| The symbiotic tree legume, mesquite, forms two lateral root systems when able to access surface and ground water and both systems have associated nitrogen fixing bacteria. Contribution to ecosystem productivity requires mesquite's renewal of symbiosis under extreme, changing conditions.; Rhizobial and bradyrhizobial populations associated with surface and deep rooting systems of a mature Sonoran Desert mesquite woodland were isolated by four to six meters of dry soil for centuries. To determine whether differences in microsymbiont physiology were paralleled genetically, we assessed their host ranges, symbiotic gene region polymorphisms, and plasmid patterns. Restriction enzyme-digested genomic DNA of isolates from the two mesquite root zones showed nif, nod, and ndv-hybridization patterns corresponding to their source root zone. The patterns indicated that all isolates of either genus, Rhizobium and Bradyrhizobium, were related, but different, with soil depth. Plasmid profile analyses revealed differences, most prominently, in lesser deep bradyrhizobial plasmid DNA content. Identical host ranges were consistent with relatedness among isolates. We concluded these were clonal populations that varied under alternating symbiotic and free-living selections within soil compartments.; Towards understanding host-specific nodulation of the unimproved woody legume, mesquite, we used marker rescue strategies to isolate DNA clones of mesquite-symbiont nodulation regions. Mesquite selected cloned rhizobial DNA from the same genomic region by extending the nodulation range of broad host Rhizobium sp. strain NGR234 or restoring nodulation to Nod-Rhizobium sp. strain HW27c. In those clones, a nodD locus and adjacent nod box sequence were organized similarly to common nodulation regions of other micosymbionts. Whether containing nodD, or not, subcloned regions carried by non-nodulating strains did not confer mesquite nodulation, In contrast, cloned bradyrhizobial genomic DNA lacking nodD similarity extended the host range of Rhizobium sp. strain NGR234 without restoring nodulation to Rhizobium sp. strain HW27c.; We found genetic parallels to physiological variation among populations that extends knowledge of how selection for the symbiotic life cycle interacts with longterm microbial survival under extreme conditions. Molecular analysis of the mesquite-bacterial association gives insight to the nodulation process, well-characterized for temperate crops, in an unimproved, tropical tree symbiosis. |
