A collection of twenty-eight strains isolated from root nodules of Astragalus inopinatus Boriss. and Astragalus umbellatus Bunge originating from the Kamchatka Peninsula (Russian Federation) was obtained for the first time. Analysis of the 16S ribosomal RNA gene sequence revealed a significant diversity of isolates belonging to 4 genera of the order Hyphomicrobiales: Mesorhizobium, Rhizobium, Bosea and Tardiphaga. The presence of phenotypically and taxonomically different strains (Bosea + Tardiphaga, Bosea + Mesorhizobium, Tardiphaga + Mesorhizobium, Bosea + Rhizobium) in some nodules of both plants was also observed. The symbiotic efficiency of strains isolated from A. inopinatus plants in the nodulation assay showed that most isolates of the genus Mesorhizobium and Rhizobium can form a nitrogen-fixing symbiosis with A. inopinatus, leading to a significant increase in plant biomass compared to the non-inoculated control.

The paper studies the genetic diversity of microorganisms isolated from root nodules of wild populations of the legumes Lathyrus pratensis L., Vicia cracca L., Trifolium repens L., and Astragalus schelichowii Turcz. collected near Norilsk (Arctic Russia). The taxonomic position of the 19 isolates obtained was determined by sequencing the 16S rRNA gene (rrs). The isolates were assigned to four genera of the order Hyphomicrobiales: Rhizobium, Pararhizobium, Bosea and Tardiphaga. Nine fast-growing isolates belonged to the genera Rhizobium and Pararhizobium. ITS region sequencing clarified the species identity of 6 rhizobial isolates. Isolates from V. cracca nodules were identified as Pararhizobium sp., P. herbae. Microsymbionts from T. repens were assigned to Rhizobium sp. and to the species R. beringeri, while isolates from L. pratensis were identified as P. herbae and R. beringeri. Symbiotic nodA and nodC genes were found in the Rhizobium strains P8/5-2, P9/1-1 and P9/3-2 from L. pratensis and T. repens nodules, nodA gene was detected in the Pararhizobium strains P7/3-1, P7/4-1, P7/5-1 from V. cracca nodules. Nine out of ten slow-growing isolates were assigned to the genus Bosea. Three isolates from the nodules of V. cracca were assigned to the species B. psychrotolerans. Six isolates from the nodules of A. schelichowii were identified as B. vaviloviae, B. lathyri and Bosea sp. The strain P22/3-5 isolated from the nodule of A. schelichowii was identified as Tardiphaga robiniae. Simultaneous presence of strains belonging to different genera of the order Hyphomicrobiales was detected in the nodules of V. cracca and A. schelichowii.

Guar (Cyamopsis tetragonoloba (L.) Taub.) is an important legume plant used as food and industrial crop. Guar forms nitrogen-fixing symbiosis with nodule bacteria (rhizobia) supplying the plant with this important nutrient element. The ability to form symbiosis with rhizobia of various taxonomic groups of genera Bradyrhizobium and Ensifer is a common trait of guar with cowpea and soybean. Pot experiments were performed to compare nodulation and response of guar, cowpea and soybean to inoculation with nine strains isolated from guar nodules and belonging to the above-mentioned genera. Strains assigned to species Bradyrhizobium archetypum and Bradyrhizobium valentinum were isolated from nodules of guar for the first time. Nodule number and biomass on the inoculated guar roots significantly varied depending on the strain from 15 to 40 nodules per plant and from 20 to 54 mg dry weight (DW)/plant, respectively. Eight strains formed nitrogen-fixing symbiosis with cowpea. Nodule number and biomass on the cowpea roots varied depending on the strain from 19 to 67 nodules per plant and from 3 to 61 mg DW/plant, respectively. Significant differences among strains were also observed in acetylene reduction activity and positive effects of inoculation on shoot biomass and nitrogen content in shoots of plant species. However, symbiosis with soybean was inefficient, since only few small white nodules were observed and no effects were found on plant growth. Correlations between the symbiotic parameters in guar and cowpea dependent on the rhizobia strain were found and discussed in view of mechanisms underlying formation and functioning of nitrogen-fixing symbiosis. Several studied strains offer promise as inoculants for guar and cowpea and recommended for further field trials.

The ability of nine bacterial strains of the genus Rhizobium and Mesorhizobium, isolated from nodules of various species of legumes Oxytropis, Astragalus, Lathyrus, Vicia and Hedysarum growing in Arctic Yakutia, to form nitrogen-fixing symbiosis with Trifolium repens L., Medicago sativa L., Oxytropis adamsiana (Trautv.) Jurtzev and Astragalus frigidus L. under conditions of sterile test-tube experiment was studied. The ability of eight strains to form both ineffective and nitrogen-fixing nodules depending on the inoculation variant of legume plants was shown. The ability of O. adamsiana to form an effective symbiosis with three strains isolated from leguminous plants of the genera Hedysarum, Oxytropis and Astragalus allows us to classify it as promising for bioremediation of disturbed lands and creation of highly productive agrophytocenoses in various soil and climatic conditions of the Arctic.

Bacterial strains isolated from root nodules of the legume plant Hedysarum arcticum B. Fedtsch growing on Samoylov Island in the Lena River delta (Arctic zone of Yakutia) were assigned to the genera Rhizobium (family Rhizobiaceae) and Mesorhizobium (Phyllobacteriaceae) of the order Hyphomicrobiales (class Alphaproteobacteria) according to the rrs gene sequencing data. According to phylogenetic analysis of concatemers of the atpD, dnaK, gyrB, and rpoB genes, the strains belonged to the species Rhizobium giardinii and Mesorhizobium norvegicum. The strains were shown to be facultative psychrotrophs growing at 5 and 28°C. These microsymbionts are promising for further study of their symbiotic efficiency regarding other forage legume species, with an aim to establish highly productive agrophytocenoses in the Far North.

The combined effect of Hg and Cd on the growth, elemental composition, root exudation and interactions with rhizobia of pea SGE and its mutant SGECdt was studied in hydroponics and sand. The tolerance mechanisms of legume-rhizobia symbiosis to heavy metals are discussed.