The mechanism of root nodule formation in leguminous plants can be explained by the following steps:
- The roots of legume plants secrete lectin protein which attracts specific species of Rhizobium towards it.
- The root hair becomes hook-like under the influence of hormone secreted by the plant root and the ‘Nod’ factor secreted by the Rhizobium.
- The cell wall of hook-shaped root hair breaks near the tip and the bacterial cells enter into root hair along with mucilaginous substance.
- The plasma membrane of root hair becomes folded and forms infection thread which penetrates the cortex region of root.
- The bacteria continue to divide and induce polyploidy in the nucleus of cells of outer cortex.
- These cells divide continuously and form a nodule.
- IAA secreted by the bacteria cause growth in the size of the nodule and soon vascular tissue differentiates in it which gets connected with the main vascular tissue of root.
- The functional nodules appear pink in colour due to the presence of leghaemoglobin
