共查询到20条相似文献,搜索用时 46 毫秒
1.
The effect of nitrate on N 2 fixation and the assimilation of fixed N 2 in legume nodules was investigated by supplying nitrate to well established soybean ( Glycine max L. Merr. cv Bragg)- Rhizobium japonicum (strain 3I1b110) symbioses. Three different techniques, acetylene reduction, 15N 2 fixation and relative abundance of ureides ([ureides/(ureides + nitrate + α-amino nitrogen)] × 100) in xylem exudate, gave similar results for the effect of nitrate on N 2 fixation by nodulated roots. After 2 days of treatment with 10 millimolar nitrate, acetylene reduction by nodulated roots was inhibited by 48% but there was no effect on either acetylene reduction by isolated bacteroids or in vitro activity of nodule cytoplasmic glutamine synthetase, glutamine oxoglutarate aminotransferase, xanthine dehydrogenase, uricase, or allantoinase. After 7 days, acetylene reduction by isolated bacteroids was almost completely inhibited but, except for glutamine oxoglutarate aminotransferase, there was still no effect on the nodule cytoplasmic enzymes. It was concluded that, when nitrate is supplied to an established symbiosis, inhibition of nodulated root N 2 fixation precedes the loss of the potential of bacteroids to fix N 2. This in turn precedes the loss of the potential of nodules to assimilate fixed N 2. 相似文献
2.
Hydrogen evolution from root nodules has been reported to make N 2 fixation by some legume- Rhizobium symbiotic systems inefficient. We have surveyed the extent of H 2 evolution and estimated relative efficiencies of nodules of Austrian winter peas formed by 15 strains of R. leguminosarum. Their rates of H 2 evolution in air were about 30% of the rates of H 2 evolution under an atmosphere in which N 2 was replaced by Ar. Relative efficiency values based on C 2H 2 reduction rates ranged from 0.55 to 0.80. With some of the strains, hydrogenase activities were demonstrated in intact nodules and in bacteroids, but the levels of activity were insufficient to recycle all the H 2 evolved by the nitrogenase system. In both intact nodules and bacteroids the hydrogenase is less sensitive to O 2 damage than the nitrogenase system, so H 2 uptake capacity was observed in intact nodules by suppressing the nitrogenase-dependent H 2 evolution with an atmosphere containing a high O 2 concentration, and in bacteroids by using aerobically prepared bacteroid suspensions. The hydrogenase activity of both was dependent on O 2 consumption. A K
mfor H 2 of near 4 M was determined in suspension of bacteroids from nodules formed by strains 128C53 and 128C56. 相似文献
3.
Malate oxidation supported C 2H 2 reduction by bacteroids isolated from Sesbania rostrata stem nodules. Optimal activity reached 7.5 nanomoles per minute per milligram of dry weight and was in the same order of magnitude as that observed with succinate but always required a lower O 2 tension. Malate dehydrogenase (EC 1.1.1.37), purified 66-fold from bacteroids, actively oxidized malate ( Km = 0.19 millimolar). Malic enzyme (EC 1.1.1.39) from Sesbania bacteroids had a lower affinity for malate ( Km = 2.32 millimolar). Both enzymes exclusively required NAD + as cofactor and required an alkaline pH for optimal activity. 2-Oxoglutarate and oxalate, inhibiting malate dehydrogenase and malic enzyme, respectively, were used to specifically block each malate oxidation pathway in bacteroids. The predominance of malate dehydrogenase activity to support bacteroid N 2 fixation was demonstrated. The inhibition of O 2 consumption by 2-oxoglutarate confirmed the importance of the malate dehydrogenase pathway in malate oxidation. It is proposed that the utilization of malate, with regard to O 2, is important in a general strategy of this legume to maintain N 2 fixation under O 2 limited conditions. 相似文献
4.
1. Cell-free extracts prepared from soya-bean nodule bacteroids produced HD from D 2 in the presence of dithionite, an ATP-generating system and nitrogen. 2. Crude extracts of bacteroids or of Azotobacter vinelandii showed some background D 2 exchange when any one of these was omitted. 3. Partial purification of bacteroid extracts diminished this background activity and gave increased D 2 exchange and nitrogen fixation. 4. Although increasing pN 2 stimulated both reactions, the apparent Km (N 2) for nitrogen fixation was much higher than the apparent Km (N 2) for D 2 exchange when partially purified bacteroid extracts were used. 5. Carbon monoxide was a competitive inhibitor of nitrogen fixation by partially purified bacteroid extracts, but D 2 exchange was inhibited in a non-competitive fashion. 6. These results are discussed in relation to the possible existence of enzyme-bound intermediates of nitrogen fixation. 相似文献
5.
Oxyleghemoglobin was used to supply low concentrations of O 2 to H 2-oxidizing bacteroids from Rhizobium japonicum USDA 122 DES. The H 2 oxidation system of these bacteroids was capable of effectively utilizing O 2 at the low concentrations of O 2 expected to be found in soybean nodules. Apparent Km values of approximately 10 nanomolar O 2 have been calculated for the oxyhydrogen reaction. These values include the Km values for both H 2 oxidation and endogenous substrate oxidation. Even in the presence of oxyleghemoglobin, H 2 additions stimulated C 2H 2 reduction, reduced the rate of endogenous respiration and maintained the ATP contents of bacteroids. In our reconstituted oxyleghemoglobin and bacteriod system, we estimate that the H 2 oxidation system is capable of recycling all of the H 2 evolved during the N 2 fixation process. 相似文献
6.
BackgroundIn symbiotic legume nodules, endosymbiotic rhizobia (bacteroids) fix atmospheric N 2, an ATP-dependent catalytic process yielding stoichiometric ammonium and hydrogen gas (H 2). While in most legume nodules this H 2 is quantitatively evolved, which loss drains metabolic energy, certain bacteroid strains employ uptake hydrogenase activity and thus evolve little or no H 2. Rather, endogenous H 2 is efficiently respired at the expense of O 2, driving oxidative phosphorylation, recouping ATP used for H 2 production, and increasing the efficiency of symbiotic nodule N 2 fixation. In many ensuing investigations since its discovery as a physiological process, bacteroid uptake hydrogenase activity has been presumed a single entity. Methodology/Principal Findings
Azorhizobium caulinodans, the nodule endosymbiont of Sesbania rostrata stems and roots, possesses both orthodox respiratory ( exo-)hydrogenase and novel ( endo-)hydrogenase activities. These two respiratory hydrogenases are structurally quite distinct and encoded by disparate, unlinked gene-sets. As shown here, in S. rostrata symbiotic nodules, haploid A. caulinodans bacteroids carrying single knockout alleles in either exo- or- endo-hydrogenase structural genes, like the wild-type parent, evolve no detectable H 2 and thus are fully competent for endogenous H 2 recycling. Whereas, nodules formed with A. caulinodans exo-, endo-hydrogenase double-mutants evolve endogenous H 2 quantitatively and thus suffer complete loss of H 2 recycling capability. More generally, from bioinformatic analyses, diazotrophic microaerophiles, including rhizobia, which respire H 2 may carry both exo- and endo-hydrogenase gene-sets. Conclusions/SignificanceIn symbiotic S. rostrata nodules, A. caulinodans bacteroids can use either respiratory hydrogenase to recycle endogenous H 2 produced by N 2 fixation. Thus, H 2 recycling by symbiotic legume nodules may involve multiple respiratory hydrogenases. 相似文献
7.
In vivo CO 2 fixation and in vitro phosphoenolpyruvate (PEP) carboxylase levels have been measured in lupin ( Lupinus angustifolius L.) root nodules of various ages. Both activities were greater in nodule tissue than in either primary or secondary root tissue, and increased about 3-fold with the onset of N 2 fixation. PEP carboxylase activity was predominantly located in the bacteroid-containing zone of mature nodules, but purified bacteroids contained no activity. Partially purified PEP carboxylases from nodules, roots, and leaves were identical in a number of kinetic parameters. Both in vivo CO 2 fixation activity and in vitro PEP carboxylase activity were significantly correlated with nodule acetylene reduction activity during nodule development. The maximum rate of in vivo CO 2 fixation in mature nodules was 7.9 nmol hour −1 mg fresh weight −1, similar to rates of N 2 fixation and reported values for amino acid translocation. 相似文献
8.
Efficiency of symbiotic nitrogen fixation in legumes depends on bringing together the processes of N 2 fixation, assimilation of its products, supply of nitrogenase with energy, and development of nodule tissue and cellular structures. Coordination of these processes could arise from the evolutionary old functions of the nodules associated with deposition of the products of photosynthesis governed by systemic signals traveling between the above-ground organs and the roots. Further increase in symbiotic efficiency was associated with a pronounced ability to fix N 2 by intracellular bacteroids that lost capability to propagate (as observed in galegoid legumes from the tribes Viciae, Trifolieae, and Galegae producing indeterminate nodules). However, efficiency of these symbioses is restricted by a slow removal from the nodules of the products of N 2 fixation, which are assimilated along the same amide pathway as nitrogen compounds arriving from the soil. In legumes from the tribe Phaseoleae, such a restriction was overcome owing to a particular way of nitrogen assimilation via its incorporation into ureides (in determinate nodules). Development of symbioses where specialization of bacteroids in symbiotic fixation of atmospheric nitrogen is combined with its ureide assimilation will make it possible to produce new forms of plants highly efficient in symbiotic nitrogen fixation. 相似文献
9.
A derivative of Rhizobium japonicum (strain 122 DES) has been isolated which forms nodules on soybeans that evolve little or no H 2 in air and efficiently fixes N 2. Bacteroids isolated from nodules formed by strain 122 DES took up H 2 with O 2 as the physiological acceptor and appeared to be typical of those R. japonicum strains that possess the H 2 uptake system. The hydrogenase system in soybean nodules is located within the bacteroids and activity in macerated bacteroids is concentrated in a particulate fraction. The pH optimum for the reaction is near 8.0 and apparent K
m values for H 2 and O 2 are 2 M and 1 M, respectively. The H 2 oxidizing activity of a suspension of 122 DES bacteroids was stable at 4°C for at least 4 weeks and was not particularly sensitive to O 2. Neither C 2H 2 nor CO inhibited O 2 dependent H 2 uptake activity.Non-physiological electron acceptors of positive oxidation reduction potential also supported H 2 uptake by bacteroids. The rate of H 2 uptake with phenazine methosulfate as the acceptor was greater than that with O 2. When methylene blue, triphenyltetrazolium, potassium ferricyanide or dichlorophenolindophenol were added to bacteriod suspensions, without preincubation, rates of H 2 uptake were supported that were lower than those in the presence of O 2. Preincubation of the bacteroids with acceptors increased the rates of H 2 uptake. No H 2 evolution was observed from reaction mixtures containing bacteroid suspensions and reduced methyl or benzyl viologens. Of a series of carbon substrates added to bacteroid suspensions only acetate, formate or succinate at concentrations of 50 mM resulted in 20% or greater inhibition of H 2 oxidation.The H 2 uptake capacity of isolated 122 DES bacteroids (expressed on a dry bacteroid basis) was at least 10-fold higher than the rate of the nitrogenase reaction in nodules expressed on a comparable basis. Since about 1 mol of H 2 is evolved for every mol of N 2 reduced during the N 2 fixation reaction, these observations explain why soybean nodules formed by strain 122 DES and other strains with high H 2 uptake activities have a capacity for recycling all the H 2 produced from the nitrogenase reaction.Abbreviations PMS
PHenazine methosulfate
- MB
Methylene blue 相似文献
10.
Nitrogenase (EC 1.7.99.2) activity in pea ( Pisum savitum) nodules formed after infection with Rhizobium leguminosarum (lacking uptake hydrogenase) was measured as acetylene reduction, H 2 evolution in air and H 2 evolution in Ar:O 2. With detached roots the relative efficiency, calculated from acetylene reduction, showed a decrease (from 55 to below 0%) with increasing temperature. With excised nodules and isolated bacteroids similar results were obtained. However, the relative efficiency calculated from H 2 evolution in Ar:O 2 was unaffected by temperature. Measurements on both excised nodules and isolated bacteroids showed a marked difference between acetylene reduction and H 2 evolution in Ar:O 2 with increased temperature, indicating that either acetylene reduction or H 2 evolution in Ar:O 2 are inadequate measures of nitrogenase activity at higher temperature. 相似文献
11.
Heat evolved by isolated soybean ( Glycine max cv Clark) nodules was measured to estimate more directly the metabolic cost associated with the symbiotic N 2 fixation system. A calorimeter constructed by modifying standard laboratory equipment allowed measurement on 1 gram of detached nodules under a controlled gas stream. Simultaneous gas balance and heat output determinations were made. There was major heat output by nodules for all of the nitrogenase substrates tested (H+, N2, N2O, and C2H2) further establishing the in vivo energy inefficiency of biological N2 fixation. Exposure to a short burst of 100% O2 partially inactivated nitrogenase to permit calculations of heat evolved per mole of substrate reduced. The specific rate of heat evolution for H+ reductions was 171 ± 6 kilocalories per mole H2 evolved in an Ar-O2 atmosphere, that for N2 fixation was 784 ± 26 kilocalories per mole H2 evolved and N2 fixed, and that for C2H2 reduction was 250 ± 12 kilocalories/mole C2H4 formed. When the appropriate thermodynamic parameters are taken into account for the different substrates and products, a ΔH′ of −200 kilocalories per mole 2e− is shown to be associated with active transfer of electrons by the nitrogenase system. These values lead to a calculated N2 fixation cost of 9.5 grams glucose per gram N2 fixed or 3.8 grams C per gram N2, which is in close agreement with earlier calculations based on nodular CO2 production. 相似文献
12.
Rhizobium japonicum 122 DES bacteroids from soybean nodules possess an active H 2-oxidizing system that recycles all of the H 2 lost through nitrogenase-dependent H 2 evolution. The addition of 72 μM H 2 to suspensions of bacteroids increased O 2 uptake 300% and the rate of C 2H 2 reduction 300 to 500%. The optimal partial pressure of O 2 was increased, and the partial pressure of O 2 range for C 2H 2 reduction was extended by adding H 2. A supply of succinate to bacteroids resulted in effects similar to those obtained by adding H 2. Both H 2 and succinate provided respiratory protection for the N 2-fixing system in bacteroids. The oxidation of H 2 by bacteroids increased the steady-state pool of ATP by 20 to 40%. In the presence of 50 mM iodoacetate, which caused much greater inhibition of endogenous respiration than of H 2 oxidation, the addition of H 2 increased the steady-state pool of ATP in bacteroids by 500%. Inhibitor evidence and an absolute requirement for O 2 indicated that the H 2-stimulated ATP synthesis occurred through oxidative phosphorylation. In the presence of 50 mM iodoacetate, H 2-dependent ATP synthesis occurred at a rate sufficient to support nitrogenase activity. The addition of H 2 to H 2 uptake-negative strains of R. japonicum had no effect on ATP formation or C 2H 2 reduction. It is concluded that the H 2-oxidizing system in H 2 uptake-positive bacteroids benefits the N 2-fixing process by providing respiratory protection of the O 2-labile nitrogenase proteins and generating ATP to support maximal rates of C 2H 2 reduction by oxidation of the H 2 produced from the nitrogenase system. 相似文献
13.
Mesorhizobium loti is a Gram negative bacterium that induces N 2-fixing root nodules on the model legume Lotus japonicus. Proteomic analysis in M. loti indicated that 3-phosphoglycerate dehydrogenase (EC. 1.1.1.95, PHGDH) protein content was 2.2 times higher in bacteroids than in cultured bacteria. A M. loti mutant (STM5) with a transposon insertion in the PHGDH gene, mll3875, showed an absolute dependence on serine or glycine in minimal medium for growth. When L. japonicus plants were infected with STM5, the roots formed nodules in numbers comparable to those formed by wild type M. loti; however, the nodules showed very low acetylene reduction activity, and significant starch granule accumulation was observed in the uninfected cells. In such nodules, vast necrosis occurred in the central tissue of the nodules, although bacteroids were detected in the infected cell of the nodules. These data indicate that serine or glycine biosynthesis by PHGDH is important for maintaining symbiosis and nitrogen fixation in L. japonicus nodules. 相似文献
14.
Symbiotic rhizobia differentiate physiologically and morphologically into nitrogen-fixing bacteroids inside legume host nodules. The differentiation is apparently terminal in some legume species, such as peas ( Pisum sativum) and peanuts ( Arachis hypogaea), likely due to extreme cell swelling induced by the host. In other legume species, such as beans ( Phaseolus vulgaris) and cowpeas ( Vigna unguiculata), differentiation into bacteroids, which are similar in size and shape to free-living rhizobia, is reversible. Bacteroid modification by plants may affect the effectiveness of the symbiosis. Here, we compare symbiotic efficiency of rhizobia in two different hosts where the rhizobia differentiate into swollen nonreproductive bacteroids in one host and remain nonswollen and reproductive in the other. Two such dual-host strains were tested: Rhizobium leguminosarum A34 in peas and beans and Bradyrhizobium sp . 32H1 in peanuts and cowpeas. In both comparisons, swollen bacteroids conferred more net host benefit by two measures: return on nodule construction cost (plant growth per gram nodule growth) and nitrogen fixation efficiency (H 2 production by nitrogenase per CO 2 respired). Terminal bacteroid differentiation among legume species has evolved independently multiple times, perhaps due to the increased host fitness benefits observed in this study.Legume-rhizobia interactions vary widely across a diverse paraphyletic group of soil bacteria known for symbiotic nitrogen fixation inside root nodules of over 18,000 species of legumes throughout the world ( Lewis et al., 2005). In several legume species, rhizobial cells are induced to swell during their differentiation into nitrogen-fixing bacteroids ( Oono et al., 2010). These legume species belong to five different major papilionoid clades (inverted repeat-lacking clade, genistoids, dalbergioids, mirbelioids, and millettioids), a pattern suggestive of convergent evolution. Swelling apparently leads to terminal differentiation; swollen bacteroids no longer divide normally ( Zhou et al., 1985). In other legume host species, bacteroid differentiation is less extreme, leading to nonswollen bacteroids. Nonswollen bacteroids are similar in shape and size to free-living rhizobia and divide normally once outside of their nodules. The proximate mechanisms for host-imposed bacteroid swelling have been investigated (Van de Velde et al., 2010), but what drove the repeated evolution of this trait? The multiple independent origins of host traits causing bacteroids to swell suggest that swollen bacteroids may provide more net benefit to legumes. Could the swelling of bacteroids improve nitrogen fixation efficiency (e.g. nitrogen fixed relative to carbon cost)? In this study, we compare symbiotic efficiencies of rhizobia in legume hosts that are evolutionarily diverged but share a common effective rhizobial strain, whose bacteroids are swollen in one host and nonswollen in the other.Variations among host species in benefits and costs of symbiosis with rhizobia are not commonly explored ( Thrall et al., 2000) because legume species typically nodulate with only one group of rhizobia (e.g. Sinorhizobium sp. in Medicago), although some legumes and some rhizobia are more promiscuous. Rhizobium sp. NGR234 has the largest known host range but does not fix nitrogen effectively with any legume species currently recognized to induce swelling of rhizobial bacteroids ( Pueppke and Broughton, 1999). Some Sinorhizobium fredii strains apparently fix nitrogen in certain cultivars of soybean ( Glycine max; hosting nonswollen bacteroids) and alfalfa ( Medicago sativa; hosting swollen bacteroids; Hashem et al., 1997), but our efforts to replicate these results did not lead to successful nodulation. Therefore, we studied two strains, a transgenic strain that nodulates beans ( Phaseolus vulgaris) and peas ( Pisum sativum) and a second wild strain harvested from cowpeas ( Vigna unguiculata) that also nodulates peanuts ( Arachis hypogaea). Beans and cowpeas are both within the Phaseolid group and do not induce terminal differentiation of rhizobial bacteroids. Peas and peanuts both host terminally differentiated bacteroids but are in distant clades and likely have different genetic origins for traits that induce terminal differentiation ( Oono et al., 2010). Also, the swollen bacteroids in peas are branched while those in peanuts are spherical.Differences in symbiotic qualities between swollen and nonswollen bacteroids have been previously explored in peanuts and cowpeas by Sen and Weaver (1980, 1981, 1984), who also hypothesized that swollen bacteroids are more beneficial to the host plant than nonswollen ones. They found 1.5 to 3 times greater acetylene reduction by nitrogenase (as well as plant nitrogen) per nodule mass in peanuts than in cowpeas at multiple nodule ages ( Sen and Weaver, 1980). Acetylene reduction per bacteroid was also greater in peanuts than in cowpeas when measuring whole nodules, but this difference disappeared when isolated bacteroids were assayed ( Sen and Weaver, 1984). They concluded that swelling of peanut bacteroids per se was not responsible for the higher rate of nitrogen fixation per bacteroid. They suggested that in cowpea nodules, with greater numbers of smaller bacteroids per nodule volume, availability of oxygen to each bacteroid might be restricted such that the rate of oxidative phosphorylation, necessary for nitrogen fixation, is reduced. Fixation rates per bacteroid may be different between hosts due to nodule gas permeability or bacteroid crowding within nodules. However, fixation efficiency (nitrogen fixed per carbon respired) would not necessarily be affected by these and may be more important for the host than the rate of fixation.Rhizobial performances are often compared by measuring the symbiotic benefits, e.g. rates of acetylene reduction or plant growth ( Sen and Weaver, 1984; Hashem et al., 1997; Lodwig et al., 2005), but rarely by measuring the symbiotic costs, e.g. carbon consumed or respired. Up to 25% of a legume’s net photosynthate may be required for nitrogen fixation by rhizobia ( Minchin et al., 1981). Faster fixation rates (mol nitrogen per s) can be beneficial for hosts, but carbon costs can also be important. Rhizobia that fix more nitrogen per carbon respired could free more carbon for other functions, including the option of supporting more nodules with the same amount of photosynthate. If legumes are sometimes carbon limited, then improved carbon-use efficiency could enhance plant fitness. Measuring both benefits and costs is therefore key to an accurate understanding of the symbiotic performance of a rhizobial strain.While we recognize the many physiological differences between peas and beans or peanuts and cowpeas, the fact that terminal differentiation induced by host legumes evolved multiple times independently ( Oono et al., 2010) suggests there may be some consistent host symbiotic benefit, such as improved fixation efficiency. Here, we measured the efficiency of each of two strains as swollen bacteroids in one host and nonswollen bacteroids in another. We measured nitrogenase activity as hydrogen (H 2) production in an N 2-free atmosphere ( Layzell et al., 1984; Witty and Minchin, 1998), and compared it to carbon dioxide (CO 2) respiration to estimate return on nodule operation cost. We also compared host biomass growth per total nodule mass growth to estimate return on nodule construction cost. To further assess carbon allocation to the different types of bacteroids, we also measured the average amounts per bacteroid of polyhydroxybutyrate (PHB), an energy storage compound that can comprise up to 50% of bacteroid dry weight (Trainer and Charles, 2006). A greater PHB accumulation per bacteroid may require a decreased allocation of carbon for nitrogenase activity within the bacteroids, and hence, less plant growth per carbon invested in bacteroids. We demonstrate that peas and peanuts that host swollen bacteroids have higher fixation efficiency as well as greater plant return on nodule construction than beans and cowpeas, respectively, nodulated with the same rhizobial strains. PHB was not consistently correlated with plant:nodule growth efficiency with the tested strains. These findings show that swollen bacteroids can indeed provide greater benefits to their legume hosts. 相似文献
15.
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
16.
Dissimilatory reduction of ionic nitrogen oxides to gaseous forms such as nitrous oxide or nitrogen can be carried out by free living or symbiotic forms of some strains of Rhizobium meliloti. In this paper we investigate whether bacteroid denitrification plays a role in the alleviation of the inhibitory effects of nitrate on nitrogen fixation both in bacteroid incubations as in whole nodules. The presence of a constitutive nitrate reductase (NR) activity in isolated bacteroids caused nitrite accumulation in the incubation medium, and acetylene reduction activity in these bacteroids was progressively inhibited, since nitrite reductase (NiR) activity was unable to reduce all the nitrite produced by NR and denitrification occurred slowly. Even nodules infiltrated with nitrate and nitrite failed to increase gaseous forms of nitrogen substantially, indicating that nitrite availability was not limiting denitrification by bacteroids. In spite of the low rates of bacteroidal denitrification, the effect of nodule denitrification on the inhibition of nitrogen fixation by nitrate in whole plants was tested. For that purpose, lucerne plants ( Medicago sativa L. cv. Aragon) were inoculated with two Rhizobium meliloti strains: 102-F-65 (non denitrifying) and 102-F-51 (a highly denitrifying strain). After a seven days nitrate treatment, both strains showed the same pattern of inhibition, and it occurred before any nitrate or nitrite accumulation within the nodules could be detected. This observation, together with the lack of alleviation of the ARA inhibition in the denitrifying strain, and the limited activity of dissimilatory nitrogen reduction present in these bacteroids, indicate a role other than nitrite detoxification for denitrification in nodules under natural conditions. 相似文献
17.
Rates of respiratory CO 2 loss and nitrogenase activities of H 2 uptake-negative mutant strains and H 2 uptake-positive revertant strains of Rhizobium japonicum have been investigated. Two-dimensional gel protein patterns of bacteroids formed by inoculation of soybeans ( Glycine max L.) with these two strains show that they are closely related and revealed only one obvious difference between them. On the basis of molecular weight standards, it was concluded that the missing protein spot in the H 2 uptake-negative mutant strain could be caused by a failure of the mutant to synthesize hydrogenase. Nodules formed by the H 2 uptake-negative mutant strain evolved respiratory CO 2 at a rate of about 10% higher than that of nodules formed by the H 2 uptake-positive revertant strain. During short-term experiments employed, rates of both C 2H 2 reduction and 15N 2 fixation varied considerably among replicate samples and no statistically significant differences between mutant and revertant strains were observed. It was observed that increasing the partial pressure of O 2 over nodules significantly decreased the proportion of nitrogenase electrons allocated to H +. 相似文献
18.
Background and AimsLegumes overcome nitrogen limitations by entering into a mutualistic symbiosis with N 2-fixing bacteria (rhizobia). Fully compatible associations (effective) between Trifolium spp. and Rhizobium leguminosarum bv. trifolii result from successful recognition of symbiotic partners in the rhizosphere, root hair infection and the formation of nodules where N 2-fixing bacteroids reside. Poorly compatible associations can result in root nodule formation with minimal (sub-optimal) or no (ineffective) N 2-fixation. Despite the abundance and persistence of strains in agricultural soils which are poorly compatible with the commercially grown clover species, little is known of how and why they fail symbiotically. The aims of this research were to determine the morphological aberrations occurring in sub-optimal and ineffective clover nodules and to determine whether reduced bacteroid numbers or reduced N 2-fixing activity is the main cause for the Sub-optimal phenotype. MethodsSymbiotic effectiveness of four Trifolium hosts with each of four R. leguminosarum bv. trifolii strains was assessed by analysis of plant yields and nitrogen content; nodule yields, abundance, morphology and internal structure; and bacteroid cytology, quantity and activity. Key ResultsEffective nodules (Nodule Function 83–100 %) contained four developmental zones and N 2-fixing bacteroids. In contrast, Sub-optimal nodules of the same age (Nodule Function 24–57 %) carried prematurely senescing bacteroids and a small bacteroid pool resulting in reduced shoot N. Ineffective-differentiated nodules carried bacteroids aborted at stage 2 or 3 in differentiation. In contrast, bacteroids were not observed in Ineffective-vegetative nodules despite the presence of bacteria within infection threads. ConclusionsThree major responses to N 2-fixation incompatibility between Trifolium spp. and R. l. trifolii strains were found: failed bacterial endocytosis from infection threads into plant cortical cells, bacteroid differentiation aborted prematurely, and a reduced pool of functional bacteroids which underwent premature senescence. We discuss possible underlying genetic causes of these developmental abnormalities and consider impacts on N 2-fixation of clovers. 相似文献
19.
Fertilizer studies in Mexico indicate that coffee production can be stimulated by added nitrogen. One traditional method of coffee cultivation employs leguminous trees for shade, but these species may also play an important role in coffee production by biologically fixing nitrogen. The presence and importance of nitrogen fixation was evaluated in four systems: coffee only, coffee plus the leguminous shade tree Inga jinicuil Schletchter, coffee plus the leguminous tree Inga vera H.B. and K., and coffee plus banana and orange trees. In all systems coffee leaves with epiphylls, wood litter, soil, roots, and root nodules were assayed for nitrogen fixing activity with the acetylene reduction technique. All components of these systems exhibited activity except roots. Total apparent fixation was highest in the Inga jinicuil site, and equivalent to >40 kg N ha ?1 yr ?1 assuming a 3∶1 C 2H 2∶N 2 ratio. The activity was primarily associated with Inga jinicuil nodules. Apparent fixation in the other three sites was less than 1 kg N ha ?1 yr ?1. Nitrogen fixed in the I. jinicuil site was 53% of the average amount of fertilizer nitrogen applied annually, suggesting that fixation by non-crop legumes can be an important nitrogen source for coffee agro-ecosystems. 相似文献
20.
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
|