Sinorhizobium Strains Research Articles

The model legume Medicago truncatula A17 is poorly matched for N2 fixation with the sequenced microsymbiont Sinorhizobium meliloti 1021

Medicago truncatula (barrel medic) A17 is currently being sequenced as a model legume, complementing the sequenced root nodule bacterial strain Sinorhizobium meliloti 1021 (Sm1021). In this study, the effectiveness of the Sm1021-M. truncatula symbiosis at fixing N(2) was evaluated. N(2) fixation effectiveness was examined with eight Medicago species and three accessions of M. truncatula with Sm1021 and two other Sinorhizobium strains. Plant shoot dry weights, plant nitrogen content and nodule distribution, morphology and number were analysed. Compared with nitrogen-fed controls, Sm1021 was ineffective or partially effective on all hosts tested (excluding M. sativa), as measured by reduced dry weights and shoot N content. Against an effective strain, Sm1021 on M. truncatula accessions produced more nodules, which were small, pale, more widely distributed on the root system and with fewer infected cells. The Sm1021-M. truncatula symbiosis is poorly matched for N(2) fixation and the strain could possess broader N(2) fixation deficiencies. A possible origin for this reduction in effectiveness is discussed. An alternative sequenced strain, effective at N(2) fixation on M. truncatula A17, is Sinorhizobium medicae WSM419.

Improving lucerne nodulation at low pH: contribution of rhizobial and plant genotype to the nodulation of lucerne seedlings growing in solution culture at pH 5

The effect of rhizobial strain and lucerne genotype on the nodulation of lucerne seedlings growing in solution cultures maintained at pH 5 was measured in two greenhouse experiments. Sinorhizobium strain SRDI291 increased the percentage of plants nodulated when compared with the commercial inoculant strain RRI128 (89% cf. 45% and 86% cf. 23% in the two experiments, respectively). Similarly, the mean number of nodules per nodulated plant (NNP) was greatest in the SRDI291 treatments (3.5 cf. 1.7 and 3.1 cf. 1.8). Plant genotype had fewer, but still significant effects on nodulation. Overall, lucerne genotype CRCSA29 had the highest percentage of plants forming nodules and most NNP (63% and 2.9); genotype L97a had the lowest (46% and 1.9). Greater variation in NNP occurred when the lucerne genotypes were inoculated with RRI128. Most notable was the better nodulation of CRCSA29 with 2.8 NNP compared with only 1.2 NNP for the two unselected genotypes (SARDI Ten and L97a). The lucerne genotypes typically had variable NNP, as exemplified in CRCSA29 inoculated with RRI128 where it ranged from 0–14, providing the opportunity for further plant selection. The work demonstrates that there remains substantial opportunity to increase the potential of lucerne to nodulate at low pH. Gains would appear to be most easily made by changing the strain of rhizobia that is used for inoculation.

Effects of rhizodeposition of non-transgenic and transplastomic tobaccos on the soil bacterial community

The effect of root-released compounds of transplastomic tobacco (Nicotiana tabacum) on the soil bacterial community structure, and their potential to support horizontal gene transfer (HGT) to bacteria have been studied. Soil microcosms were exposed to root-released compounds collected from transplastomic and non-transgenic tobacco cultivars. Cluster analysis of automated ribosomal intergenic spacer analysis (ARISA) profiles of the soil bacterial community after 48 h incubation grouped the transgenic cultivar apart from the non-transgenic, indicating that it had a rhizodeposition pattern different from the parental plants. However, these differences were less than between the two non-transgenic tobacco cultivars studied. NMR characterization of the root-released compounds showed some differences in chemical fingerprinting pattern between the transplastomic and the parental cultivar. However, the effect on bacterial community structure was transient, and tended to disappear after 96 h of incubation. The potential of root-released compounds as a source of transforming DNA for bacteria was investigated by using four potential recipient species. No transformants were obtained following exposure of all the recipients to the root-released compounds. Root-released compounds amended to transgene donor DNA decreased the transformation frequency of Acinetobacter baylyi strain ADP1200, while Azospirillum, Agrobacterium, and Sinorhizobium strains failed to develop competence also in the presence of an external added transgene source. Detection of plastid sequences by PCR suggested that a very low amount of fragmented plastid donor DNA was present in the root-released compounds.

Nodulation of Cyclopia spp. (Leguminosae, Papilionoideae) by Burkholderia tuberum

Species of the genus Burkholderia, from the Betaproteobacteria, have been isolated from legume nodules, but so far they have only been shown to form symbioses with species of Mimosa, sub-family Mimosoideae. This work investigates whether Burkholderia tuberum strains STM678 (isolated from Aspalathus carnosa) and DUS833 (from Aspalathus callosa) can nodulate species of the South African endemic papilionoid genera Cyclopia (tribe Podalyrieae) and Aspalathus (Crotalarieae) as well as the promiscuous legume Macroptilium atropurpureum (Phaseoleae). Bacterial strains and the phylogeny of their symbiosis-related (nod) genes were examined via 16S rRNA gene sequencing. Seedlings were grown in liquid culture and inoculated with one of the two strains of B. tuberum or with Sinorhizobium strain NGR 234 (from Lablab purpureus), Mesorhizobium strain DUS835 (from Aspalathus linearis) or Methylobacterium nodulans (from Crotalaria podocarpa). Some nodules, inoculated with green fluorescence protein (GFP)-tagged strains, were examined by light and electron microscopy coupled with immunogold labelling with a Burkholderia-specific antibody. The presence of active nitrogenase was checked by immunolabelling of nitrogenase and by the acetylene reduction assay. B. tuberum STM678 was also tested on a wide range of legumes from all three sub-families. Nodules were not formed on any of the Aspalathus spp. Only B. tuberum nodulated Cyclopia falcata, C. galioides, C. genistoides, C. intermedia and C. pubescens. It also effectively nodulated M. atropurpureum but no other species tested. GFP-expressing inoculant strains were located inside infected cells of C. genistoides, and bacteroids in both Cyclopia spp. and M. atropurpureum were immunogold-labelled with antibodies against Burkholderia and nitrogenase. Nitrogenase activity was also shown using the acetylene reduction assay. This is the first demonstration that a beta-rhizobial strain can effectively nodulate papilioinoid legumes. Papilionoid legumes from widely different tribes can be nodulated by beta-rhizobia, forming both indeterminate (Cyclopia) and determinate (Macroptilium) nodules.

Context dependence in the coevolution of plant and rhizobial mutualists

Several mechanisms are expected to rapidly rid mutualisms of genetic variation in partner quality. Variation for mutualist quality, however, appears to be widespread. We used a model legume-rhizobium mutualism to test for evidence that context-dependent selection may maintain variation in partner quality. In a greenhouse experiment using 10 natural populations of Medicago truncatula and two strains of Sinorhizobium medicae, we detected significant genotype x genotype (G x G) interactions for plant fitness, indicating that the most beneficial rhizobium strain depends on the host genotype. In a second experiment using a subset of the plant populations used in the first experiment, we detected significant G x G interactions for both plant and rhizobium fitness. Moreover, the plant population with which rhizobium strains gained the greatest benefit depended on the nitrogen environment. Finally, we found that in a high nitrogen environment, all plant populations had lower fitness when inoculated with a 1:1 mixture of strains than with the worse single strain alone, suggesting that nitrogen shifts the exchange of benefits in favour of rhizobia. Our data suggest that genotype, nitrogen and biotic dependency might contribute to the maintenance of genetic variation in mutualist quality when coupled with spatial or temporal heterogeneity in the environment.

Diverse bacteria isolated from root nodules of Trifolium, Crotalaria and Mimosa grown in the subtropical regions of China

To analyze the diversity and relationships of rhizobia in the subtropical and tropical zones of China, we characterized 67 bacterial strains isolated from root nodules of five legume species in the genera Trifolium, Crotalaria and Mimosa . PCR-amplified 16S rDNA RFLP, numerical taxonomy, SDS-PAGE of whole cell proteins, sequencing of 16S rDNA and DNA-DNA hybridization grouped the isolates into 17 lineages belonging to Bradyrhizobium, Mesorhizobium, Rhizobium, Sinorhizobium and Burkholderia, as well as a non-symbiotic group of Agrobacterium. The Rhizobium group contained twenty strains isolated from Mimosa pudica, Crotalaria pallida and two species of Trifolium. Fifteen of them were R. leguminosarum. Twenty-one strains isolated from four species of Trifolium, Crotalaria and Mimosa were classified into five groups of Bradyrhizobium, including B. japonicum. Agrobacterium group composed of 20 isolates from Mimosa pudica, C. pallida and Trifolium fragiferum. In addition, several strains of Sinorhizobium and Mesorhizobium associated with Trifolium and Burkholderia associated with Mimosa pudica were also identified. The predominance of Bradyrhizobium in the nodules of Trifolium was a novel finding and it demonstrated that the nodule microsymbionts might be selected by both the geographic factors and the legume hosts.

Effect of lucerne seed bacterization (Medicago sativaL.) on yield components in ecological cultivation

Biennial field experiments of parameter comparison for noninoculated and inoculated lucerne in ecological cultivation lead to conclusion that green mass yield was very significantly higher in inoculated lucerne, and dry matter yield was statistically very significantly higher in both research years. Nitrogen concentration in above ground part of plant material in both research years was significantly higher in inoculated lucerne. Protein yield of inoculated lucerne was two times higher (very significant) than of lucerne that was not inoculated with highly effective strain of nodule bacteria. From all above mentioned, we concluded that pre-sowing inoculation of lucerne seed with effective strain Sinorhizobium meliloti is necessary for obtaining higher, more sustainable yields. Besides, nitrogen fixed through root nodules of leguminous plants is cheaper, more natural and efficient for utilization by host plant. Rational use of soils and the maintenance of their multifunctionality is a key issue in sustainable systems.

Inactivation of thenodHgene inSinorhizobiumsp. BR816 enhances symbiosis withPhaseolus vulgarisL.

Sulfate modification on Rhizobium Nod factor signaling molecules is not a prerequisite for successful symbiosis with the common bean (Phaseolus vulgaris L.). However, many bean-nodulating rhizobia, including the broad host strain Sinorhizobium sp. BR816, produce sulfated Nod factors. Here, we show that the nodH gene, encoding a sulfotransferase, is responsible for the transfer of sulfate to the Nod factor backbone in Sinorhizobium sp. BR816, as was shown for other rhizobia. Interestingly, inactivation of nodH enables inoculated bean plants to fix significantly more nitrogen under different experimental setups. Our studies show that nodH in the wild-type strain is still expressed during the later stages of symbiosis. This is the first report on enhanced nitrogen fixation by blocking Nod factor sulfation.

Relationships between rhizobial diversity and host legume nodulation and nitrogen fixation in tropical ecosystems

With recent advances in rhizobial phylogeny, questions are being asked as to how an ecological framework can be developed so that rhizobial classification and diversity could have greater practical applications in enhancing agricultural productivity in tropical ecosystems. Using the results of studies on tropical rhizobia which nodulate agroforestry tree legumes, three ecological aspects of rhizobial biodiversity were used to illustrate how its potential can be exploited. The results showed that legumes nodulate with diverse rhizobial types, thus contributing to the success of legumes in colonising a wide range of environments. There was an apparent shift in the relative dominance of rhizobia populations by different rhizobial types as soil pH changed. The Rhizobium tropici-type rhizobia were predominant under acidic conditions, Mesorhizobium spp. at intermediate pH and Sinorhizobium spp. under alkaline conditions. The R. tropici-type rhizobia were the most effective symbiotic group on all the host legumes. However, strains of Sinorhizobium spp. were as effective as the R. tropici types in N2-fixation on Gliricidia sepium, Calliandra calothyrsus and Leucaena leucocephala; while Mesorhizobium strains were equally as effective as the R. tropici types on Sesbania sesban. Classification of rhizobia based on phenotypic properties showed a broad correlation with groupings based on 16S rRNA sequence analysis, although a few variant strains nested with the dominant groups in most of the clusters. Some of the phenotypic characters that differentiated different rhizobial groups are highlighted and a case is made for the need to standardise this method.

Distribution of Medicago Species and Their Microsymbionts in a Saline Region of Algeria

We studied symbiosis of Medicago ciliaris and Medicago polymorpha, two legumes of forage and ecological importance in Algeria, especially in saline soil regions. We report the spatial distribution of the two species and their microsymbionts along salinity gradient transects in the Sebkha of Misserghin (Algeria, North Africa). Seeds and root nodules were sampled from 10 sites. Twenty-seven rhizobial strains were isolated from root nodules and characterized as fast-growers and slime-producers on yeast mannitol agar. By partial sequencing of the gene coding for the 16 S ribosomal RNA, they were found to be affiliated to Rhizobium, Sinorhizobium, and Agrobacterium but several strains had sequences with separate positions.Interestingly one of these was further assigned to Phyllobacterium. Opposite to rhizobia, the distribution of the two Medicago species varied along the salinity gradient, M. ciliaris being dominant in the low NaCl concentration zones and M. polymorpha dominant in the most saline zones. Tolerance to salinity, of both bacterial and plant partners, was studied under laboratory conditions, showing that plants are susceptible to NaCl concentrations of 50 mM, 15-fold lower than that of their associated rhizobia (800 mM).

Salinity-induced accumulation of poly-β-hydroxybutyrate in rhizobia indicating its role in cell protection

Poly β-hydroxybutyrate (PHB) is an energy and carbon storage material accumulated in response to the limitation of an essential nutrient. The effect of different salt concentrations on growth and PHB accumulation of four different Sinorhizobium strains was examined. Irrespective of the strain, a defined trend in the accumulation of PHB inside the cells was observed. While minimum PHB content was accumulated at low or zero salinity, maximum was observed by the salt-tolerant strains at higher salt concentrations. This suggests a definite role for PHB in cell protection in saline conditions.

The Coexistence of Symbiosis and Pathogenicity-Determining Genes in Rhizobium rhizogenes Strains Enables Them to Induce Nodules and Tumors or Hairy Roots in Plants

Bacteria belonging to the family Rhizobiaceae may establish beneficial or harmful relationships with plants. The legume endosymbionts contain nod and nif genes responsible for nodule formation and nitrogen fixation, respectively, whereas the pathogenic strains carry vir genes responsible for the formation of tumors or hairy roots. The symbiotic and pathogenic strains currently belong to different species of the genus Rhizobium and, until now, no strains able to establish symbiosis with legumes and also to induce tumors or hairy roots in plants have been reported. Here, we report for the first time the occurrence of two rhizobial strains (163C and ATCC11325T) belonging to Rhizobium rhizogenes able to induce hairy roots or tumors in plants and also to nodulate Phaseolus vulgaris under natural environmental conditions. Symbiotic plasmids (pSym) containing nod and nif genes and pTi- or pRi-type plasmids containing vir genes were found in these strains. The nodD and nifH genes of the strains from this study are phylogenetically related to those of Sinorhizobium strains nodulating P. vulgaris. The virA and virB4 genes from strain 163C are phylogenetically related to those of R. tumefaciens C58, whereas the same genes from strain ATCC 11325T are related to those of hairy root-inducing strains. These findings may be of high relevance for the better understanding of plant-microbe interactions and knowledge of rhizobial phylogenetic history.

Effect of salinity on root-nodule conductance to the oxygen diffusion in the Medicago truncatula– Sinorhizobium meliloti symbiosis

In order to determine the effect of salinity on the nodule conductance, oxygen uptake by the nodulated roots was measured by registering the concentration of O 2 as a function of time in a tight incubator of known volume containing the nodulated roots of Medicago truncatula. Four lines, namely TN8.20 and TN6.18, originated from local populations, F83005.5 originated from Var (France) and Jemalong 6, a cultivar from Australia, were hydroponically grown in 250 ml glass bottles under semi-controlled conditions in a glasshouse, after germination and inoculation with the strain Sinorhizobium meliloti 2011. The saline treatment was applied gradually to reach 75 mM after 2 weeks. Results show that oxygen uptake increased significantly with salinity in TN6.18 and F83005.5, but not in Jemalong nor in TN8.20. Without salt, Jemalong showed a significantly higher O 2 uptake of 240 μmol O 2 per h per plant than the mean of 130 μmol O 2 per h per plant for other lines. Salinity increased significantly the nodule conductance in all genotypes. This salt effect was significantly higher for TN6.18 than for TN8.20, and for Jemalong than for F83005.5. Without salt there was less genotypic variation in nodule conductance in the range of 5–8 μm s –1 for F83005.5 and TN8.20, respectively. Thus the sensitivity to salinity appears to be associated with an increase in nodule conductance that supports the increased respiration of N 2-fixing nodules under salinity.

Production of amino acids by Rhizobium, Mesorhizobium and Sinorhizobium strains in chemically defined media.

Five strains of Rhizobium spp, one strain of Mesorhizobium loti and two strains of Sinorhizobium meliloti were tested for their ability to grow in chemically-defined medium lacking growth factor. Qualitative and quantitative production of aspartic acid, serine, glutamic acid, glycine, histidine, threonine, arginine, alanine, proline, cysteine, tyrosine, valine, methionine, lysine, isoleucine, leucine, and phenylalanine was determined by the use of mannitol as sole carbon source. Strains of Rhizobium spp. and Sinorhizobium sp. produced all the amino acids analysed with the exception of cysteine and high biological levels of serine, glycine and alanine were detected after 2 days of culture in mineral medium. Strain U226 of M. loti only produced small amounts of amino acids and glutamic acid, histidine, arginine, cysteine, methionine, lysine and phenylalanine was not liberated into the media.

Nitrogen fixation and growth of annual Medicago–Sinorhizobium associations at low temperature

Annual Medicago (medics) represents a great potential of forage production in areas suffering from shortage of livestock feed; however, low winter temperature delays the growth and reduces the nitrogen fixation of medics in Mediterranean dry areas with cool to cold winter. In this context, the identification of medics able to fix nitrogen and produce dry matter at low temperature is necessary to extend the grazing period. The experiment was carried out in controlled growth chambers at two low temperatures 7 and 9 °C, temperature known to limit growth and N 2 fixation of Medics in the field. Three accessions were used, two of Medicago aculeata, accessions 80 (A) and 5099 (B) and one of M. rigidula, accession 716 (R). Each accession was inoculated with Sinorhizobium strains appropriate for nodulation under low temperature ( Sinorhizobium meliloti strain M620 with accession A, strain M508 with B and strain BZI with R). The percentage of nitrogen in the shoots derived from atmosphere was quantified using 15 N dilution technique with barley as reference plant. Results demonstrated that responses of accessions depend on temperature, nitrogen and inoculated strain. Among the accessions tested, M. aculeata 5099 was more adapted to temperature and grew faster than the others at the lowest temperature. Strain M53 performed better with M. rigidula 716 than with M. aculeata 80 and strain M620 with M. aculeata 5099 than with M. aculeata 80 for yield and N nutrition of plants. M508 for M. aculeata 5099 and BZI for M. rigidula 716 leaded to more N fixed per plant. Nitrogen fixation in inoculated treatment of accession M. aculeata × M508 accounted for 88 and 78% of total plant N at 9 and 7 °C versus 49 and 51% for accession M. rigidula × M53. It was concluded that the selection of associations medics × Sinorhizobium is a promising way to improve yield and N 2 fixation at low temperature in areas where N fertilization cannot be possible.

Instability of Cryptic Plasmids in Strain Sinorhizobium meliloti P108 in the Course of Symbiosis with Alfalfa Medicago sativa

Instability of cryptic plasmids in Sinorhizobium meliloti laboratory strains SXM1, DM7-R, and P108 as well as in their clones isolated from nodules of alfalfa grown during a long-term microvegetation experiment (120 days) was studied. The isolated clones of strains SXM1 and DM7-R manifested stable inheritance of plasmids, whereas 12.7–14.0% of clones with changed plasmid profile were detected in a population of clones from strain P108. These segregants were designated as P108c. Segregants P108c exhibited significantly decreased symbiotic effectiveness, nitrogenase activity, and the competitiveness with respect to alfalfa, compared to the original strain P108. It was established that a 80-kb deletion occurred in a larger of two cryptic plasmids (240 and 230 kb) of segregants P108c. It was concluded that genetic rearrangements are possible in rhizobial clones that did not undergo structural transformation and retained viability in the nodule during the natural vegetation period of alfalfa.

Evaluation of the roles of two compatible solutes, glycine betaine and trehalose, for the Acacia senegal–Sinorhizobium symbiosis exposed to drought stress

Acacia senegal (Mimosoideae) is a leguminous, nitrogen-fixing tree that grows in arid areas of Africa and the Near East. In this work, we studied the effects of drought stress on the development of symbiosis between A. senegal seedlings and Sinorhizobium arboris. We also evaluated if two exogenous compatible solutes, glycine betaine and trehalose, are advantageous for the A. senegal–Sinorhizobium symbiosis and if these solutes are capable of protecting two Sinorhizobium strains from salt stress (NaCl) and osmotic stress induced by polyethylene glycol (PEG 6000). A. senegal seedlings exposed to severe drought stress developed more root hairs than plants grown under moderate stress. After inoculation with the GUS marked S. arboris strain HAMBI 2180 the hairs were deformed but infection threads occurred only occasionally. Non-typically deformed hairs were dwarfed and swollen. Severely stressed roots contained less nodules but more nodule initials than moderately stressed ones. Nodules formed showed a lowered glucuronidase activity and signs of premature senescence. The numbers of culturable rhizobia in soil mix were reduced from 107 to 106 CFU g−1. Thirty days after inoculation severely drought-stressed A. senegal seedlings were wilted. Regarding endogenous glycine betaine, A. senegal appeared to be a non-accumulator plant but was able to translocate foliar-applied glycine betaine into roots. Both glycine betaine (0.01 M) and trehalose (0.01, 0.05 and 0.09 M) protected cell cultures of the wild type S. arboris strain HAMBI 1552 and S. saheli strain HAMBI 1496 from osmotic stress (9 and 17% PEG). In the case of salt stress only trehalose had a favourable effect. Application of 0.0003 M glycine betaine or trehalose into A. senegal soil mix exposed to severe drought stress maintained the numbers of culturable rhizobia at the same level as in moderately stressed soils. The presence of glycine betaine in the soil mix also helped A. senegal seedlings survive under severe drought.

Effect of thorium on the growth and capsule morphology of Bradyrhizobium.

The thorium effect on Bradyrhizobium growth was assayed in liquid media. Th4+ inhibited the growth of Bradyrhizobium (Chamaecytisus) BGA-1, but this effect decreased in the presence of suspensions of live or dead bacterial cells. Th4+ induced the formation of a gel-like precipitate when added to a dense suspension of B. (Chamaecytisus) BGA-1 cells. Viable Bradyrhizobium cells remained in suspension after precipitate formation. Thorium was recovered in the precipitate, in which polysaccharide, lipopolysaccharide and proteins were also found. After Th4+ addition, the morphology of B. (Chamaecytisus) BGA-1 or Bradyrhizobium japonicum USDA 110 sedimented cells studied by scanning electron microscopy changed from an entangled network of capsulated bacteria to uncapsulated individual cells and an amorphous precipitate. Energy-dispersive X-ray spectroscopy showed that thorium was mainly in the amorphous fraction. Precipitate was also formed between B. (Chamaecytisus) BGA-1 and Al3+, which was also toxic to this bacterium. Precipitate induced by Th4+ or Al3+ was found in all Bradyrhizobium and Sinorhizobium strains tested, but not in Rhizobium, Salmonella typhimurium, Aerobacter aerogenes or Escherichia coli. These results suggest a specific defence mechanism based on metal precipitation by extracellular polymers.

Identification of a transmissible plasmid from an Argentine Sinorhizobium meliloti strain which can be mobilised by conjugative helper functions of the European strain S. meliloti GR4

We describe in this work the identification and the conjugal properties of two cryptic plasmids present in the strain Sinorhizobium meliloti LPU88 isolated from an Argentine soil. One of the plasmids, pSmeLPU88b (22 kb), could be mobilised from different S. meliloti strains to other bacteria by conjugation only if the other plasmid, pSmeLPU88a (139 kb), was present. This latter plasmid, however, could not be transferred via conjugation (frequency <10 −9 transconjugants per recipient) contrasting with the conjugal system from the previously described strain GR4, where one plasmid is mobilisable and a second one (helper) is self-transmissible. Despite the differences between the two systems, the conjugative helper functions present in the cryptic plasmids of strain GR4 were active in the mobilisation of plasmid pSmeLPU88b from strain LPU88. Contrasting with this, plasmid pSmeLPU88b was not mobilised by the helper functions of the broad-host-range plasmid RP4. Eckhardt gel analysis showed that none of the plasmids from strain GR4 were excluded in the presence of plasmid pSmeLPU88b suggesting that they all belong to different incompatibility groups for replication. The small plasmid from strain LPU88, pSmeLPU88b, was only able to replicate in members of the Rhizobiaceae family such as Rhizobium leguminosarum, Rhizobium tropici and Agrobacterium tumefaciens, but not in Escherichia coli or Pseudomonas fluorescens. The observation suggests that most likely plasmid pSmeLPU88b was not received from a phylogenetically distant bacterium.

Influence of pH and calcium on the growth, polysaccharide production and symbiotic association of Sinorhizobium meliloti SEMIA 116 with alfalfa roots

The Sinorhizobium -legume interaction is sensitive to a number of environmental factors, soil acidity being one of the most important. In the typic Hapludoll soil of the central-southern region of Cordoba (Argentine) it was found that the nodulation of alfalfa ( Medicago sativa L.) roots was impaired with a reduction in shoot dry matter under conditions of soil acidity (pH 5.5) Our results showed that the addition of lime as dolomitic limestone at a concentration of 1 t ha-1 to acid soil caused a significant increase in the nodulation of alfalfa roots inoculated with the strain Sinorhizobium meliloti SEMIA 116 (recommended inoculum for alfalfa) in the greenhouse experiments. The success of the lime treatment may be related not only to an increase in the pH values but also to an increase in the Ca concentration, improving the growth of S. meliloti and its nodulation ability under acidic conditions. In this study, we also demonstrated an increment in the bacterial growth rate as well as in the production of exopolysaccharides and lipopolysaccharides under low pH (5.5) and a high Ca concentration (5 mM) in the culture medium.