Physiologic and metabolic interactions in the soybean/bradyrhizobium japonicum symbiosis

Abstract

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] The agronomically vital, nitrogen-fixing symbioses between soybean and the soil bacterium Bradyrhizobium japonicum is a model system for the study of plant-microbe interactions. Its complexity extends from pre-infection and continues throughout the process. Emerging roots are the sites of infection and it was discovered that B. japonicum must compete for colonization with other inhabitants of the soil and those from the germinating seed. Through elimination of these parameters, attachment kinetics is found to occur in several phases, some lasting for only seconds, with minimal contact time resulting in maximal infection. After contact, the bacteria invade the root and form nodules on the root surface. As the nodule ages, the bacteria within these pockets begin the energy intensive process of providing nitrogen to the plant in exchange for a carbon and energy source. The effects of the environmental toxin trifluoroacetic acid (a breakdown product of the newest refrigerants: HCFCs) on this process demonstrate how important the health of the host plant is. B. japonicum is very resistant to the effects of the highly water soluble environmental toxin, however the soybean plant was not. The energy requirements of this process demand a healthy system and that nutrient transfer between the partners to be rapid and efficient. The possibility of carbon recycling during nitrogen fixation was also examined, demonstrating further the important role of alanine, pyruvate and possibly ethanol during peak nitrogen fixation. Most importantly, the knowledge gained from this study supports the theory of bacteroid alanine excretion during the soybean-Bradyrhizobium japonicum symbiosis.