The rhizobia live as free-living soil bacteria or in symbiosis with leguminous plants. The success of these organisms in each milieu involves the ability to sense the environment to assess the availability of nutrients, and to optimize cellular systems for their acquisition. Iron in the rhizosphere is mostly inaccessible due to low solubility, and microorganisms must compete for this limited nutrient. Rhizobia belong to the alpha-Proteobacteria, a diverse taxonomic group that includes numerous species that form close or intracellular associations with eukaryotic hosts in a symbiotic or pathogenic context.

Thus, in addition to their agricultural and economic importance, rhizobia are model organisms that have given new insights into related, but less tractable animal pathogens. In particular, genetic control of iron homeostasis in the rhizobia and other alpha-Proteobacteria has moved away from the Fur paradigm to an iron sensing mechanism responding to the metal indirectly. Moreover, utilization of heme as an iron source is not unique to animal pathogens, but is an acquisition strategy employed by the rhizobia with some interesting novel features.

Further reading: Iron Uptake and Homeostasis in Microorganisms

Labels: Iron transporters, Iron uptake in Rhizobia, Iron uptake systems, Iron-homeostasis, Iron-metabolism, Iron-uptake, Proteobacteria, Rhizobia, Rhizobium