Mark the correct sequence of steps for the development of root nodules in legumes

Options

A. Formation of infection thread
B. Division of cortical and pericyclic cells.
C. Curing of root hairs.
D. Leghaemoglobin synthesis.

Answer

To understand the correct sequence of steps for the development of root nodules in legumes, we can break down the process into a series of steps:

### Step-by-Step Solution:

1. **Exposure of Root Hairs to Rhizobia**: The process begins when the root hairs of leguminous plants (like peas) come into contact with the Rhizobia bacteria present in the soil.

2. **Curling of Root Hairs**: Upon contact with the Rhizobia, the root hairs begin to curl. This curling is crucial as it facilitates the entry of the bacteria into the root.

3. **Formation of Infection Thread**: Following the curling of the root hairs, an infection thread is formed. This thread acts as a conduit through which the bacteria can enter the root cells.

4. **Transfer of Bacteria to Adjacent Cells**: The bacteria then travel through the infection thread and are transferred to adjacent cells, including the cortical and pericycle cells of the root.

5. **Division of Cells**: Once inside the root cells, the bacteria stimulate the division of these cells, leading to the formation of nodules.

6. **Transformation into Bacterioids**: The bacteria undergo a transformation into a form known as bacterioids, which are specialized for nitrogen fixation.

7. **Nodule Formation**: The enlarged root cells, now containing the bacterioids, form the root nodules.

8. **Synthesis of Leghemoglobin**: Finally, leghemoglobin is synthesized within the nodules. This molecule acts as an oxygen scavenger, creating an anaerobic environment that is essential for the functioning of the Rhizobia, which are sensitive to oxygen.

### Correct Sequence:
1. Exposure of root hairs to Rhizobia
2. Curling of root hairs
3. Formation of infection thread
4. Transfer of bacteria to adjacent cells
5. Division of cells
6. Transformation into bacterioids
7. Nodule formation
8. Synthesis of leghemoglobin