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

Abstract

ABSTRACTThe diversity of bacteria nodulatingAeschynomene americanaL. in Thailand was determined from phenotypic characteristics and multilocus sequence analysis of the 16S rRNA gene and 3 housekeeping genes (dnaK,recA, andglnB). The isolated strains were nonphotosynthetic bacteria and were assigned to the genusBradyrhizobium, in whichB. yuanmingensewas the dominant species. Some of the other species, includingB. japonicum,B. liaoningense, andB. canariense, were minor species. These isolated strains were divided into 2 groups—nod-containing and divergentnod-containing strains—based on Southern blot hybridization and PCR amplification ofnodABCgenes. The divergentnodgenes could not be PCR amplified and failed to hybridizenodgene probes designed fromB. japonicumUSDA110, but hybridized to probes from other bradyrhizobial strains under low-stringency conditions. The grouping based on sequence similarity ofnodgenes was well correlated with the grouping based on that ofnifHgene, in which thenod-containing and divergentnod-containing strains were obviously distinguished. The divergentnod-containing strains and photosynthetic bradyrhizobia shared closenifHsequence similarity and an ability to fix nitrogen in the free-living state. Surprisingly, the strains isolated fromA. americanacould nodulateAeschynomeneplants that belong to different cross-inoculation (CI) groups, includingA. afrasperaandA. indica. This is the first discovery of bradyrhizobia (nonphotosynthetic andnod-containing strain) originating from CI group 1 nodulating roots ofA. indica(CI group 3). An infection process used to establish symbiosis onAeschynomenedifferent from the classical one is proposed.