Mutual host plant relationships in two groups of legume root nodule bacteria (Rhizobium spp.)

Summary Eighteen strains of Rhizobium lupini were shown to form effectively nitrogen-fixing root nodules in Lotus uliginosus whose rhizobia appeared culturally similar to R. lupini. Six strains formed effective or (in one case) semi-effective nodules in Ornithopus sativus.Evidence has been found for the existence of an extensive cross-inoculation group involving the genera Lupinus, Ornithopus, Lotus, Anthyllis, Caragana, Astragalus, Ononis, Genista, Mimosa and probably others. The rhizobia include fastgrowing as well as slow-growing strains.Dedicated to Professor C. B. Van Niel on the occasion of his 70 th birthday.     

Effect of oxygen availability on nitrogen fixation by two Lotus species under flooded conditions

The pasture legumes Lotus uliginosus (Schk.) and Lotus corniculatus (L.), known to differ in their tolerance to flooding, were inoculated with Rhizobium loti and flooded for 60 d while subjected to two levels of dissolved pO2: 0.241 and 0.094 mol ml^-1. L. uliginosus showed significantly greater growth (shoot and root) and N2 fixation under both pO2s, compared to L. corniculatus, although growth and N2 fixation by L. corniculatus was not affected by the low pO2. Surprisingly, in L. uliginosus., growth, nodulation and N2 fixation were all increased by low pO2 while nodulation of L. corniculatus where low pO2 plants showed a significant increase over that of the higher pO2 plants while L. uliginosus plants showed a decline. Root porosity of L. uliginosus doubled in the low pO2-treatment from a mean of 14.5% in high pO2 roots to 28.5%, whereas that of L. corniculatus was relatively unaffected by pO2, being 7% and 9% for high and low pO2 plants, respectively. The structure of nodules differed little between species and treatments, although nodules/nodulated roots from the L. uliginosus plants had particularly profuse lenticels and aerenchyma. However, L. corniculatus nodules, especially those grown in the lower pO2 showed signs of early senescence with vacuolation of infected cells and green coloration when cut open. Leghaemoglobin (Lb) concentrations in nodules from both species were unaffected by low pO2, although that of L. corniculatus nodules, regardless of pO2, was significantly greater than L. uliginosus. Concentrations of the intercellular glycoprotein recognized by the monoclonal antibody MAC265 were significantly reduced in nodules from the low pO2 treatment in both species. Immunogold labelling showed that the MAC265 antigen was localized primarily within intercellular spaces within nodule cortices from both Lotus species. A marked decrease in deposition of the MAC265 antigen within the cortices of L. uliginosus nodules grown in the lower pO2, is discussed in terms of the relative abilities of the two Lotus spp. to maintain an O2 supply to the N2-fixing bacteroids within submerged nodules.Keywords: Lotus uliginosus, Lotus corniculatus, N2 fixation, flooding, oxygen.      

Effect of plant nutrient supply on nodule effectiveness and rhizobium strain competition for nodulation ofLotus pedunculatus

Summary The effect of nutrient supply on nodule formation and competition between Rhizobium strains for nodulation ofLotus pedunculatus was studied. Limiting plant growth by decreasing the supply of nutrients in an otherwise nitrogen-free medium, increased the size but decreased the number and the nitrogenase activity of nodules formed by a fast-growing strain of Lotus Rhizobium (NZP2037). In contrast decreasing nutrient supply caused only a small decline in the size, number and nitrogenase activity of nodules formed by a slow-growing strain (CC814s). Providing small quantities of NH₄NO₃ (50 to 250 g N) to plants grown with a normal supply of other nutrients stimulated nodule development by both Rhizobium strains and increased the nitrogenase activity of the NZP2037 nodules. Differences in the level of effectiveness (nitrogen-fixing ability) of nodules formed by different Rhizobium strains on plants grown with a normal supply of nutrients were less apparent when the plants were grown with decreased nutrient supply or when the plants were supplied with low levels of inorganic N.Inter-strain competition for nodulation ofL. pedunculatus between the highly effective slow-growing strain CC814s and 7 other fast- and slow-growing strains, showed CC814s to form 42 to 100% of the nodules in all associations. The greater nodulating competitiveness of strain CC814s prevailed despite changes in the nutrient supply to the host plant. A tendency was observed for partially effective Lotus Rhizobium strains to become more competitive in nodule formation when plant growth was supplemented with low levels of inorganic nitrogen.     

Group antigens in slow-growing rhizobia

Summary Internal group antigens of several slow-growing and fast-growing Rhizobium strains were tested by gel-diffusion against antisera to three strains of Rhizobium japonicum. At least one, generally two common antigens were found in 13 strains of R. japonicum, 4 strains of R. lupini, 4 strains isolated from cowpea and two slow-growing strains isolated from Lotus. Forty-six fast-growing rhizobia (including two from Lotus and 4 from Leucaena leucocephala) were clearly distinguished from the slow-growing strains in tests with the same antisera. They were wholly negative (9) or gave a much weaker non-identical line with one antiserum (24 strains), two antisera (8) or three antisera (5). The 5 strains of agrobacteria grouped with the fast-growing rhizobia.     

Genetic diversity and symbiotic compatibility among rhizobial strains and Desmodium incanum and Lotus spp. plants

This work aimed to evaluate the symbiotic compatibility and nodulation efficiency of rhizobia isolated from Desmodium incanum, Lotus corniculatus, L. subbiflorus, L. uliginosus and L. glaber plants by cross-inoculation. Twelve reference strains and 21 native isolates of rhizobia were genetically analyzed by the BOX-PCR technique, which showed a high genetic diversity among the rhizobia studied. The isolates were also characterized based on their production of indolic compounds and siderophores, as well as on their tolerance to salinity. Fifteen of the 33 rhizobia analyzed were able to produce indolic compounds, whereas 13 produced siderophores. All the tested rhizobia were sensitive to high salinity, although some were able to grow in solutions of up to 2% NaCl. Most of the native rhizobia isolated from L. uliginosus were able to induce nodulation in all plant species studied. In a greenhouse experiment using both D. incanum and L. corniculatus plants, the rhizobia isolate UFRGS Lu2 promoted the greatest plant growth. The results demonstrate that there are native rhizobia in the soils of southern Brazil that have low host specificity and are able to induce nodulation and form active nodules in several plant species.     

Characterization of Strains unlike Mesorhizobium loti That Nodulate Lotus spp. in Saline Soils of Granada,Spain

Lotus species are forage legumes with potential as pastures in low-fertility and environmentally constrained soils, owing to their high persistence and yield under those conditions. The aim of this work was the characterization of phenetic and genetic diversity of salt-tolerant bacteria able to establish efficient symbiosis with Lotus spp. A total of 180 isolates able to nodulate Lotus corniculatus and Lotus tenuis from two locations in Granada, Spain, were characterized. Molecular identification of the isolates was performed by repetitive extragenic palindromic PCR (REP-PCR) and 16S rRNA, atpD, and recA gene sequence analyses, showing the presence of bacteria related to different species of the genus Mesorhizobium: Mesorhizobium tarimense/Mesorhizobium tianshanense, Mesorhizobium chacoense/Mesorhizobium albiziae, and the recently described species, Mesorhizobium alhagi. No Mesorhizobium loti-like bacteria were found, although most isolates carried nodC and nifH symbiotic genes closely related to those of M. loti, considered the type species of bacteria nodulating Lotus, and other Lotus rhizobia. A significant portion of the isolates showed both high salt tolerance and good symbiotic performance with L. corniculatus, and many behaved like salt-dependent bacteria, showing faster growth and better symbiotic performance when media were supplemented with Na or Ca salts.Legumes can establish nitrogen-fixing associations with Gram-negative soil bacteria collectively known as rhizobia. Although the symbiotic relationships among rhizobia and many legume species of agricultural importance have been intensively studied, relatively little is known about the symbiotic bacteria of certain plant genera. Lotus is a genus of legumes that includes 125 to 130 species of herbs and small shrubs, mainly distributed in the Northern Hemisphere. Several Lotus species, particularly Lotus corniculatus, Lotus uliginosus, and Lotus tenuis, are used as pasture forage worldwide and are included by phylogenetic studies in the same clade as the model legume Lotus japonicus (4). Until recently, bacteria nodulating Lotus included both intermediate-growing (mesorhizobia) and slow-growing bacteria (12, 16). The mesorhizobia can form effective symbioses with certain Lotus spp. (group I, e.g., L. corniculatus, L. tenuis, or L. japonicus) but form tumor-like structures that do not contain bacteria on L. uliginosus, Lotus subbiflorus, and Lotus angustissimus (group II Lotus spp.) (21, 24). On the other hand, slow-growing strains are usually efficient with Lotus group II species but form no nodules or form inefficient nodules in group I species (12). However, there are rare exceptions to this rule, like strain NZP2037, that can form effective symbioses with both groups of Lotus spp. (23, 25, 28). Furthermore, fast-growing Ensifer meliloti bv. lancerottense strains have been shown to be the symbionts of Lotus lancerottensis but are unable to fix nitrogen with either group I or group II Lotus spp. (19).No apparent relationship exists between the phylogenetic position of Lotus spp. and the type of rhizobia associated. For instance, L. uliginosus and L. angustissimus, which are efficiently nodulated by the bradyrhizobia, are clustered in the same clade as L. corniculatus, L. tenuis, and L. japonicus (clade B) (4), species associated with mesorhizobia. In contrast L. subbiflorus, usually associated with the same rhizobia as L. uliginosus, is clustered in a different clade.The narrow-host-range rhizobia associated with L. corniculatus and other Lotus species were initially classified as Rhizobium loti (13). Later, when the genus Mesorhizobium was created, R. loti was reclassified as Mesorhizobium loti (14), which is considered the type species. Besides the expected differences between the moderate- and the slow-growing Lotus rhizobia, large variabilities in nitrogen-fixing effectiveness (23) as well as in total DNA-DNA hybridization (3, 6) and phylogeny (5, 40) have been shown among the “meso-growing” rhizobia strains classified as M. loti, indicating that they do not form a homogeneous group. Indeed, one of the best-characterized strains of M. loti, strain MAFF303099, has been reclassified as Mesorhizobium huakuii biovar loti (35). In fact, diverse rhizobia have recently been reported to establish symbiosis with Lotus group I species. For instance, bacteria belonging to the newly described species Mesorhizobium gobiense and Mesorhizobium tarimense, were isolated from Lotus frondosus and L. tenuis in China (10). Also, rhizobia assigned to different genera (Rhizobium, Mesorhizobium, Agrobacterium, and Aminobacter) have recently been reported as symbionts of L. tenuis in the Salado River Basin in Argentina (7). While these recent reports indicate that bacteria nodulating Lotus spp. are diverse, their symbiotic genes are rather homogeneous. In fact, most isolates from Argentina and China, regardless their taxonomic assignment, had symbiotic genes closely related to M. loti (7, 10).Soil salinity is a serious and expanding threat to agricultural productivity. Improving crop productivity in saline soils requires selection of well-adapted plant genotypes and, in the case of legumes, highly efficient rhizobial partners adapted to soil conditions. As part of the Euro-South American cooperation project LOTASSA (http://www.lotassa.com/), and aiming to isolate and select for salt-tolerant bacteria able to establish efficient symbiosis with forage Lotus spp., we explored the diversity of Lotus rhizobia in two different locations of Granada province, Spain, where the presence of native Lotus spp. had previously been reported (30).     

Response of Lotus rhizobia to acidity and aluminium in liquid culture and in soil

Measurements of multiplication in liquid culture indicated that fast-growing Lotus rhizobia (Rhizobium loti) were tolerant of acidity and aluminium (at least 50 μM A1 at pH 4.5). Slow-growing Lotus rhizobia (Bradyrhizobium sp. (Lotus)) were less tolerant of acidity but equally tolerant of A1. Both genera were able to nodulateLotus pedunculatus in an acid soil (pH 4.1 in 0.01M CaCl₂) and the slow-growing strains were more effective than the fast-growing strains in this soil over 30 days.     

Selecting improved Lotus nodulating rhizobia to expedite the development of new forage species

Aims

In the past decades the increasing focus by Australian pasture development programs on the genus Lotus has seen the evaluation of many species previously untested in Australia. In field trials, nodulation failure was commonplace. This work was undertaken to select effective symbionts for Lotus to ensure further agronomic evaluation of the genus was not compromised. The symbiotic needs of Lotus ornithopodioides were a particular focus of the studies.

Methods

Glasshouse experiments were undertaken to evaluate symbiotic relationships between 15 Lotus spp and 23 strains of nodulating Mesorhizobium loti. This was followed by evaluation of elite rhizobial strains for their ability to persist and form nodules under field conditions.

Results

Complex symbiotic interactions were recorded between strains of lotus rhizobia and the different species of Lotus. Notably, the rhizobia that are currently provided commercially in Australia for the inoculation of Lotus corniculatus (strain SU343) and Lotus uliginosus (strain CC829) did not form effective symbioses with the promising species L. ornithopodioides and L. maroccanus. No strain we evaluated was compatible with all the Lotus species, however several strains with a broad host range were identified. WSM1293 and WSM1348 were the most effective strains on L. ornithopodioides and L. peregrinus.These strains were also moderately effective on L. corniculatus (79 and 52% of SU343), less effective on L. maroccanus (26 and 49% of SRDI110) but were ineffective on L. uliginosus. The latter species overall had very specific rhizobial needs. Both WSM1293 and WSM1348 produced adequate levels of nodulation when inoculated on L. ornithopodioides, over two seasons at three field sites.

Conclusions

Effective and persistent strains are now available that should allow the un-compromised evaluation of many of the contemporary Lotus species in the field. Selecting a strain for use in commercial inoculants will be more problematic, given the very large host-strain interactions for nitrogen fixation. Here, the balance of Lotus species which are adopted by farmers will have a strong bearing on which rhizobial strains are progressed to commerce.     

Extracellular polysaccharides of the genusRhizobium

This paper reports an investigation of the extracellular polysaccharides produced by 26 strains ofRhizobium andAgrobacterium. Strains ofRhizobium leguminosarum andR. phaseoli produced a water-soluble polysaccharide containing glucose, glucuronic acid and 4-0-methylglucuronic acid. These substances were also identified in the polysaccharide of a single strain fromLotus uliginosus. Glucose was the only detectable component in the polysaccharide produced by strains ofAgrobacterium radiobacter andA. tumefaciens. The polysaccharides obtained from slow-growing rhizobia were not freely water-soluble. Glucose, mannose, rhamnose, galactose and 4-0-methylglucuronic acid were identified as components of this extracellular material.These results are related to previous studies on rhizobial taxonomy and to the infection process in legumes.     

Development of resistance in slow-growing rhizobia to 1-naphthyl-N-methyl carbamate (sevin)

Summary Different strains of rhizobia (isolated fromLotus corniculatus andVigna unguiculata) andRhizobium meliloti adapt to sevin (50 g/ml). The number of transfers (20–31) and days of incubation (80–130) during which different strains of rhizobia develop resistance varied. The results of reversion of resistance to sevin, experiments showed that the resistance developed was stable. Rate of growth was faster in resistant strains but their final cell numbers were less than those of sensitive strains. Dehydrogenase activity increased with the development of resistance to sevin, except in strain D-467. With the development of resistance to sevin, total lipids and phospholipids decreased, glycolipids increased and neutral lipids varied. Presence of glycerol, sodium oleate and sodium acetate (known to stimulate lipid production) and flavin mononucleotide and wheat germ lipases (known to decrease lipid production) in the culture medium did not change the growth pattern and lipids of the sevin resistant and sensitive strains of rhizobia.     

Effect of Inoculation with Various Rhizobia Strains on the Fatty Acid Composition of Peribacteroid and Bacteroid Membranes of the Nodule Symbiosomes

The fatty acid (FA) composition of bacteroid and peribacteroid membranes was studied in the symbiotic pairs differing in their nitrogen-fixing efficiency; the results are compared with the FA composition of plasmalemma and free-living rhizobia. The experiments involved lupine plants inoculated with strains of Bradyrhizobium lupini359a (Nod⁺Fix⁺) and 400 (Nod⁺Fix L) manifesting high and low nitrogen-fixing efficiency, respectively, and broad bean plants inoculated with strains of Rhizobium leguminosarum97 (Nod⁺Fix⁺) and 87 (Nod⁺Fix L) of high and low nitrogen-fixing efficiency, respectively. We showed that the rhizobia of the strains 359a and 97 were able to form nodules with peribacteroid membranes containing FA mainly or exclusively of plant origin. These strains were able to develop effective symbiotic pairs with legume plants. The use of strains 400 and 87 resulted in the formation of nodules with peribacteroid membranes containing typical bacterial (branched-chain) FAs; these strains were characterized by an ineffective symbiosis.     

Some antigenic properties of cultured cell and bacteroid forms of fast- and slow-growing strains of Lotus rhizobia.

Immunodiffusion cross-reactions of 62 fast- and 76 slow-growing of Lotus rhizobia with antisera to four of the fast-growing and five of the slow-growing strains were studied. No sharing of antigens by both fast- and slow-growing strains was found. Somatic antigens were very strain specific with only eight of the fast-growing and five of the slow-growing strains tested having somatic antigens identical to those of one or more of the strains of the same group used for antisera production. In contrast, internal antigens were shared by all fast-growing strains and with seven exceptions by all slow-growing strains. Antigens of cultured rhizobia, and bacteroids from nodules formed on different legumes by the same strain of Rhizobium, were similar. However, incontrast to cultured cells, bacteroids generally required no pretreatment (heat or ultrasonic disruption) to give a strong somatic antigen reaction in immunodiffusions.     

Phage sensitivity and host range of Rhizobium strains isolated from root nodules of temperate legumes

Twenty-five Rhizobium strains were isolated from root nodules of Astragalus spp. (10), Hedysarum alpinum (7), Glycyrrhiza pallidiflora (3) and Ononis arvensis (5). The sensitivity of these strains to bacteriophages of Rhizobium loti, R. meliloti, R. galegae and R. leguminosarum was studied. Phages specific to R. loti strains were shown to induce the phage lysis of several Astragalus, Hedysarum and Ononis rhizobia. Ten R. loti strains tested for nodulation abilities on the plant hosts under investigation were able to develop nitrogen-fixing nodules on the Ononis arvensis roots. On the other hand, rhizobia from Ononis and Glycyrrhiza could form an effective symbiosis with Lotus corniculatus plants, so these bacteria are considered to belong to the Rhizobium loti taxon. Bacterial strains isolated from Astragalus and Hedysarum were observed to cross-nodulate their plant hosts as well as Oxytropis campestris, Glycyrrhiza uralensis and Ononis arvensis plants, whereas they could not nodulate Lotus plants. It is concluded that these Rhizobium strains comprise a cross-inoculation group related to Rhizobium loti. ei]{gnR O D}{fnDixon}     

Zur Transformierbarkeit der Knöllchenbakterien

Summary We did not succeed in transforming the host specificity of rhizobia reliably, in spite of the use of DNA preparations from 3 different rhizobia species (Rh. meliloti, Rh. trifolii, Rh. leguminosarum) and of 35 acceptor strains representative for 7 rhizobia species. Very few positive results could not be repeated. Rhizobia from ineffective nodules on alfalfa plants of the first inoculation test following the transformation procedure formed effective nodules during a second plant passage. Reisolates from these nodules exhibiting a rough type of growth differ from the smooth colonies of the donator strain as well as of the acceptor strains. These rough growing rhizobia agree with the donator strain but not with the acceptor strains as to the base composition of their deoxyribonucleic acids. Therefore we assume that some cells of the donator have survived the DNA preparation and the 24 h sterilization by ethanol, and being only weakened have formed at first ineffective but secondly—by way of regeneration—effective nodules.Pseudonodules of unknown origin on alfalfa roots may be very similar to ineffective bacteria root nodules. They consist mainly of parenchymatic tissue partly with one or several vascular bundles. Because these exuberances were found also on the roots of uninoculated control plants, they did not result from a transformation process.     

Discrimination of Rhizobium japonicum,Rhizobium lupini,Rhizobium trifolii,Rhizobium leguminosarum and of bacteriods by uptake of 2-ketoglutaric acid,glutamic acid and phosphate

Rhizobium strains (one each of Rh.japonicum, Rh. lupini, Rh. leguminosarum) take up 2-ketoglutaric acid in general much faster and from lower concentrations in the medium than strains of Escherichia coli, Bacillus subtilis and Chromobacterium violaceum. A strain of Enterobacter aerogenes, however, is more similar to some Rhizobium strains. The same strains of Rhizobium take up also phosphate much faster and from lower concentrations than the other bacteria tested. 4 strains of Rh. lupini proved to be significantly different from 4 strains of Rh. trifolii in taking up l-glutamic acid from three to ten times lower concentration within 5 h. A similar difference was noticed between 5 strains of Rh. leguminosarum and 2 strains of Rh. japonicum for the uptake of 2-ketoglutaric acid and of l-glutamic acid. Isolated bacteriods from nodules of Glycine max var. Chippeway have a reduced uptake capacity for glutamic acid and for 2-ketoglutaric acid during the first 10–12 h, but reach the same value after 24 h as free living Rh. japonicum cells. The differences in the uptake kinetics are independent of cell concentration. The group II Rhizobium strains (Rh. japonicum and Rh. lupini, slow growing Rhizobium) are characterized by a rapid uptake of glutamic acid to a lowremaining concentration of 1–3×10^-7 M and an uptake of 2-ketoglutaric acid to a remaining concentration of 2–5×10^-7 M. The group I Rhizobium strains (Rh. trifolii and Rh. leguminosarum, fast growing Rhizobium), can be characterized by a much slower uptake of both substances with a more than ten times higher concentration of both metabolites remaining in the medium after the same time.     

Flagellar characteristics ofRhizobium species

Summary The flagellation and growth characteristics of 82 strains ofRhizobium were studied. The strains were originally isolated from the root nodules of 19 genera and 35 species of leguminous plants. Two morphological types of bacteria were found which differed mainly in the nature of their flagellation. The one type shows a most unusual and unique flagellation with single subpolar flagella of wavelength averaging from 1.9 to 2.2 microns. The other type shows peritrichous flagellation with usually one and, less often, several flagella per flagellated individual. The flagellar wavelength of the latter type averaged from 1.3 to 1.6 microns. Most strains of both types were rather poorly flagellated. An almost perfect correlation was found between the type of flagellation and the growth rate in peptone-mannitol medium. The subpolar types grew relatively slowly and the peritrichous types relatively rapidly. Some strains of the subpolar type showed flagellar variants with multiple flagella of very short wavelength in addition to the normal subpolar flagellum. A few individuals showed the short wavelength flagella only.     

The high diversity of <Emphasis Type="Italic">Lotus corniculatus</Emphasis> endosymbionts in soils of northwest Spain

The diversity of rhizobia that establish symbiosis with Lotus corniculatus has scarcely been studied. Several species of Mesorhizobium are endosymbionts of this legume, including Mesorhizobium loti, the type species of this genus. We analysed the genetic diversity of strains nodulating Lotus corniculatus in Northwest Spain and ten different RAPD patterns were identified among 22 isolates. The phylogenetic analysis of the 16S rRNA gene showed that the isolated strains belong to four divergent phylogenetic groups within the genus Mesorhizobium. These phylogenetic groups are widely distributed worldwide and the strains nodulate L. corniculatus in several countries of Europe, America and Asia. Three of the groups include the currently described Mesorhizobium species M. loti, M. erdmanii and M. jarvisii which are L. corniculatus endosymbionts. An analysis of the recA and atpD genes showed that our strains belong to several clusters, one of them very closely related to M. jarvisii and the remanining ones phylogenetically divergent from all currently described Mesorhizobium species. Some of these clusters include L. corniculatus nodulating strains isolated in Europe, America and Asia, although the recA and atpD genes have been sequenced in only a few L. corniculatus endosymbionts. The results of this study revealed great phylogenetic diversity of strains nodulating L. corniculatus, allowing us to predict that even more diversity will be discovered as further ecosystems are investigated.     

Conserved plasmid/chromosome sequences in fast- and slow-growing rhizobia that nodulate the same plant

Apart from the ability to nodulate legumes, fast-and slow-growing rhizobia have few bacteriological traits in common. Given that there is only one pathway to nodulation, DNA sequences conserved in fast- and slow-growing organisms that nodulate the same host should be strongly enriched in infectivity genes. We tested this hypothesis with seven fast-growing and five slow-growing strains that produced responses varying from fully effective nodulation through various ineffective associations to non-nodulation on four different hosts (Lotus pedunculatus, Lupinus nanus, Macroptilium atropurpureum, and Vigna unguiculata). When restriction enzyme digested total DNA from 10 of the strains was separately hybridized with nick-translated plasmid DNA isolated from 4 fast-growing strains, variable but significant homologies were found with all 10 strains. Part of this homology was shown to be associated with the nifKDH genes for nitrogenase and part with putative nodulation genes carried on pC2, a cosmid clone containing a 37 kbp region of the large sym plasmid present in the fast-growing broad-host range Rhizobium sp. strain NGR234. Analysis of the extent of homology between the plasmids of 3 fastgrowing strains (NGR234, TAL 996 and UMKL 19) able to effectively nodulate Vigna unguiculata showed them to have homologous DNA fragments totalling 47 kbp. This core homology represents less than 12% of the total coding capacity of the sym plasmid present in each of these strains.Abbreviations Sym symbiotic sequences/plasmids - nod genes required for nodulation - nod putative nod genes - nif genes required for the synthesis of the enzyme nitrogenase     

Symbiosis of Astragalus cicer with its microsymbionts: partial nodC gene sequence, host plant specificity, and root nodule structure

Astragalus cicer (cicer milkvetch) nodule bacteria were investigated for host plant specificity and partial nodC gene sequences, whilst their native host was studied for the microscopic structure of root nodules. The strains under investigation formed nodules not only on the original host but also on Astragalus glycyphyllos, Astragalus sinicus, Lotus corniculatus, and Phaseolus vulgaris. The nodules induced on the cicer milkvetch were classified as indeterminate and characterized by apical, persistent meristem, a large bacteroid region with infected and uninfected cells, and elongated bacteroids singly located inside peribacteroid membranes. By comparison of the partial nodC gene sequences of a representative strain of astragali rhizobia to those contained in the GenBank database, a close symbiotic relationship of A. cicer microsymbionts to Rhizobium sp. (Oxytropis) was found.     

Comparison of the host range of fast-growingR. japonicum strains with a fast-growing isolate from Lablab

Summary Fast-growingRhizobium japnicum strains derived from the People's Republic of China were compared with a fast-growingRhizobium isolate from Lablab for their ability to nodulate tropical legumes grown in Leonard-jars and test tube culture. Fast-growingR. japonicum strains were all effective to varying degrees in their symbiosis withVigna unguiculata. Two strains USDA 192 and USDA 201, effectively nodulatedGlycine whightii and one strain, USDA 193, effectively nodulatedMacroptilium atropurpureum. Other nodulation responses in tropical legumes were ineffective. The fast-growing isolate from Lablab was more promiscuous, effectively nodulating with a larger host range. The fast-growing Lablab strain was considered more akin, on a symbiotic basis, to the slow-growing cowpea type rhizobia than the fast-growing China strains ofR. japonicum whilst maintaining physiological characteristics of other fast-growing rhizobia.