[Occurrence of islands in genomes of Sinorhizobium meliloti native isolates].

Genomes of 184 Sinorhizobium meliloti native isolates were studied to test the occurence of islands Sme21T, Sme19T, and Sme80S previously described in the model strain Rm1021. This analysis was conducted using PCR methodology involving specific primers. It was demonstrated that, in the examined geographically distinct populations of S. meliloti from the Northern Caucasus (NCG) and the Aral Sea region (PAG), the strains containing genomic islands were observed with similar frequency (0.55 and 0.57, respectively). Island Sme80S, denoted as an island of "environmental adaptivity," was identified predominantly (frequency of 0.38) in genomes of strains which exhibited a lower level of salt tolerance and was isolated in PAG, a modern center of introgressive hybridization of alfalfa subjected to salinity. Island Sme21T designated as "ancestral" was observed in genomes of strains isolated in NCG, the primary center of host-plant biodiversity, 10-fold more often than in strains from PAG. An island Sme19T, which predominantly carries genes encoding transposases, was observed in genomes of strains in both populations with average frequency of 0.10. The analysis of linkage disequilibrium (LD) based on the assessment of probability for detection of different islands combinations in genomes revealed an independent inheritance of islands in salt-sensitive strains of various geographic origin. In contrast, the absence of this trend was noted in the majority of the examined combinations of salt-tolerant strains. It was concluded that the structure of chromosome in PAG strains which predominantly possessed a salt-sensitive phenotype was subjected to active recombinant processes, which could predetermine the intensity of microevolutionary processes in bacterial populations and facilitate an adaptation of bacteria in adverse environmental effect.

[The Principle of Genome Complementarity in the Enhancement of Plant Adaptive Capacities].

In the present work, the potential for the enhancement of the adaptive capacity of microbe-plant systems (MPSs) through the integration of the symbiosis partners' genomes is considered on the example of different types of symbiotic relationships. The accumulated data on the genetic control of interactions for both the plant and microbe, which are discussed in the paper with respect to signaling genes, suggest that it is the complementarity of genetic determinants that underlies the successful formation of MPSs. A eukaryotic genome with limited information content, which is stable throughout a generation, is complemented by a virtually unlimited prokaryotic metagenome. The microsymbiont's ability to adapt to different living conditions is based on the restructuring of the accessory genome by different mechanisms, which are likely to be activated under the influence of plants, although the details of such a regulation remain unknown. Features of the genetic control of the interaction, particularly its universal character for different symbionts, allow us to formulate a principle of genome-complementarity with respect to interacting organisms and consider it an important factor, an adaptation that enhances the abilities of M PSs for their sustainable development in natural ecosystems and for high plant productivity in agrocenoses.

[Root Nodule Bacteria Sinorhizobium meliloti: Tolerance to Salinity and Bacterial Genetic Determinants].

The theoretical and experimental data on salt tolerance of root nodule bacteria Sinorhizobium meliloti (Ensifer meliloti), an alfalfa symbiont, and on genetic determination of this feature are reviewed. Extensive data on the genes affecting adaptation of proteobacteria are provided, as well as on the groups of genes with activity depending on the osmolarity of the medium. Structural and functional polymorphism of the bet genes involved in betaine synthesis and transport in S. meliloti is discussed. The phenotypic and. genotypic polymorphism in 282 environmental rhizobial strains isolated from the centers of alfalfa diversity affected by aridity and salinity is discussed. The isolates from the Aral Sea area and northern Caucasus were shown to possess the betC gene represented by two types of alleles: the dominant A-type allele found in Rm 1021 and the less common divergent E-type allele, which was revealed in regions at the frequencies at the frequencies of 0.35 and 0.48, respectively. In the isolates with the salt-tolerant phenotype, which were isolated from root nodules and subsequently formed less effective symbioses with alfalfa, the frequency of E-type alleles was 2.5 times higher. Analysis of the nucleotide and amino acid sequences of the E-type allele of the betC gene revealed that establishment of this allele in the population was a result of positive selection. It is concluded that diversification of the functionally diverse bet genes occurring in S. meliloti affects the salt tolerance and symbiotic effectivity of rhizobia.

[Influence of salt stress on the genetically polymorphic system of Sinorhizobium meliloti-Medicago truncatula].

The impacts of salt stress (75 mM NaC1) on the ecological efficiency of the genetically polymorphic Sinorhizobium meliloti-Medicago truncatula system were studied. Its impact on a symbiotic system results in an increase of the partners' variability for symbiotic traits and of the symbiosis integrity as indicated by: a) the specificity of the partners' interactions--the nonadditive inputs of their genotypes into the variation of symbiotic parameters; and b) the correlative links between these parameters. The structure of the nodDI locus and the content correlates to the efficiency of the symbiosis between S. meliloti and M. truncatula genotypes under stress conditions more sufficiently than in the absence of stress. Correlations between the symbiotic efficiency of rhizobia strains and their growth rate outside symbiosis are expressed under stress conditions, not in the absence of stress. Under salt stress symbiotic effectiveness was decreased for M. truncatula line F83005.5, which was salt sensitive for mineral nutrition. The inhibition of symbiotic activity for this line is linked with decreased nodule formation, whereas for Jemalong 6 and DZA315.16 lines it is associated with repressed N2-fixation. It was demonstrated for the first time that salt stress impairs the M. truncatula habitus (the mass : height ratio increased 2- to 6-fold), which in the salt-resistant cultivar Jemalong 6 is normalized as the result of rhizobia inoculation.

Galega orientalis is more diverse than Galega officinalis in Caucasus--whole-genome AFLP analysis and phylogenetics of symbiosis-related genes.

Legume plants can obtain combined nitrogen for their growth in an efficient way through symbiosis with specific bacteria. The symbiosis between Rhizobium galegae and its host plant Galega is an interesting case where the plant species G. orientalis and G. officinalis form effective, nitrogen-fixing, symbioses only with the appropriate rhizobial counterpart, R. galegae bv. orientalis and R. galegae bv. officinalis, respectively. The symbiotic properties of nitrogen-fixing rhizobia are well studied, but more information is needed on the properties of the host plants. The Caucasus region in Eurasia has been identified as the gene centre (centre of origin) of G. orientalis, although both G. orientalis and G. officinalis can be found in this region. In this study, the diversity of these two Galega species in Caucasus was investigated to test the hypothesis that in this region G. orientalis is more diverse than G. officinalis. The amplified fragment length polymorphism fingerprinting performed here showed that the populations of G. orientalis and R. galegae bv. orientalis are more diverse than those of G. officinalis and R. galegae bv. officinalis, respectively. These results support the centre of origin status of Caucasus for G. orientalis at a genetic level. Analysis of the symbiosis-related plant genes NORK and Nfr5 reveals remarkable diversity within the Nfr5 sequence, although no evidence of adaptive evolution could be found.

Symbiotic and genetic diversity of Rhizobium galegae isolates collected from the Galega orientalis gene center in the Caucasus.

This paper explores the relationship between the genetic diversity of rhizobia and the morphological diversity of their plant hosts. Rhizobium galegae strains were isolated from nodules of wild Galega orientalis and Galega officinalis in the Caucasus, the center of origin for G. orientalis. All 101 isolates were characterized by genomic amplified fragment length polymorphism fingerprinting and by PCR-restriction fragment length polymorphism (RFLP) of the rRNA intergenic spacer and of five parts of the symbiotic region adjacent to nod box sequences. By all criteria, the R. galegae bv. officinalis and R. galegae bv. orientalis strains form distinct clusters. The nod box regions are highly conserved among strains belonging to each of the two biovars but differ structurally to various degrees between the biovars. The findings suggest varying evolutionary pressures in different parts of the symbiotic genome of closely related R. galegae biovars. Sixteen R. galegae bv. orientalis strains harbored copies of the same insertion sequence element; all were isolated from a particular site and belonged to a limited range of chromosomal genotypes. In all analyses, the Caucasian R. galegae bv. orientalis strains were more diverse than R. galegae bv. officinalis strains, in accordance with the gene center theory.