a. Legume plant and nitrogen-fixing bacteria, e.g., chickpea and Rhizobium or Leguminosae or Fabaceae plants and nitrogen fixing-Bacteria.
b.Steps in the formation of root nodules :
1. Root hair of the leguminous plant curls upon contact with nitrogen fixing bacterium, which secretes chemicals.
2. Nitrogen-fixing bacteria proliferate within curled root hair.
3. The plant responds by forming an infection thread of the plasma membrane, which grows inward from the infected cell of the host, separating it from the rest of the plant.
4. Cell division is stimulated in the infected tissue and more bacteria invade the newly formed cells, and a nodule is formed.
Auxin from leguminous plant cells and cytokinin from bacteria help in this cell division :
Physiological processes : Photosynthesis and respiration.