Formation of root nodules (ii) Significance of leghaemoglobin

(i) Root Nodule Formation:

Steps in Nodulation:

1. Chemotaxis: Legume roots secrete flavonoids that attract Rhizobium
2. Attachment: Bacteria adhere to root hair epidermal cells.
3. Infection Thread Formation: Bacteria release Nod factors causing root hair curling. An infection thread forms, carrying bacteria into root cortex.
4. Cortical Cell Stimulation: Bacteria release signals triggering division of cortical and pericycle cells.
5. Bacteroid Formation: Bacteria transform into bacteroids (nitrogen-fixing form).
6. Mature Nodule: Continued cell division produces a fully developed root nodule containing bacteroids, leghaemoglobin, and nitrogenase.

(ii) Significance of Leghaemoglobin:

Leghaemoglobin is a pink/red oxygen-binding pigment in root nodules.

Function: Acts as an oxygen scavenger, maintaining the anaerobic environment essential for nitrogenase activity. Nitrogenase is irreversibly inactivated by oxygen; leghaemoglobin binds O₂, allowing:

• Nodule cells to respire (generate ATP)
• Nitrogenase to fix N₂ without oxygen damage

This delicate balance ensures efficient nitrogen fixation.