Reciprocal inoculations with Bradyrhizobium sp. isolates from the North American legume Amphicarpaea bracteata (L.) Fern. (Phaseoleae-Glycininae) and from a Japanese population of its close relative Amphicarpaea edgeworthii (Benth.) var. japonica were performed to analyze relative symbiotic compatibility. Amphicarpaea edgeworthii plants formed few or no nodules with any North American bradyrhizobial strains isolated from A. bracteata, but all A. bracteata lineages formed effective
nitrogen-fixing nodules with Japanese Bradyrhizobium isolates from A. edgeworthii. However, one group of A. bracteata plants (lineage Ia) when inoculated with Japanese bradyrhizobia developed a striking leaf
chlorosis similar to that known to be caused by
rhizobitoxine. The
beta-cystathionase inhibition assay demonstrated that significant amounts of
rhizobitoxine were present in nodules formed by these Japanese bradyrhizobia. No North American bradyrhizobial isolate from A. bracteata induced
chlorosis on any plants, and the
beta-cystathionase assay failed to detect
rhizobitoxine in nodules formed by these isolates. The role of
rhizobitoxine in A. edgeworthii nodulation development was tested by inoculating plants with a Bradyrhizobium elkanii
rhizobitoxine-producing strain, USDA 61, and two mutant derivatives, RX17E and RX18E, which are unable to synthesize
rhizobitoxine. Amphicarpaea edgeworthii inoculated with wild-type USDA 61 developed >150 nodules per plant, while plants inoculated with RX17E and RX18E developed fewer than 10 nodules per plant. Thus, efficient nodule development in A. edgeworthii appears to be highly dependent on
rhizobitoxine production by Bradyrhizobium strains.