Nod factors (nodulation factors or NF), are

signaling molecule In biology, cell signaling (cell signalling in British English) or cell communication is the ability of a Cell (biology), cell to receive, process, and transmit signals with its environment and with itself. Cell signaling is a fundamental property ...

s produced by

soil bacteria Soil microbiology is the study of microorganisms in soil, their functions, and how they affect soil properties. It is believed that between two and four billion years ago, the first ancient bacteria and microorganisms came about on Earth's oceans. ...

known as

rhizobia Rhizobia are diazotrophic bacteria that fix nitrogen after becoming established inside the root nodules of legumes (Fabaceae). To express genes for nitrogen fixation, rhizobia require a plant host; they cannot independently fix nitrogen. In g ...

in response to flavonoid exudation from plants under nitrogen limited conditions. Nod factors initiate the establishment of a symbiotic relationship between legumes and rhizobia by inducing nodulation. Nod factors produce the differentiation of plant tissue in root hairs into nodules where the bacteria reside and are able to fix nitrogen from the atmosphere for the plant in exchange for photosynthates and the appropriate environment for nitrogen fixation. One of the most important features provided by the plant in this symbiosis is the production of leghemoglobin, which maintains the oxygen concentration low and prevents the inhibition of

nitrogenase Nitrogenases are enzymes () that are produced by certain bacteria, such as cyanobacteria (blue-green bacteria) and rhizobacteria. These enzymes are responsible for the reduction of nitrogen (N2) to ammonia (NH3). Nitrogenases are the only fa ...

activity.

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 In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the r ...

substitutions at the terminal or non-terminal residues. Nod factors are produced in complex mixtures differing in the following characteristics: * 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

flavonoid Flavonoids (or bioflavonoids; from the Latin word ''flavus'', meaning yellow, their color in nature) are a class of polyphenolic secondary metabolites found in plants, and thus commonly consumed in the diets of humans. Chemically, flavonoids ...

s 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.

Function

Nod factors are potentially recognized by plant receptors made of two

histidine kinase Histidine kinases (HK) are multifunctional, and in non-animal kingdoms, typically transmembrane, proteins of the transferase class of enzymes that play a role in signal transduction across the cellular membrane. The vast majority of HKs are ho ...

s with extracellular LysM domain, which have been identified in '' L. japonicus,''

soybean The soybean, soy bean, or soya bean (''Glycine max'') is a species of legume native to East Asia, widely grown for its edible bean, which has numerous uses. Traditional unfermented food uses of soybeans include soy milk, from which tofu ...

, and '' M. truncatula'' ''.'' 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. Genes ''dmi1, dmi2, and dmi3'' have also been found to aid in the establishment of interactions between ''M. truncatula'' and

arbuscular mycorrhiza An arbuscular mycorrhiza (AM) (plural ''mycorrhizae'', a.k.a. ''endomycorrhiza'') is a type of mycorrhiza in which the symbiont fungus (''AM fungi'', or AMF) penetrates the cortical cells of the roots of a vascular plant forming arbuscules. ( ...

, indicating that the two very different symbioses may share some common mechanisms. 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.

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

receptor Receptor may refer to: *Sensory receptor, in physiology, any structure which, on receiving environmental stimuli, produces an informative nerve impulse *Receptor (biochemistry), in biochemistry, a protein molecule that receives and responds to a n ...

kinase In biochemistry, a kinase () is an enzyme that catalyzes the transfer of phosphate groups from high-energy, phosphate-donating molecules to specific substrates. This process is known as phosphorylation, where the high-energy ATP molecule don ...

s that have 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 A model is an informative representation of an object, person or system. The term originally denoted the plans of a building in late 16th-century English, and derived via French and Italian ultimately from Latin ''modulus'', a measure. Models c ...

legume '' Lotus japonicus'' in 2003. They now have been isolated also from

and the model legume ''

Medicago truncatula ''Medicago truncatula'', the barrelclover, strong-spined medick, barrel medic, or barrel medick, is a small annual legume native to the Mediterranean region that is used in genomic research. It is a low-growing, clover-like plant tall with trifol ...

''. NFR5 lacks the classical

activation loop In molecular biology, an intrinsically disordered protein (IDP) is a protein that lacks a fixed or ordered three-dimensional structure, typically in the absence of its macromolecular interaction partners, such as other proteins or RNA. IDPs ran ...

in the

domain. The ''NFR5'' gene lacks

intron An intron is any Nucleic acid sequence, nucleotide sequence within a gene that is not expressed or operative in the final RNA product. The word ''intron'' is derived from the term ''intragenic region'', i.e. a region inside a gene."The notion of ...

s. First the cell membrane is depolarized and the root hairs start to swell and cell division stops. Nod factor cause the fragmentation and rearrangement of actin network, which coupled with the reinstitution of cell growth lead to the curling of the root hair around the bacteria. 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. As the infection thread grows the rhizobia travel down its length towards the site of the nodule. During this process the pericycle cells in plants become activated and cells in the inner cortex start growing and become the nodule primordium where the rhizobia infect and differentiate into bacteroids and fix nitrogen. Activation of adjacent middle cortex cells leads to the formation of nodule meristem.

Notes

{{DEFAULTSORT:Nod Factor Fabaceae Oligosaccharides Signal transduction Plant physiology

Chemical Structure

Function

Nodulation

See also

Notes