Editing Nod factor

{{short description|Signaling molecule}}
[[Image:NodSm-IV (Ac,C16-2,S).svg|right|thumb|300px|The structure of the major Nod factor produced by ''[[Sinorhizobium meliloti]]''.]]
'''Nod factors''' ('''nodulation factors''' or '''NF'''), are [[signaling molecule]]s produced by [[Soil microbiology|soil bacteria]] known as [[rhizobia]] in response to flavonoid exudation from plants under nitrogen limited conditions. Nod factors initiate the establishment of a symbiotic relationship between legumes and rhizobia by inducing [[Root nodule|nodulation.]] Nod factors produce the differentiation of plant tissue in root hairs into nodules where the bacteria reside and are able to [[Nitrogen fixation|fix nitrogen]] from the atmosphere for the plant in exchange for photosynthates and the appropriate environment for nitrogen fixation.<ref>''Stambulska, U.Y. and Bayliak, M.M., 2020. Legume-Rhizobium Symbiosis: Secondary Metabolites, Free Radical Processes, and Effects of Heavy Metals. Co-Evolution of Secondary Metabolites, pp.291-322.''</ref> One of the most important features provided by the plant in this symbiosis is the production of [[leghemoglobin]], which maintains the oxygen concentration low and prevents the inhibition of [[nitrogenase]] activity.

== Chemical Structure ==
Nod factors structurally are lipochitooligosaccharides (LCOs) that consist of an [[N-Acetylglucosamine|''N''-acetyl-<small>D</small>-glucosamine]] chain linked through β-1,4 linkage with a fatty acid of variable identity attached to a non reducing nitrogen in the backbone with various [[functional group]] substitutions at the terminal or non-terminal residues.<ref>{{cite journal | author = Spaink HP. | year = 1992 | title = Rhizobial lipo-oligosaccharides: Answers and questions | journal = Plant Molecular Biology | volume = 20 | issue = 5 | pages = 977–986 | doi = 10.1007/BF00027167 | pmid = 1463833 | bibcode = 1992PMolB..20..977S | s2cid = 30609937 | issn = 1573-5028| hdl = 1887/3736313 | hdl-access = free }}</ref>

Nod factors are produced in complex mixtures differing in the following characteristics:<ref>{{cite journal | author = Spaink HP | year = 2000 | title = Root nodulation and infection factors produced by rhizobial bacteria | journal = Annual Review of Microbiology | volume = 54 | issue = 1 | pages = 257–288| doi = 10.1146/annurev.micro.54.1.257 | pmid = 11018130 | hdl = 1887/3748257 | hdl-access = free }}</ref>

* Length of the chain can vary from three to six units of ''N''-acetyl-<small>D</small>-glucosamine with the exception of ''M. loti'' which can produce Nod factors with two unit only.
* Presence or absence of strain-specific substitutions along the chain
* Identity of the fatty acid component
* Presence or absence of unsaturated fatty acids

Nod gene expression is induced by the presence of certain [[flavonoid]]s in the soil, which are secreted by the plant and act as an attractant to bacteria and induce Nod factor production. Flavonoids activate NodD, a LysR family transcription factor, which binds to the ''nod'' box and initiates the transcription of the nod genes which encode the proteins necessary for the production of a wide range of LCOs.<ref>{{cite journal |vauthors=Zuanazzi JA, Clergeot PH, Quirion JC, Husson HP, Kondorosi Á, Ratet P | year = 1998 | title = Production of ''Sinorhizobium meliloti nod'' gene activator and repressor flavonoids from ''Medicago sativa'' roots | journal = Molecular Plant-Microbe Interactions | volume = 11 | issue = 8 | pages = 784–794| doi = 10.1094/MPMI.1998.11.8.784 | doi-access = free | bibcode = 1998MPMI...11..784Z }}</ref>

== Function ==
{{See also|Common Symbiotic Signaling Pathway}}
Nod factors are potentially recognized by plant receptors made of two [[histidine kinase]]s with extracellular [[LysM domain]], which have been identified in ''[[Lotus japonicus|L. japonicus]],'' [[soybean]], and ''[[Medicago truncatula|M. truncatula]]'' <ref name=":0">{{cite journal|vauthors=Gage DJ|date=June 2004|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|pmc=419923|pmid=15187185}}</ref>''.'' Binding of Nod factors to these receptors  depolarizes the plasma membrane of root hairs via an influx of Ca<sup>+2</sup>  which induce the expression of early nodulin (ENOD) genes and swelling of the root hairs. In M. truncatula, the signal transduction initiates by the activation of ''dmi1, dmi2'', and ''dmi3'' which lead to the deformation of root hairs, early nodulin expression, cortical cell division and bacterial infection. Additionally, ''nsp'' and ''hcl'' genes are recruited later and aid in the process of early nodulation expression, cortical cell division, and infection.<ref>{{Cite journal|last=Oldroyd|first=G|date=2001|title=Dissecting Symbiosis: Developments in Nod Factor Signal Transduction|journal=Annals of Botany|volume=87|issue=6|pages=709–718|doi=10.1006/anbo.2001.1410|doi-access=free|bibcode=2001AnBot..87..709O}}</ref> Genes ''dmi1, dmi2, and dmi3''  have also been found to aid in the establishment of interactions between ''M. truncatula'' and [[arbuscular mycorrhiza]],  indicating that the two very different symbioses may share some common mechanisms.<ref>{{cite journal|vauthors=Oláh B, Brière C, Bécard G, Dénarié J, Gough C|date=August 2005|title=Nod factors and a diffusible factor from arbuscular mycorrhizal fungi stimulate lateral root formation in Medicago truncatula via the DMI1/DMI2 signalling pathway|journal=The Plant Journal|volume=44|issue=2|pages=195–207|doi=10.1111/j.1365-313X.2005.02522.x|pmid=16212600|doi-access=free}}</ref> The end result is the nodule, the structure in which nitrogen is fixed.  Nod factors act by inducing changes in gene expression in the legume, most notable the nodulin genes, which are needed for nodule organogenesis.<ref>{{cite journal | vauthors = Govers F, Moerman M, Downie JA, Hooykaas P, Franssen HJ, Louwerse J, van Kammen A, Bisseling T | title = Rhizobium nod genes are involved in inducing an early nodulin gene. | journal = Nature | date = October 1986 | volume = 323 | issue = 6088 | pages = 564–466 | doi = 10.1038/323564a0 | bibcode = 1986Natur.323..564G | s2cid = 4324682 }}</ref>

== Nodulation ==
Rhizobia bind to host specific lectins present in root hairs which together with Nod factors lead to the formation of nodulation. Nod factors are recognized by a specific class of [[Receptor (biochemistry)|receptor]] [[kinase]]s that have  [[LysM domain]]s in their extracellular domains. The two LysM (lysin motif) receptor kinases ([[NFR1]] and [[NFR5]]) that appear to make up the Nod factor receptor were first isolated in the [[model organism|model]] legume ''[[Lotus japonicus]]'' in 2003. They now have been isolated also from [[soybean]] and the model legume ''[[Medicago truncatula]]''. NFR5 lacks the classical [[activation loop]] in the [[kinase]] domain. The ''NFR5'' gene lacks [[intron]]s. First the cell membrane is depolarized and the root hairs start to swell and cell division stops. Nod factor cause the fragmentation and rearrangement of actin network, which coupled with the reinstitution of cell growth lead to the [[Root hair curling|curling of the root hair]] around the bacteria. This is followed by the localized breakdown of the cell wall and the invagination of the plant cell membrane, allowing the bacterium to form an infection thread. As the infection thread grows the rhizobia travel down its length towards the site of the nodule. During this process the pericycle cells in plants become activated and cells in the inner cortex start growing and become the nodule primordium where the rhizobia infect and differentiate into bacteroids and fix nitrogen. Activation of adjacent middle cortex cells leads to the formation of nodule meristem.<ref name=":0" />

==See also==
*[[ENOD40]]

== Notes ==
{{reflist|2}}

{{DEFAULTSORT:Nod Factor}}
[[Category:Fabaceae]]
[[Category:Oligosaccharides]]
[[Category:Signal transduction]]
[[Category:Plant physiology]]