Bradyrhizobium ivorense sp. nov. as a potential local bioinoculant for Cajanus cajan cultures in Cote d'Ivoire

For many smallholder farmers of Sub-Saharan Africa, pigeonpea (Cajanus cajan) is an important crop to make ends meet. To ascertain the taxonomic status of pigeonpea isolates of Cote d'Ivoire previously identified as bradyrhizobia, a polyphasic approach was applied to strains CI-1B(T), CI-14A, CI-190 and CI-41S. Phylogeny of 16S ribosomal RNA (rRNA) genes placed these nodule isolates in a separate lineage from current species of the B. elkanii super Glade. In phylogenetic analyses of single and concatenated partial dnaK, glnll, gyrB, recA and rpoB sequences, the C. cajan isolates again formed a separate lineage, with strain CI-16(T) sharing the highest sequence similarity (95.2%) with B. tropiciagri SEMIA 6148(T). Comparative genomic analyses corroborated the novel species status, with 86% ANIb and 89% ANIm as the highest average nucleotide identity (ANI) values with B. elkanii USDA 76(T). Although CI-1B(T), CI-14A, CI-19D and CI-41S shared similar phenotypic and metabolic properties, growth of CI-41S was slower in/on various media. Symbiotic efficacy varied significantly between isolates, with CI-1B(T) and CI-41S scoring on the C. cajan 'Light-Brown' landrace as the most and least proficient bacteria, respectively. Also proficient on Vigna radiata (mung bean), Vigna unguiculata (cowpea, niebe) and additional C. cajan cultivars, CI-1B(T) represents a potential bioinoculant adapted to local soil conditions and capable of fostering the growth of diverse legume crops in Cote d'Ivoire. Given the data presented here, we propose the 19 C. cajan isolates to belong to a novel species called Bradyrhizobium ivorense sp. nov., with CI-1B(T) (=CCOS 1862(T)=CCMM B1296(T)) as a type strain.