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Rahnella aquatilis, a nitrogen-fixing enteric bacterium associated with the rhizosphere of wheat and maize
Abstract
In a study of dominant diazotrophic bacteria present in the rhizosphere of wheat and maize, 28 strains of Enterobacteriaceae were isolated. They were all Voges-Proskauer positive, motile at 28 °C but not at 37 °C, and they produced a Tween-80 esterase and did not exhibit decarboxylase activity. This fits well with the description of Rahnella aquatilis. The ability of these strains to reduce acetylene in pure culture and in association with their host plant and the DNA hybridization with a nifHDK probe are described. This is the first time that R. aquatilis is reported as a rhizosphere-associated bacterium and also a nitrogen fixer. Key words: Rahnella aquatilis, rhizosphere, wheat, maize, nitrogen fixation.
... Rahnella aquatilis was first studied and described in 1976 by the Pasteur Institute. Rahnella belongs to the Yersiniaceae family and is a Gram-negative, rod-shaped, fast-moving microaerophilic bacterium that is common in various environments (Adeolu, 1998), i.e. in soil, plant phyllosphere, water and food (Berge et al., 1991;Rhodes et al., 1998). There are currently 6 species of bacteria belonging to the Rahnella genus, namely R. aquatilis, R. variigena, R. inusitata, R. bruchi, R. woolbedingensis and R. victoriana. ...
... Of them, isolates UT3, UT4 and UT9 showed high acetylene-reducing activity (79-91 nmol C 2 H 4 /flacon/24h). Berge et al. (1991) reported the R. aquatilis CF1, CF3, CF4, TR01, TR02, and TR03 rhizospheric strains exerted acetylene reductase activity measuring 60-306 nmol С 2 Н 4 /tube (Berge et al., 1991). It should be noted that those strains of rhizobacteria also showed growth on the Ashby's nitro-gen-free nutrient medium, which means that their nitrogen-fixing properties have been confirmed once again. ...
... Of them, isolates UT3, UT4 and UT9 showed high acetylene-reducing activity (79-91 nmol C 2 H 4 /flacon/24h). Berge et al. (1991) reported the R. aquatilis CF1, CF3, CF4, TR01, TR02, and TR03 rhizospheric strains exerted acetylene reductase activity measuring 60-306 nmol С 2 Н 4 /tube (Berge et al., 1991). It should be noted that those strains of rhizobacteria also showed growth on the Ashby's nitro-gen-free nutrient medium, which means that their nitrogen-fixing properties have been confirmed once again. ...
As the number of people on earth increases, so does the need for food. Providing the population with environmentally friendly agricultural food is one of the urgent problems of our time. Currently, the main direction of modern organic farming is the use of biofertilizers. Bacterial preparations are capable of influencing the physiological processes of plants in small quantities, leading to increase in plant productivity. The objective of this work was studying rhizobac-teria associated with wheat roots. For this purpose, more than 100 isolates of rhizobacteria from the rhizosphere and root surface of wheat plants grown in irrigated fields of Tashkent, Syrdarya, Andijan, Kashkadarya regions. Rhizobac-teria were grown on nutrient media of Döbereiner, Ashby, Pikovsky, and Zack, and 25 isolates of associative rhizobac-teria were selected based on the characteristics of absorption of molecular nitrogen, mobilization of phosphorus and potassium. They actively dissolved Сa 3 (PO 4) 2 and KAlSiO 4 for 3 days. They were found to produce organic acids. In organic farming, nitrogen-fixing, phosphorus-and potassium-mobilizing rhizobacteria are of great practical importance , while our experiments on obtaining biological products are considered as an environmentally friendly and cost-effective way to increase crop yields. From the surface of wheat roots grown in different zones of Uzbekistan, when screening for nitrogen fixation, we selected 3 isolates with acetylene reductase activity of 79-91 nmol C 2 H 4 /flacon/24h. We determined that bacteria completely mobilized phosphate, forming 100% acid when grown in a medium containing Ca 3 (PO 4) 2 for 5 days. The ability of the bacteria to mobilize potassium was studied on a nutrient KAlSiO 4-containing medium. The bacteria were observed to mobilize potassium, forming 90-100% acid within 15 days. Based on the study of the 16S rRNA gene of bacteria, we identified rhizobacteria UT3, UT4, and UT9 as Rah-nella aquatilis.
... Rahnella was a Gram-negative, rod-shaped, facultatively anaerobic bacterium belonging to the family Yersiniaceae (Adeolu et al., 2016), and was widely distributed in a variety of environments, including soil, phyllosphere, water, seeds, food, some clinical samples, and even in American mastodon remains (Berge et al., 1991;Rhodes et al., 1998). Rahnella was first proposed in 1979, and normally consisted of six species including R. aquatilis (which was described as the type species of Rahnella), R. variigena, R. inusitata, R. bruchi, R. woolbedingensis, and R. victoriana by multilocus sequence analysis (Brady et al., 2014). ...
... Rahnella spp. were also reported to possess the capacity for nitrogen fixation (Berge et al., 1991). For, insurance, R. aquatilis HX2 was reported as a PGPR and could promote the growth of corn for the ability to fix nitrogen (Guo et al., 2012). ...
Many Rahnella strains have been widely described as plant growth-promoting rhizobacteria with the potential to benefit plant growth and protect plants from pathogens. R. aceris ZF458 is a beneficial plant bacterium isolated from swamp soil with the potential for biocontrol. Strain ZF458 has shown broad-spectrum antagonistic activities against a variety of plant pathogens and exhibited a dramatic effect on controlling Agrobacterium tumefaciens in sunflowers. The R. aceris ZF458 genome sequence contained a 4,861,340-bp circular chromosome and two plasmids, with an average G + C content of 52.20%. Phylogenetic analysis demonstrated that R. aceris ZF458 was closely related to R. aceris SAP-19. Genome annotation and comparative genomics identified the conservation and specificity of large numbers of genes associated with nitrogen fixation, plant growth hormone production, organic acid biosynthesis and pyrroloquinoline quinone production that specific to benefiting plants in strain ZF458. In addition, numerous conserved genes associated with environmental adaption, including the bacterial secretion system, selenium metabolism, two-component system, flagella biosynthesis, chemotaxis, and acid resistance, were also identified in the ZF458 genome. Overall, this was the first study to systematically analyze the genes linked with plant growth promotion and environmental adaption in R. aceris. The aim of this study was to derive genomic information that would provide an in-depth insight of the mechanisms of plant growth-promoting rhizobacteria, and could be further exploited to improve the application of R. aceris ZF458 in the agriculture field.
... These eight species contributed to the existing diversity of Rahnella with isolations of Rahnella victoriana, Rahnella variigena and Rahnella inusitata from bleeding cankers of oak; R. victoriana, R. variigena and R. woolbedingensis from asymptomatic alder and walnut; Rahnella bruchi from the gut of the Agrilus biguttatus beetle; Rahnella aceris and Rahnella laticis from sap of Acer pictum and Rahnella contaminans as a contaminant from MRSA agar plates [5][6][7]. In addition to their isolation from the natural environment, Rahnella species have been linked to nitrogen-fixation [8], metal and radionuclide sequestration [9] and biological control [10]; and more recently as possible pathogens of oak [11], poplar [12] and onion [13]. ...
Bacteria isolated from onion bulbs suffering from bacterial decay in the United States and Norway were previously shown to belong to the genus Rahnella based on partial housekeeping gene sequences and/or fatty acid analysis. However, many strains could not be assigned to any existing Rahnella species. Additionally, strains isolated from creek water and oak as well as a strain with bioremediation properties were assigned to Rahnella based on partial housekeeping gene sequences. The taxonomic status of these 21 strains was investigated using multilocus sequence analysis, whole genome analyses, phenotypic assays and fatty acid analysis. Phylogenetic and phylogenomic analyses separated the strains into five clusters, one of which corresponded to Rahnella aceris . The remaining four clusters could be differentiated both genotypically and phenotypically from each other and existing Rahnella species. Based on these results, we propose the description of four novel species: Rahnella perminowiae sp. nov. (type strain SL6 T =LMG 32257 T =DSM 112609 T ), Rahnella bonaserana sp. nov. (H11b T =LMG 32256 T =DSM 112610 T ), Rahnella rivi sp. nov. (FC061912-K T =LMG 32259 T =DSM 112611 T ) and Rahnella ecdela sp. nov. (FRB 231 T =LMG 32255 T =DSM 112612 T ).
... IAA plays an important role in the physiological processes of plants, such as cell division (Patten and Glick 2002;Rangarajan et al. 2003;Yang et al. 2009). Rahnella aquatilis, isolated from drinking water and the rhizosphere of different plants (Brenner et al. 1998;Berge et al. 1991), is one of the plant-growth-promoting rhizobacteria (PGPR) (Beazley et al. 2009;Valverde et al. 2011). The main purposes of this study were to investigate (1) the effects of R. aquatilis on the As accumulation of V. natans and (2) the role of R. aquatilis in resistance of V. natans to arsenic stress. ...
Vallisneria natans (Lour.) Hara is a suitable submerged plant for the phytoremediation of As-contaminated water. Rahnella aquatilis is one of the plant growth–promoting rhizobacteria. Influences of R. aquatilis on the arsenic accumulation and detoxification of V. natans were investigated. The results showed that As accumulation by V. natans could be significantly improved after R. aquatilis inoculated at the lower level of As (< 2 mg/L). At 0.5, 1, and 2 mg/L As levels, the As concentrations of V. natans with R. aquatilis were respectively 100.40%, 57.96%, and 22.62% higher than that of V. natans with no R. aquatilis. The concentration of As in V. natans was increased with the increasing the As concentration up to 1 mg/L, but it was decreased at 2 mg/L As. The correlation analysis showed that the As accumulated in the plant was positive correlated (R² = 0.977, p < 0.01) with indole-3-acetic acid (IAA) produced by R. aquatilis under different As levels. IAA may be the major factor affecting the As accumulation of V. natans. The results of malondialdehyde and superoxide dismutase, hydrogen peroxidase, and ascorbate peroxidase indicated that IAA produced by R. aquatilis had alleviated the arsenic stress on V. natans. The synthesis of IAA by R. aquatilis was related to the As levels. When the As was at 2 mg/L, the IAA that produced by R. aquatilis decreased and the promotion of R. aquatilis on As accumulation by V. natans reduced. However, R. aquatilis has a positive influence on the arsenic accumulation by V. natans at the lower As levels (< 2 mg/L), and it may be a potentially useful way to improve the removal of arsenic from contaminated water using submerged plants.
... Second, we saw enrichment of nitrogen-fixing Rahnella in CIT rhizospheres. Rahnella species are known to form associations with plant roots, promote plant growth, and have been found with other diazotrophs in nitrogen-fixing root nodules [23,24]. Given the low levels of nitrate and ammonia in our aquaponic systems, Rahnella strains may have promoted L. sativa growth. ...
The demand for food will outpace productivity of conventional agriculture due to projected growth of the human population, concomitant with shrinkage of arable land, increasing scarcity of freshwater, and a rapidly changing climate. While aquaponics has potential to sustainably supplement food production with minimal environmental impact, there is a need to better characterize the complex interplay between the various components (fish, plant, microbiome) of these systems to optimize scale up and productivity. Here, we investigated how the commonly-implemented practice of continued microbial community transfer from pre-existing systems might promote or impede productivity of aquaponics. Specifically, we monitored plant growth phenotypes, water chemistry, and microbiome composition of rhizospheres, biofilters, and fish feces over 61-days of lettuce ( Lactuca sativa var . crispa) growth in nitrogen-limited aquaponic systems inoculated with bacteria that were either commercially sourced or originating from a pre-existing aquaponic system. Lettuce above- and below-ground growth were significantly reduced across replicates treated with a pre-existing aquaponic system inoculum when compared to replicates treated with a commercial inoculum. Reduced productivity was associated with enrichment in specific bacterial genera in plant roots, including Pseudomonas , following inoculum transfer from pre-existing systems. Increased productivity was associated with enrichment of nitrogen-fixing Rahnella in roots of plants treated with the commercial inoculum. Thus, we show that inoculation from a pre-existing system, rather than from a commercial inoculum, is associated with lower yields. Further work will be necessary to test the putative mechanisms involved.
... Nitrogenase activity (nitrogen fixation) was also found positive and 60-306 nmol C 2 H 4 /tube (measured through acetylene reduction assay) was produced in Rahnella sp. strain CF1, CF3, CF4, TRO1, TRO2, and TRO3, and 190 nmol C 2 H 4 /tube was produced in R aquatilis type strain ATCC 33071 (Berge et al., 1991). The bacterium Rahnella aquatilis strain HX2 showed activity of plant growth promotion and acted as a natural bio-control agent as it was reported to suppress the sunflower crown gall disease caused by Agrobacterium vitis, Agrobacterium rhizogenes, and Agrobacterium tumefaciens. ...
Profenofos (PFF) is an organophosphate insecticide and used worldwide to control harmful pests and insect populations. Removal of PFF from the environment is important because of its high mammalian acute toxicity due to acetylcholinesterase enzyme inhibition. Biodegradation in this context is beneficial as it is a cost-effective and sustainable process. In the present study, microbial degradation of PFF was investigated by using a psychrotolerant bacterium Rahnella sp. PFF2. The bacterial strain was isolated from the soil samples of apple orchards situated in Kufri, Himachal Pradesh, India. Quantitative analysis through High-performance liquid chromatography revealed that the bacterium PFF2 was able to degrade 100% PFF (50 μg/ml) within 14 days at 20 °C, within 16 days at 28 °C, and within 20 days at 15 °C. GC/MS and HPLC studies showed the presence of pathway metabolites 4-Bromo-2-chlorophenol, phosphoric acid, and 3, 4 – dimethyl benzoic acid. Based on these data a probable PFF degradation pathway has been proposed. An inducible and intracellular organophosphorus hydrolase enzyme might responsible for the initial degradation process. To the best of our knowledge, the current finding is the first report of PFF degradation at both the psychrophilic and mesophilic temperature conditions by any psychrotolerant Rahnella sp. isolated from Western Himalayan regions.
... The gram-negative rhizobacterium Rahnella aquatilis was reported to promote plant growth and to act as a biocontrol agent on different crops 21,22 . In a previous study, co-inoculation of chickpea plants with the fungal pathogen F. oxysporum and R. aquatilis isolate Ra36 resulted in efficient protection from vascular wilt disease 23 , although the mode of action is currently unknown. ...
Soil-inhabiting fungal pathogens use chemical signals released by roots to direct hyphal growth towards the host plant. Whether other soil microorganisms exploit this capacity for their own benefit is currently unknown. Here we show that the endophytic rhizobacterium Rahnella aquatilis locates hyphae of the root-infecting fungal pathogen Fusarium oxysporum through pH-mediated chemotaxis and uses them as highways to efficiently access and colonize plant roots. Secretion of gluconic acid (GlcA) by R. aquatilis in the rhizosphere leads to acidification and counteracts F. oxysporum-induced alkalinisation, a known virulence mechanism, thereby preventing fungal infection. Genetic abrogation or biochemical inhibition of GlcA-mediated acidification abolished biocontrol activity of R. aquatilis and restored fungal infection. These findings reveal a new way by which bacterial endophytes hijack hyphae of a fungal pathogen in the soil to gain preferential access to plant roots, thereby protecting the host from infection.
... R.aquatilis was recognized long time ago as N 2fixing enteric bacteria isolated from rhizosphere of wheat and maize (Berge et al., 1990). Positive effect of inoculating N 2fixing Pantoea agglomerans strain on rice plant growth and photosynthate allocation was also reported previously (Feng et al., 2006). ...
Methanol oxidizing organisms not only reduce atmospheric methanol
concentration but also play a major role in enhancing methane oxidation through cross-feeding in synergism with methanotrophs. Possession of plant growth promoting (PGP) attributes in methanol oxidizing bacteria will prove to be a worthy candidate for developing novel biofertilizers having dual ability of plant growth promotion and reducing methane emission to a certain extent. We are reporting 11 plant growth promoting methanol oxidizing bacteria belonging to the
genera Rahnella, Serratia, Enterobacter and Pantoea of order Enterobacterales isolated from phyllosphere, rhizosphere and non-rhizosphere of flooded paddies. Among 11 isolates, Rahnella aquatilis ANRf177 showed significantly highest growth in AMS broth having 0.5% methanol as the sole C source. R. aquatilis MaAL105 produced significantly higher IAA and showed maximum solubilization of P and K. Pantoea sp. KAAr216 showed significantly maximum N2 fixation ability and solubilization of Zn salts. These efficient methanol
oxidizing bacteria having PGP attributes can be evaluated under field conditions.
Methane utilizing bacteria (MUB) are known to inhabit the flooded paddy ecosystem where they play an important role in regulating net methane (CH4) emission. We hypothesize that efficient MUB having plant growth-promoting (PGP) attributes can be used for developing novel bio-inoculant for flooded paddy ecosystem which might not only reduce methane emission but also assist in improving the plant growth parameters. Hence, soil and plant samples were collected from the phyllosphere, rhizosphere, and non-rhizosphere of five rice-growing regions of India at the tillering stage and investigated for efficient methane-oxidizing and PGP bacteria. Based on the monooxygenase activity and percent methane utilization on NMS medium with methane as the sole C source, 123 isolates were identified and grouped phylogenetically into 13 bacteria and 2 yeast genera. Among different regions, a significantly higher number of isolates were obtained from lowland flooded paddy ecosystems of Aduthurai (33.33%) followed by Ernakulum (20.33%) and Brahmaputra valley (19.51%) as compared to upland irrigated regions of Gaya (17.07%) and Varanasi (8.94%). Among sub-samples, a significantly higher number of isolates were found inhabiting the phyllosphere (58.54%) followed by non-rhizosphere (25.20%) and rhizosphere (15.45%). Significantly higher utilization of methane and PGP attributes were observed in 30 isolates belonging to genera Hyphomicrobium, Burkholderia, Methylobacterium, Paenibacillus, Pseudomonas, Rahnella, and Meyerozyma. M. oryzae MNL7 showed significantly better growth with 74.33% of CH4 utilization at the rate of 302.9 ± 5.58 and exhibited half-maximal growth rate, Ks of 1.92 ± 0.092 mg CH4 L⁻¹. Besides the ability to utilize CH4, P. polymyxa MaAL70 possessed PGP attributes such as solubilization of P, K, and Zn, fixation of atmospheric N and production of indole acetic acid (IAA). Both these promising isolates can be explored in the future for developing novel biofertilizers for flooded paddies.
Graphic abstract
The type strain Enterobacter agglomerans ATCC 27155 was examined for deoxyribonucleic acid (DNA) relatedness to 54 strains of the 'Erwinia herbicola-Enterobacter agglomerans complex', to 23 strains of other Erwinia and Enterobacter species, and to 50 reference strains of 49 different species in other genera of the family Enterobacteriaceae. The DNA-DNA hybridization values (at the optimal renaturation temperature; nitrocellulose filter method) showed that 24 strains were highly related to Enterobacter agglomerans ATCC 27155(T) (62 to 97% DNA relatedness) and formed a genotypic group provisionally called DNA hybridization group 27155. These strains were received as Enterobacter agglomerans (including strains of DNA hybridization groups V and XIII [D.J. Brenner, G.R. Fanning, J.K. Leete Knutson, A.G. Steigerwalt, and M.I. Krichevsky, Int. J. Syst. Bacteriol. 34:45-55, 1984]); as Erwinia herbicola, including the type strain NCPPB 2971; as Erwinia milletiae, including the type strain NCPPB 2519; and as yellow-pigmented Enterobacter strains. Numerical analysis of the protein electropherograms of these strains revealed the existence of seven protein electrophoretic groups, each showing a characteristic protein pattern. These seven groups separated at an infraspecific level and allowed assignment of 37 additional strains, receivecd as Enterobacter agglomerans, Erwinia herbicola, and Erwinia milletiae, to DNA hybridization group 27155. The resulting group hybridized below 55% DNA relatedness with all remaining phenotypic or genotypic groups of the 'Erwinia herbicola-Enterobacter agglomerans complex' and below 57% DNA relatedness with other Erwinia and Enterobacter species. DNA binding with 49 other species of the family Enterobacteriaceae was less than 38%. Since DNA hybridization group 27155 contains the type strains of Erwinia herbicola, Erwinia milletiae, and Enterobacter agglomerans, these species names are subjective synonyms, and the specific epithet agglomerans has priority. Further genotypic studies with several closely related genera are required for final placement of this species in a genus. The description of Enterobacter agglomerans, Erwinia herbicola, and Erwinia milletiae is emended.
Deoxyribonucleic acid (DNA)-DNA hybridization was performed with 10 strains belonging to the “Erwinia herbicola-Enterobacter agglomerans complex” by using the competition method on nitrocellulose filters. These strains exhibited more than 75% DNA binding to Erwinia herbicola ATCC 14589T (T = type strain) and constitute DNA hybridization group 14589 (including strains ATCC 14589T and CDC 1429-71 from DNA hybridization group III [D. J. Brenner, G. R. Fanning, J. K. Leete Knutson, A. G. Steigerwalt, and M. J. Krichevsky, Int. J. Syst. Bacteriol. 34:45-55, 1984]). The high level of genomic relatedness of these strains was confirmed by the similarities observed in their electrophoretic protein patterns. On the basis of our data, DNA hybridization group 14589 constitutes a discrete species within the family Enterobacteriaceae. Its closest relative is DNA hybridization group 27155 (41 to 53% DNA relatedness), which was previously defined and includes the type strains, among others, of Enterobacter agglomerans, Erwinia herbicola, and Erwinia milletiae (A. Beji, J. Mergaert, F. Gavini, D. Izard. K. Kersters, H. Leclerc, and J. De Ley, Int. J. Syst. Bacteriol. 38:77-88, 1988). We propose to unite DNA hybridization groups 14589 and 27155 in a single genus, Pantoea gen. nov. Pantoea agglomerans (Beijerinck 1888) comb. nov. is proposed to contain most strains of DNA hybridization group 27155 (including DNA hybridization group XIII of Brenner et al.), and its type strain is strain ATCC 27155 (= NCTC 9381 = LMG 1286). Pantoea dispersa sp. nov. is proposed to contain DNA hybridization group 14589, and its type strain is strain ATCC 14589 (= LMG 2603). Descriptions of the genus and its two species are given.
Acetylene-reducing bacteria isolated from the setts (stem cuttings used as seed pieces) and roots of two sugar cane varieties propagated aseptically from stem cuttings were identified using a computer-assisted scheme based on 75 biochemical tests. Because 106 to 108 acetylene-reducing bacteria per gram (fresh weight basis) were found in the roots, while 10 to 100 times fewer were present in the sett, we suggest that the root is the site of bacterial multiplication. Sterilization of the sett surface before planting or root sterilization at harvest reduced or completely removed acetylene-reducing bacteria and associated whole plant acetylene-reducing activity. This indicates that most of the active bacteria were on the sett and root exteriors. Setts did not exhibit acetylene-reducing activity until after emergence of the roots. Since shoot emergence was not necessary for acetylene-reducing activity, the extensive carbohydrate supply of the sett itself must have provided the carbon substrate for bacterial N2 fixation. The acetylene-reducing bacteria isolated were facultative anaerobes of the families Enterobacteriaceae and Bacillaceae. Klebsiella pneumoniae, Enterobacter cloacae, Erwinia herbicola, and Bacillus polymyxa were present inside the sett and the roots but E. herbicola was the dominant bacterium on the root exterior. No Beijerinckia spp. or Azotobacter spp. were found associated with the sett or the roots.
Soil samples from a number of tussock-grassland soils have been examined for the presence of non-symbiotic nitrogen-fixing micro-organisms and estimates made of the contribution by these organisms to the nitrogen status of tussock-grassland soils.Blue–green algae belonging to three genera (Anabaena, Nostoc, and Tolypothrix) and bacteria belonging to seven genera (and Rhodopseudomonas) were isolated, although of these only Clostridium species was found to be ubiquitous. Estimated numbers of these organisms in the soil were generally low.Acetylene-reduction assay of unamended soil samples indicated annual rates of nitrogen fixation equivalent to 0-3 kg N/ha.
A method for comparing the nitrogen-fixing activity of 40 rhizosphere bacteria was carried out using rice plantlets. Due to the large number of strains studied, the comparison was divided into different experiments. However, the same N2-fixing strain was used as a positive control. The comparison of nitrogenase activity (acetylene-reduction assay) of bacterial strains was conducted using the Dunnett's procedure. Using this method, out of the 17 different strains isolated from Bangladesh, Azospirillum lipoferum MRB16 was found to be the most efficient N2-fixing strain in association with rice cv. Nizershail. When a similar method was used to screen 23 N2-fixing strains from Egyptian soils in association with rice cv. Giza 171, A. brasilense NO40 was the most active isolate.
Antisera were prepared against three Rhizobium spp, (Cicer arietinum L.) strains H45, R18 and 46b4. Results obtained by the ELISA technique complement those obtained by immunofluorescence and revealed a broad serological diversity in the chickpea Rhizobium strains used. This serological diversity allowed us to use the immunofluorescence technique for competitiveness studies between an inoculated strain (H45) and native strains. Strain H45 formed all the nodules on plants cultivated on an acidic soil, 48 per cent of Montpellier soil and 11 per cent on Montelimar soil. The inoculum concentration was 5.2 107 bacteria per seed. On the Montpellier soil, the ratio of nodules formed by H45 increased with the inoculum concentration. The competitiveness of strain H45 against native strains of Montpellier soil is highly influenced by the host plant genotype. In each case, no cross reaction were observed between native strains of the soils studied and the antisera prepared against H45.
This study is an attempt to describe the dominant N2-fixing microflora associated with the roots of wetland rice. Rice cultivar Giza 171 was grown in a phytotron on two alluvial Egyptian soils for 8 days, a stage when the nitrogenase activity of undisturbed plants reached a level of 245 10–6 mol C2H4 h–1 g–1 dry weight of leaf. The roots and rhizosphere soils were then used for counting and isolating dominant diazotrophs. Counts and initial enrichment steps were carried out on a selective medium made of an axenic rice plantlet, the spermosphere model, incubated under 1 % acetylene. The counts were very high, exceeding 108 bacteria g–1 dry weight of rhizosphere soil. Enterobacteriaceae were dominant; most isolates were Enterobacter cloacae belonging to different biotypes in the two soils. Enterobacter agglomerans, Citrobacter freundii and Klebsiella planticola were also present as members of the dominant microflora. Azospirillum brasilense and Azospirillum lipoferum were present as well, but less abundant.