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

Abstract

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 311 b135 in growth rate, sensitivity to phage Rhj78 1, 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 0-32mS), 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 1 18 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.

Characterization of Soybean-Nodulating Rhizobial Communities and Diversity

“…Keyser et al (1984) examined the distribution of indigenous soybean-nodulating rhizobia in the United States and found that serogroup 123 was dominant in northern regions and B. elkanii in southern regions. Other studies support these results, with some exceptions in the case of alkaline soils, in which Bradyrhizobium serogroup 135 or Sinorhizobium/ Ensifer fredii predominate (Damirgi et al, 1967;Ham et al, 1971;Kowalski et al, 1974;Gross et al, 1979;Suzuki et al, 2008). Further studies are still needed, however, to examine the many environmental factors potentially affecting indigenous bradyrhizobia.…”

Insights learned from pBTAi1, a 229-kb accessory plasmid from Bradyrhizobium sp. strain BTAi1 and prevalence of accessory plasmids in other Bradyrhizobium sp. strains

“…While a few studies have reported the presence of two to four plasmids in B. japonicum strains, ranging in size from 74 to 294 kb (Gross et al, 1979;Masterson et al, 1982), the taxonomic status of some of the strains was not verified using molecular, phylogenetic tools (Gross et al, 1979;Masterson et al, 1982). Although Masterson et al (1982) showed that nod genes were not found on any plasmids they detected, no information was presented regarding the genetic composition of these plasmids, and thus the physiological role of plasmids in bradyrhizobia could not be inferred.…”

The Accidental Plant Pathologist

“…A biotype of Bradyrhizobium japonicum, indigenous to alkaline soils and competitive with commercial inoculants, was originally discovered in determining the cause of chlorosis in soybeans. The biotype ultimately was found to be highly efficacious in nitrogen fixation, despite slower growth than other strains of B. japonicum (4). Efforts to interest industry in commercializing these strains were unsuccessful owing to the limited potential market and the slow growth rate of the bacterium.…”