Does the Miocene-Pliocene relict legume Oxytropis triphylla form nitrogen-fixing nodules with a combination of bacterial strains?

Abstract

Four pairs of strains were isolated from four individual root nodules collected from different plants of the Miocene-Pliocene relict legume Oxytropis triphylla growing in the Baikal region (one nodule – one pair of strains). Identification of these strains by the 16S rRNA gene sequencing showed that one strain of each pair was fast-growing and phylogenetically closest to the type strains Phyllobacterium endophyticum PEPV15T (98.8–99.5% 16S rDNA similarity) and Phyllobacterium bourgognense STM 201T (98.8–99.3% 16S rDNA similarity). The other strain was slow-growing and closely related to the type strains Bosea vestrisii 34635T and Bosea eneae 34614T (99.5–99.8% 16S rDNA similarity). Results of the atpD gene sequencing suggested that Phyllobacterium-related isolates most probably belong to a new species of Phyllobacterium. It is known that none of the currently described strains of P. endophyticum, P. bourgognense (family Phyllobacteriaceae), B. vestrisii and B. eneae (family Bradyrhizobiaceae) can induce root nodules, but several symbiotic genes were found in the representatives of these two genera. Thus we hypothesize that the pairs of strains isolated from O. triphylla root nodules can be co-microsymbionts having complementary sets of symbiotic genes and their simultaneous presence in roots is required for efficient nodulation of the host plant. The whole genome sequencing of the studied strains is in progress to find and compare symbiotic genes.