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

Symbiotic nitrogen-fixing Rhizobia are of global significance, both in terms of their ecological relationships and their importance as an environmentally benign source of nitrogen for crop plants. These bacteria are capable of forming mutualistic relationships with a variety of legume hosts, where they convert atmospheric nitrogen to ammonia that is used to help meet the nitrogen needs of the host plant (Xu, 2010).

The results of recent studies have revealed several important insights, including the existence of extensive diversity within, as well as significant genetic differentiation between local and regional populations. The results also provide evidence for long-distance gene flow between continental populations. Several studies have also indicated the existence of low-to-intermediate levels of recombination within rhizobial populations. Fine-scale studies of specific genomic components (e.g., symbiotic plasmids) have also shown that certain genomic elements appear to be more prone to recombination than others. The results also showed that the different loci responsible for the development of the symbiosis appear to be under different forms of selection. Because most the population studies to date have focused on strains from root nodules, surprisingly little is known on the population genetics of the more numerous non-nodulating soil rhizobia. Future efforts to characterize these populations should significantly enhance our ability to manipulate rhizobial populations in agricultural ecosystems (Xu, 2010).

References:
Xu, J. (2010) Microbial Population Genetics. Caister Academic Press, Norfolk, UK.

Labels: Nitrogen-fixing, Rhizobia, Rhizobial populations