Variation in Preference for Rhizobium meliloti Within and Between Medicago sativa Cultivars Grown in Soil

Variation in nodulation preferences for Rhizobium strains within and between Medicago sativa cultivars was assessed in the greenhouse with plants grown in Leonard jars and two soils of diverse origin (Lanark and Ottawa), using inocula consisting of effective individual or paired strains of R. meliloti which could be recognized by high-concentration antibiotic resistance. The results indicated considerable variability in host preferences for R. meliloti among plants within cultivars but not between cultivars. The implications of this variation are discussed from the point of view of possible improvement of symbiotic nitrogen fixation. With one exception, the differences in nodulation success between inoculant R. meliloti strains were consistent in Leonard jars and both soils. All introduced strains formed significantly more nodules in Renfrew soil containing few native rhizobia than in Ottawa soil with a large resident R. meliloti population. Plants grown in Lanark soil without inoculation were ineffectively nodulated by native rhizobia and yielded significantly less growth than those receiving inoculation. In contrast, the yield of inoculated plants in Ottawa soil did not significantly differ from those without inoculation due to effective nodulation by native R. meliloti. The data indicated synergistic effects on yield by certain paired strain inocula relative to the same strains inoculated individually in Lanark but not in Ottawa soil or Leonard jars.     

Crop uptake and leaching losses of15N labelled fertilizer nitrogen in relation to waterlogging of clay and sandy loam soils

Summary Ammonium nitrate fertilizer, labelled with¹⁵N, was applied in spring to winter wheat growing in undisturbed monoliths of clay and sandy loam soil in lysimeters; the rates of application were respectively 95 and 102 kg N ha⁻¹ in the spring of 1976 and 1975. Crops of winter wheat, oilseed rape, peas and barley grown in the following 5 or 6 years were treated with unlabelled nitrogen fertilizer at rates recommended for maximum yields. During each year of the experiments the lysimeters were divided into treatments which were either freelydrained or subjected to periods of waterlogging. Another labelled nitrogen application was made in 1980 to a separate group of lysimeters with a clay soil and a winter wheat crop to study further the uptake of nitrogen fertilizer in relation to waterlogging. In the first growing season, shoots of the winter wheat at harvest contained 46 and 58% of the fertilizer nitrogen applied to the clay and sandy loam soils respectively. In the following year the crops contained a further 1–2% of the labelled fertilizer, and after 5 and 6 years the total recoveries of labelled fertilizer in the crops were 49 and 62% on the clay and sandy loam soils respectively. In the first winter after the labelled fertilizer was applied, less than 1% of the fertilizer was lost in the drainage water, and only about 2% of the total nitrogen (mainly nitrate) in the drainage water from both soils was derived from the fertilizer. Maximum annual loss occurred the following year but the proportion of tracer nitrogen in drainage was nevertheless smaller. Leaching losses over the 5 and 6 years from the clay and sandy loam soil were respectively 1.3 and 3.9% of the original application. On both soils the percentage of labelled nitrogen to the total crop nitrogen content was greater after a period of winter waterlogging than for freely-drained treatments. This was most marked on the clay soil; evidence points to winter waterlogging promoting denitrification and the consequent loss of soil nitrogen making the crop more dependent on spring fertilizer applications.     

Symbiotic properties of some auxotrophic mutants of Rhizobium meliloti and of their prototrophic revertants

Summary 1. Auxotrophic mutants of 2 strains ofRhizobium meliloti have been isolated: among them 4 are purine or pyrimidine-dependent (adenine, cytosine), 4 are sulfur amino acid-dependent (methionine, cysteine) and 3 glycine-dependent. 2. All of these mutants induce nodule formation on lucerne (variety ‘Du Puits’). 3. There are associations between nutritional requirements and effectiveness in nitrogen fixation on this host. All the amino acid-dependent mutants are effective. Glycine-dependent mutants are more effective than the wild-type strain, whereas cyste?ne and methionine-dependent mutants are as effective as the wild-type strain. On the other hand, purine and pyrimidine-dependent mutants are ineffective. 4. All the prototrophic revertants of ineffective auxotrophic strains are effective. Some of them are more effective than wild-type strain.     

Site specific transposition of Tn7 into a Rhizobium meliloti megaplasmid

Summary Transposon Tn7 was shown to insert specifically into the megaplasmid of different Rhizobium meliloti strains. Tn7 transposition could not be detected in other Rhizobium strains such as R. trifolii, R. leguminosarum, R. phaseoli and R. japonicum. In R. meliloti strains, two unique sites in the megaplasmid were observed into which Tn7 can transpose at different frequencies. Only one copy of Tn7 could be detected in the megaplasmid and the insertion sites for Tn7 are outside the nif and nod region. Tn7 transposition in R. meliloti showed a marked preference for sites on plasmid RP4 compared to the megaplasmid sites. Attempts to cure Tn7 from the megaplasmid were unsuccessful. This site specific transposition of Tn7 in R. meliloti provides an additional genetic tool to further manipulate this important plasmid in symbiotic nitrogen fixation.     

Fertilization regimes affect the soil biological characteristics of a sudangrass and ryegrass rotation system

The sudangrass (Sorghum sudanense) and ryegrass (Lolium multiflorum L.) rotation is an intensive and new cropping system in Central China. Nutrient management practices in this rotation system may influence soil fertility, the important aspects of which are soil biological properties and quality. As sensitive soil biological properties and quality indicators, soil microbial community activity, microbial biomass, enzyme activities, soil organic matter (SOM) and total N resulting from different fertilization regimes in this rotation system were studied through a four-year field experiment from April 2005 to May 2009. Treatments included control (CK), fertilizer phosphorus and potassium (PK), fertilizer nitrogen and potassium (NK), fertilizer nitrogen and phosphorus (NP) and a fertilizer nitrogen, phosphorus and potassium combination (NPK). Soil microbial community activities in the NK, NP and NPK treatments were significantly lower than those in the CK and PK treatments after the sudangrass and ryegrass trial. The highest microbial biomass C, microbial biomass N, SOM, total N, sucrase and urease activities were found in the NPK treatment, and these soil quality indicators were significantly higher in the NK, NP and NPK treatments than in the PK and CK treatments. Soil microbial biomass and enzyme activities were positively associated with SOM in the sudangrass and ryegrass rotation system, indicating that fertilization regimes, especially N application, reduced microbial community activity in the soil. Proper fertilization regimes will increase microbial biomass, enzyme activity and SOM and improve soil fertility.     

Response ofPhaseolus vulgaris to inoculation withRhizobium phaseoli under two tillage systems in the dominican republic

Summary Field experiments were established at three locations in the Dominican Republic to evaluate the response of field beans (Phaseolus vulgaris L. cv. Pompadour) to inoculation with selected strains ofRhizobium phaseoli. A comparison of no-tillage and conventional tillage was included to determine whether modification of rhizosphere temperature and moisture would influence the inoculation response. Yields of inoculated treatments were not statistically difference from controls and ranged from 30–80% of those obtained on N-fertilized plots. Neither tillage systems nor P fertilization influence the response to inoculation. Serological investigations indicated that the applied rhizobial strains nodulated successfully. This host/endophyte combination appears to form an ineffective symbiotic association. Inoculation trials employing indigenous and commercial strains compared with N fertilizer in the greenhouse support the conclusion from the field studies that symbiotic N₂ fixation can provide only a fraction of the plant's N requirement.     

绿肥配施氮肥对岩溶区稻田土壤微生物群落的影响

绿肥参与耕作改制是土壤培肥及作物增产的有效措施,对土壤微生物群落结构及多样性的影响至关重要。【目的】研究绿肥配施氮肥对岩溶区稻田土壤微生物群落结构的影响,阐明微生物、土壤生态环境因子及作物产量的相互关系,为岩溶稻区绿肥替代氮肥提供理论依据和数据支撑。【方法】以典型岩溶稻田土壤为研究对象,设置冬闲+不施氮肥(CK)、冬闲+氮肥(N)、绿肥+不施氮肥(M)、绿肥+氮肥(MN) 4个处理,通过3年田间定位试验,对土壤微生物进行高通量测序,解析不同施肥处理对细菌和真菌群落的影响。【结果】与CK相比,MN处理显著提高了早稻产量,提升了土壤有机质、全氮、碱解氮和速效钾含量,降低了速效磷含量。MN处理显著提高细菌群落丰富度及多样性,而真菌群落丰富度和多样性在MN处理有降低趋势。岩溶稻田土壤优势细菌类群主要为Chloroflexi、Proteobacteria和Acidobacteria等,优势真菌类群主要为Ascomycota、Basidiomycota和Zygomycota等。冗余分析(RDA)结果表明,土壤速效钾是影响土壤细菌群落组成的关键环境因子。共现网络分析结果表明,细菌-真菌群落交互关系主...     

Field experiments on nitrogen fixation by nodulated legumes

Summary Phosphate increased nitrogen uptake by lucerne appreciably on a saline soil. Nitrogenous fertiliser or inoculation with an effective strain ofRhizobium meliloti did not increase the yield significantly. In soils where indigenousRhizobium japonicum was absent inoculation increased soybean yields and the additional fixed nitrogen removed by soybeans amounted to 40 to 120 kg ha⁻¹. Gram and groundnut also responded to Rhizobium inoculation in field trials.     

Estimation of host-strain compatibility for symbiotic N-fixation between Rhizobium meliloti,several annual species of Medicago and Medicago sativa

Standard methods for estimating host-strain compatibility for N-fixation in vitro proved inadequate for examination of the symbiosis between Rhizobium meliloti and Medicago sativa, M. littoralis, M. tornataas well as for hybrids between M. littoralisand M. truncatula. When screening procedures in soil free of background R. melilotiwere adopted, nitrogen fixation appeared to be unrestricted by the screening environment. During the experiments it became clear that the single commercial inoculant strain available in Australia (CC169) was poorly effective at N-fixation with many of the lucerne and annual medic cultivars examined. Interspecific hybridisation between M. truncatulaand M. littoralis also produced genotypes that were particularly difficult to satisfy in a symbiotic sense. Despite the existence of substantial host-strain genotype interactions between the symbiotic partners, several alternative strains of R. meliloti proved to be broadly effective at N-fixation. One of these, WSM826, has since become commercially available in Australia.     

Growth and uptake of nitrogen by wheat and ryegrass in fumigated and irradiated soil

Summary Wheat and ryegrass were grown in pots containing soil that had either been irradiated, fumigated with methyl bromide, fumigated with formaldehyde, or left untreated. All pots received a basal dressing of potassium, phosphorus and magnesium; response to nitrogen was tested by applying either 0, 0.177 or 0.354 g nitrogen per pot. Irradiation increased the growth of wheat and ryegrass; uptake of nitrogen was also increased in both crops. The amount of fertilizer nitrogen equivalent to the nitrogen supplied by seeds and soil (the “N value”) can be calculated from the efficiency of uptake of fertilizer nitrogen and used to allow for the effect on crop growth of the nitrogen released by irradiated soil. With wheat the increase in growth can be attributed solely to the extra mineral nitrogen released by irradiated soil. However, ryegrass grew a little better than would have been expected if the only effect of irradiation was to increase the release of soil nitrogen. Fumigation with methyl bromide or formaldehyde increased the growth of wheat and ryegrass not given fertilizer nitrogen. However, fumigation with methyl bromide left ionic bromide in the soil, and this depressed the growth of wheat receiving fertilizer nitrogen. Formaldehyde also left residues; these influenced soil metabolism and sometimes depressed the growth of plants given fertilizer nitrogen.     

Potential role of rhizobia to enhance chickpea-growth and yield in low fertility-soils of Tunisia

Plant growth-promoting rhizobacteria are bacteria that improve plant growth and reduce plant pathogen damages. In this study, 100 nodule bacteria were isolated from chickpea, screened for their plant growth-promoting (PGP) traits and then characterised by PCR-RFLP of 16 S rDNA. Results showed that most of the slow-growing isolates fixed nitrogen but those exhibiting fast-growth did not. Fourteen isolates solubilized inorganic phosphorus, 16 strains produced siderophores, and 17 strains produced indole acetic acid. Co-culture experiments identified three strains having an inhibitory effect against Fusarium oxysporum, the primary pathogenic fungus for chickpea in Tunisia. Rhizobia with PGP traits were assigned to Mesorhizobium ciceri, Mesorhizobium mediterraneum, Sinorhizobium meliloti and Agrobacterium tumefaciens. We noted that PGP activities were differentially distributed between M. ciceri and M. mediterraneum. The region of Mateur in northern Tunisia, with clay–silty soil, was the origin of 53% of PGP isolates. Interestingly, we found that S. meliloti and A. tumefaciens strains did not behave as parasitic nodule-bacteria but as PGP rhizobacteria useful for chickpea nutrition and health. In fact, S. meliloti strains could solubilize phosphorus, produce siderophore and auxin. The A. tumefaciens strains could perform the previous PGP traits and inhibit pathogen growth also. Finally, one candidate strain of M. ciceri (LL10)—selected for its highest symbiotic nitrogen fixation and phosphorus solubilization—was used for field experiment. The LL10 inoculation increased grain yield more than three-fold. These finding showed the potential role of rhizobia to be used as biofertilizers and biopesticides, representing low-cost and environment-friendly inputs for sustainable agriculture.

     

Effect of inoculation and nitrogen application on the elemental composition ofMedicago sativa

Medicato sativa plants were grown with and without inoculation with an effectiveR. meliloti strain on nitrogenfree agar slopes. Half of the uninoculated replications were, furthermore, provided with a sufficient amount of the NH₄NO₃ fertilizer, the fertilizer-treated plants (+N plants) achieved dry matter yield equal to that of inoculated variants. The concentration and uptake of Cl, Mn, Cu, Zn, Br, Sr and Mo were found to be significantly lower in inoculated plants relative to the +N plants indicating that these composition alterations are constitutive to the induced biological nitrogen fixation.     

Effect of inoculation ofAzospirillum spp. on nitrogen accumulation by field-grown wheat

Summary Two experiments were performed to examine the effects of inoculation of field grown wheat with various Azospirillum strains. In the first experiment the soil was sterilized with methyl bromide to reduce the Azospirillum population and¹⁵N labelled fertilizer was added to all treatments. Two strains ofAzospirillum brasilense isolated from surface sterilized wheat roots and theA. brasilense type strain Sp7 all produced similar increases in grain yield and N content. From the¹⁵N and acetylene reduction data it was apparent that these increases were not due to N₂ fixation. In the second experiment performed in the same (unsterilized) soil, twoA. brasilense strains (Sp245, Sp246) and oneA. amazonense strain (Am YTr), all isolated from wheat roots, produced responses of dry matter and N content while the response to the strain Sp7 was much smaller. These data confirm earlier results which indicate that if natural Azospirillum populations in the soil are high (the normal situation under Brazilian conditions), strains which are isolated from wheat roots are better able to produce inoculation responses than strains isolated from other sources. The inoculation of a nitrate reductase negative mutant of the strain Sp245 produced only a very small inoculation response in wheat. This suggests that the much greater inoculation response of the original strain was not due to N₂ fixation but to an increased nitrate assimilation due to the nitrate reductase activity of the bacteria in the roots. Consultant Inter-American Institute for Cooperation in Agriculture IICA/EMBRAPA World Bank Project.     

Nodulation and growth ofLeucaena leucocephala (Lam.) de Wit as affected by inoculation and N fertilizer

Leonard jar, pot and field experiments examined the effects of inoculation and the influence of nitrogen fertilizer on nodulation, nitrogen fixation and growth ofLeucaena leucocephala (Lam.) de Wit at IITA, Ibadan, Nigeria. Leucaena responded to both inoculation and/or nitrogen application. Shoot growth and total N and P of inoculated plants were comparable to those of the highest N treatment, and the values were about 55% greater than those of uninoculated ones. Field data indicated that toal N yields of inoculated leucaena were increased by 50% with 40 or 80 kg ha^–1 of N fertilizer. However, N fertilizer depressed N fixation by 56% as was expected from nodule mass data. N-fixation was delayed for about 8 weeks in the plots without N. Application of small amounts of N starter (20 ppm) proved to be beneficial to satisfy the plant need during the early stage of leucaena growth. The rhizobial strains IRc 1045 and IRc 1050 were effective, competitive and survived well in the field one year after their establishment.     

Arbuscular mycorrhizal fungus enhances P acquisition of wheat (Triticum aestivum L.) in a sandy loam soil with long-term inorganic fertilization regime

The P efficiency, crop yield, and response of wheat to arbuscular mycorrhizal fungus (AMF) Glomus caledonium were tested in an experimental field with long-term (19 years) fertilizer management. The experiment included five fertilizer treatments: organic amendment (OA), half organic amendment plus half mineral fertilizer (1/2 OM), mineral fertilizer NPK, mineral fertilizer NK, and the control (without fertilization). AMF inoculation responsiveness (MIR) of wheat plants at acquiring P were estimated by comparing plants grown in unsterilized soil inoculated with G. caledonium and in untreated soil containing indigenous AMF. Without AMF inoculation, higher crop yields but lower colonization rates were observed in the NPK and two OA-inputted treatments, and NPK had significantly (P < 0.05) lower impacts on organic C and available P in soils and thereby P acquisition of wheat plants compared with OA and 1/2 OM. G. caledonium inoculation significantly (P < 0.05) increased colonization rates with the NPK and two P-deficient treatments but significantly (P < 0.05) increased vegetative biomass, crop yield, and P acquisition of wheat as well as soil alkaline phosphatase (ALP) activity, only with the NPK treatment. This gave an MIR of ca. 45% on total P acquisition of wheat plants. There were no other remarkable MIRs. It suggested that the MIR is determined by soil available P status, and rational combination of AMF with chemical NPK fertilizer can compensate for organic amendments by improving P-acquisition efficiency in arable soils.     

Estimating crop N uptake from organic residues using a new approach to the 15N isotope dilution technique

Experiments were conducted to test a new approach to the ¹⁵N isotope dilution technique for estimating crop N uptake from organic inputs. Soils were pre-labelled with ¹⁵N fertiliser and a carbon source. These were then incubated until there was stabilisation of the ¹⁵N abundance of the inorganic N pool and resumption of inorganic N concentrations. Residues were then applied to the soils and planted with ryegrass (Lolium perenneL.) to determine the nitrogen derived from the residue (Ndfr) using the isotope dilution equations. This method was compared with the direct method, i.e. where ¹⁵N-labelled residues were added to the soil and Ndfr in the ryegrass calculated directly. Estimates of percentage nitrogen derived from the residue (%Ndfr) alfalfa (Medicago sativaL.) in the ryegrass, were similar, 22 and 23% for the direct and soil pre-labelling methods, respectively, in the Wechsel sandy loam. Also, estimates of the %Ndfr from soybean (Glycine max (L.) Merr) residues in the Krumbach sandy loam were similar 34% (direct) and 36% (soil pre-labelling approach). However, in the Seibersdorf clay loam, the %Ndfr from soybean was 49% using the direct method and 61% using the soil pre-labelling method; yet Ndfr from common bean residue was 46% using the direct approach and 40% using the pre-labelling, not significantly different (P > 0.05). The soil pre-labelling approach appears to give realistic values for Ndfr. It was not possible to obtain an estimate of Ndfr using the soil pre-labelling method from the maize residues (Zea mays L.) in two of the soils, as there was no increase in the total N of the ryegrass over the growing period. This was probably due to microbial immobilisation of inorganic N, as a result of the wide C:N ratio of the residue added. The results suggest that the new soil pre-labelling method is feasible and that it is a potentially useful technique for measuring N release from a wide range or organic residues, but it requires further field-testing. This revised version was published online in June 2006 with corrections to the Cover Date.     

Strain selection for pigeon pea Rhizobium under greenhouse conditions

Pigeon pea obtains N for growth by N₂ fixation although yield generally is not improved by either the inoculation of Rhizobium or by the application of N fertilizer in Puerto Rico. Sixteen strains ofRhizobium spp., different in geographical origin, were tested for N-fixing effectiveness, determined from comparisons with uninoculated controls, N controls and the standard strain 176A22. Inoculated treatments showed significant differences in nodulation, plant dry weight and %N. Several strain x plant combinations had higher N content than the N treatment, reflecting the ubiquity of effective strains and the possible lack of response of pigeon pea to inoculation or N fertilization. Strains superior in N₂ fixation were selected for testing for symbiotic effectiveness under field conditions.     

Co-inoculation with antibiotic-producing bacteria to increase colonization and nodulation by rhizobia

A study was conducted to determine whether colonization of legume roots and nodulation byRhizobium meliloti andBradyrhizobium japonicum could be enhanced by using inocula containing microorganisms that produce antibiotics suppressing soil or rhizosphere inhabitants but not the root-nodule bacteria. An antibiotic-producing strain of Pseudomonas and one of Bacillus were isolated, and mutants ofR. meliloti andB. japonicum sp. resistant to the antibiotics were used. The colonization of the alfalfa rhizosphere and nodulation byR. meliloti were enhanced by inoculation of soil withPseudomonas sp. in soil initially containing 2.7×10⁵ R. meliloti per g. The colonization of soybean roots byB. japonicum was enhanced by inoculating soil with three cell densities ofBacillus sp., and nodulation was stimulated byBacillus sp. added at two cell densities. In some tests, the dry weights of soybeans and seed yield increased as a result of these treatments, and co-inoculation with Bacillus also increased pod formation. Inoculation of seeds withBacillus sp. and the root-nodule bacterium enhanced nodulation of soybeans and alfalfa, but colonization byB. japonicum andR. meliloti was stimulated only during the early period of plant growth. Studies were also conducted withStreptomyces griseus and isolates ofR. meliloti andB. japonicum resistant to products of the actinomycete. Nodulation of alfalfa byR. meliloti was little or not affected by the actinomycete alone; however, both nodulation and colonization were enhanced if the soil was initially amended with chitin andS. griseus was also added. Chitin itself did not affectR. meliloti. Treatments of seeds with chitin orS. griseus alone did not enhance colonization of alfalfa roots byR. meliloti or soybean roots byB. japonicum, but the early colonization of the roots by both bacterial species was promoted if the seeds received both chitin andS. griseus; this treatment also increased nodulation and dry weights of alfalfa and soybeans and the N content of alfalfa. It is suggested that co-inoculation of legumes with antibiotic-producing microorganisms and root-nodule bacteria resistant to those antibiotics is a promising means of promoting nodulation and possibly nitrogen fixation.     

Soil acidity in relation to groundnut-Bradyrhizobium symbiotic performance

Effects of soil acidity on groundnut-Bradyrhizobium symbiotic performance were studied in a potted, sandy soil in a glasshouse in Zimbabwe. The soil was limed to soil-pH levels of 5.0 and 6.5. Soil acidity negatively affected plant development, measured as leaf area and plant dry weight, while nodulation was enhanced. This acidity-enhanced nodulation was most evident when nodulation was caused by the indigenousBradyrhizobium population. Effects of soil acidity differed between groundnut cultivars andBradyrhizobium spp. strains, the former having greater importance. TwoArachis hypogaea L. Spanish-type cultivars, Falcon and Plover, performed equally well at neutral soil pH, but Falcon was more acid tolerant. Comparison of the symbiotic performance in neutral versus acid soil of twoBradyrhizobium spp. strains, MAR 411 (3G4b20) and MAR 1510 (CB 756), showed that MAR 411 performed superiorly in neutral soil, but MAR 1510 in acid soil. The indigenousBradyrhizobium population was more effective than was inoculation with strains MAR 411 or MAR 1510. Comparison of twelveBradyrhizobium spp. strains for their symbiotic performance in acid soil showed that some strains were totally ineffective under acidity stress (MAR 253, MAR 967 and MAR 1506), while others performed well.Bradyrhizobium spp. strain MAR 1576 (32 H1) ranked highest for nitrogen accumulation, plant dry weight and leaf area, with strains MAR 1555 (TAL 11) and MAR 1510 following closely. Nitrate fertilisation of groundnut plants led to soil alkalinisation, while nitrogen fixation resulted in soil acidification. Soil acidity in combination with soil sterilisation gave rise to symptoms associated with Al and Mn toxicity.     

Ammonia assimilation and nitrogen fixation in Rhizobium meliloti

Summary The enzymes involved in ammonia assimilation by Rhizobium meliloti 4l and their role in the regulation of nitrogen metabolism were studied. Glutamine synthetase (GS) and glutamate synthase (GOGAT) were present at relatively high levels in cells grown in media containing either low or high concentrations of ammonia. NADP-linked glutamate dehydrogenase could not be detected.GOGAT and GS mutants were isolated and characterised. A mutant lacking GOGAT activity did not grow even on high concentrations of ammonia, it was a glutamate auxotroph and was effective in symbiotic nitrogen fixation. The GS and assimilatory nitrate reductase activities of this mutant were not repressible by ammonia but still repressible by casamino acids. A mutant with low GS activity required glutamine for optimal growth. It was ineffective and its nitrate reductase was not inducible.These findings indicate that ammonia is assimilated via the GS/GOGAT pathway in free-living R. meliloti and bacterial GOGAT is not important in symbiosis. Furthermore, GS is suggested to be a controlling element in the nitrogen metabolism of R. meliloti.