Iron Uptake and Metabolism in the Bradyrhizobium/Soybean Symbiosis

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Iron-containing proteins figure prominently in the nitrogen- fixing symbioses between bacteria and their respective plant hosts. Although iron is abundant in the soil, the acquisition of iron is problematic due to its low solubility at biological pH under aerobic conditions. The study of iron acquisition as it pertains to economically important symbioses is directed at answering three questions: 1) how do rhizobial cells acquire iron as free-living microorganisms where they must compete for this nutrient with other soil inhabitants 2) how do rhizobia acquire iron as symbionts and 3) does iron regulate gene expression? Studies are continuing on the utilization of the siderophores citrate, ferrichrome and rhodoturulate as iron sources by B. japonicum as well as on the iron regulation of genes involved in iron uptake. This research: 1) further characterizes biochemically the acquisition of iron by both free-living and symbiotic cells of B. japonicum, 2) identifies and characterizes genes involved in iron acquisition, including those which encode iron-inducible outer membrane proteins, and 3) investigates the control of gene expression by iron. The increasing world demand for protein and the increasing expense of producing nitrogen fertilizer has resulted in a widespread interest in biological nitrogen fixation. Although the nitrogen-fixing Bradyrhizobium/legume symbioses supply much of the fixed nitrogen for agriculture in the temperate regions of the world, many aspects of the symbioses are still poorly understood. The importance of iron in animal/pathogen interactions is now well established; the role of this metal in the nitrogen-fixing rhizobial/legume symbioses is just beginning to be explored. The research proposed here provides for a better understanding of the role of iron in regulating the symbiotic interaction. Such an understanding elucidates how rhizobia survive in the soil as well as how they interact with their plant host to establish an effective symbiosis. This is essential to any efforts to engineer a better symbiosis and for the ultimate extension of the symbiosis to other plant hosts.