Rhizobium acaciae sp. nov., a new nitrogen-fixing symbiovar isolated from root nodules of Acacia saligna in Tunisia

Abstract

Three bacterial strains, 1AS11T, 1AS12 and 1AS13, members of the new symbiovar salignae and isolated from root nodules of Acacia saligna grown in Tunisia, were characterized using a polyphasic approach. All three strains were assigned to the Rhizobium leguminosarum complex on the basis of rrs gene analysis. Phylogenetic analysis based on 1734 nucleotides of four concatenated housekeeping genes (recA, atpD, glnII and gyrB) showed that the three strains were distinct from known rhizobia species of the R. leguminosarum complex and clustered as a separate clade within this complex. Phylogenomic analysis of 92 up-to-date bacterial core genes confirmed the unique clade. The digital DNA–DNA hybridization and blast-based average nucleotide identity values for the three strains and phylogenetically related Rhizobium species ranged from 35.9 to 60.0% and 87.16 to 94.58 %, which were lower than the 70 and 96% species delineation thresholds, respectively. The G+C contents of the strains were 60.82–60.92 mol% and the major fatty acids (>4 %) were summed feature 8 (57.81 %; C18 : 1 ω7c) and C18 : 1 ω7c 11-methyl (13.24%). Strains 1AS11T, 1AS12 and 1AS13 could also be differentiated from their closest described species (Rhizobium indicum, Rhizobium laguerreae and Rhizobium changzhiense ) by phenotypic and physiological properties as well as fatty acid content. Based on the phylogenetic, genomic, physiological, genotypic and chemotaxonomic data presented in this study, strains 1AS11T, 1AS12 and 1AS13 represent a new species within the genus Rhizobium and we propose the name Rhizobium acaciae sp. nov. The type strain is 1AS11T (=DSM 113913T=ACCC 62388T).