Aluminium being one of the most abundant elements is very toxic for plants causing inhibition of nutrient uptake and productivity. The aim of this study was to evaluate the potential of microbial consortium consisting of arbuscular mycorrhizal fungus (AMF), rhizobia and PGPR for counteracting negative effects of Al toxicity on four pea genotypes differing in Al tolerance. Pea plants were grown in acid soil supplemented with AlCl₃ (pH_KCl = 4.5) or neutralized with CaCO₃ (pH_KCl = 6.2). Inoculation increased shoot and/or seed biomass of plants grown in Al-supplemented soil. Nodule number and biomass were about twice on roots of Al-treated genotypes after inoculation. Inoculation decreased concentrations of water-soluble Al in the rhizosphere of all genotypes grown in Al-supplemented soil by about 30%, improved N₂ fixation and uptake of fertilizer ¹⁵N and nutrients from soil, and increased concentrations of water-soluble nutrients in the rhizosphere. The structure of rhizospheric microbial communities varied to a greater extent depending on the plant genotype, as compared to soil conditions and inoculation. Thus, this study highlights the important role of symbiotic microorganisms and the plant genotype in complex interactions between the components of the soil-microorganism-plant continuum subjected to Al toxicity.

Combined inoculation of legumes with rhizobia and plant growth-promoting rhizobacteria or endophytes is a known technique for increasing the efficiency of nitrogen-fixing symbiosis and plant productivity. The aim of this work was to expand knowledge about the synergistic effects between commercial rhizobia of pasture legumes and root nodule bacteria of relict legume species. Pot experiments were performed on common vetch (Vicia sativa L.) and red clover (Trifolium pratense L.) co-inoculated with the participation of the corresponding commercial rhizobial strains (R. leguminosarum bv. viciae RCAM0626 and R. leguminosarum bv. trifolii RCAM1365) and seven strains isolated from nodules of relict legumes inhabiting the Baikal Lake region and the Altai Republic: Oxytropis popoviana, Astragalus chorinensis, O. tragacanthoides and Vicia costata. The inoculation of plants with combinations of strains (commercial strain plus the isolate from relict legume) had a different effect on symbiosis depending on the plant species: the increase in the number of nodules was mainly observed on vetch, whereas increased acetylene reduction activity was evident on clover. It was shown that the relict isolates differ significantly in the set of genes related to different genetic systems that affect plant–microbe interactions. At the same time, they had additional genes that are involved in the formation of symbiosis and determine its effectiveness, but are absent in the used commercial strains: symbiotic genes fix, nif, nod, noe and nol, as well as genes associated with the hormonal status of the plant and the processes of symbiogenesis (acdRS, genes for gibberellins and auxins biosynthesis, genes of T3SS, T4SS and T6SS secretion systems). It can be expected that the accumulation of knowledge about microbial synergy on the example of the joint use of commercial and relict rhizobia will allow in the future the development of methods for the targeted selection of co-microsymbionts to increase the efficiency of agricultural legume–rhizobia systems.

A collection of rhizobial strains isolated from root nodules of the narrowly endemic legume species Oxytropis erecta, O. anadyrensis, O. kamtschatica, and O. pumilio originating from the Kamchatka Peninsula (Russian Federation) was obtained. Analysis of the 16S ribosomal RNA gene sequence showed a significant diversity of isolates belonging to families Rhizobiaceae (genus Rhizobium), Phyllobacteriaceae (genera Mesorhizobium, Phyllobacterium), and Bradyrhizobiaceae (genera Bosea, Tardiphaga). A plant nodulation assay showed that only strains belonging to genus Mesorhizobium could form nitrogen-fixing nodules on Oxytropis plants. The strains M. loti 582 and M. huakuii 583, in addition to symbiotic clusters, possessed genes of the type III and type VI secretion systems (T3SS and T6SS, respectively), which can influence the host specificity of strains. These strains formed nodules of two types (elongated and rounded) on O. kamtschatica roots. We suggest this phenomenon may result from Nod factor–dependent and –independent nodulation strategies. The obtained strains are of interest for further study of the T3SS and T6SS gene function and their role in the development of rhizobium-legume symbiosis. The prospects of using rhizobia having both gene systems related to symbiotic and nonsymbiotic nodulation strategies to enhance the efficiency of plant-microbe interactions by expanding the host specificity and increasing nodulation efficiency are discussed.

The phenomenon of rhizobial synergy was investigated to increase the efficiency of nitrogen-fixing symbiosis of alfalfa (Medicago varia Martyn), common vetch (Vicia sativa L.) or red clover (Trifolium pratense L.). These plants were co-inoculated with the respective commercial strains Sinorhizobium meliloti RCAM1750, Rhizobium leguminosarum RCAM0626 or R. leguminosarum RCAM1365 and with the strains Mesorhizobium japonicum Opo-235, M. japonicum Opo-242, Bradyrhizobium sp. Opo-243 or M. kowhaii Ach-343 isolated from the relict legumes Oxytropis popoviana Peschkova and Astragalus chorinensis Bunge. The isolates mentioned above had additional symbiotic genes (fix, nif, nod, noe and nol) as well as the genes promoting plant growth and symbiosis formation (acdRS, genes associated with the biosynthesis of gibberellins and auxins, genes of T3SS, T4SS and T6SS secretion systems) compared to the commercial strains. Nodulation assays showed that in some variants of co-inoculation the symbiotic parameters of plants such as nodule number, plant biomass or acetylene reduction activity were increased. We assume that the study of microbial synergy using rhizobia of relict legumes will make it possible to carry out targeted selection of co-microsymbionts to increase the efficiency of agricultural legume–rhizobia systems.

We report the draft genome sequence of Cupriavidus sp. strain D39, associated with the roots of pea plants. The genome is characterized by a GC content of 63.62% and a total length of 7.7 Mbp and contains several putative genes associated with resistance to metals and plant growth promotion.

Here, we present the draft genome sequence of Rhizobium sp. strain RCAM05973 which was isolated from a Cyamopsis tetragonoloba (guar) root nodule. The genome contains 6,937,221 bp in 2 contigs and has a GC content of 60%.