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Nitrogen Fixation: Genes Involved and the Infection Process



BY: Lorato Lekgari | Category: Biology | Submitted: 2010-07-19 18:38:40

Nitrogen is a macro-nutrient that is needed in higher concentrations by plants. It is an important constituent of cell components of proteins and its related molecules such as amino acids and nucleic acids. If a plant is deficient in Nitrogen its growth becomes stunted. Most soils in nature are deficient in Nitrogen and thus limiting plant productivity. The atmosphere contains about 78% by volume of molecular nitrogen. However plants do not absorb nitrogen in this form, it is taken up by plants in the form of nitrates or ammonium. Thus the highly stable triple bonds in the nitrogen molecule have to be broken down to produce nitrates or ammonium, which are directly available to plants. This requires very high temperatures of about 200°C and atmospheric pressure of about 200 atmospheres to be achieved in the presence of catalyst to combine the nitrogen with hydrogen to produce ammonium. This reaction or rather process is referred to as nitrogen fixation.

Nitrogen Fixation

Nitrogen can be fixed in nature by lightning, photochemical reaction or biologically. Lightning accounts for 8% of fixed nitrogen, photochemical reactions fix about 2% of the fixed nitrogen into the soil. Biological nitrogen fixation then fixes 90% of nitrogen into the soil. This happens through bacteria or blue-green algae either in free living conditions or by forming symbiotic relationships with plants. They fix nitrogen into ammonium. Nitrogen fixing bacteria and the genes involved, especially looking at communication between the plant and the bacteria in their mutually gaining relationship will be discussed in this article.

The Genes Involved

The most common symbiotic relationship where nitrogen fixing is extensively researched and documented is in the plant family Leguminosae with a group of bacteria collectively referred as rhizobia. This happens under anaerobic conditions catalysed by the enzyme nitrogenase in specialised structures called nodules. Specific genes come into play for the symbiotic relationship to be established. This happens via plant nodule specific genes called nodulin (Nod) whereas the rhizobial ones are called nodulation genes (nod). Rhizobial nod genes can be common such as nodA, nodA, nodB and nodC or they can be host specific as is the case for nodE, nodF, nodH, nodP and nodQ. There is also the nodD gene which is constitutively expressed and its protein activates the other nod genes.

The Infection Process

The rhizobia infect the plants via perceivement of flavonoids that the legumes produce leading to the activation of the rhizobial nod protein which then induces the transcription of the other nod genes. Nodulation proteins needed for the biosynthesis of lipochitin oligosaccharides signalling molecules called the Nod factors which have a chitin β- →4-linke N-acetyl-D-glucosamine backbone with fatty acid chain on the C-2 position of the non-reducing sugar are then coded for. The nodA, nodB and nodC genes encode for the enzymes NodA, NodB and NodC which synthesises the Nod factors. NodA catalyses the addition of fatty acyl chain, NodB removes the acetyl group from the terminal non-reducing sugar and NodC links N-acetyl-D-glucosamine monomers. The host specific genes then come into play to modify the fatty acids and addition of the groups. The host specific NodE and NodF factors do this by determining the length and degree of the saturation of the fatty acyl chains and the other Nod factors such as NodL add specific substitutions at the non-reducing sugar moieties of the chitin backbone. The action of these genes then works to specific legume hosts responding to specific Nod factors.

The stepwise events


1. Rhizobia respond to chemical attractants released by legumes by binding to an emerging root hair and Nod factors are activated
2. The root hairs then start to curl and the rhizobia start to multiply inside the coils
3. The wall of the root hair then degrades due to infection and an infection thread is formed from the Golgi apparatus of root cells.
4. The infection thread grows until it reaches the end of the cell and then its membrane fuses with the root hair cells membrane.
5. The rhizobia are released into the apoplast resulting on the other cells being infected.
6. This extends the infection thread until it reaches target cells where the plant membrane vesicles that enclose the bacterial cells are released forming nodules.

Conclusion

If these plant Nod genes can be cloned into other crop plants that do not have such successful relationship, if any, with the rhizobia in nature, the application of Nitrogen to agricultural land will be minimised if at all necessary as the plants will be getting all its nitrogen needs from the bacterial fixation. The nod genes have been isolated in research labs and thus their sequences are known and it has been established that they are othologous to arbuscular mycorrhizae which form symbiotic relationship with about 80% of higher plants thus giving information on the evolution of the symbiosis between legumes and nitrogen fixing bacteria. This is what plant bioengineers are interested in doing though it is not easy to achieve.

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