Sinorhizobium medicae

Sinorhiozbium medicae is a gram-negative, Nitrogen-fixing, rod-shaped bacterium. It can be free living or a symbiont of leguminous plants in root nodules. S.medicae was first isolated from root nodules on plants in the Medicago genus.^[1] Some strains of S.medicae, like WSM419, are aerobic. They are chemoorganotrophic mesophiles that prefer temperatures around 28 °C. In addition to their primary genome, these organisms also have three known plasmids, sized 1,570,951 bp, 1,245,408 bp and 219,313 bp.^[2]

Colonies of Sinorhiozbium medicae are mucoid and donut-shaped and can be viewed here.

Phylogeny/Taxonomy

The genus is sometimes referred to as Ensifer (the older term) instead of Sinorhizobium. Two major subgroups include S.medicae strain A321 and S. medicae strain WSM419. this phylogenetic tree shows relatedness to the rest of the Rhizobiaceae family based on 16s rRNA gene sequences.

Ecological role and symbiosis with legumes

This bacterium is often involved in mutualistic relationships with legumes. It performs atmospheric nitrogen fixation for the plants and in exchange it receives organic carbon through the process of rhizodeposition. Free-living bacteria become housed inside specialized root cells in root nodules, which creates anaerobic microhabitat in which efficient N-fixation can occur. This mutualism has been observed with many plant species, including Medicago polymorpha and Medicago truncatula plants from around the world.^[3] The extent of the mutualism may be dependent upon soil pH, as it was though that acidity can constrain Sinorhizobium medicae. Current thinking is that acid resistance, particularly in strain WSM419, can be transferred on plasmids.^[4]

Agricultural Importance

Because S. medicae associates well with plants in the Medicago genus and increases plants growth, crop fields are commonly inoculated with the bacteria in addition to or in replacement of synthetic fertilizers.^[5]

References