Rhizobia

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Soybean root nodules, each containing billions of Bradyrhizobium bacteria

Rhizobia (from the Greek words rhiza = root and bios = Life) are soil bacteria that fix nitrogen (diazotrophy) after becoming established inside root nodules of legumes (Fabaceae). The rhizobia cannot independently fix nitrogen, and requires a plant host. Morphologically they are generally gram negative, motile, non-sporulating rods.

1 History
2 Taxonomy
3 Importance in agriculture
4 Symbiosis
- 4.1 Infection and signal exchange
- 4.2 Nodule formation and functioning
5 Other diazotrophs
6 External links
7 References

[edit] History

The first species (R. leguminosarum) was identified in 1889, and all further species placed in the Rhizobium genus. However, more advanced methods of analysis have revised this classification and now there are many in other genera. Rhizobium is still sometimes used as the singular of rhizobia. 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.

[edit] Taxonomy

Rhizobia consist of 57 species found in 12 genera.^[1] Most belong to the Rhizobiales, a probably-monophyletic group of proteobacteria. Within that group, however, they are scattered among several different families:

Family	Genera
Rhizobiaceae	Rhizobium (including Allorhizobium), Sinorhizobium/Ensifer
Bradyrhizobiaceae	Bradyrhizobium
Hyphomicrobiaceae	Azorhizobium, Devosia
Phyllobacteriaceae	Mesorhizobium, Phyllobacterium
Brucellaceae	Ochrobactrum
Methylobacteriaceae	Methylobacterium
Burkholderiaceae	Burkholderia, Cupriavidus
Oxalobacteraceae	Herbaspirillum

These groups also include a variety of other bacteria. For instance, the plant pathogen Agrobacterium is a closer relative of Rhizobium than the Rhizobia that nodulate soybean (and may not really be a separate genus). The genes responsible for the symbiosis with plants, however, may be closer than the organisms themselves, acquired by horizontal transfer (via bacterial conjugation) rather than from a common ancestor.

[edit] 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.

[edit] Symbiosis

Rhizobia are unique because they live in a symbiotic relationship with legumes. Common crop and forage legumes are peas, beans, clover, and soy.

[edit] 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.

[edit] 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.

[edit] 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 associated with plants include the Gram positive Frankia that form symbiotic nodules in actinorhizal plants and several cyanobacteria (Nostoc, Clostridium) associated with aquatic ferns (= Azolla), 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.