1. Nitrogen fixation
The process of conversion of free atmospheric nitrogen into nitrogen compounds which may be absorbed and utilized by plants is called nitrogen fixation. Biotic fixation can be, a symbiotic where free-living microbes such as Azotobacter, Clostridium (Bacteria) Nostoc and Anabaena. (Blue-green algae) carry out fixation and, symbiotic where Rhizobium and Bradyrhizobium bacteria form nodule in the root of leguminous plants and carry out fixation.
- The process of conversion of atmospheric nitrogen into organic or inorganic nitrogenous compounds by the microorganisms is called biotic nitrogen fixation.
- These microorganisms are called Di-azotrophs because they convert atmospheric dinitrogen (N ≡ N) in the form of nitrogen compounds.
- In this type of fixation, the microbes establish a mutualistic relationship with the plant.
- Rhizobium and Bradyrhizobium SPS enters the roots of Leguminous plants and develop root nodules where they fix atmospheric nitrogen in nitrogen compounds.
- Similarly Azorizobium SPS. form stem nodules in Sesbania SPS. and carry out nitrogen fixation.
- The microbe Frankia forms nitrogen-fixing nodules in Almus (A non-leguminous plant).
- In the leaves of Azolla (a pteridophyte) fixation is carried out by Anabaena SPS.
Mechanism of Bacterial infection and Nodulation:
Rhizobium is a Gram-negative, rod-shaped bacterium found in soil. The bacterium accumulates near the roots of a specific leguminous plant and induces nodule formation. The establishment of a symbiotic relationship between the root and Rhizobium is a complex process. Formation of nodules is completed by the following steps:
- To begin with, the roots of the leguminous plant secrete a specific glycoprotein (Sugar binding protein)-lectin. This lectin attracts specific Rhizobium SPS. towards roots.
- Under the influence of plant hormone, auxin and cytokinins secreted by the root and a Nod factor secreted by the bacterium attached to root hair, curling of root hair are induced and the root hair becomes hook-shaped.
- The cell wall near the tip of the hook-shaped root hair becomes damaged (breaks) and through the broken wall the bacterial cells enter the root hair along with mucilaginous substance.
- After entry into the root hair, these bacteria are called bacteroids.
- The plasma membrane of the invaded root hair becomes folded and forms a thread-like structure called infection threads. The bacteroid keeps on multiplying in the infection thread and the infection thread penetrates the cortex cells.
- As the infection thread penetrates the cortex cells, the amount of DNA in the nucleus of the cells of outer cortex increases due to polyploidy induced in the cells of outer cortex.
- These polyploid cells produce the nodule by repeated cell divisions. The size of the nodules increases by the impact of IAA (Indole acetic acid) secreted by the bacterial cells.
- After the formation of the nodule, vascular tissue develops in it and connects with the vascular tissue of the root. The bacteroid present in the active and functional nodule contains a pigment called leg haemoglobin in the peri-bacteroid membrane.
- Due to this pigment, these nodules appear pink coloured. The nonfunctional (dead) nodules are white or light yellow coloured.
The active nodules of leguminous plants (which fix nitrogen) have a pink or red coloured pigment called leg haemoglobin.
- This pigment absorbs oxygen during nitrogen fixation.
- The nitrogen-fixing enzyme nitrogenase is highly sensitive to the presence of oxygen and is active only under anaerobic conditions (in the absence of ) and becomes inactive in the presence of oxygen.
- Thus by absorbing , leg haemoglobin prevents inactivation of the nitrogenase enzyme.
- Hence in the absence of leg haemoglobin, fixation of nitrogen is not possible because oxygen inactivates nitrogenase enzyme.
2. Nodulin Protein:
- This protein can be of several forms depending upon the structure of nodule and nitrogen and carbohydrate metabolism.
Note: Symbiotic fixation is the outcome of mutual cooperation and expression of genes found in the genome of two symbiotics.
- In fixation of atmospheric into nitrogen compounds the Nod gene of the host and Nod, Nif and Fix genes of the bacterium play a special role.a
- “The Nod gene plays an important role in nodule formation whereas Nif and Fix genes play an important role in fixation”
