Rhizobium esperanzae sp nov., a N2-fixing root symbiont of Phaseolus vulgaris from Mexican soils

Common bean (Phaseolus vulgaris L.) is the most important legume consumed worldwide; its genetic origins lie in the Mesoamerican (main centre) and Andean regions. It is promiscuous in establishing root-nodule symbioses; however, in the centres of origin/domestication, the predominant association is with Rhizobium etli. We have previously identified a new lineage (PEL-3) comprising three strains (CNPSo 661, CNPSo 666 and CNPSo 668(T)) isolated from root nodules of common bean in Mexico, and that have now been analysed in more detail. Sequences of the 16S rRNA gene positioned the three strains in a large clade including R. etli. Multilocus sequence analysis (MLSA) with four housekeeping genes (recA, glnII, gyrB and rpoA) positioned the three strains in a clade distinct from all other described species, with 100% bootstrap support, and nucleotide identity (NI) of the four concatenated genes with the closest species R. etli was 95.0 %. Average nucleotide identity (ANI) values of the whole genome of CNPSo 668(T) and the closest species, R. etli, was 92.9 %. In the analyses of the symbiotic genes nifH and nodC, the strains comprised a cluster with other rhizobial symbionts of P. vulgaris. Other phenotypic and genotypic traits were determined for the new group and our data support the description of the three CNPSo strains as a novel species, for which the name Rhizobium esperanzae is proposed. The type strain is CNPSo 668(T) (= UMR 1320(T) = Z87-8(T) = LMG 30030(T) = U 10001(T)), isolated from a common-bean nodule in Mexico.