Describe the stages of infection and nodulation in rhizobium-legume symbiosis with respect to nitrogen fixation (15 points).
To describe the stages of infection and nodulation in rhizobium-legume symbiosis with respect to nitrogen fixation, we need to understand the basics of this process.
1. Recognition and attachment: The process begins when the rhizobium bacteria come in contact with the root hairs of the legume plant. The plant releases specific chemicals called flavonoids, which attract and initiate the colonization by rhizobia. In response, the rhizobia produce nod factors, which act as molecular signals that help them attach and invade the root hair cells.
2. Infection and entry: After attachment, the rhizobia penetrate the root hair cells through an infection thread. This thread is formed by the intion of the plant plasma membrane around the attached bacteria. The rhizobia are then guided towards the developing nodule primordia.
3. Nodule formation: Inside the root hair cells, the rhizobia undergo dramatic transformations to become bacteroids, which are specialized forms capable of nitrogen fixation. The plant cells form a nodule primordium by dividing and differentiating to create a specialized environment that supports bacterial colonization. The bacteria multiply and colonize the nodule cells to form infected cells.
4. Nodule development: The infected cells continue to differentiate into specialized cells called nitrogen-fixing cells or symbiosomes. These cells host the bacteroids and provide them with the necessary conditions for nitrogen fixation. The symbiosomes contain specific proteins, called leghemoglobins, which bind and transport oxygen, thereby creating a low-oxygen environment favorable for the nitrogenase enzyme to function properly.
5. Nitrogen fixation: The rhizobium bacteroids within the symbiosomes use an enzyme called nitrogenase to convert atmospheric nitrogen (N₂) into ammonia (NH₃), which can then be utilized by the host legume plant for its growth and development. This process of biological nitrogen fixation is vital for the nitrogen nutrition of legumes and has a significant impact on soil fertility and agriculture.
In summary, the stages of infection and nodulation in rhizobium-legume symbiosis involve recognition and attachment, infection and entry, nodule formation, nodule development, and nitrogen fixation. This intricate process allows legume plants to form a symbiotic relationship with rhizobia, leading to enhanced nitrogen availability for the plant and contributing to sustainable agriculture.
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.
The stages of infection and nodulation in rhizobium-legume symbiosis, specifically with respect to nitrogen fixation, can be summarized as follows:
1) Recognition and Attachment: In the first stage, the roots of the legume release specific flavonoid compounds into the rhizosphere. These flavonoids act as chemical signals, attracting compatible rhizobia to the legume roots. The rhizobia, in response to the flavonoids, produce and release Nod factors, which are specific signaling molecules. These Nod factors are recognized by the legume root hairs and initiate the formation of infection threads.
2) Infection Thread Formation: The Nod factors induce curling of the legume root hairs, resulting in the formation of infection threads. The infection threads grow through cell layers towards the root cortex. The rhizobia divide and move in these infection threads, ultimately reaching the base of the root hairs.
3) Infection of Root Cortex: Once the infection threads reach the base of the root hairs, the rhizobia are released into the root cortex. Here, the rhizobia reside in specialized structures called infection threads, where they are protected from plant defenses and provided with nutrients from the plant. This process is mediated by plant-derived compounds called lectins, which recognize and bind to specific carbohydrate moieties on the rhizobial surface.
4) Nodule Development: The infection threads continue to grow and branch within the root cortical cells, leading to the formation of nodules. Nodule formation is a highly coordinated process between the legume and the rhizobia. Inside the nodules, differentiated plant cells, called bacteroids, are formed from the rhizobia. These bacteroids are able to fix atmospheric nitrogen into a biologically useful form that can be utilized by the plant.
5) Nitrogen Fixation: Within the infected nodule cells, the bacteroids interact with the legume's nitrogenase enzyme complex, which allows them to convert atmospheric nitrogen into ammonia, an essential nutrient for plant growth. The ammonia is then assimilated into various organic compounds, including amino acids, which can be utilized by the plant for its metabolic needs.
Overall, the symbiotic relationship between rhizobia and legumes involves a series of intricate steps that result in the formation of nodules and subsequent nitrogen fixation. This process plays a vital role in biological nitrogen cycling and contributes to the overall productivity and sustainability of legume plants.