Diversity of rhizobia nodulating Lotus corniculatus grown in northern and southern regions of Uruguay

Diversity of rhizobia nodulating Lotus corniculatus grown in geographical regions with different rainfall regimes in northern and southern Uruguay, was estimated using 168 root nodule isolates. ERIC-PCR analysis revealed no correlation between observed fingerprints and the geographical origin of isolates. Despite the commercial strain U510 has been used for decades to inoculate L. corniculatus, none of the isolates corresponded to this strain. Phylogenetic analyses using 16S rRNA and atpD genes, and ITS sequences clustered all the isolates within genus Mesorhizobium. A great majority of the isolates likely belong to the species M. huakuii, as does the commercial strain U510. The remaining isolates were closely related to either M. septentrionale or M. caraganae. Although no M. loti-like bacteria were identified, all isolates carried symbiotic genes closely related to M. loti and other narrow host range Lotus rhizobia. A significant portion of the Uruguayan isolates were as efficient as the reference strain U510 in symbiosis with L. corniculatus. A few of the isolates were also capable of nitrogen fixation in symbiosis with L. uliginosus, albeit with lower efficiency than reference strains. Our results indicate that rhizobia nodulating L. corniculatus in Uruguay are genetically and phenotypically diverse, and that the commercial strain U510 is probably not adapted to survive the Uruguayan edaphoclimatic conditions.

► The diversity of rhizobia nodulating L. corniculatus was not correlated with the geographical origins of the isolates that were selected for their different rainfall regimes.
► No Mesorhizobium loti-like bacteria were identified but all isolates carried symbiotic genes related to M. loti.
► The commercial inoculant could not be isolated from the sites explored despite it has been continuously used for decades.
► Effective strains of Mesorhizobium sp. so effective in L. corniculatus as the commercial inoculant were isolated.