Siderophore-mediated iron metabolism in growth and nitrogen fixation by alfalfa nodulated with Rhizobium meliloti

This research was initiated to examine the significance of the high-affinity iron (Fe) transport system in the nitrogen (N)-fixing activity of Rhizobium meliloti bacteroides in nodules of alfalfa, Medicago sativa L. Iron was supplied as Fe-chelate at a concentration of 10 mu M for nodulation and plant growth. Nitrogen-fixation measurements by the acetylene reduction assay were conducted on plants with adequate Fe in the hydroponic medium. Mutants of R. meliloti strain 1021 that were defective in rhizobactin 1021 synthesis produced nodules on alfalfa but fixed insignificant amounts of dinitrogen. Mutants of R. meliloti constitutive for rhizobactin 1021 synthesis also produced nodules; however, the levels of dinitrogen fixed were low. Nodules produced by a strain of R. meliloti defective in rhizobactin-Fe uptake and subject to siderophore induction were found to fix dinitrogen at levels similar to nodules produced by wild type bacteria. Iron availability to R. meliloti in developing nodules was critical since bacterial strains unable to produce rhizobactin 1021 could not sustain plant growth in N-free medium. Additionally, there was some evidence that overproduction of rhizobactin 1021 was detrimental to the symbiotic system.