Showing papers on "Nitrogen fixation published in 1995"
TL;DR: This review describes the early steps of the interaction between rhizobia and legumes that result in the formation of a nitrogen-fixing nodule and focuses on the role of specific lipooligosaccharides secreted by Rhizobia in the induction of these early steps.
Abstract: that are expressed before the onset of nitrogen fixation and are involved in infection and nodule development. The products of the late nodulin genes are involved in the interaction with the endosymbiont and in the metabolic specialization of the nodule (Nap and Bisseling, 1990). In the first part of this review, we describe the early steps of the interaction between rhizobia and legumes that result in the formation of a nitrogen-fixing nodule. We focus on the role of specific lipooligosaccharides secreted by rhizobia in the induction of these early steps. In the second part, we describe nodule functioning and compare actinorhizal and legume nodules.
680 citations
TL;DR: It is indicated that only one lineage of closely related taxa achieved the underlying genetic architecture necessary for symbiotic nitrogen fixation in root nodules.
Abstract: Of the approximately 380 families of angiosperms, representatives of only 10 are known to form symbiotic associations with nitrogen-fixing bacteria in root nodules. The morphologically based classification schemes proposed by taxonomists suggest that many of these 10 families of plants are only distantly related, engendering the hypothesis that the capacity to fix nitrogen evolved independently several, if not many, times. This has in turn influenced attitudes toward the likelihood of transferring genes responsible for symbiotic nitrogen fixation to crop species lacking this ability. Phylogenetic analysis of DNA sequences for the chloroplast gene rbcL indicates, however, that representatives of all 10 families with nitrogen-fixing symbioses occur together, with several families lacking this association, in a single clade. This study therefore indicates that only one lineage of closely related taxa achieved the underlying genetic architecture necessary for symbiotic nitrogen fixation in root nodules.
475 citations
TL;DR: Some aspects of the physiological ecology of cyanobacteria are discussed by taking a microbial mat as an example, where the majority of microbial mats are built and dominated by cyarsobacteria which are primary producers at the basis of the microbial foodweb in microbial mats.
Abstract: In this review some aspects of the physiological ecology of cyanobacteria are discussed by taking a microbial mat as an example. The majority of microbial mats are built and dominated by cyarsobacteria which are primary producers at the basis of the microbial foodweb in microbial mats. These micro-scale ecosystems are characterized by steep and fluctuating physico-chemical gradients of which those of light, oxygen and sulphide are the most conspicuous. Light is strongly attenuated in the sediment, and owing to constant sedimentation, the mat-forming cyanobacteria have to move upwards towards the light. However, at the sediment surface, light intensity, particularly in the u.v. part of the spectrum, is often deleterious. The gliding movement of the cyanobacteria, with photo- and chemotaxis, allows the organism to position itself in a thin layer at optimal conditions. The organic matter produced by cyanobacterial photosynthesis is decomposed by the ruicrobial community. Sulphate-reducing bacteria are important in the end-oxidation of the organic matter. These organisms are obligate anaerobes and produce sulphide. Gradients of sulphide and oxygen move up and down in the sediment as a response to diurnal variations of light intensity. Cyanobacteria, therefore, are sometimes exposed to large concentrations of the extremely toxic sulphide. Some species are capable of sulphide-dependent anoxygenic photosynthesis. Other cyanobacteria show increased rates of oxygenic photosynthesis in the presence of sulphide and have mechanisms to oxidize sulphide while avoiding sulphide toxicity. Iron might play an important role in this process. Under anoxic conditions in the dark, mat-forming cyanobacteria switch to fermentative metabolism. Many species are also capable of fermentative reduction of elemental sulphur to sulphide. The gradients of sulphide and oxygen are of particular importance for nitrogen fixation. Very few microbial mats are formed by heterocystous cyanobacteria, which are best adapted to diazntrophic growth. However, these organisms probably cannot tolerate greater concentrations of sulphide or anoxic conditions or both. Under such conditions non-heterocystous cyanobacteria become dominant as diazotrophs. These organisms avoid conditions of oxygen supersaturation. In the ecosystem, nitrogen fixation and photosynthesis might be separated temporally as well as spatially. In addition, non-heterocystous diazotrophic cyanobacteria have mechanisms at the subcellular level to protect the oxygen-sensitive nitrogenase from inaction. CONTENTS Summary 1 I. Introduction 2 II. Microbial mats 3 III. Cyanobacteria in light gradients 7 IV. Dark metabolism 10 V. Interactions with sulphide 13 VI. Nitrogen fixation 16 VII. References 28.
404 citations
TL;DR: This paper considers ways and means by which populations of root-nodule bacteria can be manipulated ecologically, agronomically, edaphically and genetically to improve legume productivity and, as a consequence, soil fertility.
Abstract: Inputs of biologically fixed nitrogen derived from the symbiotic relationship between legumes and their root-nodule bacteria into terrestrial ecosystems amount to at least 70 million metric tons per year. It is obvious that this enormous quantity will need to be augmented as the world's population increases and as the natural resources that supply fertilizer nitrogen diminish. This objective will be achieved through the development of superior legume varieties, improvement in agronomic practice, and increased efficiency of the nitrogen fixation process itself by better management of the symbiotic relationship between plant and bacteria. This paper considers ways and means by which populations of root-nodule bacteria, established and introduced, can be manipulated ecologically, agronomically, edaphically and genetically to improve legume productivity and, as a consequence, soil fertility.
344 citations
TL;DR: The sugarcane/endophytic diazotroph association is the first efficient N2-fixing system to be discovered associated with any member of the gramineae and should serve as a foundation for the improvement/development of similar N2 -fixing systems in wetland rice and other cereal crops.
Abstract: 15N isotope and N balance studies performed over the last few years have shown that several Brazilian varieties of sugarcane are capable of obtaining over 60% of their nitrogen (<150 kg N ha-1 year-1) from biological nitrogen fixation (BNF). This may be due to the fact that this crop in Brazil has been systematically bred for high yields with low fertilizer N inputs. In the case of wetland rice, N balance experiments performed both in the field and in pots suggest that 30 to 60 N ha-1 crop-1 may be obtained from plant-associated BNF and that different varieties have different capacities to obtain N from this source. 15N2 incorporation studies have proved that wetland rice can obtain at least some N from BNF and acetylene reduction (AR) assays also indicate differences in N2-fixing ability between different rice varieties. However in situ AR field estimates suggest plant-associated BNF inputs to be less than 8 kg N ha-1 crop-1. The problems associated with the use of the 15N dilution technique for BNF quantification are discussed and illustrated with data from a recent study performed at EMBRAPA-CNPAB. Although many species of diazotrophs have been isolated from the rhizosphere of both sugarcane and wetland rice, the recent discovery of endophytic N2-fixing bacteria within roots, shoots and leaves of both crops suggests, at least in the case of sugarcane, that these bacteria may be the most important contributors to the observed BNF contributions. In sugarcane both Acetobacter diazotrophicus and Herbaspirillum spp. have been found within roots and aerial tissues and these microorganisms, unlike Azospirillum spp. and other rhizospheric diazotrophs, have been shown to survive poorly in soil. Herbaspirillum spp. are found in many graminaceous crops, including rice (in roots and aerial tissue), and are able to survive and pass from crop to crop in the seeds. The physiology, ecology and infection of plants by these endophytes are fully discussed in this paper. The sugarcane/endophytic diazotroph association is the first efficient N2-fixing system to be discovered associated with any member of the gramineae. As yet the individual roles of the different diazotrophs in this system have not been elucidated and far more work on the physiology and anatomy of this system is required. However, the understanding gained in these studies should serve as a foundation for the improvement/development of similar N2-fixing systems in wetland rice and other cereal crops.
319 citations
TL;DR: The rate of increase in nitrogen deposition is not linear over the 90-year period, and the increases were negligible over the period 1880-1915, however, during the period 1950 to 1990 the data suggest an increase inNitrogen deposition of 2 kg N ha(-1) every 10 years.
265 citations
TL;DR: It was concluded that inoculation with bacterial mixtures provided a more balanced nutrition for the plants and the improvement in root uptake of nitrogen and phosphorus was the major mechanism of interaction between plants and bacteria.
Abstract: Pot experiments were carried out to investigate the effect of inoculation with pure and mixed cultures of nitrogen fixers Azospirillum lipoferum 137, Arthrobacter mysorens 7 and the phosphate-solubilizing strain Agrobacterium radiobacter 10 on growth and mineral nutrition of two barley cultivars. A significant positive effect on grain yield both of the studied barley cultivars was obtained after inoculation with mixtures of A. lipoferum 137 + A. radiobacter 10 and A. lipoferum 137 + A. mysorens 7 only. The acetylene reduction activity on roots or in batch culture was significantly higher when A. lipoferum 137 and A. radiobacter 10 were combined. Using 15N isotope dilution technique it was established that these mixed cultures significantly increased the accumulation of nitrogen fertilizer in the plants. The strain A. radiobacter 10 promoted a better accumulation of phosphorus fertilizer by plants and A. mysorens 7 increased the total phosphorus content in plant tissues. The maximum positive effect of joint inoculation on plant development was observed when the combined nitrogen in soil was in short supply. It was concluded that inoculation with bacterial mixtures provided a more balanced nutrition for the plants and the improvement in root uptake of nitrogen and phosphorus was the major mechanism of interaction between plants and bacteria. The introduced bacteria were able to colonize actively the rhizoplane of barley. No interspecific competition or antagonism were established between components of the bacterial mixtures in the rhizoplane. The strains A. mysorens 7 and A. radiobacter 10 improved viability of A. lipoferum 137 when the plants were grown in acid soil. Field experiments carried out on 3 barley cultivars confirmed the assertion that inoculation with mixed cultures significantly increases the grain yield and nitrogenous nutrition of plants as compared with single cultures.
190 citations
TL;DR: A range of management options can be applied to legumes growing in farming systems to manipulate N2 fixation and improve the N benefits to agriculture and agroforestry.
Abstract: Atmospheric N2 fixed symbiotically by associations between Rhizobium spp. and legumes represents a renewable source of N for agriculture. Contribution of legume N2 fixation to the N-economy of any ecosystem is mediated by: (i) legume reliance upon N2 fixation for growth, and (ii) the total amount of legume-N accumulated. Strategies that change the numbers of effective rhizobia present in soil, reduce the inhibitory effects of soil nitrate, or influence legume biomass all have potential to alter net inputs of fixed N. A range of management options can be applied to legumes growing in farming systems to manipulate N2 fixation and improve the N benefits to agriculture and agroforestry.
168 citations
TL;DR: Data from long-term experiments with model grassland ecosystems, consisting of monocultures or mixtures of perennial ryegrass and white clover, grown under elevated CO2 under free-air or field-like conditions, supports the following hypothesis: symbiotic N2 fixation in legumes will be enhanced under elevatedCO2, and this enhancement of N1 fixation will result in a larger N-input to the grassland ecosystem.
Abstract: The response of plants to elevated CO2 is dependent on the availability of nutrients, especially nitrogen It is generally accepted that an increase in the atmospheric CO2 concentration increases the C:N ratio of plant residues and exudates This promotes temporary N-immobilization which might, in turn, reduce the availability of soil nitrogen In addition, both a CO2 stimulated increase in plant growth (thus requiring more nitrogen) and an increased N demand for the decomposition of soil residues with a large C:N will result under elevated CO2 in a larger N-sink of the whole grassland ecosystem One way to maintain the balance between the C and N cycles in elevated CO2 would be to increase N-import to the grassland ecosystem through symbiotic N2 fixation Whether this might happen in the context of temperate ecosystems is discussed, by assessing the following hypothesis: i) symbiotic N2 fixation in legumes will be enhanced under elevated CO2, ii) this enhancement of N2 fixation will result in a larger N-input to the grassland ecosystem, and iii) a larger N-input will allow the sequestration of additional carbon, either above or below-ground, into the ecosystem Data from long-term experiments with model grassland ecosystems, consisting of monocultures or mixtures of perennial ryegrass and white clover, grown under elevated CO2 under free-air or field-like conditions, supports the first two hypothesis, since: i) both the percentage and the amount of fixed N increases in white clover grown under elevated CO2, ii) the contribution of fixed N to the nitrogen nutrition of the mixed grass also increases in elevated CO2 Concerning the third hypothesis, an increased nitrogen input to the grassland ecosystem from N2 fixation usually promotes shoot growth (above-ground C storage) in elevated CO2 However, the consequences of this larger N input under elevated CO2 on the below-ground carbon fluxes are not fully understood On one hand, the positive effect of elevated CO2 on the quantity of plant residues might be overwhelming and lead to an increased long-term below-ground C storage; on the other hand, the enhancement of the decomposition process by the N-rich legume material might favour carbon turn-over and, hence, limit the storage of below-ground carbon
155 citations
TL;DR: These rhizobial isolates are indigenous to Kenyan soils and yet have marked similarities to previously described Rhizobium species from other continents.
Abstract: Rhizobia were isolated from two Kenyan soils with pHs of 4.5 and 6.8 and characterized on the basis of their host ranges for nodulation and nitrogen fixation, colony morphologies, restriction fragment fingerprints, and hybridization with a nifH probe. The populations of rhizobia nodulating Phaseolus vulgaris in the two soils were similar in numbers and in effectiveness of N(inf2) fixation but were markedly different in composition. The population in the Naivasha soil (pH 6.8) was dominated by isolates specific in host range for nodulation to P. vulgaris; these all had multiple copies, in most cases four, of the structural nitrogenase gene nifH. Only one of the isolates from this soil formed effective nodules on Leucaena leucocephala, and this isolate had only a single copy of nifH. By contrast, the population in the acid Daka-ini soil (pH 4.5) was composed largely of broad-host-range isolates which had single copies of nifH. The isolates from the Daka-ini soil which were specific to P. vulgaris generally had three copies of nifH, although one isolate had only two copies. These rhizobial isolates are indigenous to Kenyan soils and yet have marked similarities to previously described Rhizobium species from other continents.
01 Feb 1995
TL;DR: Through the use of N balance and 15N techniques it has been shown that in the case of lowland rice, several tropical pasture grasses and especially sugar cane, the contributions of biological N2 fixation (BNF) are of agronomic significance.
Abstract: Over the last 20 years many new species of N2-fixing bacteria have been discovered in association with grasses, cereals and other non-nodulating crops. Virtually all of these bacteria are microaerophylic, fixing N2 only in the presence of low partial pressures of oxygen. Until a few years ago much attention was focussed on members the genus Azospirillum and it was assumed that N2 fixation was restricted to the rhizosphere or rhizoplane of the host plants. Through the use of N balance and 15N techniques it has been shown that in the case of lowland rice, several tropical pasture grasses and especially sugar cane, the contributions of biological N2 fixation (BNF) are of agronomic significance.
TL;DR: In laboratory culture systems, inoculation of straws with cellulolytic and diazotrophic microorganisms has resulted in significant increases in N2 fixation in comparison to uninoculated controls and gains of N of up to 72 mg N fixed g-1 straw consumed have been obtained, indicating the potential of inoculation to improve N gains in composts that can then be used as biofertilisers.
Abstract: Much of the crop residues, including cereal straw, that are produced worldwide are lost by burning. Plant residues, and in particular straw, contain large amounts of carbon (cellulose and hemicellulose) which can serve as substrates for the production of microbial biomass and for biological N2 fixation by a range of free-living, diazotroph-ic bacteria. Microorganisms with the dual ability to utilise cellulose and fix N2 are rare, but some strains that utilize hemicellulose and fix N2 have been found. Generally, cellulolysis and diazotrophy are carried out by a mixed microbial community in which N2-fixing bacteria utilise cellobiose and glucose produced from straw by cellulolytic microorganisms. N2-fixing bacteria include heterotrophic and phototrophic organisms and the latter are apparently more prominent in flooded soils such as rice paddies than in dryland soils. The relative contributions of N2 fixed by heterotrophic diazotrophic bacteria compared with cyanobacteria and other phototrophic bacteria depend on the availability of substrates from straw decomposition and on environmental pressures. Measurements of asymbiotic N2 fixation are limited and variable but, in rice paddy systems, rates of 25 kg N ha-1 over 30 days have been found, whereas in dryland systems with wheat straw, in situ measurements have indicated up to 12 kg N ha-1 over 22 days. Straw-associated N2 fixation is directly affected by environmental factors such as temperature, moisture, oxygen concentration, soil pH and clay content as well as farm management practices. Modification of managements and use of inoculants offer ways of improving asymbiotic N2 fixation
TL;DR: It is demonstrated that infection and/or early nodule development were very sensitive to low RZT, such that only a few days at 19°C, instead of 21°C can make important differences in the time to onset of N 2 fixation and, subsequent N accumulation and plant growth.
01 Feb 1995
TL;DR: Three main approaches are appropriate to increase N2 fixation: clonal selection of trees combined with vegetative propagation, inoculation with effective rhizobium orFrankia strains, and proper fertilization (especially P).
Abstract: The N2-fixing potential (NFP) (i.e. the amount of fixed N2 in a constraint-free environment) of N2-fixing trees (NFTs) varies with the genotype. The NFP can be higher than 30-50 g N2 fixed tree−1 year−1 in the most active species, be they leguminous trees such asAlbizia lebbeck, Gliricidia sepium andLeucaena leucocephala, or actinorhizal trees such asCasuarina equisetifolia. The actual amount of nitrogen fixed (ANF) (i.e. the amount of N2 fixed in the field) is lower than the NFP or even nil because of various constraints, especially drought, nutrient deficiencies, excess of available N and pathogenic nematodes. As tree litters are mineralized, the amount of available N in the soil increases with time, this process leading to the cessation of N2 fixation in aging plantations. When the mineralization rate is slowed down or inhibited, N2 fixation can continue. NFTs improve the N status of soils, but the transfer of fixed N to associated plants is not always ensured. Three main approaches are appropriate to increase N2 fixation: clonal selection of trees combined with vegetative propagation, inoculation with effective rhizobium orFrankia strains, and proper fertilization (especially P). In the absence of major environmental constraints, a positive response to inoculation is expected only when specific (non-promiscuous) NFTs are grown in sites where the density of compatible rhizobia is low or nil. The potentialities of NFTs are far from being fully exploited. Further investigations are proposed and the economics of NFT management is briefly discussed.
Journal Article•
TL;DR: Nitrogen fixation and colonization by associative cyanobacteria in the aerial roots (pneumatophores) of black mangrove trees was evaluated in situ at Balandra lagoon, Baja California Sur, Mexico, for 18 consecutive months, showing seasonal and diurnal fluctuations.
Abstract: Nitrogen fixation and colonization by associative cyanobacteria in the aerial roots (pneumatophores) of black mangrove trees was evaluated in situ at Balandra lagoon, Baja California Sur, Mexico, for 18 consecutive months. Year-round vertical zonation of cyanobacterial colonization was determined along the pneumatophores. The bottom part close to the sediment was colonized mainly by nonheterocystous, filamentous cyanobacteria resembling Lyngbya sp. and Oscillatoria sp. The central zone was colonized mainly by filaments resembling Microcoleus sp. and the upper part was colonized by coccoidal cyanobacteria within defined colonies resembling Aphanothece sp. mixed with undefined filamentous cyanobacteria. Two of the cyanobacteria (Microcoleus sp. and Anabaena sp.) isolated from the pneumatophore were diazotrophs. Massive biofilm production along the pneumatophores was evident throughout the observation period. The surrounding sediment was seasonally dominated by heterocystous Anabaena sp. Glass and dead-wood surfaces incubated for 18 months in the pneumatophore vicinity showed no zonation in the colonization pattern, although they were heavily colonized. In situ N2 fixation showed seasonal and diurnal fluctuations. N2 fixation was low during winter, increased in early summer, and reached its peak in midsummer. N2 fixation in the summer showed diurnal peaks: one in the morning until midday and the second in the late afternoon. N2 fixation was at its lowest levels near midnight. Light and water temperature are probably primary environmental factors governing N2 fixation on the pneumatophores.
TL;DR: The high sensitivity of the system both to the initial value of the soil organic matter nitrogen pool, and to any photosynthetic competitive advantage, is investigated.
TL;DR: Results show that heavy metals in soil, and the resulting concentration in plants, had no effect on any of the parameters examined, when pH was maintained at 6.0 or higher.
Abstract: Effects of sludge-borne heavy metals on plant growth, nodulation, and nitrogen (N 2 ) fixation of alfalfa (Medicago saliva L.), white clover (Trifolium repens L.), and red clover (Trifolium pratense L.) were examined. Plants were grown in the greenhouse in pots containing soil (Typic Paleudults) obtained from plots where heat-treated and Nu-Earth sludges were applied in 1976 and 1978, respectively. Two soil pH regimes (low and high) were examined for each of the treatments. Nitrogen fertilizer (NH 4 NO 3 ) was applied to one-half the pots and each legume was inoculated with the appropriate Rhizobium culture. Soil pH and sludge type significantly affected uptake of metals with phytotoxicity observed in the Nu-Earth, low pH soil. Nodulation was reduced, but not always completely eliminated in all low pH treatments, including the controls. Symbioses were generally ineffective in low pH treatments. In soils where pH was above 6.0, there was a significant increase in shoot weight and total shoot N with sludge addition. Plants derived significant quantities of N from soil, where sludge was applied many years ago. When pH was maintained at 6.0 or higher the results show that heavy metals in soil, and the resulting concentration in plants, had no effect on any of the parameters examined.
TL;DR: In this article, Nitrogen fixation and colonization by associative cyanobacteria in the aerial roots (pneumatophores) of black mangrove trees were evaluated in situ at Balandra lagoon, Baja California Sur, Mexico.
Abstract: Nitrogen fixation and colonization by associative cyanobacteria in the aerial roots (pneumatophores) of black mangrove trees was evaluated in situ at Balandra lagoon, Baja California Sur, Mexico, f...
TL;DR: The first report of the artificial inoculation of cyanobacteria on marine mangroves is reported, to the best of the authors' knowledge, and the level of N2 fixation in the presence of the plant was significantly higher than the amount of nitrogen fixed by a similar quantity of cyanOBacteria on a N-free growth medium.
Abstract: An isolate of the filamentous cyanobacterium Microcoleus sp. was obtained from black mangrove aerial root (pneumatophore) and inoculated onto young mangrove seedlings to evaluate N2-fixation and root- colonization capacities of the bacterium under in vitro conditions in closed-system experiments. N2 fixation (acetylene reduction) gradually increased with time and reached its peak 5 days after inoculation. Later, it decreased sharply. The level of N2 fixation in the presence of the plant was significantly higher than the amount of nitrogen fixed by a similar quantity of cyanobacteria on a N-free growth medium. The main feature of this root colonization was the gradual production of a biofilm in which the cyanobacterial filaments were embedded. Visible biofilm production increased with time until it completely covered the entire root system of the plant. The in-and-out movement of cyanobacterial filaments from the biofilm probably allows colonization of uncolonized portions of the root several days after the initial inoculation. This is, to the best of our knowledge, the first report of the artificial inoculation of cyanobacteria on marine mangroves.
01 Feb 1995
TL;DR: No information is available on the mechanism of xylem loading with amides or ureides, a process that may play a key role in avoiding accumulation of amino acids in infected nodule cells, but both transport processes, however, may determine the efficiency of N2 fixation by legumes.
Abstract: Legumes may feed on three different sources of nitrogen: nitrate, ammonium, and, due to symbiotic N2 fixation, atmospheric dinitrogen. In all three cases ammonium is finally assimilated by the glutamine synthetase (GS)/glutamate synthase (GOGAT) system. NH 4 + produced by nitrogenase in symbiosomes of legume nodules is released into the host cell cytosol where it is incorporated into amino acids and amides. The release of NH 4 + into the cytosol appears to occur purely by diffusion. Therefore, the activity of the GS/GOGAT enzymes is decicive to avoid product inhibition of nitrogenase by NH 4 + . No information is available on the mechanism of xylem loading with amides or ureides, a process that may play a key role in avoiding accumulation of amino acids in infected nodule cells. The same applies to phloem unloading of sucrose. Both transport processes, however, may determine the efficiency of N2 fixation by legumes.
TL;DR: Evidence presented here suggests a need to select faba bean genotypes that are tolerant to salt stress, and activity of the ammonium assimilation enzymes glutamine synthetase and glutamate synthase, and soluble protein content, were reduced by salinity in all genotypes tested.
Abstract: Fifteen genotypes of faba bean (Vicia faba L.) were inoculated with salt-tolerant Rhizobium leguminosarum biovar. viciae strain GRA 19 in solution culture with 0 (control) and 75 mM NaCl added immediately after transplanting. Genotypes varied in their tolerance of high levels of NaCl. Physiological parameters (dry weight of shoot and root, number and dry weight of nodules) were not affected by salinity in lines VF46, VF64 and VF112. Faba bean line VF60 was sensitive to salt stress. Host tolearance appeared to be a major requisite for nodulation and N2 fixation under salt stress. Tolerant line VF112 sustained nitrogen fixation under saline conditions. Activity of the ammonium assimilation enzymes glutamine synthetase and glutamate synthase, and soluble protein content, were reduced by salinity in all genotypes tested. Evidence presented here suggests a need to select faba bean genotypes that are tolerant to salt stress.
TL;DR: It is concluded that a Nif2‐type system operates in vegetative cells of non‐heterocystous and some, but not all, heterocyst‐forming filamentous cyanobacteria and is environmentally regulated by the levels of both oxygen and combined nitrogen in the habitat.
Abstract: Two different fdxH genes (fdxH1, fdxH2) have been isolated from the nitrogen-fixing, heterocyst-forming cyanobacterium Anabaena variabilis ATCC 29413. They are part of two different nif gene clusters, nif1 and nif2. fdxH1 encodes the [2Fe-2S] ferredoxin that is known as the direct electron donor to nitrogenase in heterocysts, and is very similar to FdxH from Anabaena sp. PCC 7120. FdxH2 has more residues in common and shares its oxygen sensitivity with the single FdxH from the non-heterocystous, filamentous cyanobacterium Plectonema boryanum PCC 73110. The latter expresses nitrogenase early (< or = 3-4h) after nitrogen depletion in vegetative cells and exclusively under anaerobic conditions. fdxH2 and the nif2 genes of Anabaena 29413 are also transcribed < or = 4 h after onset of nitrogen-stepdown, exclusively under anaerobic growth conditions and long before functional heterocysts appear. At this time, no fdxH1 and nif1 gene transcription was observed. It occurred later and was associated with nitrogen fixation under aerobic conditions, i.e. within heterocysts. fdxH2 and nifHDK2 were not transcribed during aerobic, nitrogen-fixing growth. In addition, neither was an fdxH2-type gene found nor an anaerobically and early inducible Nif2 system detectable in Anabaena 7120. These data reveal that in filamentous cyanobacteria two different Nif systems have evolved based on molybdenum nitrogenases. It is concluded that a Nif2-type system operates in vegetative cells of non-heterocystous and some, but not all, heterocyst-forming filamentous cyanobacteria. It is environmentally regulated by the levels of both oxygen and combined nitrogen in the habitat. To simultaneously allow for oxygen-evolving photosynthesis and oxygen-sensitive nitrogen fixation, the Nif1-type system probably branched from an ancestral Nif2-type system and has evolved for an exclusive operation within heterocysts. Accordingly, its expression has become an obligate late event in the developmental programme of heterocyst differentiation, irrespective of aerobic or anaerobic growth conditions.
TL;DR: The discovery of novel subcellular structures related to bacterial nitrogen fixation in the strictly respiratory diazotrophic bacterium Azoarcus sp.
Abstract: We report the discovery of novel subcellular structures related to bacterial nitrogen fixation in the strictly respiratory diazotrophic bacterium Azoarcus sp. BH72, which was isolated as an endophyte from Kallar grass. Nitrogenase is derepressed under microaerobic conditions at O2 concentrations in the micromolar range. With increasing O2 deprivation, bacteria can develop into a hyperinduced state, which is characterized by high specific rates of respiration and efficient nitrogen fixation at approximately 30 nM O2. Ultrastructural analysis of cells in the course of hyperinduction revealed that complex intracytoplasmic membrane systems are formed, which consist of stacks of membranes and which are absent under standard nitrogen-fixing conditions. The iron protein of nitrogenase was highly enriched on these membranes, as evidenced by immunohistochemical studies. Membrane deficiency in NifH/K- mutants, a deletion mutant in the nifK gene and the character of NH+4-grown cells suggested, in concert with the membrane localization of nitrogenase, that these structures are specialized membranes related to nitrogen fixation. We propose the term 'diazosomes' for them. Development of intracytoplasmic membranes coincides with the appearance of a high-molecular-mass form of the iron protein of nitrogenase, which was detectable in membrane fractions. Mutational analysis, and determination of the N-terminal amino acid sequence indicate that the nifH gene product is covalently modified by a mechanism probably different from adenosine diphosphoribosylation. Development of diazosomes in nitrogen-fixing cells can be induced in pure cultures and in co-culture with a fungus isolated from the rhizosphere of Kallar grass.
TL;DR: Although soil compaction made the quantification difficult, at the end of the experiment it was estimated that there was at least an extra 800 kg N ha-1 in the total soil nitrogen under Lucerne compared to strips of Phalaris aquatica grown between the lucerne plots.
Abstract: Nodulation, N2 fixation (estimated by 15N natural abundance methods) and dry matter production were studied in a lucerne (Medicago sativa) crop managed for hay production at Ginninderra Experiment Station, A.C .T. Measurements were taken in the year of establishment and during two subsequent growing seasons. There were three treatments: (1) no inoculation and no annual fertilizer applied, (2) initial inoculation and superphosphate applied annually, (3) no inoculation, superphosphate applied annually and ammonium sulfate periodically. Before planting and after each growth season, soil was analysed for extractable mineral nitrogen, total nitrogen and the 15N natural abundance of this nitrogen, to the depth explored by lucerne roots. Before planting, no appropriate root-nodule bacteria (Rhizobium meliloti) were detected in the soil and initially plants were nodulated only in the inoculated treatment. Thereafter nodulation increased on the other treatments. Eight months after sowing there were no differences between treatments in numbers of R. meliloti g-l soil or in nodulation. In the third growing season, almost 30 kg ha-1 (dry wt) of nodules were recovered to a depth of 25 cm. These nodules were primarily located on fine, ephemeral roots and many appeared to be renewed after cutting of the lucerne. In the year of establishment, dry matter yields (0% moisture) totalled 3 to 4 t ha-1 in three hay cuts. In succeeding years, total yields were in the range 10 to 13 t ha-1 in four or five cuts per season. Nitrogen removed in the harvested lucerne reached 340 to 410 kg N ha-lyr-l in the second and third years and between 65 and 96% of this N arose from N2 fixation, depending on the method of calculation used. Poorer dry matter production and N2 fixation in treatment 1 in the third growing season was attributed to an insufficient supply of available phosphorus. Fixed N removed in Lucerne hay from treatment 2 totalled at least 640 kg N ha-1 in the three years of the experiment. Also, there were substantial increases in soil nitrogen due to lucerne growth. Although soil compaction made the quantification difficult, at the end of the experiment it was estimated that there was at least an extra 800 kg N ha-1 in the total soil nitrogen under lucerne compared to strips of Phalaris aquatica grown between the lucerne plots. It was concluded that lucerne contributed at least the same amount of fixed nitrogen to the soil as was being removed in the harvested hay.
TL;DR: This study suggests that the composition of R. meliloti populations sampled by the traditionally used host legume may not be representative of that actually present in soil, and uses a highly species specific DNA probe derived from ISRm5 to investigate this possibility.
Abstract: Insertion sequence (IS) hybridization was used to define the structure of a population of Rhirobium meliloti isolated directly from soil and from nodules of Medicago sativn (alfalfa) and Melilotus alba (sweet clover) grown under controlled conditions and inoculated with a suspension of the same soil. The detection of R. meliloti isolated from soil on agar plates was facilitated by use of a highly species specific DNA probe derived from ISRm5. All R. meliloti o'btained directly from soil proved to be symbiotic (i.e. noddated and fixed nitrogen with alfalfa). Analysis of 293 R. meliloti isolates revealed a total of 17 distinct IS genotypes of which 9,9 and 15 were from soil, M. alba and M. satina, respectively; 8 genotypes were common to soil and both plant species. The frequency of R. meliloti genotypes from soil differed markedly from that sampled from nodules of both legume species: 5 genotypes represented about 90% of the isolates from soil whereas a single genotype predominated among isolates from nodules accounting for more than 55% of the total. The distribution of genotypes differed between M. satiaa and M. alba indicating species variation in nodulation preferences for indigenous R. meliloti. The data are discussed in the context of competition for nodalation of the host plant and the selection of Rhizobium strains for use in legume inoculants. This study has ecological implications and suggests that the composition of R meliloti populations sampled by the traditionally used host legume may not be representative of that actually present in soil.
01 Feb 1995
TL;DR: The four commonly used methods for measuring biological nitrogen fixation (BNF) in plants are: the total nitrogen difference (TND) method, acetylene reduction assay (ARA) technique, xylem-solute (or ureide production) method and the use of 15N labelled compounds.
Abstract: The four commonly used methods for measuring biological nitrogen fixation (BNF) in plants are: the total nitrogen difference (TND) method, acetylene reduction assay (ARA) technique, xylem-solute (or ureide production) method and the use of 15N labelled compounds.
TL;DR: In this paper, Nitrogen management in organic agriculture is discussed focussing on strategies maximizing nitrogen input (source: N2-fixation) and minimizing nitrate losses onsite (field losses) and off-site (losses from manure heaps).
Abstract: Nitrogen management in organic agriculture is discussed focussing on strategies maximizing nitrogen input (source: N2-fixation) and minimizing nitrate losses on-site (field losses) and off-site (losses from manure heaps). Due to a strong relationship between grain yield/amount of N in grains and N2-fixation in pulse-crops or total dry matter yield and N2-fixation in fodder legumes, e.g. grass/clover mixtures, nitrogen fixation can be maximized by selecting the best site-adapted (best yielding) species, varieties or mixtures, respectively. Strategies to reduce nitrate losses are based on N-sinks, that is, plants taking up soil-borne and residual nitrogen during or after growth of legumes efficiently, e.g. brassica-underseeds in faba beans, catch crops, or following crops with great sink capacity for nitrogen. In order to keep pre-winter mineralization as low as possible (minimizing N-source) turn over of grass/clover can be postponed or tillage intensity can be reduced in depth or frequency. Gaseo...
TL;DR: This paper reports the isolation and characterization of a NADH-GOGAT gene from alfalfa (Medicago sativa L.), the first GOGat gene to be isolated from a eukaryote, and indicates that NADh-GogAT plays a central role in the functioning of effective root nodules.
Abstract: Glutamate synthase (GOGAT), a key enzyme in ammonia (NH+4) assimilation, occurs as two forms in plants: a ferredoxin-dependent form (Fd-GOGAT) and an NADH-dependent form (NADH-GOGAT). These enzymes are encoded by distinct genes as evidenced by their cDNA and deduced amino acid sequences. This paper reports the isolation and characterization of a NADH-GOGAT gene from alfalfa (Medicago sativa L.), the first GOGAT gene to be isolated from a eukaryote. RNase protection and primer extension experiments map the transcription start site of NADH-GOGAT to nearly identical positions. The transcribed region of this gene, 12,214 bp, is comprised of 22 exons separated by 21 introns. The 2.7 kbp region 5' from the translation initiation site confers nodule-specific reporter gene activity when used in a chimeric beta-glucuronidase (GUS) construct and transformed into Lotus corniculatus and Medicago sativa. Both infected and uninfected cells display GUS activity. The abundance of NADH-GOGAT transcripts increases substantially in developing nodules of plants infected with effective rhizobia. However, this increase is not observed when nodules are induced by a variety of ineffective rhizobial strains. Thus, unlike many other plant genes involved in root nodule NH+4 assimilation, high levels of NADH-GOGAT expression are strictly associated with effective nodules indicating that NADH-GOGAT plays a central role in the functioning of effective root nodules. An alfalfa Fd-GOGAT PCR product showing greater than 85% identity to maize Fd-GOGAT was isolated and used to investigate the contribution of this enzyme to NH+4 assimilation in nodules. Fd-GOGAT mRNA was abundant in leaves and cotyledons but was not detected in alfalfa root nodules. Fd-GOGAT in alfalfa does not appear to play a significant role in symbiotic N2 fixation.