VARIATION OF CLONAL, MESQUITE-ASSOCIATED RHIZOBIAL AND BRADYRHIZOBIALPOPULATIONS FROM SURFACE AND DEEP SOILS BY SYMBIOTIC GENE REGION RESTRICTION-FRAGMENT-LENGTH-POLYMORPHISM AND PLASMID PROFILE ANALYSIS

Genetic characteristics of 14 Rhizobium and 9 Bradyrhizobium mesquite (Prosopis glandulosa)-nodulating strains isolated from surface (0- to 0.5-m) and deep (4- to 6-m) rooting zones were determined in order to examine the hypothesis that surface- and deep-soil symbiont population s were related but had become genetically distinct during adaptation t o contrasting soil conditions. To examine genetic diversity, Southern blots of PstI-digested genomic DNA were sequentially hybridized with t he nodDABC region of Rhizobium meliloti, the Klebsiella pneumoniae nif HDK region encoding nitrogenase structural genes, and the chromosome-l ocalized ndvB region of R. meliloti. Plasmid profile and host plant no dulation assays were also made. Isolates from mesquite nodulated beans and cowpeas but not alfalfa, clover, or soybeans. Mesquite was nodula ted by diverse species of symbionts (R. meliloti, Rhizobium leguminosa rum by. phaseoli, and Parasponia bradyrhizobia). There were no differe nces within the groups of mesquite-associated rhizobia or bradyrhizobi a in cross-inoculation response. The ndvB hybridization results showed the greatest genetic diversity among rhizobial strains. The pattern o f ndvB-hybridizing fragments suggested that surface and deep strains w ere clonally related, but groups of related strains from each soil dep th could be distinguished. Less variation was found with nifHDK and no dDABC probes. Large plasmids (>1,500 kb) were observed in all rhizobia and some bradyrhizobia. Profiles of plasmids of less than 1,000 kb we re related to the soil depth and the genus of the symbiont. We suggest that interacting selection pressures for symbiotic competence and fre e-living survival, coupled with soil conditions that restrict genetic exchange between surface and deep-soil populations, led to the observe d patterns of genetic diversity.