Question

Trace the events starting from the coming in contact of Rhizobium to a leguminous root till nodule formation.

Add a note on importance of leg hemoglobin.

Answer 1

a. The root hair of the leguminous plants are surrounded by Rhizobium.

Multiplication of Rhizobium:

The bacteria get stimulated to multiply as plant roots start secreting certain chemical signals like flavonoids. After multiplication, the bacteria surround the root hair completely and colonise near them.


Curling of root hair:

After colonisation, the root hair gets curled.


Formation of the infection thread:

An infection thread is formed which extends up to the cortex through which Rhizobium enters and invades the cortex.



Formation of Nitrogen-fixing cells:

The invasion of the cortex by Rhizobium leads to the differentiation of the cortical cells into specialised nitrogen-fixing cells. This leads to the appearance of the nodules. The nodule starts enlarging and extending, thereby invading the pericycle. Additionally, it also protrudes outwards to form a bulging/ swelling.


Formation of vascular connections:

The bacteria that inhabit the nodules perform nitrogen fixation and allow these nitrogen derivatives to get discharged into the vascular system and get transported throughout the plant through xylem.


b. Importance of leg-haemoglobin:

Nitrogenase enzyme required for nitrogen fixation gets inactivated in the presence of oxygen. Leg-haemoglobin acts as an oxygen-scavenger and protects the enzyme nitrogenase by maintaining an anaerobic environment around it.