Isolation and Characterization of Rhizobia Effective with Maackia amurensis

Abstract

Our objectives were to test whether Maackia amurensis Rupr. & Maxim. nodulates and fixes N and to characterize the N-fixing bacteria effective with this host. Soil samples were collected near diverse legume trees at arboreta and public gardens in the United States, Canada, and China. Seedlings of M. amurensis were grown for 6 weeks in a low-N, sterile medium and inoculated with soil samples. At harvest, nodules were found on the lateral and upper portions of root systems. Bacteria were isolated from nodules and subculture. Roots of seedlings inoculated with all 11 of these isolates nodulated and freed N, confirming that the isolates were rhizobial bacteria. Growth of isolates in axenic culture generally was poor when single sources of C were provided. Generation times of the isolates ranged from 6 to 10 hours, and all isolates raised the pH of culture media. Isolates were highly resistant to several antibiotics, showed no 6-phosphogluconate dehydrogenase (6PGD) or β-galactosidase activity, and were highly sensitive to NaCl. These results provide the first evidence that M. amurensis has the capacity to form N-fixing symbioses with rhizobial bacteria and indicate that the bacteria are Bradyrhizobium sp.