ITS Polymorphism of Salt Tolerant and Salt Sensitive Native Isolates of Sinorhizoblum meliloti Symbionts of Alfalfa, Clover and Fenugreek Plants

Polymorphism of rrsrrl sequence of ribosomal operons (intergenic sequence, ITS) was studied among 81 isolates of Sinorhizobium meliloti (AK001–AK210) derived from the collection of alfalfa nodulating bacteria of the Laboratory of genetics of ARRIAM, by using speciesspecific primers FGPS1490/ FGPL132VM. Isolates were obtained from nodules of different species of wild host plants from Medicago, Melilotus and Trigonella genera grown in salinized NorthWestern region of Kazakhstan. The typical structure of ITS, similar to that of test strain Rm1021, was dominant in native rhizobia population, while in one third of the isolates (33.3%) this sequence was different. Among the latter, the ITS type of strain AK83 (RCAM00182) was dominant. Here, we show for the first time that isolates with reduced level of salttoler ance had more diverse intergenic sequences of rrnoperons. No phylogenetic separation was observed between isolates grouped on the basis of their tolerance or sensitivity towards 0.6 M NaCl. However, the fre quency of divergent ITS types within the two groups of rhizobia depended on the host symbiotic preference observed in natural environment, allowing to speculate about the existence of a chromosome types specific for S. meliloti isolates with differential salt tolerance. In conclusion, we propose that in the area subjected to secondary salinization, which are also the centre of introgressive hybridization of alfalfa, microevolutionary processes, affecting rrnoperons and associated with salt adaptation, are also occurring in symbiotic root nod ule bacteria populations.