Bacterial community studies and novel Bradyrhizobium and Rhizobium strains from drought-tolerant legumes grown in Namibia

AimsAnalysis of the microbiota associated with root nodules of five species of drought-tolerant legumes grown in Namibia. These legumes were Lablab purpureus, Vigna radiata, Vigna unguiculata, Macrotyloma uniflorum, and Vigna aconitifolia. Methods16S rRNA gene amplicon sequencing analysis and isolation of rhizobial and non-rhizobial bacterial strains from root nodules was performed. Plant growth-promoting traits were assessed on some isolates and the genomes of four rhizobial strains were sequenced. ResultsThe microbiota analysis revealed Bradyrhizobium as the most prevalent genus in the root nodules, while most of the non-rhizobial community included members of Bacillus genus. In addition, a strain of Variovorax paradoxus was also isolated from V. unguiculata, representing the first documented isolation and characterization report in Namibia. In vitro phenotypic characterization of the non-rhizobial nodule endophytes indicated that they possessed several plant growth-promoting traits. The four rhizobial strains isolated were taxonomically affiliated to the genera Bradyrhizobium and Rhizobium. Genome sequencing revealed that these strains possibly belong to novel species since they share only a limited similarity (dDDH values<70%) to already known species. Impact StatementThis study demonstrated that drought-tolerant legume species harbour microbial diversity from Namibian soil, which allowed us to increase our knowledge on plant-associated bacteria and their possible use in sustainable farming systems, as potential bioinoculants for crop production and for soil revitalization. Utilizing locally isolated bacterial strains as microbial bioinoculants maximizes their compatibility with the local conditions increasing the potential for positive effects on plant health and ecosystem functioning.