Editing Root nodule (section)

==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)
|}