Allomesorhizobium aridi sp. nov., a novel symbiotic nitrogen-fixing species isolated from root nodules of Astragalus armatus growing in lead mine tailings in the Middle Atlas, Morocco

A novel rhizobial species, Allomesorhizobium aridi sp. nov., is described based on six strains (AaZ2, AaZ7, AaZ11, AaZ16 ^T , AaZ37 and AaZ58) isolated from root nodules of Astragalus armatus Willd. plants growing on lead-enriched mine tailings in the Middle Atlas region of Morocco. Following the recent reclassification of the genus Mesorhizobium [], the novel species belongs to the genus Allomesorhizobium . All the strains nodulate their original host plant and possess a nodC nodulation gene copy. Phylogenetic analysis of multilocus sequence analysis indicated that the strains are more closely related to Allomesorhizobium camelthorni CCNWXJ404 ^T and Allomesorhizobium alhagi CCNWXJ122 ^T . The whole-genome sequencing of the representative strain AaZ16 ^T revealed a genome size of 6.6 Mb with a G+C content of 62.5 mol%. Digital DNA–DNA hybridization values, average nt identity and phylogenomic analysis of 81 single-copy core gene sequences confirmed that strain AaZ16 ^T is distinct from closely related Allomesorhizobium species. AaZ16 ^T is a Gram-negative, motile, rod-shaped bacterium with optimal growth at pH 7. It tolerates lead concentrations up to 490 µg ml ⁻¹ but is sensitive to salinity. The strain forms effective symbioses with Astragalus gombiformis Pomel and Sulla spinosissima (L.) B.H.Choi & H.Ohashi. Symbiotic efficiency ranged from 60.5 to 66.6%, and the inoculation efficiency index ranged from 69 to 95%, providing indirect evidence of nitrogen fixation. The major fatty acid is ante-iso-pentadecanoic acid (C _15:0 ante-iso). The type strain is AaZ16 ^T (=CCMM B1353 ^T =DSM 120679 ^T ). This species is adapted to arid, metal-contaminated environments and represents a new lineage within the recently circumscribed genus Allomesorhizobium .