Iron uptake and metabolism in the rhizobia/legume symbioses

Iron-containing proteins figure prominently in the nitrogen-fixing symbioses between bacteria of the genera Azorhizovium, Bradyrhizobium and Rhizobium 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 these 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 the plant hosts acquire enough iron for the symbiosis and 3) how do rhizobia acquire iron as symbionts? Production and/or utilization of ferric-specific ligands (siderophores) has now been documented in the laboratory for a number of rhizobial species, but there is limited information on whether production and/or untilization occurs either in the soil or in planta. Studies with rhizobial mutants which can no longer produce and/or utilize siderophores should address whether siderophores contribute to functional symbioses. In addition, the ability to produce and/or utilize siderophores may affect the outcome of both interstrain and interspecific competition in the rhizosphere and in bulk soil. Some progress has been made at documenting the effects of iron deficiency on nodule development. Studies are also underway to determine whether, in addition to its central structural role, iron may also play a regulatory role in the symbioses. This review is an attempt to give an overview of the field, and hopefully will stimulate further research on the iron nutrition of these symbioses which account for such a significant proportion of the world's biologically fixed nitrogen.