Nod factor
Encyclopedia
Nodulation factors are signaling molecule
s 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
kinase
s that have so-called LysM domain
s 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 intron
s.
Nod gene expression is induced by the presence of certain flavanoids in the soil, which are sectreted by the plant to attract the bacteria . These chemicals induce the formation of NodD, which in turn activates other genese in 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 invagiantion 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 organogensis .
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.
Signaling molecule
A signaling molecule is a chemical involved in transmitting information between cells. Such molecules are released from the cell sending the signal, cross over the gap between cells by diffusion, and interact with specific receptors in another cell, triggering a response in that cell by activating...
s produced by bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
known as rhizobia
Rhizobia
Rhizobia are soil bacteria that fix nitrogen after becoming established inside root nodules of legumes . Rhizobia require a plant host; they cannot independently fix nitrogen...
during the initiation of nodules
Root nodule
Root nodules occur on the roots of plants that associate with symbiotic nitrogen-fixing bacteria. Under nitrogen-limiting conditions, capable plants form a symbiotic relationship with a host-specific strain of bacteria known as rhizobia...
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
Leghemoglobin
Leghemoglobin is a nitrogen or oxygen carrier, because naturally occurring oxygen and nitrogen interact similarly with this protein; and a hemoprotein found in the nitrogen-fixing root nodules of leguminous plants. But nitrogen is necessary for the cycle to occur...
to carry away any oxygen that would inhibit nitrogenase
Nitrogenase
Nitrogenases are enzymes used by some organisms to fix atmospheric nitrogen gas . It is the only known family of enzymes that accomplish this process. Dinitrogen is quite inert because of the strength of its N-N triple bond...
activity.
Nod factors structurally are lipochitooligosaccharides (LCOs) that consist of an acylated
Acylation
In chemistry, acylation is the process of adding an acyl group to a compound. The compound providing the acyl group is called the acylating agent....
chitin
Chitin
Chitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
oligomeric backbone with various functional group
Functional group
In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction regardless of the size of the molecule it is a part of...
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
Receptor (biochemistry)
In biochemistry, a receptor is a molecule found on the surface of a cell, which receives specific chemical signals from neighbouring cells or the wider environment within an organism...
kinase
Kinase
In chemistry and biochemistry, a kinase is a type of enzyme that transfers phosphate groups from high-energy donor molecules, such as ATP, to specific substrates, a process referred to as phosphorylation. Kinases are part of the larger family of phosphotransferases...
s that have so-called LysM domain
Protein domain
A protein domain is a part of protein sequence and structure that can evolve, function, and exist independently of the rest of the protein chain. Each domain forms a compact three-dimensional structure and often can be independently stable and folded. Many proteins consist of several structural...
s 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
Model organism
A model organism is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the organism model will provide insight into the workings of other organisms. Model organisms are in vivo models and are widely used to...
legume Lotus japonicus
Lotus japonicus
Lotus japonicus is a wild legume that belongs to family Fabaceae. Members of this family are very diverse, constituting about 20,000 species. They are of significant agricultural and biological importance as many of the legume species are rich sources of protein and oil and can also fix atmospheric...
in 2003. They now have been isolated also from soybean
Soybean
The soybean or soya bean is a species of legume native to East Asia, widely grown for its edible bean which has numerous uses...
and the model legume Medicago truncatula
Medicago truncatula
Medicago truncatula is a small legume native to the Mediterranean region that is used in genomic research. It is a low-growing, clover-like plant 10–60 cm tall with trifoliate leaves. Each leaflet is rounded, 1–2 cm long, often with a dark spot in the center...
. NFR5 lacks the classical activation loop in the kinase domain. The NFR5 gene lacks intron
Intron
An intron is any nucleotide sequence within a gene that is removed by RNA splicing to generate the final mature RNA product of a gene. The term intron refers to both the DNA sequence within a gene, and the corresponding sequence in RNA transcripts. Sequences that are joined together in the final...
s.
Nod gene expression is induced by the presence of certain flavanoids in the soil, which are sectreted by the plant to attract the bacteria . These chemicals induce the formation of NodD, which in turn activates other genese in 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 invagiantion 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 organogensis .
Interestingly, at least three plant genes which are stimulated by Nod factors are also involved in the arbuscular mycorrhizal
Arbuscular mycorrhiza
An arbuscular mycorrhiza is a type of mycorrhiza in which the fungus penetrates the cortical cells of the roots of a vascular plant....
symbiosis . The addition of certain Nod factors enhances arbuscular mycorrhizal colonization, indicating that the two very different symbioses may share some common mechanisms.