Rhizobial microsymbionts of the narrowly endemic Oxytropis species growing in Kamchatka are characterized by significant genetic diversity and possess a set of genes that are associated with T3SS and T6SS secretion systems and can affect the development of symbiosis.

A collection of rhizobial strains isolated from root nodules of the narrowly endemic legume species Oxytropis erecta, O. anadyrensis, O. kamtschatica and O. pumilio originated from the Kamchatka Peninsula (Russian Federation) was obtained. Analysis of the 16S rRNA gene sequence showed a significant diversity of isolates belonging to families Rhizobiaceae (genus Rhizobium), Phyllobacteriaceae (genera Mesorhizobium, Phyllobacterium) and Bradyrhizobiaceae (genera Bosea, Tardiphaga). Plant nodulation assay showed that only strains belonging to the 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 T3SS and T6SS secretion systems, 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 non-symbiotic nodulation strategies to enhance the efficiency of plant-microbe interactions by expanding the host specificity and increasing nodulation efficiency are discussed.