A relationship is an association or connection between two or more entities and associated mutual contact. An association is regarded as symbiotic if the parties to the relationship coexist for prolonged periods of time and derive benefits. Many mutualisms encompass intricate control mechanisms and exhibit reciprocal usefulness. Leguminous plants of the Fabaceae family have a certain kind of association with Rhizobium bacteria that exists freely in soil and invades root nodules. Legumes include alfalfa, clover, beans, peanuts and lupines. Both the legume and the bacteria can exist separately, are facultative.
Nitrogen fixation, a bacteria-plant partnership, can however be achieved when the two entities work in coordination. The bacteria intrude into the plant occasioning the appearance of root nodules owing to the increase of plant cells. The bacteria are always alienated from the plant cytoplasm by a protective membrane. Both organisms benefit from this mutual relationship. This relationship allows legumes to synthesize proteins (http://users. rcn. com/jkimball. ma. ultranet/BiologyPages/N/NitrogenFixation. html). Plants require nitrogen to manufacture nucleic acids, amino acids, proteins and numerous cellular nitrogenous components.
Nitrogen in form of gas (N2) is very much abundant in the atmosphere- more then 78% composition. The existence of a triple bond between two nitrogen atoms makes it unavailable fro use by plants. To be utilized by plants, nitrogen must be “fixed” or, joined in the structure of nitrate ions (NH4) or ammonium (NO3). Rock weathering supplies nitrate ions in very minute quantities that cannot influence the availability of nitrogen. This makes nitrogen to always remain the limiting factor for growth in all environments. Nitrogen fixation involves the conversion of inert atmospheric nitrogen gas into biologically functional nitrates.
This process can only be naturally facilitated by bacteria. The nitrates are absorbed by the plant roots. About 25-75 pounds of nitrogen per acre is generated in a natural ecosystem. A cropping system can produce a couple of hundred pounds in a year. The bacteria Rhizoid can initiate nodule development by different mechanisms: Rhizoid can invade and infect root hairs resulting in thread formation as is the case in beans and clovers, entry via lacerations or locations where lateral root has been damaged as happens in Stylosanthes and peanuts or invasion of root primordial through plant stems as is the case in Sesbania.
Infection commences with the attachment of the Rhizoid to young root hairs. The Rhizoid caps the root hair tip and assumes an orientation facing the host. The root hair disfigures and curls and the point of infection is hydrolyzes to allow invasion by the Rhizoid. The Rhizoid produce a nod feature which, in conjunction with receptors in the legume root hairs, facilitates nitrogen fixation. The bacteria penetrate root epithelial cells then move into the cortex. The route is through a channel in the cells that extends through the cortex cell-after-cell. This infection line is developed by the root cells and not by the Rhizoid.
Once the infection line attains cell-depth in the cortex, it erupts and endocytosis takes place in the bacteria with the resultant production of endosomes. A number of mitosis cycles occur in the cell which becomes polyploid. Infection occurs about 5 to 6 days after rhizobial infection (http://users. rcn. com/jkimball. ma. ultranet/BiologyPages/N/NitrogenFixation. html). The cells of the cortex rapidly divide into a nodule owing to the transfer of cytokinins from epidermal to cortex cells. Rapid multiplication and disfiguration with loss of motility of the Rhizoid occur in the nodule cells.
The nodules, now called bacteroids, may occupy an entire cell and nitrogen fixation ensues. The plant discards nodules that have cease nitrogen fixation. Observable changes accompany root hair invasion and infection. These include the proliferation of host cell in the root cortex next to contaminated root hairs. A nodule primordium develops in indeterminate nodules. Rhizoid is then poured into adjusted cells of the root cortex where they are protected from the host’s defense mechanism by a peribacteroid membrane produced by the legume.
Active nitrogen fixation occurs eight to fifteen days after infection of root hairs with Rhizoid. In the process of infection, several proteins not existent in either the host or the bacteria in isolation are produced. Early nodulin proteins are produced about 6 hours after inoculation and are thought to assist nodule development. Late nodulins are associated with nodule operations and nitrogen fixation. Examples of these proteins include leghemoglobin, enzyme nitrogenase, uricase and glutamine synthetase. Determinate nodules, found in phaseolus and soy bean, are round with no conspicuous meristematic region.
Indeterminate nodules, on the other hand, are elongated and have conspicuous meristematic zone, as found in medics, peas and clovers. In indeterminate nodules, the N2 fixation-derived ammonia is transferred to the plant cells where it is changed into glutamate, glutamine and aspartate then asparagines. Asparagine is moved to the shoot. Each nodule is linked to the plant’s vascular system by phloem and xylem vessels. The growth of nodules is a well-organized plant process and is dependent on Rhizoid. Nodules house all the metabolic apparatus, including nitrogenase enzyme for nitrogen fixation.
The legume provides nourishment for the bacteroids to manufacture huge amounts of ATP required for conversion of nitrogen (N2) into ammonia (NH3). A legume provides significant amounts of photosynthates and to the Rhizoid. A soybean may supply 20-30% of its photosynthates to the bacteria at the expense of other plant processes. The Nodules require oxygen to synthesize ATP through respiration. Oxygen rapidly inactivates nitrogenase and thus the bacteroids have to balance between too little and too much oxygen. Hemoglobin (leghemoglobin), highly prevalent in the nodules, eases this task by supplying just adequate amounts of oxygen.
Leghemoglobin occurs in the nodules only and is not synthesized by either the bacteria or the legume in isolation. The plant, through complex mechanisms, facilitates the conversion of Rhizoid that cannot nitrogen fix into bacteroids with this capability. The ammonia (NH3) produced is rapidly protonated into ammonium (NH4). The purpose of leghemoglobin in root nodules is to minimize the amount of free oxygen and hence shield the nitrogenase enzyme (http://users. rcn. com/jkimball. ma. ultranet/BiologyPages/N/NitrogenFixation. html).
Nitrogenase enzyme is composed of two proteins; a molybdenum-iron protein and an iron protein. Nitrogen gas binds to nitrogenase with the initial reduction of iron protein (Fe) by ferredoxin-donated electrons. The reduced iron protein binds ATP and reduces molybdenum-iron protein, which supplies nitrogen gas with electrons forming HN=NH. Two subsequent cycles result in the reduction of HN=HN into H2N-NH2 which is then reduced to 2NH3.
References
Symbiotic nitrogen fixation, retrieved on 23 rd February 2009 from http://users. rcn. com/jkimball. ma. ultranet/BiologyPages/N/NitrogenFixation. html