Question

In root nodules of legumes, the role of leghemoglobin is
(a)To transports oxygen to root nodules
(b)To act as an oxygen scavenger
(c)To act as a catalyst in transamination
(d)To provide energy to nitrogen-fixing bacteria

Answer 1

Leghaemoglobin is structurally similar to hemoglobin. It balances the free oxygen concentration in the cytoplasm of infected plant cells to ensure root nodules are functioning properly. It acts as a ‘collector’ of oxygen.

Complete answer:
The atmospheric nitrogen is unavailable to the plants. The nitrogen-fixing bacteria convert the atmospheric nitrogen into compounds containing ammonia and make it available to the plants. Free-living and symbiotic bacteria are the two types of bacteria that help in fixing atmospheric nitrogen. Azotobacter is an example of free-living nitrogen-fixing bacteria and Rhizobium is the example of symbiotic nitrogen-fixing bacteria. Rhizobium lives in the root nodules and are in symbiotic association with the legume plants and fixes atmospheric nitrogen in. Root nodules of the legume plant contain leghaemoglobin and this function as an oxygen scavenger.

Additional Information: -The oxygen carrier and hemoprotein present in the nitrogen-fixing root nodules of the leguminous plants is called leghaemoglobin. These plants develop it when the nitrogen-fixing bacteria colonize the roots of the plant and these bacteria (specifically rhizobium) live in the symbiotic relationship with the plant. Roots not colonized by Rhizobium do not synthesize leghemoglobin.
-The presence of oxygen in the root nodules will reduce the function of oxygen-sensitive nitrogenase, which is an enzyme responsible for fixing atmospheric nitrogen, in plants colonized by rhizobium, such as alfalfa or soybeans. Leghemoglobin is shown to balance free oxygen concentrations in the cytoplasm of infected plant cells to ensure root nodules are functioning properly.
-That being said, nitrogen fixation is an extremely expensive process in terms of energy, so aerobic respiration, which needs a high concentration of oxygen, is required in the root nodule cells. Leghemoglobin maintains a free oxygen concentration that is low enough to enable nitrogenase to work, but a sufficiently high total oxygen concentration for aerobic respiration.
In simple words the 3 main functions of leghaemoglobin are:
-To promote oxygen supply to the bacteria that fix nitrogen
-To prevent the enzyme, nitrogenase from being inactivated by oxygen
-Hence leghaemoglobin presence shows strong communication between the host plant and the bacteria.
So, the correct answer is ‘To act as an oxygen scavenger’.

Note: The main purpose of the nitrogen-containing compounds is that they help the plant grow and compete with other plants. The fixed nitrogen is then released when the plant dies, making it available to other plants; and this will in turn help to fertilize the soil.