Symbiotic root nodule bacteria isolated from yam bean (Pachyrhizus erosus)

A total of 25 isolates from root nodules of yam bean (Pachyrhizus erosus L. Urban), a tuber-producing leguminous plant, were characterized. All isolates formed effective nodules mainly on lateral roots while edible tubers were developed on the taproot. The root nodules formed were identified as the typical determinate type. By an analysis of the partial sequences of the 16S rRNA gene (approximately 300 bp) of 10 strains which were selected randomly, the isolated root nodule bacteria of yam bean were classified into two different genera, Rhizobium and Bradyrhizobium. Two strains, YB2 (Bradyrhizobium group) and YB4 (Rhizobium group) were selected and used for further analyses. The generation time of each strain was shown to be 22.5 h for strain YB2 and 0.8 h for strain YB4, respectively. Differences between strains YB2 and YB4 were also reflected in the bacteroid state in the symbiosome. Symbiosome in nodule cells for the strain YB4 contained one bacteroid cell in a peribacteroid membrane, whereas a symbiosome for strain YB2 contained several bacteroid cells.