Two novel, Gram-negative, motile, rod-shaped, aerobic bacterial strains, MH17T and RD15, were isolated from the sterilized root and rhizosphere soil of rice, respectively. Phylogenetic analysis based on 16S rRNA gene sequences showed that the similarity between strains MH17T and RD15 was 100%. The isolates exhibit high sequence similarities to Rhizobium oryzae CGMCC 1.7048T (98.7%) and Rhizobium petrolearium SL-1T (97.0% and 97.1%), which supports that they belong to a novel species in the genus Rhizobium. Strains MH17T and RD15 exhibited growth at 15-45 °C, pH 5.0-11.0, 0-2.0% sodium chloride (w/v). Sequence analysis of housekeeping genes gyrB, recA, atpD, ropB, gltA showed that these two novel strains had less than 94% similarity with the known species, indicating the distinct position of MH17T and RD15 in the genus Rhizobium. The major cellular fatty acids were identified as summed feature 8 (C18:1 ω7c and/or C18:1 ω6c). Type strain MH17T had 87.5% DNA-DNA relatedness with RD15 by using the initial renaturation rate method. Based on draft genome sequences, strain MH17T showed 30.1% DNA-DNA hybridization values to R. oryzae CGMCC 1.7048T, the closely related strain, which supported that MH17T represents a novel species in the genus Rhizobium. Average nucleotide identity (ANI) between strains MH17T and RD15 were 97.8%, and strain MH17T showed 82.2% ANI value with R. oryzae CGMCC 1.7048T. The DNA G+C content was 60.4 mol% (Tm). Based on physiological, biochemical characteristic, genotypic data, strains MH17T and RD15 are concluded to represent a new species within the genus Rhizobium, for which the name Rhizobium rhizosphaerae sp. nov. is proposed. The type strain is MH17T (=ACCC 19963T = KCTC 52414T).
Keywords: Polyphasic taxonomy; Rhizobium rhizosphaerae sp. nov.; Root of rice.