to inhibition by NADP+ and proline. The K1 for proline inhibition of P5CR in bacteroid cytosol was 1.8 millimolar. The properties of P5CR in B. japonicum and bacteroid cytosol were similar. The specific activities of P5CR in the cytosolic fractions of the nodule host and the bacteroid compartment were also comparable. 相似文献
2.
Nitrogen fixation and carbon metabolism by nodules and bacteroids of pea plants under sodium chloride stress 总被引:7,自引:0,他引:7
Acetylene reduction activity (ARA) and leghemoglobin (Lb) content in nodules were sigificantly reduced when pea ( Pisum sativum L. cv. Lincoln) plants were subjected to 50 m M sodium chloride stress for 3 weeks. C2 H2 reduction activity by bacteriods isolated from pea nodules was drastically inhibited by saline stress, and malate appeared to be a more appropriate substrate than glucose or succinate in maintaining this activity. Salt added directly to the incubation mixture of bacteriods or to the culture medium of plants inhibited O2 uptake by bacteroids. Nodule cytosolic phosphoenolpyruvate carboxylase (PEPC; EC 4.1.1.31) and bacteriod malate dehydrogenase (MDH; EC 1.1.1.37) activities were strongly enhanced by salt stress. Under these conditions, malate concentration was depressed in bacteroids and cytosol, whereas total soluble sugar (TSS)content slightly increased in both fractions. The effect of salt stress on TSS and malate content suggests that the utilization of carbohydrate within nodules could be inhibited during salt stress. The inhibitory effect of NaCl on N2 fixation activity of bacteroids and to the decrease in bacteroid respiration. The stimulation of fermentative metabolism induced by salinity suggests some reduction in O2 availability within the nodule. Salt stress was also responsible for a decrease of the cytosolic protein content, specifically of leghemoglobin, in the nodules. 相似文献
3.
Proteolytic Activity in Soybean Root Nodules : Activity in Host Cell Cytosol and Bacteroids throughout Physiological Development and Senescence 总被引:6,自引:4,他引:2
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Root nodules were harvested from chamber-grown soybean (Glycine max L. Merrill cv Woodworth) plants throughout development. Apparent nitrogenase activity (acetylene reduction) peaked before seeds began to develop, but a significant amount of activity remained as the seeds matured. Nodule senescence was defined as the period in which residual nitrogenase activity was lost. During this time, soluble protein and leghemoglobin levels in the host cell cytosol decreased, and proteolytic activity against azocasein increased. Degradative changes were not detected in bacteroids during nodule senescence. Total soluble bacteroid protein per gram of nodule remained constant, and an increase in proteolytic activity in bacteroid extracts was not observed. These results are consistent with the view that soybean nodule bacteroids are capable of redifferentiation into free-living bacteria upon deterioration of the legume-rhizobia symbiosis. 相似文献
4.
ABSTRACT The present work investigates the relationships between nitrogen fixation, carbon metabolism and oxygen consumption by bacteroids of Mesorhizobium ciceri in root nodules of chick-pea plants. Its aim was to establish whether some of the compounds which accumulate under salt stress may be used as respiratory substrates by bacteroids to fuel their own metabolism and nitrogenase activity. Plants were grown in a growth chamber, and salt stress was induced by adding 50 mM NaCl to the nutrient solution at sowing. The data presented here show a rise in fermentative metabolism in nodules of chick-pea plants exposed to high salinity, and suggest that proline, lactate or ethanol, may play an important role as energy-yielding substrates for bacteroids in this plant species. The bacteroids could utilize glucose as a respiratory substrate both under control and saline conditions, while malate did not appear to be the preferred substrate in the presence of salt. 相似文献
5.
The competition between combined nitrogen and nitrogen fixation in legumes was studied after a 24 h exposure of nodulated French-beans to nitrate. Acetylene reduction by bacteroids was significantly inhibited and even nitrogenase extracted from nitrate-treated plant nodules showed reduced activity. Sensitivity to nitrate was directly related to nodule age and also increased with increasing oxygen tensions in the bacteroid incubations with or without a gas phase; it was particularly marked when glucose was used in place of succinate as energy-yielding substrate. Bacteroid respiration was also depressed by nitrate-treatment of the plants, leading to diminished acetylene reduction and this effect increased with increasing oxygen concentrations. Added oxyleghemoglobin partly restored oxygen consumption and acetylene reduction by bacteroid suspensions. 相似文献
6.
Effect of water stress on the reduction of nitrate and nitrite by soybean nodules 总被引:2,自引:1,他引:1
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The effects of water stress on nitrate reductase and nitrite reductase activities in symbiotic nodules were examined in field-grown soybean plants (Glycine max L Merr. cv Clark). The in vitro assays of enzyme activity indicated that the nodule cytosol and bacteroids contained both nitrate reductase and nitrite reductase activities. The reduction of nitrate in bacteroids increased significantly as nodule water potential declined from −0.6 to −1.4 megapascals, and then decreased when −1.8 megapascals water potential was reached. On the contrary, the reduction of nitrate in nodule cytosol was inhibited as water stress progressed. Increases in water stress intensity also caused a significant inhibition in nitrite reductase activities of bacteroids and nodule cytosol within soybean nodules. The results show that nitrate reduction occurred both in the cytosol and bacteroids of water-stressed soybean nodules. The reduction of nitrate functioned at different physiological modes in these two fractions. 相似文献
7.
Exposure of Galega orientalis plants to diamines putrescine (Put) and cadaverine (Cad) at concentrations from 0.01 to 2.0 m M significantly altered carbon and nitrogen metabolism in their root nodules. Correlative studies of bacteroid poly- β -hydroxybutyrate (PHB) content and acetylene-reduction capacity of the nodules revealed a negative relationship between these parameters. Utilisation of PHB deposits by bacteroids and high acetylene reduction activity was observed when applying low diamine concentrations. The increase in PHB accumulation in response to high diamine levels was accompanied by a considerable decline in nodule nitrogenase activity. Supplying isolated Galega bacteroids with various diamine concentrations significantly modified bacteroid oxygen consumption, which might be associated with alterations in carbon flux to the bacteroids. Finally, modulation of the bacteroid content upon Put and Cad treatment was examined. The results are discussed in terms of possible causes of the diamine-induced changes in nodule metabolism. 相似文献
8.
Poly-beta-hydroxybutyrate Utilization by Soybean (Glycine max Merr.) Nodules and Assessment of Its Role in Maintenance of Nitrogenase Activity 总被引:1,自引:0,他引:1
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Soybean (Glycine max) nodule bacteroids contain high concentrations of poly-β-hydroxybutyrate and possess a depolymerase system that catalyzes the hydrolysis of the polymer. Changes in poly-β-hydroxybutyrate content and in activities of nitrogenase, β-hydroxybutyrate dehydrogenase, and isocitrate lyase in nodule bacteroids were investigated under conditions in which the supply of carbohydrate from the soybean plants was interrupted. The poly-β-hydroxybutyrate content of bacteroids did not decrease appreciably until the carbohydrate supply from the host plants was limited by incubation of excised nodules, incubation of plants in the dark, or by senescence of the host plant. Isocitrate lyase activity in bacteroids was not detected until poly-β-hydroxybutyrate utilization appeared to begin. The presence of a supply of poly-β-hydroxybutyrate in nodule bacteroids was not sufficient for maintenance of high nitrogenase activity under conditions of limited carbohydrate supply from the host plant. An unusually high activity of β-hydroxybutyrate dehydrogenase was observed in bacteroid extracts but no significant change in the activity of this enzyme was observed as a result of apparent utilization of poly-β-hydroxybutyrate by nodule bacteroids. 相似文献
9.
Glenn G. Preston Carolyn Zeiher Judy D. Wall David W. Emerich 《Applied microbiology》1989,55(1):165-170
Acetyl coenzyme A (acetyl-CoA) synthetase and acetate kinase were localized within the soluble portion of Bradyrhizobium japonicum bacteroids, and no appreciable activity was found elsewhere in the nodule. The presence of each acetate-activating enzyme was confirmed by separation of the two enzyme activities on a hydroxylapatite column, by substrate dependence of each enzyme in both the forward and reverse directions, by substrate specificity, by inhibition patterns, and also by identification of the reaction products by C18 reverse-phase high-pressure liquid chromatography. Phosphotransacetylase activity, found in the soluble portion of the bacteroid, was dependent on the presence of potassium and was inhibited by added sodium. The greatest acetyl-CoA hydrolase activity was found in the root nodule cytosol, although appreciable activity also was found within the bacteroids. The combined specific activities of acetyl-CoA synthetase and acetate kinase-phosphotransacetylase were approximate to that of the pyruvate dehydrogenase complex, thus providing B. japonicum with sufficient capacity to generate acetyl-CoA. 相似文献
10.
Some Enzymes of Hydrogen Peroxide Metabolism in Leaves and Root Nodules of Medicago sativa 总被引:7,自引:0,他引:7
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Leaves and nodules (bacteroids and cytosol) of alfalfa (Medicago sativa L. cv Aragon) plants inoculated with Rhizobium meliloti strain 102F51 have been analyzed for the presence of the enzymes superoxide dismutase (SOD, EC 1.15.1.1), catalase (EC 1.11.1.6), and peroxidase (EC 1.11.1.7). All three fractions investigated (leaves, bacteroids, and nodular cytosol) show Cu,Zn-SOD activity. Besides, the bacteroids and cytosol of nodules possess CN−-insensitive SOD activities. Studies of SOD inactivation with H2O2 indicate that, very likely, a Mn-SOD is present in the bacteroids, and suggest that the cytosol contain both Mn-SOD and Fe-SOD. Bacteroids show high catalase activity but lack peroxidase. By contrast, the nodule cytosol exhibits an elevated peroxidase activity as compared with the foliar tissue; this activity was completely inhibited by 50 to 100 micromolar KCN. The significantly lower contents of H2O2 and malondialdehyde (a product of lipid peroxidation) in nodules with respect to those in leaves reveal that the above-mentioned bacteroid and cytosol enzymes act in an efficient and combined manner to preserve integrity of nodule cell membranes and to keep leghemoglobin active. 相似文献
11.
Summary A series of investigations were conducted with the objective of elucidating natural pathways of electron transport from respiratory
processes to the site of N2 fixation in nodule bacteroids. A survey of dehydrogenase activities in a crude extract of soybean nodule bacteroids revealed
relatively high activities of NAD-specific β-hydroxybutyrate and glyceraldehyde-3-phosphate dehydrogenases. Moderate activities
of NADP-specific isocitrate and glucose-6-phosphate dehydrogenases were observed. By use of the ATP-dependent acetylene reduction
reaction catalyzed by soybean bacteroid nitrogenase, and enzymes and cofactors from bacteroids and other sources, the following
sequences of electron transport to bacteroid nitrogenase were demonstrated: (1) H2 to bacteroid nitrogenase in presence of a nitrogenase-free extract ofC. pasteurianum; (2) β-hydroxybutyrate to bacteroid nitrogenase in a reaction containing β-hydroxybutyrate dehydrogenase, NADH dehydrogenase,
NAD and benzyl viologen; (3) β-hydroxybutyrate dehydrogenase, to nitrogenase in reaction containing NADH dehydrogenase, NAD
and either FMN or FAD; (4) light-dependent transfer of electrons from ascorbate to bacteroid nitrogenase in a reaction containing
photosystem I from spinach chloroplasts, 2,6-dichlorophenolindophenol, and either azotoflavin from Azotobacter or non-heme
iron protein from bacteroids; (5) glucose-6-phosphate to bacteroid nitrogenase in a system that included glucose-6-phosphate
dehydrogenase, NADP, NADP-ferredoxin reductase from spinach, azotoflavin from Azotobacter and bacteroid non-heme iron protein.
The electron transport factors, azotoflavin and bacteroid non-heme iron protein, failed to function in the transfer of electrons
from an NADH-generating system to bacteroid nitrogenase. When FMN or FAD were added to systems containing azotoflavin and
bacteroid non-heme iron protein, electrons apparently were transferred to the flavin-nucleotides and then nitrogenase without
involvement of azotoflavin and bacteroid non-heme iron protein.
Evidence is available indicating that nodule bacteroids contain flavoproteins analogous to Azotobacter, azotoflavin, and spinach
ferredoxin-NADP reductase. It is concluded that physiologically important systems involved in transport of electrons from
dehydrogenases to nitrogenase in bacteroids very likely will include relatively specific electron transport proteins such
as bacteroid non-heme iron protein and a flavoprotein from bacteroids that is analogous to azotoflavin. 相似文献
12.
Vasily I. Romanov Bahar R. Hajy-Zadeh Boris F. Ivanov Grigory L. Shaposhnikov WacŁav L. Kretovich 《Phytochemistry》1985,24(10):2157-2160
On feeding 14CO2 to the shoots of lupine (25 mCi per plant) 30 min was the minimal time needed to determine the incorporation of label into bacteroid compounds. The predominant incorporation, exhibited in all root, nodule and bacteroid samples after 30 min exposure, was into sucrose (45–90% of the corresponding fraction radioactivity) of the neutral fraction; into malate (30–40%) of the acid fraction; into aspartic acid and asparagine (60–80% in sum) of the basic fraction. The composition of carbon compounds containing the greatest amount of 14C in the cytosol of nodules and in bacteroids was similar. Their radioactivity after 30 min exposure was for bacteroids (nCi per g of bacteroid fr. wt): sucrose 5.73, glucose 1.00, malate 0.15, succinate 0.11; for the nodule cytosol (nCi per g of nodule fr. wt): sucrose 200.00, glucose 8.40, malate 9.34, succinate 8.50. Thus it was demonstrated that in lupine, sucrose is the main photoassimilate entering not only into nodules but also into bacteroids. The biosynthesis of aspartic acid and asparagine occurs during nitrogen fixation in bacteroids. 相似文献
13.
Well-nodulated soybean ( Glycine max L. Merr. cv. Akisengoku) plants were allowed to assimilate 13 CO2 . Plant cytosol and bacteroid fractions were isolated from nodules, and the kinetics of [13 C]-labelling of soluble carbohydrates, organic acids and amino acids were investigated.
The concentrations of all metabolites, with the exception of trehalose and 3-hydroxy-butyrate, were 10- to 1000-fold higher in plant cell cytosol than in bacteroids. The major portion of trehalose was found in bacteroids and 3-hydroxybutyrate only in bacteroids. Sucrose was most highly labelled with13 C in nodules, and the levels and time-course of labelling of sucrose were in good agreement with those of respired CO2 from the nodules. The levels and time-courses of labelling of sucrose were closely similar in cytosol and bacteroids. Glucose was less labelled than sucrose and the level of labelling was consistently higher in cytosol than in bacteroids. The levels of [13 C]-labelling of organic acids and amino acids in nodules were lower than those of sucrose and of respired CO2 . Tricarboxylic acid cycle intermediates, particularly succinate, were considerably less labelled in bacteroids than in the cytosol. All amino acids detected were also much more rapidly labelled in the cytosol. The results are discussed in relation to the utilization and possible compartmentation of carbon substrates in nodule tissues. 相似文献
The concentrations of all metabolites, with the exception of trehalose and 3-hydroxy-butyrate, were 10- to 1000-fold higher in plant cell cytosol than in bacteroids. The major portion of trehalose was found in bacteroids and 3-hydroxybutyrate only in bacteroids. Sucrose was most highly labelled with
14.
Metabolism of trehalose, α,d-glucopyranosyl-α,d-glucopyranoside, was studied in nodules of Bradyrhizobium japonicum-Glycine max [L.] Merr. cv Beeson 80 symbiosis. The nodule extract was divided into three fractions: bacteroid soluble protein, bacteroid fragments, and cytosol. The bacteroid soluble protein and cytosol fractions were gel-filtered. The key biosynthetic enzyme, trehalose-6-phosphate synthetase, was consistently found only in the bacteroids. Trehalose-6-phosphate phosphatase activity was present both in the bacteroid soluble protein and cytosol fractions. Trehalase, the most abundant catabolic enzyme was present in all three fractions and showed two pH optima: pH 3.8 and 6.6. Two other degradative enzymes, phosphotrehalase, acting on trehalose-6-phosphate forming glucose and glucose-6-phosphate, and trehalose phosphorylase, forming glucose and β-glucose-1-phosphate, were also detected in the bacteroid soluble protein and cytosol fractions. Trehalase was present in large excess over trehalose-6-phosphate synthetase. Trehalose accumulation in the nodules would appear to be predicated on spatial separation of trehalose and trehalase. 相似文献
15.
Addition of NO− 3 rapidly induced senescence of root nodules in alfalfa ( Medicago sativa L. cv. Aragon). Loss of nodule dry matter began at the lowest NO− 3 concentration (10 m M ) but degradation of bacteroid proteins was only detected when nodules were supplied with NO− 3 concentrations above 20 m M .
Bacteroids from Rhizobium meliloti contained high specific activities of nitrate reductase (NR) and nitrite reductase (NiR). Both enzymes were presumably substrate-induced although substantial enzyme activities were present in the absence of NO− 3 Typical specific activities for soluble NR and NiR of bacteroids under NO− 3 free conditions were 1.2 and 1.4 μmol (mg protein)−1 h−1 , respectively. In the presence of NO− 3 , the specific activity of NR was considerably greater than that of NiR, thus causing NO− 2 accumulation in bacteroids. Nitrite levels in the bacteroids were linearly correlated with specific activities of NR and NiR, indicating that NO− 2 is formed by bacteroid NR and that this NO− 2 in turn, induces bacteroid NiR. Accumulation of NO− 2 within bacteroids also indicates that NO− 2 inhibits nodule activity after feeding plants with NO− 3 相似文献
Bacteroids from Rhizobium meliloti contained high specific activities of nitrate reductase (NR) and nitrite reductase (NiR). Both enzymes were presumably substrate-induced although substantial enzyme activities were present in the absence of NO
16.
A two‐week salt treatment (NaCl, 100 m M ) induced a 50% inhibition of acetylene reduction activity (ARA) of faba bean ( Vicia faba L. var. minor cv. Soravi) nodules, associated with a large increase in the nodule pool of amino acids. The concentration of proline in the different nodule compartments was determined after calculating their respective volumes from their areas on electron micrographs. The proline concentration exhibited a large increase, especially in the cytosol where its amount was 8‐fold enhanced under salt stress, whereas the low proline content of bacteroids was less affected. Increase of proline concentration in faba bean nodules subjected to salt stress was correlated with an enhancement of the cytosolic Δ1 ‐pyrroline‐5‐carboxylate synthetase (EC 2.7.2.11 + EC 1.2.1.41; P5CS) activity. Experiments with purified symbiosome preparations showed that the greatest proline content occurred in the peribacteroid space (PBS), where proline was the most abundant amino acid, with a concentration reaching 15.3 m M under salt stress. Proline accumulation in the PBS resulted both from a diffusive transport from the host cell to the symbiosomes through the peribacteroid membrane (PBM) and from the very low rate of uptake by faba bean bacteroids. This accumulation could be partly responsible for the 1.7‐fold enlargement of the symbiosome volume observed in salt‐stressed nodules. In incubations of bacteroids, isolated from salt‐stressed or unstressed plants and supplied with O2 by purified oxyleghemoglobin, addition of proline stimulated neither O2 consumption nor ARA. These results were consistent with proline playing a role as osmoticum, rather than energy source for bacteroid N2 fixation in amide‐exporting legumes such as faba bean. 相似文献
17.
C. Arrese-Igor J. I. García-Plazaola A. Hernández P. M. Aparicio-Tejo 《Physiologia plantarum》1990,80(2):185-190
Plants of lucerne ( Medicago sativa L. cv. Aragón) inoculated with several strains of Rhizobium meliloti were supplied with a low level of nitrate (5 m M ). After 1 week, normalised nodule mass, obtained by dividing nodule weight by shoot weight, was decreased by one-fourth. This result closely paralleled the bacteroid protein content of nodules, whereas the cytosolic content remained constant. Nitrate reductase activity (NRA, EC 1.7.99.4) of bacteroids increased rapidly after nitrate supply, with actual rates being highly dependent on the Rhizobium strain. The expression of cytosolic NR (EC 1.6.6.1) also varied depending on the bacterial strain but was largely insensitive to nitrate feeding. Nitrite reductase activity (NiRA, EC 1.7.2.2) of either bacteroid or plant origin was independent of the R. meliloti strain. Activation occurred after 3 and 7 days, respectively, of nitrate feeding. Significant amounts of nitrite were obtained throughout the experimental period from buffered extracts of both bacteroids and cytosol of nodules. However, when these nodules were ground in the presence of inhibitors of enzyme activity, nitrite was only found in nodules containing strain 102-F-51 after 1 week of treatment. These results agree with the recent hypothesis that nitrite plays a role in a secondary stage of nodule damage by nitrate. We propose that NiRA rather than NRA can be used as an internal probe of nitrate access to the infected region of nodules. 相似文献
18.
《Plant Physiology and Biochemistry》1999,37(9):685-692
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. 相似文献
19.
Enzymatic regulation of pyricline nucleotide formation, under symbiotic and non-symbiotic conditions, was analyzed using soybeans (Glycine max L. cv. 'Akisengoku') and rhizobia (Bradyrhizobia japonicum strain A1017), respectively. It was found that levels of pyridine nucleotides in bacteroids in root nodules were different from those in free-living cells of rhizobia. This difference was associated with differences in activities of enzymes involved in the pathway from L-tryptophan to NAD and NADP. That is, these activities were lower in bacteroids than in free-living bacteria and lower in the nodule cytosol than in root extracts. The optimum pH for NAD synthetase in bacteroids, was 9.0. Additionally, the optimum pH for ATP-nicotinamide mononucleotide (NMN) adenyltransferase, final step enzyme in NAD formation, was estimated to be 7.6. In the bacteroid fraction, the K(m) of NAD synthetase (22 microM) was approximately 1/22 of that of ATP-NMN adenyltransferase (482 microM). Vmax values were estimated to be almost in the same order for both NAD synthetase and ATP-NMN adenyltransferase. This is the first report on the formation of pyridine nucleotides originating from L-tryptophan in bacteroids in soybean nodules and free-living bacteria. 相似文献
20.
The inhibitory effect exerted by water stress on acetylene reductionactivity (ARA) by nodulated roots of faba beans (Vicia fabaL.) was correlated with a 40% decline in the organic acid poolof nodule cytosol. Oxalate concentration was lowered (55%)whereas a stimulation of the bacteroid oxalate oxidase concomitantlyoccurred. This enzyme was characterized by an optimal activityat pH 8 but, as in higher plants, exhibited a Km for oxalateof 1.4 mM and an inhibition by substrate excess. Oxalate providedto bacteroid incubations supported C2H2 reduction up to 2.5mM whereas higher concentrations were strongly inhibitory. Incontrast, purified symbiosomes incubated with oxyleghaemoglobinreduced C2H2 in the presence of oxalate concentrations up to10 mM. The peribacteroid membrane (PBM), in controlling theoxalate flux to the bacteroids avoided the substrate inhibitionwhich would limit its efficiency. Thus, oxalate present in highconcentration in faba bean nodules could play a role as complementarysubstrate for bacteroids slowing down the nitrogen fixationdecline induced by water restricted conditions. Key words: Faba bean, water stress, oxalate, acetylene reduction, bacteroid 相似文献