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High-throughput sequencing of the amplicon gene library revealed variations in the population structure of clover rhizobia (Rhizobium leguminosarum bv. trifolii) upon transition from soil into the root nodules of the host plant (Trifolium-hybridum). Analysis of rhizobial-diversity using the nodA gene revealed 3258 and 1449 nucleotide sequences (allelic-variants) for the soil and root nodule population, respectively. They were combined into 29 operational taxonomic units (OTU) according to the 97% identity level; 24 OTU were. foundin the soil population, 12 were present in the root nodulepopulation, and 7 were common. The predominant OTE13 (77.4 and 91.5% of the soil and root nodule populations, respectively) contained 155 and -200 variants of the soil and root nodule populations, respectively, with the nucleotide diversity increasing significantly upon the "soil-->root" transition. The "moving window" approach was used to reveal the sites of the nodA gene in which polymorphism, including that associated with increased frequency of non-synonymous substitution frequency, increased sharply upon transition from soil into root nodiles. PCR analysis of the IGS genotypes of individual strains revealed insignificant changes in rhizobial diversity upon transition from soil into root nodules. These results indicate that acceleration of rhizobial evolutioin in the course of symbiosis may be associated with development of highly polymorphic virulent:subpopulations subjected to directional selection in the "plant-soil" system.

Two hundred forty-three isolates of alfalfa root-nodule bacteria ( Sinorhizobium meliloti) were obtained from nodules and soils sampled in the northern Aral region, experiencing secondary salinization. Isolates obtained from nodules (N isolates) were significantly more salt-tolerant than those from soils (T isolates) when grown in a liquid medium with 3.5% NaCl. It was found that wild species of alfalfa, melilot, and trigonella preferably formed symbioses with salt-tolerant root-nodule bacteria in both salinized and nonsalinized soils. Only two alfalfa species, Medicago falcata and M. trautvetteri, formed efficient symbioses in soils contrasting in salinity. The formation of efficient symbiosis with alfalfa in the presence of 0.6% NaCl was studied in 36 isolates (N and T) differing in salt tolerance and symbiotic efficiency. Fifteen isolates formed efficient symbioses in the presence of salt. The increase in the dry weight of the plants was 25–68% higher than in the control group. The efficiency of symbiotic interaction under salinization conditions depended on the symbiotic efficiency of the isolates under standard conditions but did not correlate with the source of root-nodule bacteria (soil or nodule) or their salt tolerance. The results indicate that the strains of root-nodule bacteria forming efficient symbioses under salinization conditions can be found. [ABSTRACT FROM AUTHOR]

Instability of cryptic plasmids in Sinorhizobium meliloti laboratory strains SXM1, DM7-R, and P108 as well as in their clones isolated from nodules of alfalfa grown during a long-term microvegetation experiment (120 days) was studied. The isolated clones of strains SXM1 and DM7-R manifested stable inheritance of plasmids, whereas 12.7–14.0% of clones with changed plasmid profile were detected in a population of clones from strain P108. These segregants were designated as P108c. Segregants P108c exhibited significantly decreased symbiotic effectiveness, nitrogenase activity, and the competitiveness with respect to alfalfa, compared to the original strain P108. It was established that a 80-kb deletion occurred in a larger of two cryptic plasmids (240 and 230 kb) of segregants P108c. It was concluded that genetic rearrangements are possible in rhizobial clones that did not undergo structural transformation and retained viability in the nodule during the natural vegetation period of alfalfa. [ABSTRACT FROM AUTHOR]