Identification of Legume Nodule Bacteroids by the Fluorescent Antibody Reaction

Polyclonal antisera production by immunization with mixed cell antigens of different rhizobial species

Somatic antigens of Bradyrhizobium japonicum, Rhizobium sp. (Cicer arietinum) and Rhizobium sp. (Leucaena leucocephala) were prepared as standard, single-species type from cultured cells. Equal numbers of the cells of these rhizobia were then combined to obtain a mixed-rhizobial-species antigen preparation. Rabbits were immunized either with the standard, single-species type or with the mixed-rhizobial-species antigen preparations. The antisera developed from the mixed antigen immunization contained antibodies for all three rhizobial species, detectable at agglutination titres of over 800. The mixed-rhizobial-species antisera were made species specific by cross-absorption. The cross-absorbed and the mixed-rhizobial-species antisera were generally similar in quality for strain identification by agglutination, fluorescent antibodies, immunoblot and ELISA. A 66% reduction in cost was estimated for the production of antisera by immunization with mixed-rhizobial-species antigen.

Identification of inoculant strains and naturalized populations ofRhizobium leguminosarum bvtrifolii using complementary methodologies

Introduced clovers (Trifolium repens andTrifolium subterraneum) fall to persist more than two years in Uruguayan improved pastures. Three strains of naturalizedRhizobium leguminosarum bvtrifolii, isolated from persistent clover pastures inoculated several years ago, and two commercial inoculant strains have been identified. Serological techniques (enzyme-linked immunosorbent assay-ELISA) and total cell protein pattern have been selected as sensitive and reliable methods for strain identification. Indirect ELISA was suitable for detecting rhizobia in cultures or in nodules. Two of the naturalized strains had distinct protein profiles but they could not be differentiated by the immunological method with a polyclonal antibody.

Localization of Bacteria and Hemoglobin in Root Nodules of Parasponia andersonii Containing Both Bradyrhizobium Strains and Rhizobium leguminosarum biovar trifolii

Dual occupancy of Parasponia andersonii nodules with different Bradyrhizobium strains and Rhizobium leguminosarum biovar trifolii was frequently obtained when two strains were inoculated into plants grown aseptically in tubes. Since reisolates of Bradyrhizobium strains from dually occupied nodules acquired the ability to nodulate Trifolium repens , the spatial relationship of the two species of bacteria during nodule initiation and development was investigated and their proximity was demonstrated. By using light microscopy and electron microscopy and immunogold labeling, R. leguminosarum biovar trifolii NGR66 inoculated alone onto P. andersonii produced small ineffective nodules, with bacteria embedded in matrix material in intercellular spaces and in a few nonliving host cells rather than in infection threads (CP299). In dual infections, the two bacterial species were shown to be adjacent to one another in the matrix of nodule intercellular spaces and in some host nodule cells. However, when two different Bradyrhizobium strains occupied a single nodule, they were located in different lobes of the same nodule. Immunogold labeling showed that Parasponia hemoglobin was localized in the cytoplasm of young infected nodule cells. This suggests that the nitrogen-fixing phase of Parasponia nodule cells is short-lived and correlates with previous acetylene reduction data from nodule slices. Hemoglobin was associated only with areas of nodule tissue infected with the effective nitrogen-fixing strain CP299 and absent from areas infected with R. leguminosarum biovar trifolii .

Competition by Bradyrhizobium Strains for Nodulation of the Nonlegume Parasponia andersonii

Bradyrhizobium strains isolated from the nonlegume Parasponia spp. formed a group of strains that were highly competitive for nodulation of P. andersonii when paired with strains isolated from legumes. Strains from legumes, including those of similar effectiveness to NGR231 and CP283, were not able to form nodules as single occupants on P. andersonii in the presence of Parasponia strains. However, NGR86, an isolate from Macroptilium lathyroides , jointly occupied one-third of the nodules formed with each of the three strains isolated from Parasponia spp. Time taken for nodules to appear may have influenced the outcome of competition, since CP283 and all isolates from legumes were slow to nodulate P. andersonii . Among the Parasponia strains, competitiveness for nodulation of P. andersonii was not associated with effectiveness of nitrogen fixation. The highly effective strain CP299 was a poor competitor when paired with the least effective strain NGR231. CP283 was the least competitive of the Parasponia strains but was still able to dominate nodules when paired with legume isolates. Dual occupancy was high, up to 67% when the inoculum contained CP299 and CP273. Both the Muc + and Muc - types of CP283 form a symbiosis of similar effectiveness and were similarly competitive at high inoculation densities, but the Muc - form was more competitive at low inoculum densities. Both forms frequently occupied the same nodule. Bradyrhizobium strains isolated from Parasponia spp. may have specific genetic information that favor their ability to competitively and effectively infect plants in the genus Parasponia (Ulmaceae) outside the Leguminosae.

Production and Use of Monoclonal Antibodies for Identification of Strains of Rhizobium trifolii

We produced a monoclonal antibody against Rhizobium trifolii 162×95. This antibody in cell culture supernatant was used in an indirect enzyme-linked immunosorbent assay to differentiate strain 162×95 from naturalized strains in the Appalachian region. Nodules crushed in 0.1 to 0.2 ml of phosphate-buffered saline and used to charge enzyme-linked immunosorbent assay plates gave strong absorbance readings. Heat-inactivated and noninactivated portions of 162×95 cultures were strongly reactive, indicating that the antigen is probably a carbohydrate. Of 10 strains from California, where 162×95 was isolated, 6 strongly cross-reacted with the antibody. The cellular protein patterns in a sodium dodecyl sulfate-polyacrylamide gradient gel of cross-reactive strains were essentially identical. A Western blot analysis indicated that the antibody was against a 19.8-kilodalton band. The Western blot analysis also revealed that the polyvalent antiserum contained other strongly reacting antibodies with molecular weights of approximately 20,000, indicating the possibility that other monoclonal antibodies to detect strain 162×95 may be produced. However, the available antibody has been shown to be useful for short-term experiments. Based upon protein profiles and immunological reactions, there are 4 or 5 California strains rather than 10.

Application of immunodiffusion to the identification of Rhizobium meliloti strains competing for nodulation on Medicago sativa

The immunodiffusion technique was successfully used to unambiguously recognize four strains of Rhizobium meliloti in a study of competition for nodulation with Medicago sativa cv. Apollo inoculated with two-, three- and four-strain mixtures. The serological reactions of all R. meliloti strains revealed no significant changes following plant passage indicating that the antigens involved in immunodiffusion were stable. R. meliloti 102F70 formed 50% or more of the nodules on M. sativa inoculated with two-, three- and four-strain mixtures. The remaining three strains were less competitive and produced similar proportions of nodules (14-20%) on plants inoculated with three- and four-strain mixtures. Cases of mixed-strain occupancy of nodules involving either two of three strains were detected in a sub-sample of nodules. The data also indicated considerable variation in the proportions of strains in the nodules of individual plants.

Variations in Ability of Rhizobium japonicum Strains To Nodulate Soybeans and Maintain Fixation in the Presence of Nitrate

This study investigated differences in sensitivity to nitrate of soybean ( Glycine max cv. Davis) symbioses with 16 different Rhizobium japonicum strains. When nitrate (20 mM) was added to established symbioses, there were no significant differences in the degree of inhibition of acetylene reduction for any of the 16 strains. When nitrate was present during the establishment of nodules, high levels of nitrate (10 mM) were equally inhibitory on all symbioses, whereas specific strain effects appeared at low (0.5 mM) to medium (2.0 mM) levels of nitrate. At 1.5 mM nitrate in solution culture, the days to emergence of nodules varied from less than 10 (CB:1809, Nit61A118) to more than 16 (11 of 16 strains). In a clay-pot trial maintained at the low nitrate level (0.5 mM), symbioses with CB:1809 increased total nodule mass by 30% relative to nitrate-free controls. In the presence of 2.0 mM nitrate, CB:1809 maintained total nodule mass. For the remaining 6 strains tested, total nodule mass decreased to below the levels of the nitrate-free controls. In a separate clay pot trial, CB:1809 increased its competitive ability relative to USDA:110 when nitrate was added. If no nitrate was added, CB:1809 occupied 0.97 times as many nodules as USDA:110; when 10 mM nitrate was added, CB:1809 occupied 1.75 times as many nodules as USDA:110. Attempts to select nitrogen-adapted substrains of R. japonicum through sequential isolation and infection of plants grown on nitrate were not successful.

A strain marker in Rhizobium trifolii based on the absorption of congo-red

It is widely recognized that most rhizobia absorb congo-red weakly when grown on agar media containing this dye. In contrast to this generalized behaviour, an ineffective strain of Rhizobium trifolii (Coryn) has been found which strongly absorbs the dye, producing growth of a very deep-red colour on yeast-extract mannitol agar, supplemented with congo-red (0.025 g/l). Assays for dye absorption characteristics on this medium with 21 strains of R. trifolii showed that Coryn could be readily differentiated from all the other strains examined. Supplementary fluorescent antibody tests with Coryn confirmed the reliability of differential absorption of congo-red as a strain-marker system. Data are presented which demonstrate the potential of this characteristic as a strain identification method in nodulation and competition studies under aseptic growth conditions, using mixed inocula.