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1.
In vivo CO 2 fixation activity and in vitro phosphoenolpyruvate carboxylase activity were demonstrated in effective and ineffective nodules of alfalfa ( Medicago sativa L.) and in the nodules of four 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 activity. 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. The distribution of radioactivity in the nodules of control plants (unharvested, 0 kilograms 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 only 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% CO2) 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. 相似文献
2.
Our aim was to determine whether fixation of inorganic carbon (C i), due to phospho enolpyruvate carboxylase activity, is limited by the availability of C i in the cytoplasm of maize ( Zea mays L.) root tips. Rates of C i uptake and metabolism were measured during K 2SO 4 treatment, which stimulates dark C i fixation. 13C i uptake was followed by 13C-nuclear magnetic resonance (NMR); 5 millimolar K 2SO 4 had no significant effect on 13C i influx. The contribution of respiratory CO 2 production to cytoplasmic HCO 3− was measured using in vivo 13C-NMR and 1H-NMR of cell extracts; K 2SO 4 treatment had no effect on respiratory CO 2 production. The concentration of cytoplasmic HCO 3− was estimated to be approximately 11 millimolar, again with K 2SO 4 having no significant effect. These experiments allowed us to determine the extent to which extracellularly supplied 14C i was diluted in the cytoplasm by respiratory CO 2 and thereby measure phospho enolpyruvate (PEP) carboxylase activity in vivo using 14C i. PEP carboxylase activity in root tips was enhanced approximately 70% over controls within 12 minutes of the addition of 5 millimolar K 2SO 4. The activity of carbonic anhydrase, which provides PEP carboxylase with C i, was determined by saturation transfer 13C-NMR to be more than 200 times that of PEP carboxylase in vivo. The regulation of PEP carboxylase in K 2SO 4-treated roots is discussed. 相似文献
3.
The dependence of alfalfa ( Medicago sativa L.) root and nodule nonphotosynthetic CO 2 fixation on the supply of currently produced photosynthate and nodule nitrogenase activity was examined at various times after phloem-girdling and exposure of nodules to Ar:O 2. Phloemgirdling was effected 20 hours and exposure to Ar:O 2 was effected 2 to 3 hours before initiation of experiments. Nodule and root CO 2 fixation rates of phloem-girdled plants were reduced to 38 and 50%, respectively, of those of control plants. Exposure to Ar:O 2 decreased nodule CO 2 fixation rates to 45%, respiration rates to 55%, and nitrogenase activities to 51% of those of the controls. The products of nodule CO 2 fixation were exported through the xylem to the shoot mainly as amino acids within 30 to 60 minutes after exposure to 14CO 2. In contrast to nodules, roots exported very little radioactivity, and most of the 14C was exported as organic acids. The nonphotosynthetic CO 2 fixation rate of roots and nodules averaged 26% of the gross respiration rate, i.e. the sum of net respiration and nonphotosynthetic CO 2 assimilation. Nodules fixed CO 2 at a rate 5.6 times that of roots, but since nodules comprised a small portion of root system mass, roots accounted for 76% of the nodulated root system CO 2 fixation. The results of this study showed that exposure of nodules to Ar:O 2 reduced nodule-specific respiration and nitrogenase activity by similar amounts, and that phloem-girdling significantly reduced nodule CO 2 fixation, nitrogenase activity, nodule-specific respiration, and transport of 14C photoassimilate to nodules. These results indicate that nodule CO 2 fixation in alfalfa is associated with N assimilation. 相似文献
4.
Products of the nodule cytosol in vivo dark [ 14C]CO 2 fixation were detected in the plant cytosol as well as in the bacteroids of pea ( Pisum sativum L. cv “Bodil”) nodules. The distribution of the metabolites of the dark CO 2 fixation products was compared in effective (fix +) nodules infected by a wild-type Rhizobium leguminosarum (MNF 300), and ineffective (fix −) nodules of the R. leguminosarum mutant MNF 3080. The latter has a defect in the dicarboxylic acid transport system of the bacterial membrane. The 14C incorporation from [ 14C]CO 2 was about threefold greater in the wild-type nodules than in the mutant nodules. Similarly, in wild-type nodules the in vitro phospho enolpyruvate carboxylase activity was substantially greater than that of the mutant. Almost 90% of the 14C label in the cytosol was found in organic acids in both symbioses. Malate comprised about half of the total cytosol organic acid content on a molar basis, and more than 70% of the cytosol radioactivity in the organic acid fraction was detected in malate in both symbioses. Most of the remaining 14C was contained in the amino acid fraction of the cytosol in both symbioses. More than 70% of the 14C label found in the amino acids of the cytosol was incorporated in aspartate, which on a molar basis comprised only about 1% of the total amino acid pool in the cytosol. The extensive 14C labeling of malate and aspartate from nodule dark [ 14C]CO 2 fixation is consistent with the role of phospho enolpyruvate carboxlase in nodule dark CO 2 fixation. Bacteroids from the effective wild-type symbiosis accumulated sevenfold more 14C than did the dicarboxylic acid transport defective bacteroids. The bacteroids of the effective MNF 300 symbiosis contained the largest proportion of the incorporated 14C in the organic acids, whereas ineffective MNF 3080 bacteroids mainly contained 14C in the amino acid fraction. In both symbioses a larger proportion of the bacteroid 14C label was detected in malate and aspartate than their corresponding proportions of the organic acids and amino acids on a molar basis. The proportion of 14C label in succinate, 2-oxogultarate, citrate, and fumarate in the bacteroids of the wild type greatly exceeded that of the dicarboxylate uptake mutant. The results indicate a central role for nodule cytosol dark CO 2 fixation in the supply of the bacteroids with dicarboxylic acids. 相似文献
6.
Detached roots and nodules of the N 2-fixing species, Albus glutinosa (European black alder), actively assimilate CO 2. The maximum rates of dark CO 2 fixation observed for detached nodules and roots were 15 and 3 micromoles CO 2 fixed per gram dry weight per hour, respectively. The net incorporation of CO 2 in these tissues was catalyzed by phosphoenolpyruvate carboxylase which produces organic acids, some of which are used in the synthesis of the amino acids, aspartate, glutamate, and citrulline and by carbamyl phosphate synthetase. The latter accounts for approximately 30 to 40% of the CO 2 fixed and provides carbamyl phosphate for the synthesis of citrulline. Results of labeling studies suggest that there are multiple pools of malate present in nodules. The major pool is apparently metabolically inactive and of unknown function while the smaller pool is rapidly utilized in the synthesis of amino acids. Dark CO 2 fixation and N 2 fixation in nodules decreased after treatment of nodulated plants with nitrate while the percentage of the total 14C incorporated into organic acids increased. Phosphoenolpyruvate carboxylase and carbamyl phosphate synthetase play key roles in the synthesis of amino acids including citrulline and in the metabolism of N 2-fixing nodules and roots of alder. 相似文献
7.
The in vivo
14CO 2 fixation assay and xylem sap analysis showed that in Sesbania rostrata the transport of fixed nitrogen from stem nodules was in the amide form. The majority of nitrogen was transported as asparagine.
The close relationship between nodule phosphoenolpyruvate carboxylase and nitrogenase activities suggested that nodule CO 2 fixation contributed directly to nitrogen assimilation in stem nodules of S. rostrata. 相似文献
8.
The effect of short- and long-term changes in shoot carbon-exchange rate (CER) on soybean ( Glycine max [L.] Merr.) root nodule activity was assessed to determine whether increases in photosynthate production produce a direct enhancement of symbiotic N 2 fixation. Shoot CER, root + nodule respiration, and apparent N 2 fixation (acetylene reduction) were measured on intact soybean plants grown at 700 microeinsteins per meter per second, with constant root temperature and a 14/10-hour light/dark cycle. There was no diurnal variation of root + nodule respiration or apparent N 2 fixation in plants assayed weekly from 14 to 43 days after planting. However, if plants remained in darkness following their normal dark period, a significant decline in apparent N 2 fixation was measured within 4 hours, and decreasing CO 2 concentration from 320 to 90 microliters CO 2 per liter produced diurnal changes in root nodule activity. Increasing shoot CER by 87, 84, and 76% in 2-, 3-, and 4-week-old plants, respectively, by raising the CO 2 concentration around the shoot from 320 to 1,000 microliters CO 2 per liter, had no effect on root + nodule respiration or acetylene-reduction rates during the first 10 hours of the increased CER treatment. When the CO 2-enrichment treatment was extended in 3-week-old plants, the only measured parameter that differed significantly after 3 days was shoot CER. After 5 days of continuous CO 2 enrichment, root + nodule respiration and acetylene reduction increased, but such changes reflected an increase in root nodule mass rather than greater specific root nodule activity. The results show that on a 24-hour basis the process of symbiotic N 2 fixation in soybean plants grown under controlled environmental conditions functioned at maximum capacity and was not limited by shoot CER. Whether N 2-fixation capacity was limited by photosynthate movement to root nodules or by saturation of metabolic processes in root nodules is not known. 相似文献
9.
The pathway of carbon assimilation in greening roots was compared to the pathway in leaves of Lens culinaris seedlings by means of labelling distribution analysis among the products of 14CO 2 fixation in vivo, and in vitro with ribulose 1,5-diphosphate as the substrate. In green leaves, CO 2 fixation via ribulose 1,5-diphosphate carboxylase predominated largely while, in green roots, this carboxylase activity and the phosphoenolpyruvate carboxylase contributed almost equally to the whole in vivo CO 2 fixation. A participation of the activities of both carboxylases according to the double carboxylation pathway in the synthesis of dicarboxylic acids (malate and aspartate) was demonstrated in vitro after 48 h of greening in roots but seemed to be absent in in vivo experiments. 相似文献
10.
Photosynthetic data collected from Pisum sativum L. and Phaseolus vulgaris L. plants at different stages of development were related to symbiotic N 2 fixation in the root nodules. The net carbon exchange rate of each leaf varied directly with carboxylation efficiency and inversely with the CO 2 compensation point. Net carbon exchange of the lowest leaves reputed to supply fixed carbon to root nodules declined in parallel with H 2 evolution from root nodules. The decrease in H 2 evolution also coincided with the onset of flowering but preceded the peak in N 2 fixation activity measured by acetylene-dependent ethylene production. A result of these changes was that the relative efficiency of N 2 fixation in peas increased to 0.7 from an initial value of 0.4. The data reveal that attempts to identify photosynthetic contributions of leaves to root nodules will require careful timing and suggest that the relative efficiency of N 2 fixation may be influenced by source-sink relationships. 相似文献
11.
Ribulose-1,5-bisphosphate carboxylase activity was found in endosperm of germinating castor bean seed Ricinus communis and was localized in proplastids. The endosperm carboxylase has been extensively purified and is composed of two different subunits. The molecular weights of the native carboxylase and its subunits were 560,000, 55,000, and 15,000 daltons, respectively. The Michaelis-Menten constants, Km, for the endosperm carboxylase with respect to ribulose 1,5-bisphosphate, bicarbonate, CO 2, and magnesium in millimolar are 0.54, 13.60, 0.92, and 0.57, respectively. The endosperm carboxylase was activated by Mg 2+ and HCO 3−. The preincubation of the carboxylase with 1 millimolar HCO 3− and 5 millimolar MgCl 2 resulted in activation by low and inhibition by high concentrations of 6-phosphogluconate. In studies of dark 14CO2 fixation by endosperm slices, [14C]malate and [14C]citrate were the predominantly labeled products after 30 seconds of exposure of the tissue to H14CO3−. In pulse-chase experiments, 87% of the label is malate, and citrate was transferred to sugars after a 60-minute chase with a small amount of the label appearing in the incubation medium as 14CO2. The minimal incorporation of the label from 14CO2 into phosphoglyceric acid indicated a lack of the endosperm ribulose-1,5-bisphosphate carboxylase participation in the endosperm's CO2 fixation system. The activities of key Calvin cycle enzymes were examined in the endosperms and cotyledons of dark-grown castor bean seedlings. Many of these autotrophic enzymes develop in the dark in these tissues. The synthesis of ribulose-1,5-bisphosphate carboxylase in the nonphotosynthetic endosperms is not repressed in the dark, and high levels of enzymic activity appear with germination. All of the Calvin cycle enzymes are present in the castor bean endosperm except NADP-linked glyceraldehyde 3-P dehydrogenase, and the absence of this dehydrogenase probably prevents the functioning of these series of reactions in dark CO2 fixation. 相似文献
12.
Kinetic properties of soybean net photosynthetic CO 2 fixation and of the carboxylase and oxygenase activities of purified soybean ( Glycine max [L.] Merr.) ribulose 1, 5-diphosphate carboxylase (EC 4.1.1.39) were examined as functions of temperature, CO 2 concentration, and O 2 concentration. With leaves, O 2 inhibition of net photosynthetic CO 2 fixation increased when the ambient leaf temperature was increased. The increased inhibition of CO 2 fixation at higher temperatures was caused by a reduced affinity of the leaf for CO 2 and an increased affinity of the leaf for O 2. With purified ribulose 1,5-diphosphate carboxylase, O 2 inhibition of CO 2 incorporation and the ratio of oxygenase activity to carboxylase activity increased with increased temperature. The increased O 2 sensitivity of the enzyme at higher temperature was caused by a reduced affinity of the enzyme for CO 2 and a slightly increased affinity of the enzyme for O 2. The similarity of the effect of temperature on the affinity of intact leaves and of ribulose 1,5-diphosphate carboxylase for CO 2 and O 2 provides further evidence that the carboxylase regulates the O 2 response of photosynthetic CO 2 fixation in soybean leaves. Based on results reported here and in the literature, a scheme outlining the stoichiometry between CO 2 and O 2 fixation in vivo is proposed. 相似文献
13.
The exchange properties of the activator CO 2 of spinach ribulose-1,5-bisphosphate carboxylase/oxygenase were characterized both in vitro with the purified enzyme, and in situ within isolated chloroplasts. Carboxyarabinitol-1,5-bisphosphate, a proposed reaction intermediate analog for the carboxylase activity of the enzyme, was used to trap the activator CO 2 on the enzyme both in vitro and in situ. Modulation of ribulose-1,5-bisphosphate carboxylase/oxygenase activity in intact chloroplasts during a light/dark cycle was associated with a similar modulation in carboxyarabinitol-1,5-bisphosphate-trapped CO 2. The exchange kinetics of the activator CO 2 were monitored by activation of the enzyme to steady state in the presence of 12CO 2, followed by addition of 14CO 2 and determination of the amount of labeled CO 2 trapped on the enzyme by carboxyarabinitol-1,5-bisphosphate. Rate constants ( Kobs) for exchange with both the purified enzyme (0.45 min −1) and in illuminated chloroplasts (0.18 min −1) were comparable to the observed rate constants for enzyme activation under the two conditions. A similar exchange of the activator CO 2 was not observed in chloroplasts in the dark. Kinetic analysis of the exchange properties of the purified enzyme were consistent with an equilibrium between active and inactive forms of the enzyme during steady state activation. 相似文献
14.
Summary Ribulose diphosphate (RuDP) and (PEP) phosphoenolpyruvate carboxylase enzyme activities were studied in young, mature, and senescent Portulaca oleracea leaves. While the absolute amount of both the C 3 (RuDP) and C 4 (PEP) carboxylase is less in senescent leaves than in mature leaves, RuDP carboxylase activity is reduced to a lesser degree. In senescent leaves, PEP carboxylase activity equals 10% of that in mature tissue, but RuDP carboxylase is 27% of that in mature leaves. The same ontogenetic series was also used to determine photorespiration rates and responses to several gas treatments. Young and mature leaves were unaffected by changes in the light regime or oxygen concentrations, and exhibited typical C 4-plant light/dark 14CO 2 evolution ratios. Senescent leaves, on the other hand, have photorespiration ratios similar to C 3-plants. In addition, senescent leaves were affected by minus CO 2, 100% O 2 and N 2 in a manner expected of C 3-plants, but not C 4-plants. These results are discussed in terms of a relative increase in activity of the C 3 cycle in later developmental stages in this plant.Abbreviation RuDP
ribulose diphosphate
- PEP
phosphoenolpyruvate
- PGA
phosphoglyceric acid 相似文献
15.
Young leaves of salt-depleted Aeluropus litoralis Parl. plants show CO 2 fixation by the C 3-carbon fixation pathway. No detectable activity of phosphoenol pyruvate (PEP) carboxylase was found. When A. litoralis plants were exposed to a NaCl solution, the leaves showed a high activity of PEP carboxylase as well as a significant CO 2 fixation by the C 4-pathway. — Also in Zea mays L. and Chloris gayana Kunth., the presence of NaCl in the medium influences the balance between phosphoenol pyruvate carboxylase and ribulose-1,5-diphosphate carboxylase. 相似文献
16.
We examined the interactive effects of elevated CO 2, soil phosphorus (P) availability, and soil drought on nodulation, nitrogenase activity, and biomass allocation in Alnus hirsuta and Alnus maximowiczii. Potted seedlings were grown in either ambient or elevated CO 2 (36 Pa and 72 Pa CO 2), with different levels of P (7.7 and 0.77 mgP pot ?1 week ?1 for high-P and low-P, respectively) and water supply in a natural daylight phytotron. Measurements of nitrogenase activity by an acetylene reduction assay failed to reveal significant effects of the treatments in any species. In high-P, nodule biomass increased under elevated CO 2 and decreased under drought. In low-P, nodule biomass decreased substantially compared to high-P, but the effect of elevated CO 2 on nodule biomass was unclear. Soil drought increased the partitioning of biomass into nodules, especially in A. hirsuta. These results suggest that with high P availability, elevated CO 2 could promote N 2 fixation by increasing nodule biomass even under drought. On the other hand, if soil P is limiting, elevated CO 2 may not enhance N 2 fixation because of the suppression of growth. 相似文献
17.
Water potentials of leaves and nodules of broad bean ( Vicia faba L.) cultivated on a sandy mixture were linearly and highly ( r2 = 0.99) correlated throughout a water deprivation of plants. A decrease of 0.2 megapascal of the nodule water potential (Ψ nod) induced an immediate 25% inhibition of the highest level of acetylene reduction of broad bean nodules attached to roots. This activity continued to be depressed when water stress increased, but the effect was less pronounced. Partial recovery of optimal C 2H 2 reduction capacity of mildly water stressed nodules (Ψ nod = −1.2 megapascals) was possible by increasing the external O 2 partial pressure up to 60 kilopascals. The dense packing of the cortical cells of nodules may be responsible for the limitation of O 2 diffusion to the central tissue. Bacteroids isolated from broad bean nodules exhibited higher N 2 fixation activity with glucose than with succinate as an energy-yielding substrate. Bacteroids from stressed nodules appeared more sensitive to O 2, and their optimal activity declined with increasing nodule water deprivation. This effect could be partly due to decreased bacteroid respiration capacity with water stress. Water stress was also responsible for a decrease of the cytosolic protein content of the nodule and more specifically of leghemoglobin. The alteration of the bacteroid environment appears to contribute to the decline in N 2 fixation under water restricted conditions. 相似文献
18.
Photosynthetic properties of photoautotrophic suspensions cultured in a minimal growth medium have been evaluated to determine whether changes have occurred in ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity, phosphoenol-pyruvate (PEP) carboxylase activity, chlorophyll content, or culture growth. Five photoautotrophic lines Amaranthus powellii, Datura innoxia, Glycine max, Gossypium hirsutum, and a Nicotiana tabacum-Nicotiana glutinosa fusion hybrid were grown in a medium without organic carbon other than phytohormones, and without vitamins. These photoautotrophic lines had total Rubisco activities ranging from 85 to 266 micromoles CO 2 fixed per milligram chlorophyll hour −1, with percent activation of Rubisco ranging from 16 to 53%. Inclusion of protease inhibitors in the homogenization buffer did not result in higher Rubisco activity. PEP carboxylase activity for cells cultured in minimal medium was found to range from 16 to 146 micromoles CO 2 per milligram chlorophyll hour −1, with no higher activity in the C 4Amaranthus cells compared with PEP carboxylase activity in the C 3 species assayed. Rubisco-to-PEP carboxylase ratios ranged from 2.2 to 1 up to 9.4 to 1. Chlorophyll contents increased in all but the Nicotiana cell line, and all of the photoautotrophic culture lines were capable of growth in vitamin-free medium with the exception of SB-P, which requires thiamine. 相似文献
19.
NaCl treated Mesembryanthemum crystallinum plants exhibit a Crassulacean acid metabolism. The activity of phosphoenolpyruvate (PEP) carboxylase, the enzyme responsible for CO 2 dark fixation, depends on leaf age showing maximum activity in mature leaves. Electrophoresis revealed that the young leaves possess only two protein bands with PEP carboxylase activity, while older leaves have 3 bands. The removal of NaCl from the soil resulted in the disappearance of the 3rd band obtained after electrophoresis and a decline in the total activity of the PEP carboxylase. The reintroduction of NaCl at the same concentration as before did not restore the activity of the PEP carboxylase nor did it restore the initial electrophoretic band pattern. 相似文献
20.
Background and AimsNitrogen fixation in legumes requires tight control of carbon and nitrogen balance. Thus, legumes control nodule numbers via an autoregulation mechanism. ‘Autoregulation of nodulation’ mutants super-nodulate are thought to be carbon-limited due to the high carbon-sink strength of excessive nodules. This study aimed to examine the effect of increasing carbon supply on the performance of super-nodulation mutants. MethodsWe compared the responses of Medicago truncatula super-nodulation mutants ( sunn-4 and rdn1-1) and wild type to five CO 2 levels (300–850 μmol mol −1). Nodule formation and nitrogen fixation were assessed in soil-grown plants at 18 and 42 d after sowing. Key ResultsShoot and root biomass, nodule number and biomass, nitrogenase activity and fixed nitrogen per plant of all genotypes increased with increasing CO 2 concentration and reached a maximum at 700 μmol mol −1. While the sunn-4 mutant showed strong growth retardation compared with wild-type plants, elevated CO 2 increased shoot biomass and total nitrogen content of the rdn1-1 mutant up to 2-fold. This was accompanied by a 4-fold increase in nitrogen fixation capacity in the rdn1-1 mutant. ConclusionsThese results suggest that the super-nodulation phenotype per se did not limit growth. The additional nitrogen fixation capacity of the rdn1-1 mutant may enhance the benefit of elevated CO 2 for plant growth and N 2 fixation. 相似文献
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