Enhancing the Competitiveness and Effectiveness of Soybean Bradyrhizobia Inoculant with Rhizobiophages

摘要

Soybean productivity is often limited by the failure of Bradyrhizobium japonicum inoculant strain(s) to compete with relatively mediocre bacterial strains already present in the soil. Initially, we conducted field trials in the Delmarva region of the United States to study the interactions between genotypes of forage and seed soybean, and elite Bradyrhizobium japonicum strains TA11NOD~+, 532C (SEMIA 5039), NS-1, and SOY 212. The competitive ability of the introduced inoculant bacteria, versus the native bacteria, in nodulating soybean was also examined. Soybean genotypes varied significantly in their response to bradyrhizobia inoculants in the presence of the native bradyrhizobia. Bacterial viruses, (termed rhizobiophages), specific for B. japonicum were isolated from the rhizosphere soils of soybean, and their host ranges were determined. Selected rhizobiophages were found to reduce the competitive ability of indigenous native B. japonicum strains in nodulating soybean. These phages were used as biocontrol agents to reduce the number of susceptible B. japonicum strains in the soil in greenhouse and field experiments. Soybean seeds, cv. Williams, seeds were coated with B. japonicum strain USDA 110 and phage virulent to the native ineffective bradyrhizobia strains that were highly competitive in nodule formation, and planted in the greenhouse. The results showed that nodule occupancy by undesirable indigenous bacteria strains was decreased by 44% while nodule occupancy by superior introduced strains increased by about 55%. When soybean seeds were inoculated with a combinations of a phage lytic to the native bradyrhizobia strain and with a phage resistant inoculant strain, nodulation and soybean growth were significantly increased by the introduced strain. Our studies indicate that the use of rhizobiophages as biocontrol agents requires the identification of symbiotically competent, rhizobiophage resistant bradyrhizobia with demonstrated ability to promote growth and yield of their specific soybean hosts. Therefore, it is concluded from this study that simultaneous inoculation of soybean with effective rhizobiophage-resistant B. japonicum strains and phages virulent to the predominant ineffective strains in the soil will enhance soybean growth, productivity and sustainability.