Array ( [0] => {{Short description|Nitrogen fixing soil bacteria}} [1] => {{About|the generic term that includes species in other genera|the bacterial genus|Rhizobium}} [2] => [3] => [[Image:Soybean-root-nodules.jpg|thumb|right|Root nodules, each containing billions of ''[[Rhizobiaceae]]'' bacteria]] [4] => '''Rhizobia''' are [[diazotrophic]] [[bacteria]] that [[Nitrogen fixation|fix nitrogen]] after becoming established inside the [[root nodule]]s of [[legume]]s ([[Fabaceae]]). To express genes for [[nitrogen fixation]], rhizobia require a [[plant]] [[Host (biology)|host]]; they cannot independently fix [[nitrogen]].{{Cite journal |last=Zahran |first=Hamdi Hussein |date=1999-12-01 |title=Rhizobium-legume symbiosis and nitrogen fixation under severe conditions and in an arid climate |journal=Microbiology and Molecular Biology Reviews |volume=63 |issue=4 |pages=968–989, table of contents |doi=10.1128/MMBR.63.4.968-989.1999 |issn=1092-2172 |pmc=98982 |pmid=10585971}} In general, they are [[gram-negative bacteria|gram negative]], [[Motility|motile]], non-[[sporogenesis|sporulating]] rods. [5] => [6] => Rhizobia are a "group of soil bacteria that infect the roots of legumes to form [[root nodules]]".{{cite journal|last1=Herridge|first1=David|title=Rhizobial Inoculants|journal=GRDC|date=2013}} Rhizobia are found in the soil and, after infection, produce nodules in the [[legume]] where they fix nitrogen gas (N2) from the atmosphere, turning it into a more readily useful form of nitrogen. From here, the nitrogen is exported from the nodules and used for growth in the legume. Once the legume dies, the nodule breaks down and releases the rhizobia back into the soil, where they can live individually or reinfect a new legume host. [7] => [8] => == History == [9] => The first known [[species]] of rhizobia, ''[[Rhizobium leguminosarum]]'', was identified in 1889, and all further species were initially placed in the ''[[Rhizobium]]'' [[genus]]. Most research has been done on [[crop]] and [[forage]] legumes such as [[clover]], [[alfalfa]], [[bean]]s, [[pea]]s, and [[soybean]]s; more research is being done on North American legumes.{{citation needed|date=April 2016}} [10] => [11] => == Taxonomy {{efn|As with many bacterium classifications, taxonomy work is still in progress as new genetic data and discoveries re-shuffle the existing phylogenetic tree}} == [12] => [13] => Rhizobia are a [[paraphyletic]] group that fall into two [[Class (biology)|classes]] of [[Pseudomonadota]]—the [[alphaproteobacteria]] and [[betaproteobacteria]]. As shown below, most belong to the order [[Hyphomicrobiales]], but several rhizobia occur in distinct bacterial orders of the Pseudomonadota.{{cite web|url=http://www.rhizobia.co.nz/taxonomy/rhizobia.html|access-date=2013-12-02|title=Current taxonomy of rhizobia|archive-url=https://web.archive.org/web/20130604070051/http://www.rhizobia.co.nz/taxonomy/rhizobia|archive-date=2013-06-04|url-status=live}} [14] => {{Cite web |last=Weir |first=Bevan |year=2016 |title=The current taxonomy of rhizobia |url=https://rhizobia.nz/taxonomy/rhizobia |access-date=2023-11-18}}{{cite web|url=http://www.rhizobia.co.nz/taxonomy/not-rhizobia|access-date=2013-12-02|title=Bacteria confused with rhizobia, including ''Agrobacterium'' taxonomy|archive-url=https://web.archive.org/web/20131203164833/http://www.rhizobia.co.nz/taxonomy/not-rhizobia|archive-date=2013-12-03|url-status=live}}{{cite web|url=http://edzna.ccg.unam.mx/rhizobial-taxonomy/node/4|access-date=2013-12-02|title=Taxonomy of legume nodule bacteria (rhizobia) and agrobacteria|archive-url=https://web.archive.org/web/20181017011839/http://edzna.ccg.unam.mx/rhizobial-taxonomy/node/4|archive-date=2018-10-17|url-status=live}} [15] => [16] => {| [17] => |- valign=top [18] => | [19] => '''[[Alphaproteobacteria]]''' [20] => :'''[[Hyphomicrobiales]] (syn. [[Rhizobiales]])''' [21] => ::'''[[Nitrobacteraceae]]''' [22] => :::''[[Bosea (bacteria)|Bosea]]'' [23] => [24] => [25] => [26] => [27] => [28] => :::''[[Bradyrhizobium]]'' [29] => ::::''[[Bradyrhizobium arachidis|B. arachidis]]'' [30] => [31] => ::::''[[Bradyrhizobium canariense|B. canariense]]'' [32] => ::::''[[Bradyrhizobium cytisi|B. cytisi]]'' [33] => ::::''[[Bradyrhizobium daqingense|B. daqingense]]'' [34] => ::::''[[Bradyrhizobium denitrificans|B. denitrificans]]'' [35] => ::::''[[Bradyrhizobium diazoefficiens|B. diazoefficiens]]'' [36] => ::::''[[Bradyrhizobium elkanii|B. elkanii]]'' [37] => ::::''[[Bradyrhizobium huanghuaihaiense|B. huanghuaihaiense]]'' [38] => ::::''[[Bradyrhizobium iriomotense|B. iriomotense]]'' [39] => ::::''[[Bradyrhizobium japonicum|B. japonicum]]'' [40] => ::::''[[Bradyrhizobium jicamae|B. jicamae]]'' [41] => ::::''[[Bradyrhizobium lablabi|B. lablabi]]'' [42] => ::::''[[Bradyrhizobium liaoningense|B. liaoningense]]'' [43] => ::::''[[Bradyrhizobium pachyrhizi|B. pachyrhizi]]'' [44] => ::::''[[Bradyrhizobium rifense|B. rifense]]'' [45] => ::::''[[Bradyrhizobium yuanmingense|B. yuanmingense]]'' [46] => ::'''[[Brucellaceae]]''' [47] => :::''[[Ochrobactrum]]'' [48] => ::::''[[Ochrobactrum cytisi|O. cytisi]]'' [49] => ::::''[[Ochrobactrum lupini|O. lupini]]'' [50] => ::'''[[Hyphomicrobiaceae]]''' [51] => :::''[[Devosia]]'' [52] => ::::''[[Devosia neptuniae|D. neptuniae]]'' [53] => ::'''[[Methylobacteriaceae]]''' [54] => :::''[[Methylobacterium]]'' [55] => ::::''[[Methylobacterium nodulans|M. nodulans]]'' [56] => :::''[[Microvirga]]'' [57] => ::::''[[Microvirga lotononidis|M. lotononidis]]'' [58] => ::::''[[Microvirga lupini|M. lupini]]'' [59] => ::::''[[Microvirga zambiensis|M. zambiensis]]'' [60] => | [61] => ::'''[[Phyllobacteriaceae]]''' [62] => :::''[[Aminobacter]]'' [63] => ::::''[[Aminobacter anthyllidis|A. anthyllidis]]'' [64] => :::''[[Mesorhizobium]]'' [65] => ::::''[[Mesorhizobium abyssinicae|M. abyssinicae]]'' [66] => ::::''[[Mesorhizobium albiziae|M. albiziae]]'' [67] => ::::''[[Mesorhizobium alhagi|M. alhagi]]'' [68] => ::::''[[Mesorhizobium amorphae|M. amorphae]]'' [69] => ::::''[[Mesorhizobium australicum|M. australicum]]'' [70] => ::::''[[Mesorhizobium camelthorni|M. camelthorni]]'' [71] => ::::''[[Mesorhizobium caraganae|M. caraganae]]'' [72] => ::::''[[Mesorhizobium chacoense|M. chacoense]]'' [73] => ::::''[[Mesorhizobium ciceri|M. ciceri]]'' [74] => ::::''[[Mesorhizobium gobiense|M. gobiense]]'' [75] => ::::''[[Mesorhizobium hawassense|M. hawassense]]'' [76] => ::::''[[Mesorhizobium huakuii|M. huakuii]]'' [77] => ::::''[[Mesorhizobium loti|M. loti]]'' [78] => ::::''[[Mesorhizobium mediterraneum|M. mediterraneum]]'' [79] => ::::''[[Mesorhizobium metallidurans|M. metallidurans]]'' [80] => ::::''[[Mesorhizobium muleiense|M. muleiense]]'' [81] => ::::''[[Mesorhizobium opportunistum|M. opportunistum]]'' [82] => ::::''[[Mesorhizobium plurifarium|M. plurifarium]]'' [83] => ::::''[[Mesorhizobium qingshengii|M. qingshengii]]'' [84] => ::::''[[Mesorhizobium robiniae|M. robiniae]]'' [85] => ::::''[[Mesorhizobium sangaii|M. sangaii]]'' [86] => ::::''[[Mesorhizobium septentrionale|M. septentrionale]]'' [87] => ::::''[[Mesorhizobium shangrilense|M. shangrilense]]'' [88] => ::::''[[Mesorhizobium shonense|M. shonense]]'' [89] => ::::''[[Mesorhizobium tamadayense|M. tamadayense]]'' [90] => ::::''[[Mesorhizobium tarimense|M. tarimense]]'' [91] => ::::''[[Mesorhizobium temperatum|M. temperatum]]'' [92] => [93] => ::::''[[Mesorhizobium tianshanense|M. tianshanense]]'' [94] => :::''[[Phyllobacterium]]'' [95] => [96] => [97] => [98] => [99] => [100] => ::::''[[Phyllobacterium sophorae|P. sophorae]]'' [101] => ::::''[[Phyllobacterium trifolii|P. trifolii]]'' [102] => [103] => | [104] => ::'''[[Rhizobiaceae]]''' [105] => :::''[[Rhizobium (genus)|Rhizobium]]'' [106] => [107] => ::::''[[Rhizobium alamii|R. alamii]]'' [108] => ::::''[[Rhizobium cauense|R. cauense]]'' [109] => ::::''[[Rhizobium cellulosilyticum|R. cellulosilyticum]]'' [110] => ::::''[[Rhizobium daejeonense|R. daejeonense]]'' [111] => [112] => ::::''[[Rhizobium etli|R. etli]]'' [113] => ::::''[[Rhizobium fabae|R. fabae]]'' [114] => ::::''[[Rhizobium gallicum|R. gallicum]]'' [115] => ::::''[[Rhizobium grahamii|R. grahamii]]'' [116] => ::::''[[Rhizobium hainanense|R. hainanense]]'' [117] => ::::''[[Rhizobium halophytocola|R. halophytocola]]'' [118] => ::::''[[Rhizobium indigoferae|R. indigoferae]]'' [119] => ::::''[[Rhizobium leguminosarum|R. leguminosarum]]'' [120] => ::::''[[Rhizobium leucaenae|R. leucaenae]]'' [121] => ::::''[[Rhizobium loessense|R. loessense]]'' [122] => ::::''[[Rhizobium lupini|R. lupini]]'' [123] => ::::''[[Rhizobium lusitanum|R. lusitanum]]'' [124] => ::::''[[Rhizobium mesoamericanum|R. mesoamericanum]]'' [125] => ::::''[[Rhizobium mesosinicum|R. mesosinicum]]'' [126] => ::::''[[Rhizobium miluonense|R. miluonense]]'' [127] => ::::''[[Rhizobium mongolense|R. mongolense]]'' [128] => ::::''[[Rhizobium multihospitium|R. multihospitium]]'' [129] => ::::''[[Rhizobium oryzae|R. oryzae]]'' [130] => ::::''[[Rhizobium petrolearium|R. petrolearium]]'' [131] => ::::''[[Rhizobium phaseoli|R. phaseoli]]'' [132] => ::::''[[Rhizobium pisi|R. pisi]]'' [133] => ::::''[[Rhizobium qilianshanense|R. qilianshanense]]'' [134] => ::::''[[Rhizobium sullae|R. sullae]]'' [135] => ::::''[[Rhizobium taibaishanense|R. taibaishanense]]'' [136] => ::::''[[Rhizobium tibeticum|R. tibeticum]]'' [137] => ::::''[[Rhizobium tropici|R. tropici]]'' [138] => ::::''[[Rhizobium tubonense|R. tubonense]]'' [139] => ::::''[[Rhizobium vallis|R. vallis]]'' [140] => ::::''[[Rhizobium yanglingense|R. yanglingense]]'' [141] => :::''[[Agrobacterium]]'' [142] => ::::''[[Agrobacterium nepotum|A. nepotum]]'' [143] => ::::''[[Agrobacterium pusense|A. pusense]]'' [144] => :::''[[Allorhizobium]]'' [145] => ::::''[[Allorhizobium undicola|A. undicola]]'' [146] => | [147] =>
[148] => :::''[[Pararhizobium]]'' [149] => ::::''[[Pararhizobium giardinii|P. giardinii]]'' [150] => ::::''[[Pararhizobium helanshanense|P. helanshanense]]'' [151] => ::::''[[Pararhizobium herbae|P. herbae]]'' [152] => ::::''[[Pararhizobium sphaerophysae|P. sphaerophysae]]'' [153] => :::''[[Neorhizobium]]'' [154] => ::::''[[Neorhizobium alkalisoli|N. alkalisoli]]'' [155] => ::::''[[Neorhizobium galegae|N. galegae]]'' [156] => ::::''[[Neorhizobium huautlense|N. huautlense]]'' [157] => ::::''[[Neorhizobium vignae|N. vignae]]'' [158] => :::''[[Shinella]]'' [159] => ::::''[[Shinella kummerowiae|S. kummerowiae]]'' [160] => :::''[[Ensifer (bacterium)|Ensifer]]'' (syn. ''[[Sinorhizobium]]'') [161] => ::::''[[Sinorhizobium abri|E. abri]]'' [162] => ::::''[[Ensifer adhaerens|E. adhaerens]]'' [163] => ::::''[[Ensifer americanus|E. americanus]]'' [164] => ::::''[[Sinorhizobium arboris|E. arboris]]'' [165] => ::::''[[Sinorhizobium chiapanecum|E. chiapanecum]]'' [166] => ::::''[[Ensifer fredii|E. fredii]]'' [167] => ::::''[[Ensifer garamanticus|E. garamanticus]]'' [168] => ::::''[[Sinorhizobium indiaense|E. indiaense]]'' [169] => ::::''[[Sinorhizobium kostiense|E. kostiense]]'' [170] => ::::''[[Sinorhizobium kummerowiae|E. kummerowiae]]'' [171] => ::::''[[Ensifer medicae|E. medicae]]'' [172] => ::::''[[Ensifer meliloti|E. meliloti]]'' [173] => ::::''[[Ensifer mexicanus|E. mexicanus]]'' [174] => [175] => ::::''[[Ensifer numidicus|E. numidicus]]'' [176] => ::::''[[Ensifer psoraleae|E. psoraleae]]'' [177] => ::::''[[Sinorhizobium saheli|E. saheli]]'' [178] => ::::''[[Ensifer sesbaniae|E. sesbaniae]]'' [179] => ::::''[[Ensifer sojae|E. sojae]]'' [180] => ::::''[[Sinorhizobium terangae|E. terangae]]'' [181] => [182] => ::'''[[Xanthobacteraceae]]''' [183] => :::''[[Azorhizobium]]'' [184] => ::::''[[Azorhizobium caulinodans|A. caulinodans]]'' [185] => ::::''[[Azorhizobium doebereinerae|A. doebereinerae]]'' [186] => | [187] => '''[[Betaproteobacteria]]''' [188] => :'''[[Burkholderiales]]''' [189] => ::'''[[Burkholderiaceae]]''' [190] => [191] => [192] => [193] => :::''[[Cupriavidus]]'' [194] => ::::''[[Cupriavidus taiwanensis|C. taiwanensis]]'' [195] => :::''[[Paraburkholderia]]'' [196] => ::::''[[Paraburkholderia caribensis|P. caribensis]]'' [197] => ::::''[[Paraburkholderia diazotrophica|P. diazotrophica]]'' [198] => ::::''[[Paraburkholderia dilworthii|P. dilworthii]]'' [199] => ::::''[[Paraburkholderia mimosarum|P. mimosarum]]'' [200] => ::::''[[Paraburkholderia nodosa|P. nodosa]]'' [201] => ::::''[[Paraburkholderia phymatum|P. phymatum]]'' [202] => ::::''[[Paraburkholderia piptadeniae|P. piptadeniae]]'' [203] => ::::''[[Paraburkholderia rhynchosiae|P. rhynchosiae]]'' [204] => ::::''[[Paraburkholderia sabiae|P. sabiae]]'' [205] => ::::''[[Paraburkholderia sprentiae|P. sprentiae]]'' [206] => ::::''[[Paraburkholderia symbiotica|P. symbiotica]]'' [207] => ::::''[[Paraburkholderia tuberum|P. tuberum]]'' [208] => [209] => [210] => [211] => |} [212] => [213] => These groups include a variety of non-[[Symbiosis|symbiotic]] bacteria. For instance, the plant [[pathogen]] ''[[Agrobacterium]]'' is a closer relative of ''Rhizobium'' than the ''Bradyrhizobium'' that nodulate soybean.{{cite journal |last1=Sullivan |first1=John T. |last2=Ronson |first2=Clive W. |date=11 December 1997 |title=Evolution of rhizobia by acquisition of a 500-kb symbiosis island that integrates into a phe-tRNA gene |journal=PNAS |volume=95 |issue=9 |pages=5145–5149 |bibcode=1998PNAS...95.5145S |doi=10.1073/pnas.95.9.5145 |pmc=20228 |pmid=9560243 |doi-access=free}} [214] => [215] => == Importance in agriculture == [216] => [217] => [[Image:Rhizobia nodules on Vigna unguiculata.jpg|thumb|right|Rhizobia nodules on ''[[Vigna unguiculata]]'']] [218] => [219] => Although much of the nitrogen is removed when [[protein]]-rich [[Cereal|grain]] or [[hay]] is [[harvest]]ed, 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]] [[Crop rotation|rotation schemes]] or in some less-[[Industrialisation|industrialized]] countries.{{cite web | url = http://www.bionewsonline.com/y/what_is_rhizobia.htm | archive-url = https://archive.today/20120720012116/http://www.bionewsonline.com/y/what_is_rhizobia.htm | url-status = dead | archive-date = 2012-07-20 | title = What is Rhizobia | access-date = 2008-07-01 }} [[Nitrogen]] is the most commonly deficient nutrient in many soils around the world and it is the most commonly supplied plant nutrient. The supply of nitrogen through [[fertilizers]] has severe [[Fertilizer#Risks of fertilizer use|environmental concerns]]. [220] => [221] => Specific strains of rhizobia are required to make functional nodules on the roots able to fix the N2.{{cite web|last1=Rachaputi|first1=Rao|last2=Halpin|first2=Neil|last3=Seymour|first3=Nikki|last4=Bell|first4=Mike|title=rhizobium inoculation|url=https://www.daff.qld.gov.au/__data/assets/pdf_file/0005/58946/Rhizobium-brochure.pdf|publisher=GRDC|access-date=2015-04-23|archive-url=https://web.archive.org/web/20141129121754/https://www.daff.qld.gov.au/__data/assets/pdf_file/0005/58946/Rhizobium-brochure.pdf|archive-date=2014-11-29|url-status=live}} Having this specific rhizobia present is beneficial to the legume, as the N2 fixation can increase crop yield.{{cite book|last1=Catroux|first1=Gerard|last2=Hartmann|first2=Alain|last3=Revillin|first3=Cecile|title=Trends in rhizobium inoculant production and use|date=2001|publisher=Kluwer Academic Publishers|location=Netherlands|pages=21–30}} Inoculation with rhizobia tends to increase yield.{{cite book|last1=Purcell|first1=Larry C.|last2=Salmeron|first2=Montserrat|last3=Ashlock|first3=Lanny|title=Arkansas Soybean Production Handbook - MP197|date=2013|chapter=Chapter 5|chapter-url=http://www.uaex.edu/publications/pdf/mp197/chapter5.pdf|publisher=University of Arkansas Cooperative Extension Service|location=Little Rock, AR|page=5|url=http://www.uaex.edu/publications/mp-197.aspx|access-date=21 February 2016|archive-url=https://web.archive.org/web/20160304011452/http://www.uaex.edu/publications/mp-197.aspx|archive-date=4 March 2016|url-status=live}} [222] => [223] => Legume inoculation has been an agricultural practice for many years and has continuously improved over time. 12–20 million hectares of soybeans are inoculated annually. An ideal inoculant includes some of the following aspects; maximum efficacy, ease of use, compatibility, high rhizobial concentration, long shelf-life, usefulness under varying field conditions, and survivability.{{cite web|last1=Shrestha|first1=R|last2=Neupane|first2=RK|last3=Adhikari|first3=NP|title=Status and Future Prospects of Pulses in Nepal|url=http://www.doanepal.gov.np/downloadfile/Current%20Status%20SAARC_paper-Nepal_1320838291.pdf|publisher=Government of Nepal|access-date=2015-04-23|archive-url=https://web.archive.org/web/20150706223103/http://www.doanepal.gov.np/downloadfile/Current%20Status%20SAARC_paper-Nepal_1320838291.pdf|archive-date=2015-07-06|url-status=live}}{{cite book|last1=Bennett|first1=J. Michael|last2=Hicks|first2=Dale R.|last3=Naeve|first3=Seth L.|last4=Bush Bennett|first4=Nancy |title=The Minnesota Soybean Field Book|date=2014|publisher=University of Minnesota Extension|location=St Paul, MN|page=79|url=http://www.extension.umn.edu/agriculture/soybean/docs/minnesota-soybean-field-book.pdf|access-date=21 February 2016|archive-url=https://web.archive.org/web/20130930151502/http://www1.extension.umn.edu/agriculture/soybean/docs/minnesota-soybean-field-book.pdf|archive-date=30 September 2013|url-status=dead}} [224] => [225] => These inoculants may foster success in legume cultivation.{{cite book|last1=Stephens|first1=J.H.G|last2=Rask|first2=H.M|title=Inoculant production and formulation|date=2000|publisher=MicroBio RhizoGen Corporation|location=Saskatoon|pages=249–258}} As a result of the nodulation process, after the harvest of the crop, there are higher levels of soil nitrate, which can then be used by the next crop. [226] => [227] => == Symbiotic relationship == [228] => Rhizobia are unique in that they are the only nitrogen-fixing bacteria living in a [[symbiosis|symbiotic]] relationship with [[legumes]]. Common crop and forage legumes are peas, beans, clover, and soy. [229] => [230] => ===Nature of the mutualism=== [231] => The legume–rhizobium [[symbiosis]] is a classic example of [[Mutualism (biology)|mutualism]]—rhizobia supply ammonia or [[amino acids]] to the plant and, in return, receive organic acids (mainly [[Malic acid|malate]] and [[Succinic acid|succinate]], which are [[dicarboxylic acid]]s) as a carbon and energy source. However, because several unrelated strains infect each individual plant, a classic [[tragedy of the commons]] scenario presents itself. Cheater strains may hoard plant resources such as [[polyhydroxybutyrate]] for the benefit of their own [[reproduction]] without fixing an appreciable amount of [[nitrogen]].{{cite journal|last1=Ratcliff|first1=W.C.|last2=Kadam|first2=S.V.|last3=Denison|first3=R.F.|title=Poly-3-hydroxybutyrate (PHB) supports survival and reproduction in starving rhizobia|journal=FEMS Microbiology Ecology|date=2008|volume=65|issue=3|pages=391–399|doi=10.1111/j.1574-6941.2008.00544.x|pmid=18631180|doi-access=free|bibcode=2008FEMME..65..391R }} Given the costs involved in nodulation and the opportunity for rhizobia to cheat, it may be surprising that this symbiosis exists. [232] => [233] => ===Infection and signal exchange=== [234] => {{Further|Symbiosome|Common Symbiotic Signaling Pathway}} [235] => The formation of the symbiotic relationship involves a signal exchange between both partners that leads to mutual recognition and the development of symbiotic structures. The most well understood mechanism for the establishment of this symbiosis is through intracellular infection. Rhizobia are free living in the soil until they are able to sense [[flavonoid]]s, derivatives of 2-phenyl-1.4-benzopyrone, which are secreted by the roots of their host plant, triggering the accumulation of a large population of cells and eventually attachment to [[root hair]]s.{{Cite book|title=Brock biology of microorganisms|last1=Martinko |first1=John M.|last2=Bender|first2=Kelly S.|last3=Buckley|first3=Daniel H. |last4=Stahl |first4=David Allan|year=2015|publisher=Pearson |isbn=9780321897398|oclc=857863493}}{{Cite journal|last1=Maj|first1=Dominika|last2=Wielbo|first2=Jerzy|last3=Marek-Kozaczuk|first3=Monika|last4=Skorupska|first4=Anna|date=2010-01-01|title=Response to flavonoids as a factor influencing competitiveness and symbiotic activity of Rhizobium leguminosarum|journal=Microbiological Research|volume=165|issue=1|pages=50–60|doi=10.1016/j.micres.2008.06.002|issn=1618-0623|pmid=18678476|doi-access=}} These flavonoids then promote the DNA binding activity of NodD, which belongs to the LysR family of transcriptional regulators and triggers the secretion of [[nod factor]]s after the bacteria have entered the root hair. [[Nod factor]]s trigger a series of complex developmental changes inside the root hair, beginning with [[root hair curling]] and followed by the formation of the infection thread, a cellulose lined tube that the bacteria use to travel down through the root hair into the root cells.{{Cite journal|last=Gage|first=Daniel J.|date=2017-05-12|title=Infection and Invasion of Roots by Symbiotic, Nitrogen-Fixing Rhizobia during Nodulation of Temperate Legumes|journal=Microbiology and Molecular Biology Reviews|volume=68|issue=2|pages=280–300|doi=10.1128/MMBR.68.2.280-300.2004|issn=1092-2172|pmc=419923|pmid=15187185}} The bacteria then infect several other adjacent root cells. This is followed by continuous cell proliferation, resulting in the formation of the [[root nodule]]. A second mechanism, used especially by rhizobia that infect aquatic hosts, is called crack entry. In this case, no root hair deformation is observed. Instead, the bacteria penetrate between cells through cracks produced by lateral root emergence.{{Cite journal|last1=Morgante|first1=Carolina|last2=Angelini|first2=Jorge|last3=Castro|first3=Stella|last4=Fabra|first4=Adriana|date=2005-08-01|title=Role of rhizobial exopolysaccharides in crack entry/intercellular infection of peanut|journal=Soil Biology and Biochemistry|volume=37|issue=8|pages=1436–1444|doi=10.1016/j.soilbio.2004.12.014}} [236] => [237] => Inside the nodule, the bacteria differentiate morphologically into [[Symbiosome#In the plant|bacteroids]] and fix atmospheric nitrogen into [[ammonium]] using the enzyme [[nitrogenase]]. [[Ammonium]] is then converted into amino acids like [[glutamine]] and [[asparagine]] before it is exported to the plant. In return, the plant supplies the bacteria with [[carbohydrate]]s in the form of organic acids. The plant also provides the bacteroid oxygen for [[cellular respiration]], tightly bound by [[Leghemoglobin|leghaemoglobins]], plant proteins similar to human [[hemoglobin]]s. This process keeps the nodule oxygen poor in order to prevent the inhibition of [[nitrogenase]] activity. [238] => [239] => Recently, a ''[[Bradyrhizobium]]'' strain was discovered to form nodules in ''[[Aeschynomene]]'' without producing nod factors, suggesting the existence of alternative communication signals other than nod factors, possibly involving the secretion of the plant hormone cytokinin.{{Cite journal|last1=Okazaki|first1=Shin|last2=Tittabutr|first2=Panlada|last3=Teulet|first3=Albin|last4=Thouin|first4=Julien|last5=Fardoux|first5=Joël|last6=Chaintreuil|first6=Clémence|last7=Gully|first7=Djamel|last8=Arrighi|first8=Jean-François|last9=Furuta|first9=Noriyuki|date=2016-01-01|title=Rhizobium–legume symbiosis in the absence of Nod factors: two possible scenarios with or without the T3SS|journal=The ISME Journal|language=en|volume=10|issue=1|pages=64–74|doi=10.1038/ismej.2015.103|issn=1751-7362|pmc=4681849|pmid=26161635|bibcode=2016ISMEJ..10...64O }} [240] => [241] => It has been observed that root nodules can be formed spontaneously in ''[[Medicago]]'' without the presence of rhizobia.{{Cite book|title=Advances in Molecular Genetics of Plant-microbe Interactions Vol. 3 Proceedings of the 7th International Symposium on Molecular Plant-microbe Interactions, Edinburgh, U.K.|date=June 1994|last1=Daniels |first1=Michael J.|last2=Downie |first2=J. Allan |last3=Osbourn |first3=Anne E.|publisher=Springer Verlag|isbn=9789401040792|oclc=968919649}} This implies that the development of the nodule is controlled entirely by the plant and simply triggered by the secretion of [[nod factor]]s. [242] => [243] => ===Evolutionary hypotheses=== [244] => ====The sanctions hypothesis==== [245] => There are two main hypotheses for the mechanism that maintains legume-rhizobium symbiosis (though both may occur in nature). The '''sanctions hypothesis''' theorizes that legumes cannot recognize the more parasitic or less nitrogen fixing rhizobia and must counter the parasitism by post-infection legume sanctions. In response to underperforming rhizobia, legume hosts can respond by imposing sanctions of varying severity to their nodules.{{cite journal|last1=Kiers|first1=E. Toby|title=Measured sanctions: legume hosts detect quantitative variation in rhizobium cooperation and punish accordingly|journal=Evolutionary Ecology Research|date=2006|volume=8|pages=1077–1086|url=http://www.tobykiers.com/wp-content/uploads/2011/12/14.-Kiers_EER_2006.pdf|access-date=23 April 2015|archive-url=https://web.archive.org/web/20160305070605/http://www.tobykiers.com/wp-content/uploads/2011/12/14.-Kiers_EER_2006.pdf|archive-date=5 March 2016|url-status=live}} [246] => These sanctions include, but are not limited to, reduction of nodule growth, early nodule death, decreased carbon supply to nodules, or reduced [[oxygen]] supply to nodules that fix less nitrogen. Within a nodule, some of the bacteria differentiate into nitrogen fixing bacteroids, which have been found to be unable to reproduce.{{cite journal | last1 = Denison | first1 = R. F. | year = 2000 | title = Legume sanctions and the evolution of symbiotic cooperation by rhizobia | journal = American Naturalist | volume = 156 | issue = 6| pages = 567–576 | doi=10.1086/316994| pmid = 29592542 | s2cid = 4404801 }} Therefore, with the development of a symbiotic relationship, if the host sanctions hypothesis is correct, the host sanctions must act toward whole nodules rather than individual bacteria because individual targeting sanctions would prevent any reproducing rhizobia from proliferating over time. This ability to reinforce a mutual relationship with host sanctions pushes the relationship toward mutualism rather than parasitism and is likely a contributing factor to why the symbiosis exists. [247] => [248] => However, other studies have found no evidence of plant sanctions.{{cite journal | last1 = Marco | first1 = D. E. | last2 = Perez-Arnedo | first2 = R. | last3 = Hidalgo-Perea | first3 = A. | last4 = Olivares | first4 = J. | last5 = Ruiz-Sainz | first5 = J. E. | last6 = Sanjuan | first6 = J. | year = 2009 | title = A mechanistic molecular test of the plant-sanction hypothesis in legume-rhizobia mutualism | journal = Acta Oecologica-International Journal of Ecology | volume = 35 | issue = 5 | pages = 664–667 | doi = 10.1016/j.actao.2009.06.005 | url = https://zenodo.org/record/848802 | bibcode = 2009AcO....35..664M | access-date = 2017-08-27 | archive-url = https://web.archive.org/web/20170827125626/https://zenodo.org/record/848802 | archive-date = 2017-08-27 | url-status = live }} [249] => [250] => ====The partner choice hypothesis==== [251] => The '''partner choice hypothesis''' proposes that the plant uses prenodulation signals from the rhizobia to decide whether to allow nodulation, and chooses only noncheating rhizobia. There is evidence for sanctions in soybean plants, which reduce rhizobium reproduction (perhaps by limiting oxygen supply) in nodules that fix less nitrogen.Kiers ET, Rousseau RA, West SA, Denison RF 2003. Host sanctions and the legume–rhizobium mutualism. ''[[Nature (journal)|Nature]]'' 425 : 79-81 Likewise, wild lupine plants allocate fewer resources to nodules containing less-beneficial rhizobia, limiting rhizobial reproduction inside.{{Cite journal |last=Simms |first=Ellen L |last2=Taylor |first2=D. Lee |last3=Povich |first3=Joshua |last4=Shefferson |first4=Richard P |last5=Sachs |first5=J.L |last6=Urbina |first6=M |last7=Tausczik |first7=Y |date=2006-01-07 |title=An empirical test of partner choice mechanisms in a wild legume–rhizobium interaction |url=https://royalsocietypublishing.org/doi/10.1098/rspb.2005.3292 |journal=Proceedings of the Royal Society B: Biological Sciences |language=en |volume=273 |issue=1582 |pages=77–81 |doi=10.1098/rspb.2005.3292 |issn=0962-8452 |pmc=1560009 |pmid=16519238}} This is consistent with the definition of sanctions, although called "partner choice" by the authors. Some studies support the partner choice hypothesis.{{cite journal | last1 = Heath | first1 = K. D. | last2 = Tiffin | first2 = P. | year = 2009 | title = Stabilizing mechanisms in legume-rhizobium mutualism | journal = Evolution | volume = 63 | issue = 3| pages = 652–662 | pmid = 19087187 | doi = 10.1111/j.1558-5646.2008.00582.x | title-link = Mutualism (biology) | s2cid = 43500062 }} While both mechanisms no doubt contribute significantly to maintaining rhizobial cooperation, they do not in themselves fully explain the persistence of [[Mutualism (biology)|mutualism]]. The partner choice hypothesis is not exclusive from the host sanctions hypothesis, as it is apparent that both of them are prevalent in the symbiotic relationship.{{cite journal|last1=Heath|first1=Katy D.|title=Stabilizing Mechanisms in a Legume-Rhizobium Mutualism|journal=Evolution|date=12 December 2008|volume=63|issue=3|pages=652–662|doi=10.1111/j.1558-5646.2008.00582.x|pmid=19087187|s2cid=43500062}} [252] => [253] => ===Evolutionary history=== [254] => The symbiosis between nitrogen fixing rhizobia and the legume family has emerged and evolved over the past 66 million years.{{cite journal|last1=Herendeen|first1=Patrick|title=A Preliminary Conspectus of the Allon Flora from the Late Cretaceous (Late Santonian) of Central Georgia, U.S.A|journal=Annals of the Missouri Botanical Garden|date=1999|volume=86|issue=2|pages=407–471|doi=10.2307/2666182|jstor=2666182|url=https://www.biodiversitylibrary.org/part/28622}}{{cite journal |last1=Renne |first1=Paul R. |last2=Deino |first2=Alan L. |last3=Hilgen |first3=Frederik J. |last4=Kuiper |first4=Klaudia F. |last5=Mark |first5=Darren F. |last6=Mitchell |first6=William S. |last7=Morgan |first7=Leah E. |last8=Mundil |first8=Roland |last9=Smit |first9=Jan |title=Time Scales of Critical Events Around the Cretaceous-Paleogene Boundary |journal=Science |date=7 February 2013 |volume=339 |issue=6120 |pages=684–687 |doi=10.1126/science.1230492 |pmid=23393261 |url=http://www.cugb.edu.cn/uploadCms/file/20600/20131028144132060.pdf |bibcode=2013Sci...339..684R |s2cid=6112274 |access-date=1 April 2018 |archive-url=https://web.archive.org/web/20170207164818/http://www.cugb.edu.cn/uploadCms/file/20600/20131028144132060.pdf |archive-date=7 February 2017 |url-status=live }} Although evolution tends to swing toward one species taking advantage of another in the form of noncooperation in the selfish-gene model, management of such symbiosis allows for the continuation of cooperation.{{cite journal|last1=Sachs|first1=Joel L.|title=The Evolution of Cooperation|journal=The Quarterly Review of Biology|date=June 2004|volume=79|issue=2|jstor=383541|doi=10.1086/383541|pages=135–160|pmid=15232949|s2cid=19830045}} When the relative fitness of both species is increased, natural selection will favor symbiosis. [255] => [256] => To understand the evolutionary history of this symbiosis, it is helpful to compare the rhizobia-legume symbiosis to a more ancient symbiotic relationship, such as that between [[endomycorrhizae|endomycorrhizae fungi]] and land plants, which dates back to almost 460 million years ago.{{cite journal|last1=Martin|first1=Parniske|author-link=Martin Parniske|title=Arbuscular mycorrhiza: the mother of plant root endosymbioses|journal=Nature Reviews Microbiology|date=2008|volume=6|issue=10|pages=763–775|doi=10.1038/nrmicro1987|pmid=18794914|s2cid=5432120}} [257] => [258] => Endomycorrhizal symbiosis can provide many insights into rhizobia symbiosis because recent genetic studies have suggested that rhizobia co-opted the signaling pathways from the more ancient endomycorrhizal symbiosis.{{cite journal|last1=Geurts|first1=René|title=Mycorrhizal Symbiosis: Ancient Signalling Mechanisms Co-opted|journal=Current Biology|date=2012|volume=22|issue=23|pages=R997–9|doi=10.1016/j.cub.2012.10.021|pmid=23218015|doi-access=free}} Bacteria secrete Nod factors and endomycorrhizae secrete Myc-LCOs. Upon recognition of the Nod factor/Myc-LCO, the plant proceeds to induce a variety of intracellular responses to prepare for the symbiosis.{{cite journal|last1=Parniske|first1=Martin|title=Intracellular accommodation of microbes by plants: a common developmental program for symbiosis and disease?|journal=Curr Opin Plant Biol|date=2000|issue=4|pages=320–328|pmid=10873847|volume=3|doi=10.1016/s1369-5266(00)00088-1|author1-link=Martin Parniske}} [259] => [260] => It is likely that rhizobia co-opted the features already in place for endomycorrhizal symbiosis because there are many shared or similar genes involved in the two processes. For example, the plant recognition gene SYMRK (symbiosis receptor-like kinase) is involved in the perception of both the rhizobial Nod factors as well as the endomycorrhizal Myc-LCOs.{{cite journal|last1=Oldroyd|first1=Giles|title=Coordinating nodule morphogenesis with rhizobial infection in legumes|journal=Annual Review of Plant Biology|date=2008|volume=59|pages=519–546|doi=10.1146/annurev.arplant.59.032607.092839|pmid=18444906}} The shared similar processes would have greatly facilitated the evolution of rhizobial symbiosis because not all the symbiotic mechanisms would have needed to develop. Instead, the rhizobia simply needed to evolve mechanisms to take advantage of the symbiotic signaling processes already in place from endomycorrhizal symbiosis. [261] => [262] => == Other diazotrophs == [263] => 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 that do associate with plants include the [[Actinomycetota|actinomycete]], ''[[Frankia]]'', which form symbiotic root nodules in [[actinorhizal plant]]s, although these bacteria have a much broader host range, implying the association is less specific than in legumes. Additionally, several [[cyanobacteria]] like ''[[Nostoc]]'' are associated with [[Azolla|aquatic ferns]], ''[[Cycas]]'', and ''[[Gunnera]]s,'' although they do not form nodules.{{Cite journal|last=Campbell|first=Douglas Houghton|date=1908-01-01|title=Symbiosis in Fern Prothallia|jstor=2455676|journal=The American Naturalist|volume=42|issue=495|pages=154–165|doi=10.1086/278916|url=https://zenodo.org/record/1431345|doi-access=free}}{{Cite journal|last1=Vagnoli|first1=L.|last2=Margheri|first2=M. C.|last3=And|first3=G. Allotta|last4=Materassi|first4=R.|date=1992-02-01|title=Morphological and physiological properties of symbiotic cyanobacteria|journal=New Phytologist|language=en|volume=120|issue=2|pages=243–249|doi=10.1111/j.1469-8137.1992.tb05660.x|issn=1469-8137|doi-access=free}} [264] => [265] => Additionally, loosely associated plant bacteria, termed [[endophyte]]s, have been reported to fix nitrogen ''in planta''.{{Cite book|title=Associative and endophytic nitrogen-fixing bacteria and cyanobacterial associations|last1=Elmerich |first1=C.|last2=Newton |first2=William E. |date=2007-01-01|publisher=Springer|isbn=9781402035418|oclc=187303797}} These bacteria colonize the intercellular spaces of leaves, stems, and roots in plants {{Cite book |title=Bacteria in Agrobiology: Plant Growth Responses |date=2011 |publisher=Springer Berlin Heidelberg |isbn=9783642203312 |editor-last=Maheshwari |editor-first=Dinesh K. |oclc=938989968}} but do not form specialized structures like rhizobia and ''[[Frankia]].'' Diazotrophic bacterial endophytes have very broad host ranges, in some cases colonizing both [[Monocotyledon|monocots]] and [[Dicotyledon|dicots]].{{Cite journal|last1=Khan|first1=Zareen|last2=Guelich|first2=Grant|last3=Phan|first3=Ha|last4=Redman|first4=Regina|last5=Doty|first5=Sharon|date=2012-10-15|title=Bacterial and Yeast Endophytes from Poplar and Willow Promote Growth in Crop Plants and Grasses|journal=ISRN Agronomy|language=en|volume=2012|pages=1–11|doi=10.5402/2012/890280|doi-access=free}} [266] => [267] => == Note == [268] => {{notelist}} [269] => [270] => == References == [271] => {{Reflist}} [272] => [273] => ==Further reading== [274] => * {{Cite journal [275] => |last1=Jones [276] => |first1= KM [277] => |title=How rhizobial symbionts invade plants: the Sinorhizobium–Medicago model [278] => |journal=Nature Reviews Microbiology [279] => |volume=5 [280] => |issue=8 [281] => |pages=619–33 [282] => |year=2007 [283] => |pmid=17632573 [284] => |doi=10.1038/nrmicro1705 [285] => |last2=Kobayashi [286] => |first2=H [287] => |last3=Davies [288] => |first3=BW [289] => |last4=Taga [290] => |first4=ME [291] => |last5=Walker [292] => |first5=GC [293] => |pmc=2766523 [294] => |display-authors=etal}} [295] => [296] => == External links == [297] => * [http://www.nature.com/nature/journal/v425/n6953/full/nature01931.html Legume sanctions maintain Rhizobium mutualism] [298] => * [https://www.rhizobia.co.nz/taxonomy/rhizobia Current list of rhizobia species] [299] => * [http://edis.ifas.ufl.edu/aa126 Nitrogen Fixation and Inoculation of Forage Legumes] [300] => [301] => [[Category:Hyphomicrobiales]] [302] => [[Category:Symbiosis]] [303] => [[Category:Nitrogen cycle]] [304] => [[Category:Soil biology]] [] => )
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Rhizobia

Rhizobia are diazotrophic bacteria that fix nitrogen after becoming established inside the root nodules of legumes (Fabaceae). To express genes for nitrogen fixation, rhizobia require a plant host; they cannot independently fix nitrogen.

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