Plasmids, Biological Properties and Efficacy of Nitrogen Fixation in Rhizobium japonicum Strains Indigenous to Alkaline Soils

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Abstract
Summary: Plasmids were isolated from strains of Rhizobium japonicum, predominantly serogroup 135, obtained from soybean nodules collected at 15 sites in Nebraska, U.S.A. In addition to their serotype, these strains were indistinguishable from R. japonicum strain 3I1b135 in growth rate, sensitivity to phage Rhj781, antibiotic sensitivities, general colony characteristics and rates of nitrogen fixation per plant. All strains occupied soil habitats with similar characteristics, including a high pH (7.2 to 8.3), relatively high conductivity (0.04 to 0.32 mS), relatively high sodium saturation (0.32 to 12.7%), low iron content (3.2 to 14.8 p.p.m.) and low manganese content (5.1 to 18.7 p.p.m.). However, agarose gel electrophoresis analysis of plasmids enabled subdivision of these extra-slow-growing strains into four groups on the basis of differences in plasmid number and size. These strains carried combinations of two or more of four plasmids, ranging in mass from 49 to 118 megadaltons and comprising approximately 20% of the total DNA per cell. Biological and symbiotic data, along with plasmid analysis, were useful in identifying a wild-type strain (RJ23A) that shows potential as a soybean inoculant in alkaline soils.