What are the steps involved in formation of a root nodule?

The formation of root nodules in leguminous plants involves several steps, including recognition and infection by nitrogen-fixing bacteria (such as Rhizobium), colonization of root tissues, and the establishment of a symbiotic relationship. Here are the general steps involved in the formation of a root nodule:

1. Chemotaxis and Recognition: Rhizobium bacteria present in the soil are attracted to the roots of host leguminous plants by chemical signals released by the plant roots. This process, known as chemotaxis, enables the bacteria to locate and attach to the surface of the root hairs.

2. Infection Thread Formation: Upon attachment to the root hairs, Rhizobium bacteria induce the formation of infection threads, which are tubular structures that penetrate the root epidermis and extend towards the inner cortical cells. The bacteria colonize the infection threads and move deeper into the root tissues.

3. Cortex Invasion: As the infection threads elongate, Rhizobium bacteria continue to proliferate and invade the cortical cells of the root, eventually reaching the inner layers of the root tissue. This process involves the degradation of cell walls and the release of bacterial enzymes to facilitate penetration and colonization.

4. Nodule Initiation: Within the cortical cells, the presence of Rhizobium bacteria triggers a series of molecular and biochemical signals that induce the formation of nodulation zones. These nodulation zones are sites of cell division and differentiation, where specialized structures called primordia begin to develop.

5. Nodule Development: The primordia undergo further growth and differentiation, eventually forming mature root nodules. Inside the nodules, the plant cells undergo extensive morphological and physiological changes to accommodate the nitrogen-fixing bacteria and establish a symbiotic relationship.

6. Symbiotic Relationship: Within the root nodules, the nitrogen-fixing bacteria (Rhizobium) differentiate into bacteroids, which are specialized forms capable of nitrogen fixation. The bacteroids and plant cells exchange nutrients and signals, facilitating nitrogen fixation by the bacteria and the assimilation of fixed nitrogen by the plant.

7. Nitrogen Fixation: Once established, the symbiotic relationship between the leguminous plant and Rhizobium bacteria enables the fixation of atmospheric nitrogen (N2) gas into ammonia (NH3), a form of nitrogen that can be utilized by the plant for growth and development. This process provides the host plant with a direct source of nitrogen for essential metabolic processes.

Overall, the formation of root nodules involves a series of coordinated interactions between the host plant and nitrogen-fixing bacteria, resulting in the establishment of a symbiotic relationship beneficial for both partners.