Iron Homeostasis in Azotobacter vinelandii.

Iron is an essential nutrient for all life forms. Specialized mechanisms exist in bacteria to ensure iron uptake and its delivery to key enzymes within the cell, while preventing toxicity. Iron uptake and exchange networks must adapt to the different environmental conditions, particularly those that require the biosynthesis of multiple iron proteins, such as nitrogen fixation. In this review, we outline the mechanisms that the model diazotrophic bacterium Azotobacter vinelandii uses to ensure iron nutrition and how it adapts Fe metabolism to diazotrophic growth.

Medicago truncatula Ferroportin2 mediates iron import into nodule symbiosomes.

Iron is an essential cofactor for symbiotic nitrogen fixation, required by many of the enzymes involved, including signal transduction proteins, O2 homeostasis systems, and nitrogenase itself. Consequently, host plants have developed a transport network to deliver essential iron to nitrogen-fixing nodule cells. Ferroportin family members in model legume Medicago truncatula were identified and their expression was determined. Yeast complementation assays, immunolocalization, characterization of a tnt1 insertional mutant line, and synchrotron-based X-ray fluorescence assays were carried out in the nodule-specific M. truncatula ferroportin Medicago truncatula nodule-specific gene Ferroportin2 (MtFPN2) is an iron-efflux protein. MtFPN2 is located in intracellular membranes in the nodule vasculature and in inner nodule tissues, as well as in the symbiosome membranes in the interzone and early-fixation zone of the nodules. Loss-of-function of MtFPN2 alters iron distribution and speciation in nodules, reducing nitrogenase activity and biomass production. Using promoters with different tissular activity to drive MtFPN2 expression in MtFPN2 mutants, we determined that expression in the inner nodule tissues is sufficient to restore the phenotype, while confining MtFPN2 expression to the vasculature did not improve the mutant phenotype. These data indicate that MtFPN2 plays a primary role in iron delivery to nitrogen-fixing bacteroids in M. truncatula nodules.