Rhizobia
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2008-07-01T16:49:41Z
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PLAGERISM the rest of the article probably also needs going over
[[Image:Soybean-root-nodules.jpg|thumb|right|Soybean root nodules, each containing billions of ''[[Bradyrhizobium]]'' bacteria]]
'''Rhizobia''' (from the [[Greek language|Greek]] words ''rhiza'' = root and ''bios'' = Life) are [[soil]] [[bacterium|bacteria]] that [[Nitrogen fixation|fix]] [[nitrogen]] ([[diazotroph]]y) after becoming established inside [[root nodule]]s of legumes ([[Fabaceae]]). The rhizobia cannot independently fix nitrogen, and requires a plant host. Morphologically they are generally [[Gram staining|gram negative]], motile, non-sporulating rods.
== 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 (agriculture)|crop]] and [[Fodder|forage]] legumes such as [[clover]], [[bean]]s, and [[soy]]. However, recently more work is occurring on North American legumes.
== Taxonomy ==
''Rhizobia'' consist of 57 species found in 12 genera.<ref name="The current taxonomy of rhizobia">{{cite web|url=http://www.rhizobia.co.nz/taxonomy/rhizobia.html|accessdate=2006-08-07|title=Current taxonomy of rhizobia}}</ref> Most belong to the [[Rhizobiales]], a probably-[[monophyletic]] group of [[proteobacteria]]. Within that group, however, they are scattered among several different families:
{| width=100%
| '''Family''' || '''Genera'''
|-
| [[Rhizobiaceae]] || ''[[Rhizobium (genus)|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 [[gene]]s responsible for the [[symbiosis]] with plants, however, may be closer than the organisms themselves, acquired by [[horizontal gene transfer|horizontal transfer]] (via [[bacterial conjugation]]) rather than from a [[common descent|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 farming|organic]] rotation schemes or some less-industrialized countries." <ref>{{cite web | url = http://www.bionewsonline.com/y/what_is_rhizobia.htm | title = What is Rhizobia | accessdate = 2008-07-01}}</ref> [[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 [[Fertilizer#Risks of fertilizer use|environmental concerns]]. [[Nitrogen fixation]] by ''Rhizobium'' is also beneficial to the [[Natural environment|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_factor]]s, 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]] (NH<sub>4</sub><sup>+</sup>), 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 ([[diazotroph]]s), 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 plant]]s and several [[cyanobacteria]] (''Nostoc'') associated with aquatic ferns (= ''[[Azolla]]''), ''[[Cycas]]'' and ''[[Gunnera]]s''. 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.
== External links ==
* [http://www.biomedcentral.com/news/20030904/02 Legume sanctions maintain Rhizobium mutualism]
* [http://www.rhizobia.co.nz/taxonomy/rhizobia.html Current list of rhizobia species]
* [http://edis.ifas.ufl.edu/AG152 Nitrogen Fixation and Inoculation of Forage Legumes]
== References ==
<references />
*{{citation
|last1=Jones
|first1= KM
|last2=et al.
|title=How rhizobial symbionts invade plants: the Sinorhizobium-Medicago model.
|journal=Nat Rev Microbiol.
|volume=5
|issue=8
|pages=619-33
|year=2007
|pmid=17632573
}}
*{{citation
|last1=Giraud
|first1= Eric
|last2=et al.
|title=Legumes symbioses: absence of Nod genes in photosynthetic bradyrhizobia.
|journal=Science
|volume=316
|issue=5829
|pages=1307-12
|year=2007
|pmid=17540897
}}
[[Category:Rhizobiales]]
[[Category:Symbiosis]]
[[Category:Nitrogen metabolism]]
[[Category:Soil biology]]
[[Category:Microbiology]]
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