Phylogeny of Bradyrhizobia from Chinese Cowpea Miscellany Inferred from 16S RRNA, AtpD, GlnII, and 16S-23S Intergenic Spacer Sequences

The cowpea (Vigna unguiculata L.), peanut (Arachis hypogaea L.), and mung bean (Vigna radiata L.) belong to a group of plants known as the "cowpea miscellany" plants, which are widely cultivated throughout the tropic and subtropical zones of Africa and Asia. However, the phylogeny of the rhizobial strains that nodulate these plants is poorly understood. Previous studies have isolated a diversity of rhizobial strains from cowpea miscellany hosts and have suggested that, phylogenetically, they are from different species. In this work, the phylogeny of 42 slow-growing rhizobial strains, isolated from root nodules of cowpea, peanut, and mung bean from different geographical regions of China, was investigated using sequences from the 16S rRNA, atpD and glnII genes, and the 16S-23S rRNA intergenic spacer. The indigenous rhizobial strains from the cowpea miscellany could all be placed in the genus Bradyrhizobium , and Bradyrhizobium liaoningense and Bradyrhizobium yuanmingense were the main species. Phylogenies derived from housekeeping genes were consistent with phylogenies generated from the ribosomal gene. Mung bean rhizobia clustered only into B. liaoningense and B. yuanmingense and were phylogenetically less diverse than cowpea and peanut rhizobia. Geographical origin was significantly reflected in the phylogeny of mung bean rhizobia. Most cowpea rhizobia were more closely related to the 3 major groups B. liaoningense, B. yuanmingense, and Bradyrhizobium elkanii than to the minor groups Bradyrhizobium japonicum or Bradyrhizobium canariense . However, most peanut rhizobia were more closely related to the 2 major groups B. liaoningense and B. yuanmingense than to the minor group B. elkanii.