Nodulation (Nod) factors are signaling molecules produced by bacteria known as rhizobia during the initiation of nodules on the root of legumes. A symbiosis is formed when legumes take up the bacteria. The rhizobia produce nitrogen for the plant, and the legumes produce leghemoglobin to carry away any oxygen that would inhibit nitrogenase activity.
Nod factors structurally are lipochitooligosaccharides (LCOs) that consist of an acylated chitin oligomeric backbone with various functional group substitutions at the terminal or non-terminal residues. The number of N-acetylglucosamine molecules vary among Nod factors; however, generally the length of a chitin backbone is from 3-5. The exact chemical structure of the Nod factor that is recognised by the plant varies between bacterial species and is the basis for host-symbiont specificity. Nod factors are recognized by a specific class of receptor kinases that have so-called LysM domains 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 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 introns.
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