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Rhizobia
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Rhizobia are soil bacteria that fix nitrogen (diazotrophy) after becoming established inside root nodules of legumes (Fabaceae). Rhizobia require a plant host; they cannot independently fix nitrogen. Morphologically, they are generally gram negative, motile, non-sporulating rods.
first species of rhizobia, R.
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Encyclopedia
Rhizobia are soil bacteria that fix nitrogen (diazotrophy) after becoming established inside root nodules of legumes (Fabaceae). Rhizobia require a plant host; they cannot independently fix nitrogen. Morphologically, they are generally gram negative, motile, non-sporulating rods.
History
The first species of rhizobia, R. leguminosarum, was identified in 1889, and all further species were initially placed in the Rhizobium genus. However, more advanced methods of analysis have revised this classification, and now there are many in other genera. Most research has been done on crop and forage legumes such as clover, beans, and soy. However, recently more work is occurring on North American legumes.
The word rhizobia comes from the Ancient Greek , "rhíza," meaning "root," and , "bios," meaning "life." The word rhizobium is still sometimes used as the singular form of rhizobia.
Taxonomy
Rhizobia are a paraphyletic group which fall into two classes of the proteobacteria—the alpha- and beta-proteobacteria. As shown below, most belong to the order Rhizobiales but several rhizobia occur in distinct bacterial orders of the proteobacteria.
These groups include a variety of non-symbiotic bacteria. For instance, the plant pathogen Agrobacterium is a closer relative of Rhizobium than the Bradyrhizobium that nodulate soybean (and may not really be a separate genus). The genes responsible for the symbiosis with plants, however, may be more closely related than the organisms themselves, acquired by horizontal transfer (via bacterial conjugation) rather than vertical gene transfer (from a common ancestor).
Importance in agriculture
Although much of the nitrogen is removed when protein-rich grain or hay is harvested, significant amounts can remain in the soil for future crops. 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. Nitrogen fixation by Rhizobium is also beneficial to the environment.
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 change 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 rizobia 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, though 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.
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 enough 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.
The legume – Rhizobia symbiosis is a classic example of mutualism — Rhizobia supply ammonia or amino acids to the plant and in return receive organic acids (principally as the dicarboxylic acids malate and succinate) as a carbon and energy source — but its evolutionary persistence is actually somewhat surprising. Because several unrelated strains infect each individual plant, any one strain could redirect resources from nitrogen fixation to its own reproduction without killing the host plant upon which they all depend. But this form of cheating should be equally tempting for all strains, a classic tragedy of the commons. It turns out that legume plants guide the evolution of Rhizobia towards greater mutualism by reducing the oxygen supply to nodules that fix less nitrogen, thereby reducing the frequency of cheaters in the next generation.
Other diazotrophs
Many other species of bacteria are able to fix nitrogen (diazotrophs), but few are able to associate intimately with plants and colonize specific structures like Legume nodules. Bacteria that do associate with plants include the actinobacteria Frankia, which form symbiotic root nodules in actinorhizal plants, and several cyanobacteria (Nostoc) associated with aquatic ferns, Cycas and Gunneras. Free-living diazotrophs are often found in the rhizosphere and in the intercellular spaces of several plants including rice and sugarcane, but in this case the lack of a specialized structure results in poor nutrient transfer efficiency compared to legume or actinorhizal nodules.
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