Elkanii Strains Research Articles

Differential symbiotic efficiency by shading of soybean nodulated by B. japonicum and B. elkanii strains

Symbiotic efficiency of soybean Glycine max L. cv. Doko nodulated by BR-33 (CB 1809) strain of Bradyrhizobium japonicum or BR-29 (29W) strain of B. elkanii was investigated under two light intensities: full sunlight or 30% of full sunlight. Plants were harvested during pod-filling and at maturity. Soybeans cultivated under full sunlight and inoculated with strain BR-33 showed higher shoot dry mass and nitrogen accumulation, nitrogenase activity, ureide-N in the xylem sap and grain yield than plants inoculated with strain BR-29, confirming the superior symbiotic effectiveness of strain BR-33. In contrast, soybean submitted to shading treatment showed no significant differences regardless of the strain used as inoculant. When submitted to shading, plants inoculated with BR-29 strain showed an increase in most of the variables observed while plants inoculated with strain BR-33 were not significantly affected by the level of shading. The results suggest that strain efficiency used as an inoculant for soybean can be different in plants cultivated either in the greenhouse or in the field. This can help to explain why the potential of hydrogenase activity to improve symbiotic effectiveness is still a matter of controversy when comparing results from different experiments.

Soybean Response to Nodulation by Wild‐Type and an Isogenic Bradyrhizobium elkanii Mutant Lacking Rhizobitoxine Production

Soybean [Glycine max (L.) Merr.] cultivars differ in their susceptibility to rhizobitoxine‐producing (RT+) bradyrhizobia, but the physiological basis for this variation is uncertain. We conducted greenhouse experiments to determine the response of six cultivars, that differ in their RT susceptibilities, to nodulation by four strains of Bradyrhizobium: USDA 61 and 94 (RT+ wild‐type B. elkanii strains), USDA 110 (RT wild‐type B. japonicum strain), and RX 18E (a Tn5‐induced mutant of USDA 61 lacking RT production in planta). At 45 d after planting, plants were rated for RT symptoms and analyzed for shoot N contents and shoot, root, and nodule biomass and RT concentrations. Foliar chlorosis was generally better correlated with shoot RT concentrations than with nodular concentrations. Strain USDA 94 generally produced more RT in planta than did USDA 61. When nodulated by USDA 61, cultivars rated as RT‐resistant often had lower nodular RT concentrations than did RT‐susceptible plants. Relative to RX 18E, nodulation by USDA 61 significantly reduced plant shoot weight for the RT‐sensitive cultivar Hutcheson and tended to reduce shoot weight for all remaining cultivars except the highly resistant SS FFR 565; decreased shoot N content by 50.5 and 17.4% in cultivar Hutcheson and Williams, respectively; and had no significant effect on nodule number and nodule weight. These results indicate that mechanisms of RT resistance may differ among the cultivar‐strain combinations examined and involve differences in RT production, RT export from the nodules, and RT detoxification during and following transport to the shoot.

Cooperative Action of Lipo-Chitin Nodulation Signals on the Induction of the Early Nodulin, ENOD2, in Soybean Roots

Various lipo-chitin molecules were tested for their ability to induce the expression of the early nodulin, ENOD2, in Glycine soja roots. When inoculated separately onto G. soja roots, LCO-V (C18:1 delta 11,Mefuc), LCO-V (C18:1 delta 9,Mefuc), LCO-V (C16:0,Mefuc), and LCO-IV (C16:0) were unable to induce ENOD2 expression, even though these compounds had previously been shown to induce root hair curling, the formation of nodule-like primordia, and induction of the early nodulin, ENOD40. ENOD2 expression, however, was induced when any two of these molecules were inoculated in combination. Thus, the lipo-chitin nodulation signals appear to act cooperatively to induce ENOD2 expression. B. japonicum strains USDA110 and USDA135 and B. elkanii strain USDA61, all symbionts of soybean, were found to produce at least two distinct nod signals ([i.e., NodBj-V[C18:1,Mefuc] and NodBj-V[C16:0,Mefuc]). These two compounds were mixed in various ratios and tested for their ability to induce ENOD2 expression. The results indicate that the former compound must be present in equivalent or excess amount in order to obtain maximum ENOD2 expression. Additional nonspecific LCOs (e.g., LCO-IV[C16:2 delta 2,9; SO3]), incapable of inducing root hair curling or cortical cell division, were tested in combination with the four active LCOs listed above. It was found that any combination of one active LCO with a nonspecific LCO was sufficient to induce ENOD2 mRNA expression. The ENOD2 mRNA expression pattern detected by in situ hybridization closely resembled that found in bacterial-induced nodules with expression detected in cortical cells between primary and secondary meristems and around the vascular strands. These data demonstrate that the cooperative action of at least two LCO nodulation signals leads to a greater progression of nodule ontogeny as demonstrated by the expression of ENOD2, a marker gene for the differentiation of nodule parenchyma.

Adhesion behaviour of Bradyrhizobium japonicum and Bradyrhizobium elkanii strains: Evidence of duality between the two species

Adhesion behaviour of Bradyrhizobium japonicum and Bradyrhizobium elkanii strains: Evidence of duality between the two species

Differences among strains of Bradyrhizobium in fatty acid–methyl ester analysis

Fatty acid–methyl ester (FAME) and two-dimensional principal component analysis of 89 strains of Bradyrhizobium, most of which were from soybean, distinguished five groups of bradyrhizobia. These included one cluster containing several isolates previously designated as Bradyrhizobium elkanii, and two related clusters containing strains previously identified as belonging to Bradyrhizobium japonicum groups IA and IB. Not all of the organisms evaluated clustered with the B. japonicum and B. elkanii strains. A number of Bradyrhizobium strains isolated from soybean in Korea and northern Thailand had FAME profiles so different from the B. japonicum and B. elkanii strains as to warrant separation at the species level. A slow-growing isolate from Lupinus also had a FAME profile very different from those of the other bradyrhizobia. Results obtained in this study were generally in agreement with those obtained using other taxonomic approaches, suggesting that FAME analysis provides a relatively simple and reliable procedure for the initial characterization of Bradyrhizobium isolates.Key words: fatty acid analysis, Bradyrhizobium taxonomy, FAME analysis, strain identification.

The structure of a novel polysaccharide produced by Bradyrhizobium species within soybean nodules

Certain strains of Bradyrhizobium japonicum and B. elkanii produce a polysaccharide within the root nodules of their legume host, soybean. These nodule polysaccharides (NPSs) were isolated and characeterized. The NPS produced by B. elkanii strains proved to be identical in glycosyl composition and linkages to the extracellular polysaccharide (EPS) of this species indicating that the NPS and EPS for B. elkanii have identical structures (W.F. Dudman, Carbohydr. Res., 66 (1978) 9–23), ▪ However, the structure of the NPS from B. japonicum proved to be quite different from that of its EPS. Methylation analysis of this NPS showed that it consists of 3-linked Gal, 3-linked Rha, 2,4-linked Rha, 4-linked Rha, and terminal 2- O-methyl GLcA in a 1:1:1:1:1 ratio. Stereochemical configurations of the glycosyl residues were determined by the preparation and analysis of trimethylsilyl (Me 3Si) (−)-2-butylglycosides. NMR spectroscopy (both 1H and 13C) showed that the Gal residue is α-linked, while all the other glycosyl residues are β-linked. Oligosaccharides produced by periodate oxidation-Smith degradation were purified, as were oligosaccharides produced by partial acid hydrolysis. Characterization of the Smith degradation products by methylation analysis, NMR spectroscopy, electrospray-mass spectrometry, and characterization of the partial acid hydrolysate oligsaccharides showed that the repeating oligosaccharide unit of the NPS has the structure, ▪.

Variability among Soybean Genotypes in Response to Nodulation by a Rhizobitoxine‐Producing Strain of Bradyrhizobia

AbstractNodulation by rhizobitoxine‐producing (RT+) strains of Bradyrhizobium elkanii (syn. Bradyrhizobium japonicum; DNA homology group II) has been shown to reduce shoot growth, N2 fixation, and seed yield of ‘Forrest’ soybean (Glycine max [L.] Merr.). We conducted greenhouse and field experiments to assess the frequency of these negative responses among selected soybean cultivars. In the greenhouse, 38 cultivars representing a range of yield potentials, maturity groups, and growth habits were inoculated with B. elkanii strain USDA 94 (RT+) or B. japonicum strain USDA 110 (RT−). Large differences in tolerance to nodulation by RT+ bradyrhizobia were noted, as estimated visually and by comparing shoot weights (40 d after planting) of plants inoculated with the two strains. For the field study, two tolerant (Stafford and SS FFR 565), two moderately sensitive (Pharaoh and Williams 82), and two highly sensitive cultivars (Forrest and Hutcheson) were chosen. Seeds sown to vermiculite under greenhouse conditions were inoculated with either USDA 94 or a suspension of soil taken from the study site containing the indigenous bradyrhizobia. After 23 d, the nodulated seedlings were transplanted to 1.8‐m2 field microplots. Shoots were harvested at the R7 growth stage and analyzed for various response parameters. Only Forrest, Hutcheson, and Pharaoh were significantly affected by inoculation with USDA 94, which decreased seed yield, pod number, vegetative growth, and the amount of N2 fixed. These results support the hypothesis that significant genotypic variability exists in the response of soybeans to nodulation by RT+ bradyrhizobia. The greenhouse screening procedure can provide useful predictions of the field response of tolerant and highly sensitive cultivars, but may not be accurate for moderately sensitive cultivars.

Near isogenic lines of soybeans as tools to identify nodulation specificmutants of Bradyrhizobium elkanii

The dominant allele Rj4 in soybean interdicts or restricts the nodulation of plants by certain strains of bacteria, most of which are classified as Bradyrhizobium elkanii, while the recessive allele permits normal nodulation with the same strains. The near isogenic lines BARC-2 (Rj4) and BARC-3 (rj4) are calculated to be 99.95% identical in their nuclear DNA, but differ specifically in the allele present at the Rj4 locus. These lines were used to identify spontaneous mutants of the Rj4-restricted Bradyrhizobium elkanii strain USDA 61 Nalr that had the ability to effectively nodulate plants of the Rj4 genotype. Of the eight rare nodules found on roots of soybean plants of the Rj4 genotype inoculated with the genetically marked strain USDA 61 Nalr, four were identified as containing mutants with the ability to overcome the effects of the Rj4 allele.