Symbiotic competence and genetic diversity of Rhizobium strains used as inoculants for alfalfa and berseem clover

To provide the necessary information for strain improvement and the development of competitive strains, characterization of some agronomically important inoculant Rhizobium strains specific for either alfalfa or berseem clover is an ongoing project in this laboratory. Plasmid DNA content and identification of symbiosis-controlling plasmids were previously determined. In this study, to determine their symbiotic competence and their apparent genetic diversity, plant tests and RFLP analysis of total DNAs were conducted. Symbiotic effectiveness varied significantly among the Rh. meliloti strains but not among the berseem clover symbionts. Only one strain, Rh. meliloti ARC 104, was symbiotically ineffective. RFLP analysis using a ribosomal RNA operon probe, with three different enzyme digests, produced four related groups which correlated with plasmid profiles. The use of a nifHD gene probe produced similar results which clustered the berseem clover microsymbionts but more clearly separated two distinct groups of Rh. meliloti. A cosmid clone, from a Rh. leguminosarum biovar trifolii T24 gene library, distinguished each strain. This new information will be useful in identifying and producing improved recombinant strains by plasmid transfer.