Dynamics of Rhizobium competition for nodulation of Pisum sativum cv. Afghanistan

Association of Rhizobium Strains with Roots of Trifolium repens

TWO TECHNIQUES WERE USED TO ASSESS THE BINDING OF RHIZOBIA TO CLOVER ROOTS: indirect counting after radiolabeling the bacteria and direct counting by using phase-contrast microscopy. Microscopic observations revealed a large variability in the number of bacteria associated with individual root hairs. This variability made unbiased counting by microscopy difficult. Systematic examination of all visible root hairs and "blind" counting of coded strains and treatments were adopted to minimize observer bias. The validity of the radiolabeling method was also examined in some detail. The reproducibility of results from this method was satisfactory. However, drawbacks of this method included its lack of sensitivity and its failure to distinguish between bacteria attached to mature root hairs, emerging root hairs, and undifferentiated epidermal cells. The method also failed to distinguish between individual bacteria and any aggregates that may be present. The ability of a number of chosen mutant strains of Rhizobium trifolii and their corresponding parent strains, as well as a number of nonhomologous strains, to bind to clover roots was assessed by using both of these methods. Our results gave no indication of specificity of R. trifolii binding to clover roots. 2-Deoxy-d-glucose did not appear to have a major inhibitory effect on the attachment of rhizobia to the host root, which suggests that lectin cross-bridging is not an obligatory step in the initiation of infection even though it may occur under some conditions. The presence or absence of the symbiotic plasmid was not correlated with bacterial adherence to the host plant root. Since host specificity functions are carried on this plasmid, our results suggest that binding of rhizobia to the legume root is not the basis of host specificity.

Host-Controlled Restriction of Nodulation by Bradyrhizobium japonicum Strains in Serogroup 110

We previously reported the identification of a soybean plant introduction (PI) genotype, PI 417566, which restricts nodulation by Bradyrhizobium japonicum MN1-1c (USDA 430), strains in serogroup 129, and USDA 110 (P. B. Cregan, H. H. Keyser, and M. J. Sadowsky, Appl. Environ. Microbiol. 55:2532-2536, 1989, and Crop Sci. 29:307-312, 1989). In this study, we further characterized nodulation restriction by PI 417566. Twenty-four serogroup 110 isolates were tested for restricted nodulation on PI 417566. Of the 24 strains examined, 62.5% were restricted in nodulation by the PI genotype. The remainder of the serogroup 110 strains tested (37.5%), however, formed significant numbers of nodules on PI 417566, suggesting that host-controlled restriction of nodulation by members of serogroup 110 is strain dependent. Analysis of allelic variation at seven enzyme-encoding loci by multilocus enzyme electrophoresis indicated that the serogroup 110 isolates can be divided into two major groups. The majority of serogroup 110 isolates which nodulated PI 417566 belonged to the same multilocus enzyme electrophoresis group. B. japonicum USDA 110 and USDA 123 were used as coinoculants in competition-for-nodulation studies using PI 417566. Over 98% of the nodules formed on PI 417566 contained USDA 123, whereas less than 2% contained USDA 110. We also report the isolation of a Tn5 mutant of USDA 110 which has overcome nodulation restriction conditioned by PI 417566. This mutant, D4.2-5, contained a single Tn5 insertion and nodulated PI 417566 to an extent equal to that seen with the unrestricted strain USDA 123. The host range of D4.2-5 on soybean plants and other legumes was unchanged relative to that of USDA 110, except that the mutant nodulated Glycine max cv. Hill more efficiently. While strain USDA 110 has the ability to block nodulation by D4.2-5 on PI 417566, the nodulation-blocking phenomenon was not seen unless strain USDA 110 was inoculated at a 100-fold greater concentration than the mutant strain.

Quantitation of adsorption of rhizobia in low numbers to small legume roots

Bacteria adsorbed in low numbers to alfalfa or clover root surfaces were counted after incubation of seedlings in mineral solution with very dilute inocula (less than 10 bacteria per ml) of an antibiotic-resistant strain under defined conditions. After specified washing, bacteria which remained adsorbed to roots were selectively quantitated by culturing the roots embedded in yeast extract-mannitol-antibiotic agar and counting the microcolonies along the root surface; the range was from about 1 bacterium per root (estimated as the most probable number) to 50 bacteria per cm of root length (by direct counting). This simple procedure can be used with any pair of small-rooted plant and antibiotic-resistant bacterium, requires bacterial concentrations comparable to those frequently found in soils, and yields macroscopic localization and distribution data for adsorbed bacteria over the root surface. The number of adsorbed bacteria was proportional to the size of the inoculum. One of every four Rhizobium meliloti cells adsorbed in very low numbers to alfalfa roots resulted in the formation of a nodule. Overall adsorption of various symbiotic and nonsymbiotic bacterial strains to alfalfa and clover roots did not reflect the specificities of these legumes for their respective microsymbionts, R. meliloti and R. trifolii.

Host-Symbiont Specificity Expressed during Early Adsorption of Rhizobium meliloti to the Root Surface of Alfalfa

Early (4 h) adsorption of Rhizobium meliloti L5-30 in low numbers to alfalfa roots in mineral solution was examined for competition with other bacterial strains. All tested competitor strains decreased the adsorption of L5-30 by extents which depended on the strain and its concentration. The decrease of adsorption by R. meliloti competitors (all of them infective in alfalfa) was nearly complete at saturation (97 to 99% decrease). All other heterologous rhizobia and Agrobacterium tumefaciens at saturating concentrations (10 to 10 per ml) decreased adsorption of L5-30 only partially, less than 60%. The differential effects of homologous and heterologous competitors indicate that initial adsorption of R. meliloti to the root surface of its host occurs in symbiont-specific as well as nonspecific modes and suggest the existence of binding sites on roots which are highly selective for the specific microsymbiont in the presence of other heterologous bacteria even in very unfavorable (less than 10) symbiont-competitor concentration ratios.

Performance of phaseolus bean rhizobia in soils from the major production sites in the Nile Delta

The symbiotic and competitive performances of two highly effective rhizobia nodulating French bean P. vulgaris were studied in silty loam and clayey soils. The experiments were carried out to address the performance of two rhizobia strains (CE3 and Ph. 163] and the mixture thereof with the two major cultivated bean cultivars in two soil types from major growing French bean areas in Egypt. Clay and silty loam soils from Menoufia and Ismailia respectively were planted with Bronco and Giza 6 phaseolus bean cultivars. The data obtained from this study indicated that rhizobial inoculation of Giza 6 cultivar in clayey soil showed a positive response to inoculation in terms of nodule numbers and dry weight. This response was also positive in dry matter and biomass accumulation by the plants. The inoculant of strain CE3 enhanced plant growth and N-uptake relative to Ph. 163. However, the mixed inoculant strains were not always as good as single strain inoculants. The competition for nodulation was assessed using two techniques namely fluorescent antibody testing (FA) and REP-PCR fingerprinting. The nodule occupancy by inoculant strain Ph. 163 in both soils occupied 30-40% and 38-50 of nodules of cultivar Bronco. The mixed inocula resulted in higher proportions of nodules containing CE3 in silty loam soil and Ph. 163 in clayey soil. The native rhizobia occupied at least 50% of the nodules on the Bronco cultivar. For cultivar Giza 6, the native rhizobia were more competitive with the inoculant strains. Therefore, we suggest using the studied strains as commercial inocula for phaseolus bean.

Characterization of the lipopolysaccharide from a Rhizobium phaseoli mutant that is defective in infection thread development

The lipopolysaccharide (LPS) from a Rhizobium phaseoli mutant, CE109, was isolated and compared with that of its wild-type parent, CE3. A previous report has shown that the mutant is defective in infection thread development, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis shows that it has an altered LPS (K. D. Noel, K. A. VandenBosch, and B. Kulpaca, J. Bacteriol. 168:1392-1462, 1986). Mild acid hydrolysis of the CE3 LPS released a polysaccharide and an oligosaccharide, PS1 and PS2, respectively. Mild acid hydrolysis of CE109 LPS released only an oligosaccharide. Chemical and immunochemical analyses showed that CE3-PS1 is the antigenic O chain of this strain and that CE109 LPS does not contain any of the major sugar components of CE3-PS1. CE109 oligosaccharide was identical in composition to CE3-PS2. The lipid A's from both strains were very similar in composition, with only minor quantitative variations. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of CE3 and CE109 LPSs showed that CE3 LPS separated into two bands, LPS I and LPS II, while CE109 had two bands which migrated to positions similar to that of LPS II. Immunoblotting with anti-CE3 antiserum showed that LPS I contains the antigenic O chain of CE3, PS1. Anti-CE109 antiserum interacted strongly with both CE109 LPS bands and CE3 LPS II and interacted weakly with CE3 LPS I. Mild-acid hydrolysis of CE3 LPS I, extracted from the polyacrylamide gel, showed that it contained both PS1 and PS2. The results in this report showed that CE109 LPS consists of only the lipid A core and is missing the antigenic O chain.

Cloning of Rhizobium leguminosarum genes for competitive nodulation blocking on peas

One type of competitive interaction among rhizobia is that between nonnodulating and nodulating strains of Rhizobium leguminosarum on primitive pea genotypes. Pisum sativum cv. Afghanistan nodulates effectively with R. leguminosarum TOM, and this can be blocked in mixed inoculations by R. leguminosarum PF2, which does not nodulate this cultivar. We termed this PF2 phenotype Cnb+, for competitive nodulation blocking. Strain PF2 contains three large plasmids including a 250-kilobase-pair symbiotic (Sym) plasmid. Transfer of this plasmid, pSymPF2, to nonblocking rhizobia conferred the Cnb+ phenotype on recipients in mixed inoculations on cultivar Afghanistan with TOM. A library of the PF2 genome constructed in the vector pMMB33 was used to isolate two cosmid clones which hybridize to pSymPF2. These cosmids, pDD50 and pDD58, overlapped to the extent of 23 kilobase pairs and conferred a Cnb+ phenotype on recipient Cnb- rhizobia, as did pSD1, a subclone from the common region.