Instability of cryptic plasmids in Sinorhizobium meliloti laboratory strains SKhM1-188, 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 SKhM1-188 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.
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