1. Recognition: In the first stage, the rhizobium bacteria and legume roots recognize each other. This recognition is facilitated by specific signaling molecules produced by both parties. For example, the legume root secretes flavonoids, which act as signals for rhizobium to initiate infection.
2. Infection: Once recognition occurs, rhizobium bacteria initiate the process of infection by penetrating the root hairs of the legume. This is made possible by the secretion of enzymes that degrade the plant cell wall. The rhizobium then enters the interior of the root hair through an infection thread, which is a tube-like structure formed by the plant cell membrane.
3. Nodule formation: The presence of the rhizobium bacteria triggers the legume plant to form specialized structures called nodules, which are swollen outgrowths on the root system. Nodule formation is initiated by the release of signaling molecules such as cytokinins and auxins by the rhizobium. These signaling molecules result in the division and differentiation of cells in the root cortex, leading to nodule formation.
4. Nodule development: Once nodules are formed, they undergo further development. The rhizobium bacteria residing in the nodules multiply and differentiate into bacteroids, which are specialized forms of rhizobium capable of nitrogen fixation. This differentiation is mediated by various factors secreted by both the plant and rhizobium.
5. Nitrogen fixation: The bacteroids within the nodules are responsible for the process of nitrogen fixation. Nitrogen fixation is the conversion of atmospheric nitrogen (N2) into ammonia (NH3), which can be utilized by the plant as a nutrient. This process is facilitated by the enzyme nitrogenase, which is produced by the bacteroids.
6. Nutrient exchange: The rhizobium-legume symbiosis is mutually beneficial because both parties exchange nutrients. The rhizobium provides the legume with fixed nitrogen, enhancing its growth and development, while the legume provides the rhizobium with carbohydrates and other nutrients required for its survival.
7. Nodule senescence: Over time, nodules undergo senescence and eventually die off. This senescence may be triggered by factors such as aging or environmental stress. As nodules senesce, the rhizobium bacteria are released back into the soil, ready to initiate new infections in other legume plants.
8. Reproduction and dispersal: Rhizobium bacteria have various mechanisms for reproduction and dispersal. They can form specialized structures called bacteroids, which are released from the dead nodules into the soil. Additionally, rhizobium can form symbiotic associations with new legume plants, leading to a cycle of infection and nodulation.
It is important to note that the stages described here are generalized and can vary depending on the specific rhizobium-legume symbiosis. Additionally, the process of nitrogen fixation is energetically costly for both the legume and rhizobium and requires optimal environmental conditions for efficient nitrogen fixation to occur.
Describe the stages of infection and nodulation in rhizobium-legume symbiosis with respect to nitrogen fixation (15 points).
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