Search

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.

The review summarizes the results of studies on the comigration of tubercular bacteria and bean plants to new habitats, which is often accompanied by a decrease in the symbiosis efficiency due to a loss of the diversity of genes responsible for the interaction. This migration may lead to a rise in new symbionts as a result of gene transfers from initial symbionts to local bacteria. It was demonstrated that typically new symbionts lack an ability for N2 fixation but are highly competitive, blocking the inoculation of bean cultures by industrial strains. The design of coadapted systems of recognition and signal interaction of partners is a perspective approach to ensure competitive advantages of efficient rhizobia strains introduced into agrocenoses, together with host plants, over inactive local strains.

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.

Compared to free-living relatives, bacterial symbionts of plants exhibit higher rates of population diversification (microevolution), as well as of speciation and formation of super-species taxa (macroevolution). Analysis of rhizobia, the N2-fixing symbionts of leguminous plants, suggests that both evolutionary modes are determined by the hosts, which have mutagenic and recombinogenic effects on their microsymbionts, apart from activating the selective processes that determine the fate of newly emerged genotypes in their populations. Migration induced by leguminous plants plays a key role in the evolution of rhizobia, including: (1) joint movement of the symbionts and their hosts into new ecological zones and (2) local circulation of the symbionts in plant‒soil systems. In the first case, allopatric speciation occurs in rhizobia due to transfer of the symbiotically specialized (sym) genes from the introduced strains into local bacteria, which are transformed into novel symbionts. In the second case, a sympatric divergence of Rhizobium populations occurs, resulting in formation of intraspecies taxa (biovars and symbiovars) contrasting in their host ranges. In the evolutionarily advanced rhizobia, Rhizobium leguminosarum and Neorhizobium galegae interacting with galegoid legumes (tribes Fabeae, Galegae, and Trifolieae), these processes are associated with diversification of the sym genes representing the accessory genome parts due to the interaction between bacteria and the hosts differing in their symbiotic affinities. In parallel, formation of new cryptic (genomic) species was revealed in local rhizobia populations, which is associated with divergence of the core genome parts. It may be caused either by genetic drift or by selection for resistance against local edaphic stresses, which in combination with disruptive selection can sufficiently enhance the rhizobia diversity and change the structures of their pangenomes. [ABSTRACT FROM AUTHOR]

The goat's rue rhizobia (Neorhizobium galegae) represent a convenient model to study the evolution and speciation of symbiotic bacteria. This rhizobial species is composed of two biovars (bv. orientalis and bv. officinalis), which form N2-fixing nodules with certain species of goat's rue (Galega orientalis and G. officinalis). The cross-inoculation between them results in the formation of nodules unable to fix nitrogen. On the basis of the data on the whole-genome sequencing, we studied the nucleotide polymorphism of 11 N. galegae strains isolated from the North Caucasus ecosystems, where G. orientalis has higher diversity than G. officinalis. The low level of differences in the polymorphism within the group of the sym genes in comparison with the nonsymbiotic genes can be associated with the active participation of host plants in the evolution of rhizobia. The intragenic polymorphism of bv. orientalis proved to be significantly higher than that of bv. officinalis. The level of polymorphism of nonsymbiotic genes was lower than that of the symbiotic genes, which are functionally more homogeneous. The divergence of the nitrogen fixation genes (nif/fix) is more pronounced than that of the nodule formation genes (nod) in the N. galegae biovars. These facts indicate the leading role of the host-specific nitrogen fixation in the evolution of the studied rizhobial species. [ABSTRACT FROM AUTHOR]

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]