Ekaterina S. Akimova

The ability of nodule bacteria to form a nitrogen-fixing legume–rhizobium symbiosis is ensured by a cluster of symbiotic genes that determine both the processes of interaction with the legume, leading to the formation of specialized structures called nodules on the roots, and the fixation of atmospheric nitrogen. The organization and localization of these genomic clusters facilitate their active participation in horizontal gene transfer. The frequency of transfer correlates with the degree of relationship between the bacteria. Recombination events lead to formation of numerous rhizobial strains with varying symbiotic efficiency. In this study, we investigated the possible mechanisms underlying the formation of genetic diversity in nodule bacteria and the involvement of the legume in this diversity. It was demonstrated that the pangenome of a rhizobial population in the rhizosphere of wild legumes may contain several allelic variants of symbiotic genes, which, upon recombination, can form various combinations. This suggests that the evolution of the symbiotic system of nodule bacteria may occur not through the formation of an optimal cluster of symbiotic genes, but through individual evolution of each gene component, which, when assembled into different mosaics, produce diverse symbiotic phenotypes of rhizobia. This allows the host plant to select the most effective microorganisms based on the changing environmental conditions.

BACKGROUND: The beginning of the life cycle of a leguminous plant in its natural habitat is usually associated with interaction with nodule bacteria in order to form a nitrogen-fixing symbiosis. In a short period of time, a plant needs to “choose” suitable microsymbionts for itself. Since a wide variety of rhizobial strains is formed in the rhizosphere of legumes, the choice made by the macrosymbiont will further influence its productivity. AIM: The purpose of our work was to compare the principles of selection by different plants of their microsymbionts at different stages of plant development. MATERIALS AND METHODS: Nodule bacteria Trifolium hybridum L. and Galegaorientalis Lam. were taken into the study. Their genetic diversity was studied by the RAPD method, a phylogenetic analysis of bacteria and their symbiotic nodC and nifH genes was carried out, and their nitrogen-fixing activity was assessed. RESULTS: It was found that the rhizobia that form nodules on the roots of the studied leguminous plants at different stages of their vegetation have certain patterns. It was found that the highest polymorphism and specific nitrogen-fixing activity are characteristic of bacteria obtained from nodules formed at the initial stage of vegetation. CONCLUSIONS: We assume that the plasticity of the rhizobia genome gives the host plant the ability to more flexibly adjust its nitrogen-fixing apparatus to changes in growing conditions.

Cadmium (Cd) stress is an obstacle for crop production, quality crops, and sustainable agriculture. An important role is played by the application of eco-friendly approaches to improve plant growth and stress tolerance. In the current study, a pre-sowing seed treatment with Rhizobium leguminosarum strains, isolated from the leguminous plants Phaseolus vulgaris (strain Pvu5), Vicia sylvatica (strain VSy12), Trifolium hybridium (strain Thy2), and T. pratense (strain TPr4), demonstrated different effects on wheat (Triticum aestivum L.) plant growth under normal conditions. Among all tested strains, Thy2 significantly increased seed germination, seedling length, fresh and dry biomass, and leaf chlorophyll (Chl) content. Further analysis showed that Thy2 was capable of producing indole-3-acetic acid and siderophores and fixing nitrogen. Under Cd stress, Thy2 reduced the negative effect of Cd on wheat growth and photosynthesis and had a protective effect on the antioxidant system. This was expressed in the additional accumulation of glutathione and proline and the activation of glutathione reductase. In addition, Thy2 led to a significant reduction in oxidative stress, which was evidenced by the data on the stabilization of the ascorbate content and the activity of ascorbate peroxidase. In addition, Thy2 markedly reduced Cd-induced membrane lipid peroxidation and electrolyte leakage in the plants. Thus, the findings demonstrated the ability of the R. leguminosarum strain Thy2, isolated from T. hybridium nodules, to exert a growth-promoting and anti-stress effect on wheat plants. These results suggest that the Thy2 strain may enhance wheat plant growth by mitigating Cd stress, particularly through improving photosynthesis and antioxidant capacity and reducing the severity of oxidative damage. This may provide a basic and biological approach to use the Thy2 strain as a promising, eco-friendly candidate to combat Cd stress in wheat production.