Rhizobium leguminosarum
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Rhizobium leguminosarum
Rhizobium is a genus of Gram-negative soil bacteria that fix nitrogen. Rhizobium forms an endosymbiotic nitrogen fixing association with roots of legumes.
| Kingdom | Bacteria |
|---|---|
| Phylum | Proteobacteria |
| Class | Alphaproteobacteria |
| Order | Rhizobiales |
| Family | Rhizobiaceae |
| Genus | Rhizobium |
| Species | R. leguminosarum |
| Binomial | Rhizobium leguminosarum |
[edit] Surface Characteristics
The bacterial surface lipopolysaccharide (LPS) is composed of three parts: (1) lipid A, (2) core polysaccharide, and (3) O antigen. The lipid A region contains two glucosamine sugar derivatives, each with three fatty acids and phosphate or pyrophosphate attached. The core polysaccharide has KDO and attached to lipid A. The side chain O is small polysaccharide chain extending outward from the core, it has several peculiar sugars and varies in composition between bacterial strains.
[edit] Importance in agriculture
This is especially important when nitrogen fertilizer is not used, as in organic rotation schemes or some less-industrialized countries. Nitrogen is the most commonly deficient nutrient in many soils around the world and it is the most commonly supplied plant nutrient. Supply of nitrogen through fertilizers has severe environmental concerns.
Symbiosis Rhizobia are unique because they live in a symbiotic relationship with legumes. Common crop and forage legumes are peas, beans, clover, and soy.
Infection and signal exchange
The symbiotic relationship implies a signal exchange between both partners that leads to mutual recognition and development of symbiotic structures. Rhizobia live in the soil where they are able to sense flavonoids secreted by the root of their host legume plant. Flavonoids trigger the secretion of Nod factors, which in turn are recognized by the host plant and can lead to root hair deformation and several cellular responses such as ion fluxes. The best known infection mechanism is called intracellular infection, in this case the rhizobia enter through a deformed root hair in a similar way to endocytosis, forming an intracellular tube called the infection thread. A second mechanism is called "crack entry", in this case no root hair deformation is observed and the bacteria penetrate between cells, through cracks produced by lateral root emergence. Later on bacteria become intracellular and an infection thread is formed like in intracellular infections.
The infection triggers cell division in the cortex of the root where a new organ, the nodule appears as a result of successive processes. Nodule formation and functioning
Nodule formation and functioning
Infection threads grow to the nodule, infect its central tissue and release the rhizobia in these cells where they differentiate morphologically into bacteroids and fix nitrogen from the atmosphere into a plant usable form, ammonium (NH4+), utilizing the enzyme nitrogenase. In return the plant supplies the bacteria with carbohydrates, proteins, and sufficient oxygen so as not to interfere with the fixation process. Leghaemoglobins, plant proteins similar to human hemoglobins help to provide oxygen for respiration while keeping the free oxygen concentration low enough not to inhibit nitrogenase activity. Recently, it was discovered that a Bradyrhizobium strain forms nodules in Aeschynomene without producing Nod factors, suggesting the existence of alternative communication signals other than Nod factors
