Editing Root nodule

{{Short description|Plant part}}
[[File:Nitrogen fixation Fabaceae en.svg|thumb|upright=1.25|A simplified diagram of the relation between the plant and the symbiotic bacteria (cyan) in the root nodules]]
'''Root nodules''' are found on the [[root]]s of [[plant]]s, primarily [[legume]]s, that form a [[symbiosis]] with [[nitrogen-fixing bacteria]].<ref name="Wagner">{{cite journal |last1=Wagner |first1=Stephen C. |date=2011 |title=Biological Nitrogen Fixation |url=https://www.nature.com/scitable/knowledge/library/biological-nitrogen-fixation-23570419 |journal=Nature Education Knowledge |volume=3 |issue=10 |page=15}}</ref> Under [[nitrogen]]-limiting conditions, capable plants form a symbiotic relationship with a host-specific strain of bacteria known as [[rhizobia]].<ref>{{cite journal |last1=Wang |first1=Qi |last2=Yang |first2=Shengming |date=2017 |title=Host-secreted antimicrobial peptide enforces symbiotic selectivity in ''Medicago truncatula'' |journal=PNAS |volume=114 |issue=26 |pages=6854–6859 |bibcode=2017PNAS..114.6854W |doi=10.1073/pnas.1700715114 |pmc=5495241 |pmid=28607058 |doi-access=free}}</ref> This process has evolved multiple times within the legumes, as well as in other species found within the [[Rosid]] clade.<ref>{{Cite journal |last1=Doyle |first1=Jeff J. |last2=Luckow |first2=Melissa A. |date=2003 |title=The Rest of the Iceberg. Legume Diversity and Evolution in a Phylogenetic Context |journal=Plant Physiology |language=en |volume=131 |issue=3 |pages=900–910 |doi=10.1104/pp.102.018150 |issn=1532-2548 |pmc=1540290 |pmid=12644643}}</ref> [[Legume]] crops include [[bean]]s, [[pea]]s, and [[soybean]]s.

Within legume root nodules, nitrogen gas (N<sub>2</sub>) from the atmosphere is converted into [[ammonia]] (NH<sub>3</sub>), which is then assimilated into [[amino acids]] (the building blocks of proteins), [[nucleotides]] (the building blocks of [[DNA]] and [[RNA]] as well as the important energy molecule [[Adenosine triphosphate|ATP]]), and other cellular constituents such as [[vitamin]]s, [[flavone]]s, and [[plant hormones|hormones]].{{citation needed|date=October 2022}} Their ability to [[Nitrogen fixation|fix gaseous nitrogen]] makes legumes an ideal agricultural organism as their requirement for nitrogen fertilizer is reduced. Indeed, high nitrogen content blocks nodule development as there is no benefit for the plant of forming the symbiosis. The energy for splitting the nitrogen gas in the nodule comes from sugar that is translocated from the leaf (a product of [[photosynthesis]]). [[Malate]] as a breakdown product of [[sucrose]] is the direct carbon source for the bacteroid. Nitrogen fixation in the nodule is very oxygen sensitive. Legume nodules harbor an iron containing protein called [[leghaemoglobin]], closely related to animal [[myoglobin]], to facilitate the diffusion of oxygen gas used in respiration.

==Symbiosis==
[[File:Symbiosis in Root Nodules.png|thumb|upright=1.25|Nitrogen is the most commonly limiting nutrient in plants. Legumes use nitrogen fixing bacteria, specifically symbiotic rhizobia bacteria, within their root nodules to counter the limitation. Rhizobia bacteria convert nitrogen gas (N<sub>2</sub>) to [[ammonia]] (NH<sub>3</sub>) in a process called [[nitrogen fixation]]. Ammonia is then assimilated into [[nucleotide]]s, [[amino acid]]s, [[vitamin]]s and [[flavone]]s which are essential to the growth of the plant. The plant root cells convert sugar into organic acids which then supply to the rhizobia in exchange, hence a symbiotic relationship between rhizobia and the legumes.]]

===Leguminous family===
Plants that contribute to N2 fixation include the [[legume]] family&nbsp;– [[Fabaceae]]&nbsp;– with taxa such as [[kudzu]], [[clover]]s, [[soybean]]s, [[alfalfa]], [[lupin]]es, [[peanut]]s, and [[rooibos]]. They contain [[symbiosis|symbiotic]] bacteria called [[rhizobia]] within the nodules, producing nitrogen compounds that help the plant to grow and compete with other plants. When the plant dies, the fixed nitrogen is released, making it available to other plants, and this helps to fertilize the [[soil]].<ref name="postgate">{{cite book |last1=Postgate |first1=John |title=Nitrogen Fixation |publisher=Cambridge University Press |year=1998 |isbn=9780521648530 |edition=3rd |location=Cambridge UK}}</ref><ref>{{Cite book |last1=Smil |first1=Vaclav |title=Cycles of Life: Civilization and the Biosphere |publisher=Scientific American Library |year=2000 |isbn=9780716750796 |language=en}}</ref> The great majority of legumes have this association, but a few genera (e.g., ''[[Styphnolobium]]'') do not. In many traditional farming practices, fields are rotated through various types of crops, which usually includes one consisting mainly or entirely of a leguminous crop such as clover, in order to take advantage of this.{{cn|date=March 2025}}

===Non-leguminous===
Although by far the majority of plants able to form nitrogen-fixing root nodules are in the legume family [[Fabaceae]], there are a few exceptions:
* [[Actinorhizal plant]]s such as [[alder]] and [[bayberry]] can form (less complex) nitrogen-fixing nodules, thanks to a symbiotic association with ''[[Frankia]]'' bacteria. These plants belong to 25 genera distributed among 8 plant families.<ref>{{cite book |first1=J. O. |last1=Dawson |chapter=Ecology of Actinorhizal Plants |title=Nitrogen-fixing Actinorhizal Symbioses |volume=6 |pages=199–234 |doi=10.1007/978-1-4020-3547-0_8 |year=2008 |publisher=Springer |series=Nitrogen Fixation: Origins, Applications, and Research Progress |isbn=978-1-4020-3540-1 |s2cid=85913801 }}</ref> According to a count in 1998, it includes about 200 species and accounts for roughly the same amount of nitrogen fixation as rhizobial symbioses. An important structural difference is that in these symbioses the bacteria are never released from the infection thread.<ref name="Doyle">{{cite journal |last1=Doyle |first1=Jeff J. |date=1998 |title=Phylogenetic perspectives on nodulation: evolving views of plants and symbiotic bacteria |journal=Trends in Plant Science |volume=3 |issue=12 |pages=473–778 |doi=10.1016/S1360-1385(98)01340-5 |doi-access=free|bibcode=1998TPS.....3..473D }}</ref>
* ''[[Parasponia]]'', a tropical genus in the [[Cannabaceae]] is also able to interact with rhizobia and form nitrogen-fixing nodules. As related plants are actinorhizal, it is believed that the plant "switched partner" in its evolution.<ref>{{Cite journal |last1=Op den Camp |first1=Rik |last2=Streng |first2=Arend |last3=De Mita |first3=Stéphane |last4=Cao |first4=Qingqin |last5=Polone |first5=Elisa |last6=Liu |first6=Wei |last7=Ammiraju |first7=Jetty S. S. |last8=Kudrna |first8=Dave |last9=Wing |first9=Rod |last10=Untergasser |first10=Andreas |last11=Bisseling |first11=Ton |last12=Geurts |first12=René |date=2011-02-18 |title=LysM-Type Mycorrhizal Receptor Recruited for Rhizobium Symbiosis in Nonlegume ''Parasponia'' |url=https://www.science.org/doi/10.1126/science.1198181 |journal=[[Science (journal)|Science]] |language=en |volume=331 |issue=6019 |pages=909–912 |doi=10.1126/science.1198181 |issn=0036-8075 |bibcode=2011Sci...331..909O |pmid=21205637 |s2cid=20501765|url-access=subscription }}</ref>

The ability to fix nitrogen is far from universally present in these families. For instance, of 122 genera in the [[Rosaceae]], only 4 [[genera]] are capable of fixing nitrogen. All these families belong to the [[order (biology)|order]]s [[Cucurbitales]], [[Fagales]], and [[Rosales]], which together with the [[Fabales]] form a ''nitrogen-fixing clade'' (NFC) of [[eurosid]]s. In this clade, Fabales were the first lineage to branch off; thus, the ability to fix nitrogen may be [[plesiomorphic]] and subsequently lost in most descendants of the original nitrogen-fixing plant; however, it may be that the basic [[genetics|genetic]] and [[physiological]] requirements were present in an incipient state in the [[last common ancestor]]s of all these plants, but only evolved to full function in some of them:{{cn|date=March 2025}}

{{Clear}}
{|
|- valign=top
|'''Family: Genera'''

[[Betulaceae]]: ''[[Alnus]]'' (alders)

[[Cannabaceae]]: ''[[Trema (plant)|Trema]]''

[[Casuarinaceae]]:
:''[[Allocasuarina]]''
:''[[Casuarina]]''
:''[[Ceuthostoma]]''
:''[[Gymnostoma]]''
|
<span style="color:white;">......</span>
|
<br />
[[Coriariaceae]]: ''[[Coriaria]]''

[[Datiscaceae]]: ''[[Datisca]]''

[[Elaeagnaceae]]:
:''[[Elaeagnus]]'' (silverberries)
:''[[Hippophae]]'' (sea-buckthorns)
:''[[Shepherdia]]'' (buffaloberries)
|
<span style="color:white;">......</span>
|
<br />
[[Myricaceae]]:
:''[[Comptonia (plant)|Comptonia]]'' (sweetfern)
:''[[Morella (plant)|Morella]]''
:''[[Myrica]]'' (bayberries)
|
<span style="color:white;">......</span>
|
<br />
[[Rhamnaceae]]:
:''[[Ceanothus]]''
:''[[Colletia]]''
:''[[Discaria]]''
:''[[Kentrothamnus]]''
:''[[Retanilla]]''
:''[[Talguenea]]''
:''[[Trevoa]]''
|
<span style="color:white;">......</span>
|
<br />
[[Rosaceae]]:
:''[[Cercocarpus]]'' (mountain mahoganies)
:''[[Chamaebatia]]'' (mountain miseries)
:''[[Dryas (plant)|Dryas]]''
:''[[Purshia]]/Cowania'' (bitterbrushes/cliffroses)
|}

==Classification==
[[Image:Medicago italica root nodules 2.JPG|right|thumb|Indeterminate nodules growing on the roots of ''Medicago italica'']]
Two main types of nodule have been described in legumes: determinate and indeterminate.<ref>{{Cite journal |last1=Crespi |first1=Martin |last2=Gálvez |first2=Susana |date=2000-06-01 |title=Molecular Mechanisms in Root Nodule Development |url= |journal=Journal of Plant Growth Regulation |language=en |volume=19 |issue=2 |pages=155–166 |doi=10.1007/s003440000023 |issn=1435-8107 |pmid=11038225 |s2cid=22216527}}</ref>

'''Determinate nodules''' are found on certain tribes of tropical legume such as those of the genera ''[[Glycine (plant)|Glycine]]'' (soybean), ''[[Phaseolus]]'' (common bean), and ''[[Vigna]]''. and on some temperate legumes such as ''[[Lotus (genus)|Lotus]]''. These determinate nodules lose meristematic activity shortly after initiation, thus growth is due to cell expansion resulting in mature nodules which are spherical in shape. Another type of determinate nodule is found in a wide range of herbs, shrubs and trees, such as ''[[Arachis]]'' ([[peanut]]). These are always associated with the axils of lateral or adventitious roots and are formed following infection via cracks where these roots emerge and not using [[root hair]]s. Their internal structure is quite different from those of the [[soybean]] type of nodule.<ref name="Sprent">{{Cite book |last=Sprent |first=Janet I. |title=Legume Nodulation: A Global Perspective |publisher=Wiley-Blackwell |year=2009 |isbn=9781444316384 |language=en |doi=10.1002/9781444316384}}</ref>

'''Indeterminate nodules''' are found in the majority of legumes from all three sub-families, whether in temperate regions or in the tropics. They can be seen in ''[[Faboideae]]'' legumes such as ''[[Pisum]]'' (pea), ''[[Medicago]]'' (alfalfa), ''[[Trifolium]]'' (clover), and ''[[Vicia]]'' (vetch) and all [[mimosoid]] legumes such as ''[[acacia]]''s, the few nodulated ''[[Caesalpinioideae|caesalpinioid]]'' legumes such as [[Chamaecrista fasciculata|partridge pea]]. They earned the name "indeterminate" because they maintain an active apical [[meristem]] that produces new cells for growth over the life of the nodule. This results in the nodule having a generally cylindrical shape, which may be extensively branched.<ref name="Sprent" /> Because they are actively growing, indeterminate nodules manifest zones which demarcate different stages of development/symbiosis:<ref>{{Cite journal |last1=Foucher |first1=Fabrice |last2=Kondorosi |first2=Eva |date=2000-08-01 |title=Cell cycle regulation in the course of nodule organogenesis in ''Medicago'' |url= |journal=Plant Molecular Biology |language=en |volume=43 |issue=5 |pages=773–786 |doi=10.1023/A:1006405029600 |pmid=11089876 |issn=1573-5028 |s2cid=11658948}}</ref><ref>{{cite journal |last1=Monahan-Giovanelli |first1=Hannah |last2=Pinedo |first2=Catalina Arango |last3=Gage |first3=Daniel J. |name-list-style= |date=2006 |title=Architecture of Infection Thread Networks in Developing Root Nodules Induced by the Symbiotic Bacterium ''Sinorhizobium meliloti'' on ''Medicago truncatula'' |journal=Plant Physiology |volume=140 |issue=2 |pages=661–670 |doi=10.1104/pp.105.072876 |pmc=1361332 |pmid=16384905}}</ref><ref>{{Cite journal |last1=Van de Velde |first1=Willem |last2=Guerra |first2=Juan Carlos Pérez |last3=Keyser |first3=Annick De |last4=De Rycke |first4=Riet |last5=Rombauts |first5=Stéphane |last6=Maunoury |first6=Nicolas |last7=Mergaert |first7=Peter |last8=Kondorosi |first8=Eva |last9=Holsters |first9=Marcelle |last10=Goormachtig |first10=Sofie |date=2006-04-28 |title=Aging in Legume Symbiosis. A Molecular View on Nodule Senescence in ''Medicago truncatula'' |url= |journal=Plant Physiology |volume=141 |issue=2 |pages=711–720 |doi=10.1104/pp.106.078691 |issn=1532-2548 |pmc=1475454 |pmid=16648219}}</ref>
[[Image:Indeterminate Nodule Zones Diagram.svg|right|thumb|Diagram illustrating the different zones of an indeterminate root nodule (see text).]]
* Zone I—the '''active meristem'''. This is where new nodule tissue is formed which will later differentiate into the other zones of the nodule.
* Zone II—the '''infection zone'''. This zone is permeated with infection threads full of bacteria. The plant cells are larger than in the previous zone and cell division is halted.
** Interzone II–III—Here the bacteria have entered the plant cells, which contain [[amyloplast]]s. They elongate and begin terminally differentiating into symbiotic, nitrogen-fixing [[Symbiosome#Differentiation|bacteroids]].
* Zone III—the '''nitrogen fixation zone'''. Each cell in this zone contains a large, central [[vacuole]] and the cytoplasm is filled with fully differentiated bacteroids which are actively [[nitrogen fixation|fixing nitrogen]]. The plant provides these cells with [[leghemoglobin]], resulting in a distinct pink color.
* Zone IV—the '''senescent zone'''. Here plant cells and their bacteroid contents are being degraded. The breakdown of the heme component of leghemoglobin results in a visible greening at the base of the nodule.

This is the most widely studied type of nodule, but the details are quite different in nodules of peanut and relatives and some other important crops such as lupins where the nodule is formed following direct infection of rhizobia through the epidermis and where infection threads are never formed. Nodules grow around the root, forming a collar-like structure. In these nodules and in the peanut type the central infected tissue is uniform, lacking the uninfected ells seen in nodules of soybean and many indeterminate types such as peas and clovers.{{cn|date=March 2025}}
{{multiple image
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 | header = Alder tree root nodule
 | image1 = A sectioned alder root nodule gall.JPG
 | alt1 = Sectioned alder root nodule
 | caption1 = Sectioned
 | image2 = An alder root nodule gall.JPG
 | alt2 = Whole alder root nodule
 | caption2 = Whole
}}

'''Actinorhizal-type nodules''' are markedly different structures found in non-legumes. In this type, cells derived from the root cortex form the infected tissue, and the prenodule becomes part of the mature nodule. Despite this seemingly major difference, it is possible to produce such nodules in legumes by a single [[homeotic]] mutation.<ref>{{cite journal |last1=Shen |first1=Defeng |last2=Xiao |first2=Ting Ting |last3=van Velzen |first3=Robin |last4=Kulikova |first4=Olga |last5=Gong |first5=Xiaoyun |last6=Geurts |first6=René |last7=Pawlowski |first7=Katharina |last8=Bisseling |first8=Ton |title=A Homeotic Mutation Changes Legume Nodule Ontogeny into Actinorhizal-Type Ontogeny |journal=The Plant Cell |date=June 2020 |volume=32 |issue=6 |pages=1868–1885 |doi=10.1105/tpc.19.00739 |pmid=32276984 |pmc=7268803|bibcode=2020PlanC..32.1868S }}</ref>

==Nodulation==
[[Image:Root-nodule01.jpg|thumb|upright=1.25|right|Cross section through a [[soybean]] root nodule. The bacterium, ''[[Bradyrhizobium|Bradyrhizobium japonicum]]'', colonizes the roots and establishes a nitrogen fixing symbiosis. This high magnification image shows part of a cell with single bacteroids within their [[symbiosome]]s. In this image, endoplasmic reticulum, dictysome and cell wall can be seen.]]
[[File:NitrogenFixingNodulesOnClover.jpg|thumb|upright|Nitrogen-fixing nodules on a clover root.]]
Legumes release [[organic compound]]s as [[secondary metabolite]]s called [[flavonoid]]s from their roots, which attract the rhizobia to them and which also activate [[Bradyrhizobium#Nod genes|''nod'' genes]] in the bacteria to produce [[nod factor]]s and initiate nodule formation.<ref>{{cite journal |last=Eckardt |first=Nancy A. |date=June 2006 |title=The Role of Flavonoids in Root Nodule Development and Auxin Transport in Medicago truncatula |journal=The Plant Cell |volume=18 |issue=7 |pages=1539–1540 |doi=10.1105/tpc.106.044768 |pmc=1488913|bibcode=2006PlanC..18.1539E }}</ref><ref name="Esseling 2003">{{cite journal |last1=Esseling |first1=John J. |last2=Lhuissier |first2=Franck G.P. |last3=Emons |first3=Anne Mie C. |date=August 2003 |title=Nod Factor-Induced Root Hair Curling: Continuous Polar Growth towards the Point of Nod Factor Application |journal=Plant Physiology |language=en |volume=132 |issue=4 |pages=1982–1988 |doi=10.1104/pp.103.021634 |issn=1532-2548 |pmc=181283 |pmid=12913154 }}</ref> These ''nod'' factors initiate '''root hair curling'''. The curling begins with the very tip of the root hair curling around the ''Rhizobium''. Within the root tip, a small tube called the infection thread forms, which provides a pathway for the ''Rhizobium'' to travel into the root epidermal cells as the root hair continues to curl.<ref>{{cite book | first1 = Joan | last1 = Slonczewski | first2 = John Watkins | last2 = Foster | title = Microbiology: An Evolving Science | isbn = 978-0393614039 | edition = Fourth | location = New York | oclc = 951925510 | year = 2017 }}</ref>

Partial curling can even be achieved by ''nod'' factor alone.<ref name="Esseling 2003"/> This was demonstrated by the isolation of ''nod'' factors and their application to parts of the root hair. The root hairs curled in the direction of the application, demonstrating the action of a root hair attempting to curl around a bacterium. Even application on lateral roots caused curling. This demonstrated that it is the ''nod'' factor itself, not the bacterium that causes the stimulation of the curling.<ref name="Esseling 2003"/>

When the nod factor is sensed by the root, a number of biochemical and morphological changes happen: [[cell division]] is triggered in the root to create the nodule, and the [[root hair]] growth is redirected to curl around the bacteria multiple times until it fully encapsulates one or more bacteria. The bacteria encapsulated divide multiple times, forming a [[microcolony]]. From this microcolony, the bacteria enter the developing nodule through the infection thread, which grows through the root hair into the basal part of the [[Epidermis (botany)|epidermis]] cell, and onwards into the [[cortex (botany)|root cortex]]; they are then surrounded by a plant-derived [[symbiosome|symbiosome membrane]] and differentiate into bacteroids that [[nitrogen fixation|fix nitrogen]].<ref>{{cite journal|last1=Mergaert|first1=P.|last2=Uchiumi|first2=T.|last3=Alunni|first3=B.|last4=Evanno|first4=G.|last5=Cheron|first5=A.|last6=Catrice|first6=O.|last7=Mausset|first7=A.-E.|last8=Barloy-Hubler|first8=F.|last9=Galibert|first9=F.|last10=Kondorosi|first10=A.|last11=Kondorosi|first11=E.|display-authors = 6|title= Eukaryotic control on bacterial cell cycle and differentiation in the Rhizobium-legume symbiosis |journal= PNAS |volume=103 |issue=13 |pages= 5230–35 |date= 2006 |doi= 10.1073/pnas.0600912103 |pmid=16547129|id= Online  |bibcode=2006PNAS..103.5230M|pmc=1458823|issn=1091-6490|doi-access=free}}</ref>

Effective nodulation takes place approximately four weeks after [[crop|crop planting]], with the size, and shape of the nodules dependent on the crop. Crops such as soybeans, or peanuts will have larger nodules than forage legumes such as red clover, or alfalfa, since their nitrogen needs are higher. The number of nodules, and their internal color, will indicate the status of nitrogen fixation in the plant.<ref>{{Cite web|url=http://www1.foragebeef.ca/$Foragebeef/frgebeef.nsf/all/frg90/$FILE/fertilitylegumefixation.pdf|title=Nitrogen Fixation and Inoculation of Forage Legumes|last=Adjei|first=M. B.|publisher=[[University of Florida]]|access-date=December 1, 2016|archive-url=https://web.archive.org/web/20161202170130/http://www1.foragebeef.ca/$Foragebeef/frgebeef.nsf/all/frg90/$FILE/fertilitylegumefixation.pdf|archive-date=December 2, 2016|url-status=dead}}</ref>

Nodulation is controlled by a variety of processes, both external (heat, acidic soils, drought, nitrate) and internal (autoregulation of nodulation, ethylene). '''Autoregulation of nodulation'''<ref name="Reid">{{cite journal |last1=Reid |first1=DE |last2=Ferguson |first2=BJ |last3=Hayashi |first3=S |last4=Lin |first4=YH |last5=Gresshoff |first5=PM |title=Molecular mechanisms controlling legume autoregulation of nodulation. |journal=Annals of Botany |date=October 2011 |volume=108 |issue=5 |pages=789–95 |doi=10.1093/aob/mcr205 |pmid=21856632|pmc=3177682 }}</ref> controls nodule numbers per plant through a systemic process involving the leaf. Leaf tissue senses the early nodulation events in the root through an unknown chemical signal, then restricts further nodule development in newly developing root tissue. The Leucine rich repeat (LRR) receptor kinases (NARK in soybean (''Glycine max''); HAR1 in ''[[Lotus japonicus]]'', SUNN in ''[[Medicago truncatula]]'') are essential for autoregulation of nodulation (AON). Mutation leading to loss of function in these AON receptor kinases leads to supernodulation or hypernodulation. Often root growth abnormalities accompany the loss of AON receptor kinase activity, suggesting that nodule growth and root development are functionally linked. Investigations into the mechanisms of nodule formation showed that the [[ENOD40]] gene, coding for a 12–13 amino acid protein [41], is up-regulated during nodule formation [3].

== Connection to root structure ==
Root nodules apparently have evolved three times within the [[Fabaceae]] but are rare outside that family. The propensity of these plants to develop root nodules seems to relate to their root structure. In particular, a tendency to develop lateral roots in response to [[abscisic acid]] may enable the later evolution of root nodules.<ref>{{Cite journal |last1=Liang |first1=Yan |last2=Harris |first2=Jeanne M. |date=2005 |title=Response of root branching to abscisic acid is correlated with nodule formation both in legumes and nonlegumes |url=https://bsapubs.onlinelibrary.wiley.com/doi/10.3732/ajb.92.10.1675 |journal=American Journal of Botany |language=en |volume=92 |issue=10 |pages=1675–1683 |doi=10.3732/ajb.92.10.1675 |issn=0002-9122 |pmid=21646084|url-access=subscription }}</ref>

==Nodule-like structures==
Some [[fungi]] produce nodular structures known as tuberculate [[ectomycorrhiza]]e on the roots of their plant hosts. ''[[Suillus tomentosus]]'', for example, produces these structures with its plant host [[lodgepole pine]] (''Pinus contorta'' var. ''latifolia''). These structures have, in turn, been shown to host [[nitrogen fixation|nitrogen fixing]] [[bacteria]], which contribute a significant amount of [[nitrogen]] and allow the pines to colonize nutrient-poor sites.<ref name=paul07> {{cite journal |journal=Annals of Botany |volume=99 |pages=1101–1109 |date=2007 |doi=10.1093/aob/mcm061 |title=Nitrogen Fixation Associated with ''Suillus tomentosus'' Tuberculate Ectomycorrhizae on ''Pinus contorta'' var. ''latifolia'' |first1=L.R. |last1=Paul |first2=B.K. |last2=Chapman |first3=C.P. |last3=Chanway |pmid=17468111 |issue=6 |pmc=3243579}}</ref>

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== Gallery ==
<gallery mode="packed">
File:Tuinboon stikstofknolletjes.jpg|Nodules on the [[Vicia faba|''Vicia Faba'']] roots.
File:Glycine max root nodules.jpg|''[[Soybean]]'' roots.
File:Robinia pseudoacacia root nodules.JPG|''[[Robinia pseudoacacia]]'' nodules
File:Closeup of a Dissected Medicago Root Nodule 2.JPG|Close up of dissected ''[[Medicago rotata|Medicago Root]]'' nodule of the [[Fabaceae]] plants family.
File:Fabaceae root nodules with Bradyrhizobium.jpg|[[Fabaceae]] family root nodules.
File:Medicago italica root nodules 1.JPG|''[[Medicago]] italica'' nodules.
File:Root tubercle legume.jpg|Cross section of the nodule.
File:Rhizobia nodules on Vigna unguiculata.jpg|[[Cowpea]] (''[[Vigna unguiculata]] spp.'') roots.
</gallery>

==See also==
* [[Root gall nematode]]
* [[Rhizobium]]
* [[Sinorhizobium]]
* [[Bradyrhizobium]]
* [[Neorhizobium]]
* [[Pararhizobium]]
* [[Common Symbiotic Signaling Pathway]]

==References==
{{Reflist|2|refs=

}}

==External links==
* [http://tolweb.org/notes/?note_id=3948 Legume root nodules at the Tree of Life Web project]
* [https://www.youtube.com/watch?v=3LlUk4EbxY8 Video and commentary on root nodules of White Clover]

{{Authority control}}

[[Category:Plant organogenesis]]
[[Category:Fabaceae]]
[[Category:Nitrogen cycle]]
[[Category:Plant roots]]
[[Category:Symbiosis]]
[[Category:Oligotrophs]]