Inoculation of soybean with Bradyrhizobium japonicum is often unsuccessful owing to the failure of inoculum strains to nodulate soybeans (Glycine max (L.) Merr.) in the presence of indigenous strains of rhizobia in soil. Previous studies have shown that it is possible to reduce nodulation with indigenous strains of rhizobia by amending the soil with a bacteriophage specific for the indigenous strain. The objective of the current study was to determine whether the coating of seed with phage affected nodule occupancy and soybean growth. A phage specific for B. japonicum USDA 469 and a symbiotically superior strain of rhizobium (B. japonicum USDA 110) were coated together onto soybean seed and planted into both greenhouse and field soil previously inoculated with B. japonicum USDA 469. The phage coated onto seed reduced nodulation by B. japonicum USDA 469 to 48% occupancy, compared with 64% for the untreated control value. Nodulation by the superior inoculum strain was increased from 48 to 82% occupancy by coating seed with the homologous phage and B. japonicum USDA 110. The rate of nitrogenase activity (on a per plant basis) was increased by coating seed with the phage and B. japonicum USDA 110. No other plant or symbiotic parameters were affected by phage coating of seed. These results indicate that the nodulation of soybeans can be significantly affected by the coating of seed with phage specific for undesirable strains of rhizobia in soil and the concurrent coating of seed with desirable strains of rhizobia. Key words: competition, rhizobiophage, rhizobia, soybeans.
Addition of heavy metals to soils from sources such as biosolid application, smelter emissions, fertilizers, and the like may produce soil solution concentrations of Zn and Cd that can potentially restrict nodulation of legume crops such as white clover. To assess the effects of Zn and Cd on the early stages of nodulation of white clover, a solution culture study was conducted. Rhizobia [Rhizobium leguminosarum bv. trifolii] of a metal‐sensitive strain (USDA 2063) and a metal‐tolerant strain (USDA 2046) were inoculated into nutrient solution with white clover seedlings containing EGTA to buffer Zn2+ and Cd2+ activities. The calculated activities of Zn2+ were p = 8.00, 5.70, 5.25, and 5.00; the calculated activities of Cd2+ were p = 10.50, 9.50, 8.75, and 8.50. Phase‐contrast microscopy was used after staining roots with methylene blue to observe root hair infection at intervals of 1, 12, 24, 72, and 168 h. Few differences were observed between the sensitive and tolerant rhizobial strains. At the lowest Zn concentration, initial attachment of bacteria to root hairs was observed within 1 h. At the highest Zn concentration, attachment was not observed until 12 h. Also, at the highest Zn concentration, a delay was observed for up to 60 h and 4 d, respectively, for the first signs of root hair curling and infection thread formation. Shoot weight decreased with increasing Zn concentration. Few Cd‐induced effects on early stages of nodule development were observed. Cadmium was more toxic to plant growth than Zn. These results show that in addition to direct effects on the macro and microsymbiont, the process of nodulation is susceptible to the toxic effects of heavy metals.
Thirty-three strains of Bradyrhizobium japonicum within serogroup 110 were examined for genotypic diversity by using DNA-DNA hybridization analyses. The analysis of the DNA from 15 hydrogen-uptakenegative strains with the bradyrhizobial uptake hydrogenase probe pHU52 showed variation in degree of homology and restriction fragment length polymorphism of EcoRI-restricted DNA. Clustering analysis of the 33 strains on the basis of DNA-DNA hybridization analysis with four restriction enzymes and with the bradyrhizobial nodulation locus, pRJUT10, as probe indicated the existence of four groups of strains, which were less than 70%o similar. Restriction digestion of genomic DNA with BamHI and DNA-DNA hybridization with pRJUT10 permitted classification of each of the strains according to a specific fingerprint pattern.
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