Question

What are the steps involved in the formation of the root nodule?

Answer 1

Hint: Nitrogen is an important constituent of protoplasm. It is the component of amino acids, proteins, enzymes, nucleotides, and nucleic acids. It is picked up as an inorganic compound and is changed into organic forms by plants and some prokaryotes.

Complete answer: There are three methods for nitrogen fixation: atmospheric, biological and industrial nitrogen fixation.
In biological nitrogen fixation, some algae, symbiotic bacteria and free-living bacteria pick up the atmospheric nitrogen, reduce it to ammonia, and combine with organic acids to form amino acids. It occurs by two methods- symbiotic and non-symbiotic.
Symbiotic bacteria are found in the root nodules of the members of the family Leguminosae. The best known nitrogen fixing symbiotic bacteria is Rhizobium leguminosarum.
The steps involved in the formation of root nodules are:
The Rhizobium bacteria divide and redivides and forms a large colony that gets attached to the root hairs and epidermal cells.
The root hairs get curled after the invasion of bacteria. Rhizobium forms an infection thread and reaches to the cortex of the thread through the infection thread.
The cortical cells are then stimulated to divide more vigorously to form nodules on the root.
The nitrogen fixation through this method requires the enzyme nitrogenase which requires an anaerobic condition for its functioning. Leghemoglobin combines with oxygen and protects nitrogenase.

Note: Neither plant nor bacterium alone can fix nitrogen in this as it is the outcome of the symbiotic relationship between the two. The nodules formed are pink in color due to the presence of leghemoglobin pigment in it.