Definition of a novel symbiovar (sv. retamae) within Bradyrhizobium retamae sp. nov., nodulating Retama sphaerocarpa and Retama monosperma

Abstract

In this paper we analyze through a polyphasic approach several Bradyrhizobium strains isolated in Spain and Morocco from root nodules of Retama sphaerocarpa and Retama monosperma. All the strains have identical 16S rRNA genes and their closest relative species is Bradyrhizobium lablabi CCBAU 23086T, with 99.41% identity with respect to the strain Ro19T. Despite the closeness of the 16S rRNA genes, the housekeeping genes recA, atpD and glnII were divergent in Ro19T and B. lablabi CCBAU 23086T, with identity values of 95.71%, 93.75% and 93.11%, respectively. These differences were congruent with DNA-DNA hybridization analysis that revealed an average of 35% relatedness between the novel species and B. lablabi CCBAU 23086T. Also, differential phenotypic characteristics of the new species were found with respect to the already described species of Bradyrhizobium. Based on the genotypic and phenotypic data obtained in this study, we propose to classify the group of strains isolated from R. sphaerocarpa and R. monosperma as a novel species named Bradyrhizobium retamae sp. nov. (type strain Ro19T=LMG 27393T=CECT 8261T). The analysis of symbiotic genes revealed that some of these strains constitute a new symbiovar within genus Bradyrhizobium for which we propose the name >retamae>, that mainly contains nodulating strains isolated from Retama species in different continents.