Genotypic analysis of isolated peanut-nodulating rhizobial strains reveals differences among populations obtained from soils with different cropping histories

Abstract

Peanut (Arachis hypogaea) is one of the most important crops of Córdoba province, Argentina; however, knowledge of the genetic diversity of peanut-nodulating rhizobial populations is fragmentary. In view of the importance of information on native rhizobial populations in relation to improved agricultural management practices, we performed genetic characterization of rhizobia isolated from root nodules of plants grown on soils with previous peanut cropping history (PH: Río Cuarto and Cabrera sites) vs. soils with no previous peanut cropping history (NPH: La Aguada and Chaján sites). Ten different 16S rRNA restriction fragment length polymorphism (RFLP) genotypes were obtained through combination of restriction patterns obtained with four endonucleases. Cluster analysis of genotypes gave two major groups at 80% similarity. Each group contained strains from mostly unrelated origins, indicating a genetic relationship among peanut-nodulating populations isolated from different peanut cropping systems. Furthermore, sequence analysis of the 16S rRNA gene demonstrated identity of isolates with strains of the genus Bradyrhizobium, and phylogenetic association among rhizobial populations from sites with different peanut cropping histories. Diversity indexes estimated for RFLP genotypes showed that populations obtained from PH soils were less diverse than those from NPH soils, suggesting that the presence of the legume results in selection of particular rhizobial strains. Higher polymorphism and diversity were revealed by enterobacterial repetitive intergenic consensus (ERIC)-PCR analysis. Peanut strains clustered at very low levels of similarity (55%). Populations of different origins clustered together, although at elevated genetic distance. ERIC-PCR results indicated the presence of different rhizobial populations in soils of Córdoba province.