Nod factor
Nod factors (nodulation factors or NF), are
Chemical Structure
Nod factors structurally are lipochitooligosaccharides (LCOs) that consist of an N-acetyl-D-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.[2]
Nod factors are produced in complex mixtures differing in the following characteristics:[3]
- Length of the chain can vary from three to six units of N-acetyl-D-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 flavonoids 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.[4]
Function
Nod factors are potentially recognized by plant receptors made of two histidine kinases with extracellular LysM domain, which have been identified in L. japonicus, soybean, and M. truncatula [5]. Binding of Nod factors to these receptors depolarizes the plasma membrane of root hairs via an influx of Ca+2 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.[6] 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.[7] 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.[8]
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