Editing Root nodule (section)

==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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 | alt2 = Whole alder root nodule
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'''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>