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 to 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.
Nod gene expression is induced by the presence of certain flavonoids in the soil, which are secreted by the plant to attract the bacteria. These chemicals induce the formation of NodD, which in turn activates other genes involved in the expression of nod factors and their secretion into the soil. Nod factors induce root-hair curling such that it envelops the bacterium. 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 and enter the root hair. 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.
Interestingly, at least three plant genes which are stimulated by Nod factors are also involved in the arbuscular mycorrhizal symbiosis. The addition of certain Nod factors enhances arbuscular mycorrhizal colonization, indicating that the two very different symbioses may share some common mechanisms.
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