Genetic Diversity, Symbiotic Evolution, and Proposed Infection Process of Bradyrhizobium Strains Isolated from Root Nodules of Aeschynomene americana L. in Thailand

<jats:title>ABSTRACT</jats:title><jats:p>The diversity of bacteria nodulating<jats:named-content content-type="genus-species">Aeschynomene americana</jats:named-content>L. in Thailand was determined from phenotypic characteristics and multilocus sequence analysis of the 16S rRNA gene and 3 housekeeping genes (<jats:italic>dnaK</jats:italic>,<jats:italic>recA</jats:italic>, and<jats:italic>glnB</jats:italic>). The isolated strains were nonphotosynthetic bacteria and were assigned to the genus<jats:named-content content-type="genus-species">Bradyrhizobium</jats:named-content>, in which<jats:named-content content-type="genus-species">B. yuanmingense</jats:named-content>was the dominant species. Some of the other species, including<jats:named-content content-type="genus-species">B. japonicum</jats:named-content>,<jats:named-content content-type="genus-species">B. liaoningense</jats:named-content>, and<jats:named-content content-type="genus-species">B. canariense</jats:named-content>, were minor species. These isolated strains were divided into 2 groups—<jats:italic>nod</jats:italic>-containing and divergent<jats:italic>nod</jats:italic>-containing strains—based on Southern blot hybridization and PCR amplification of<jats:italic>nodABC</jats:italic>genes. The divergent<jats:italic>nod</jats:italic>genes could not be PCR amplified and failed to hybridize<jats:italic>nod</jats:italic>gene probes designed from<jats:named-content content-type="genus-species">B. japonicum</jats:named-content>USDA110, but hybridized to probes from other bradyrhizobial strains under low-stringency conditions. The grouping based on sequence similarity of<jats:italic>nod</jats:italic>genes was well correlated with the grouping based on that of<jats:italic>nifH</jats:italic>gene, in which the<jats:italic>nod</jats:italic>-containing and divergent<jats:italic>nod</jats:italic>-containing strains were obviously distinguished. The divergent<jats:italic>nod</jats:italic>-containing strains and photosynthetic bradyrhizobia shared close<jats:italic>nifH</jats:italic>sequence similarity and an ability to fix nitrogen in the free-living state. Surprisingly, the strains isolated from<jats:named-content content-type="genus-species">A. americana</jats:named-content>could nodulate<jats:named-content content-type="genus-species">Aeschynomene</jats:named-content>plants that belong to different cross-inoculation (CI) groups, including<jats:named-content content-type="genus-species">A. afraspera</jats:named-content>and<jats:named-content content-type="genus-species">A. indica</jats:named-content>. This is the first discovery of bradyrhizobia (nonphotosynthetic and<jats:named-content content-type="genus-species">nod</jats:named-content>-containing strain) originating from CI group 1 nodulating roots of<jats:named-content content-type="genus-species">A. indica</jats:named-content>(CI group 3). An infection process used to establish symbiosis on<jats:named-content content-type="genus-species">Aeschynomene</jats:named-content>different from the classical one is proposed.</jats:p>