Editing Fabaceae (section)

=== Biological nitrogen fixation ===

[[File:Vicia sepium10 ies.jpg|thumb |Roots of ''[[Vicia]]'' with white root nodules visible]]
[[File:Cross-section through root nodule of (Vicia spec.).jpg|thumb |Cross-section through a root nodule of ''[[Vicia]]'' observed through a microscope]]

[[Nitrogen fixation|Biological nitrogen fixation]] (BNF, performed by the organisms called [[diazotroph]]s) is a very old process that probably originated in the [[Archean]] eon when the primitive [[atmosphere]] lacked [[oxygen]]. It is only carried out by [[Euryarchaeota]] and just 6 of the more than 50 [[Phylum|phyla]] of [[bacteria]]. Some of these lineages co-evolved together with the [[flowering plant]]s establishing the molecular basis of a mutually beneficial [[symbiosis|symbiotic]] relationship. BNF is carried out in nodules that are mainly located in the root cortex, although they are occasionally located in the stem as in ''[[Sesbania rostrata]]''. The [[spermatophyte]]s that co-evolved with [[actinorhizal]] diazotrophs (''[[Frankia]]'') or with [[rhizobia]] to establish their symbiotic relationship belong to 11 families contained within the [[Rosidae]] [[clade]] (as established by the gene molecular phylogeny of ''rbcL'', a gene coding for part of the [[RuBisCO]] enzyme in the [[chloroplast]]). This grouping indicates that the predisposition for forming nodules probably only arose once in flowering plants and that it can be considered as an ancestral characteristic that has been conserved or lost in certain lineages. However, such a wide distribution of families and genera within this lineage indicates that nodulation had multiple origins. Of the 10 families within the Rosidae, 8 have nodules formed by [[actinomyces]] ([[Betulaceae]], [[Casuarinaceae]], [[Coriariaceae]], [[Datiscaceae]], [[Elaeagnaceae]], [[Myricaceae]], [[Rhamnaceae]] and [[Rosaceae]]), and the two remaining families, [[Ulmaceae]] and Fabaceae have nodules formed by rhizobia.<ref name="Looret">{{cite journal |last1=Lloret |first1=L. |last2=Martínez-Romero |first2=E. |year=2005 |title=Evolución y filogenia de |url=http://www.medigraphic.com/espanol/e-htms/e-lamicro/e-mi2005/e-mi05-1_2/em-mi05-1_2f.htm |archive-url=https://archive.today/20130411080503/http://www.medigraphic.com/espanol/e-htms/e-lamicro/e-mi2005/e-mi05-1_2/em-mi05-1_2f.htm |url-status=dead |archive-date=2013-04-11 |journal=Rhizobium |volume=47 |issue=1–2 |pages=43–60 }}</ref><ref name="Sprent">Sprent, J. I. 2001. Nodulation in legumes. Royal Botanic Gardens, Kew, UK.</ref>

The rhizobia and their hosts must be able to recognize each other for nodule formation to commence. Rhizobia are specific to particular host species although a rhizobia species may often infect more than one host species. This means that one plant species may be infected by more than one species of bacteria. For example, nodules in ''[[Acacia senegal]]'' can contain seven species of rhizobia belonging to three different genera. The most distinctive characteristics that allow rhizobia to be distinguished apart are the rapidity of their growth and the type of root nodule that they form with their host.<ref name="Sprent" /> Root nodules can be classified as being either indeterminate, cylindrical and often branched, and determinate, spherical with prominent lenticels. Indeterminate nodules are characteristic of legumes from temperate climates, while determinate nodules are commonly found in species from tropical or subtropical climates.<ref name="Sprent" />

Nodule formation is common throughout the Fabaceae. It is found in the majority of its members that only form an association with rhizobia, which in turn form an exclusive symbiosis with the Fabaceae (with the exception of ''Parasponia'', the only genus of the 18 Ulmaceae genera that is capable of forming nodules). Nodule formation is present in all the Fabaceae sub-families, although it is less common in the Caesalpinioideae. All types of nodule formation are present in the subfamily Papilionoideae: indeterminate (with the [[meristem]] retained), determinate (without meristem) and the type included in ''Aeschynomene''. The latter two are thought to be the most modern and specialised type of nodule as they are only present in some lines of the subfamily Papilionoideae. Even though nodule formation is common in the two [[Monophyly|monophyletic]] subfamilies Papilionoideae and Mimosoideae they also contain species that do not form nodules. The presence or absence of nodule-forming species within the three sub-families indicates that nodule formation has arisen several times during the evolution of the Fabaceae and that this ability has been lost in some lineages. For example, within the genus ''Acacia'', a member of the Mimosoideae, ''A. pentagona'' does not form nodules, while other species of the same genus readily form nodules, as is the case for ''Acacia senegal'', which forms both rapidly and slow growing rhizobial nodules.