What are Root Nodules
Root nodules are specialised structures found in the roots of leguminous plants. These nodules are formed due to the symbiotic relationship between the plants and nitrogen-fixing bacteria, such as Rhizobium, which is a genus of bacteria that plays a key role in nitrogen fixation. Other bacteria involved in this process include Bradyrhizobium, Azorhizobium, and more.
Role of Root Nodules
Plants cannot directly use atmospheric nitrogen, but these bacteria convert nitrogen into ammonia, a form that plants can absorb and use for the synthesis of important macromolecules, like amino acids and nucleotides. This process is crucial for supplementing soil nitrogen levels and is considered an effective biofertilizer. By reducing the reliance on chemical fertilisers, root nodules contribute to sustainable agricultural practices. Legumes, once they decompose, release nitrogen back into the soil, benefiting other plants in the rotation cycle.
Leguminous Plants with Root Nodules
Root nodules are commonly found in plants of the Fabaceae family, including peas, beans, soybeans, alfalfa, and clover. Interestingly, some non-leguminous plants also develop root nodules, such as Parasponia (nodulated by Rhizobium), as well as alder and bayberry (nodulated by Frankia). Certain genera in the Rosaceae family can also form root nodules.
Types of Root Nodules
There are two main types of root nodules:
Indeterminate Root Nodules-
These nodules are characterised by a persistent nodule meristem, which leads to continuous growth due to cell division.
They exhibit a developmental gradient with several distinct zones:
Zone I (Active meristem)- New tissues are formed.
Zone II (Infectious zone)- Contains infectious threads with bacteria.
Zone III (Nitrogen-fixing zone)- Bacteroids are present and fix nitrogen. These cells contain a large central vacuole and leghaemoglobin, which gives the nodule its pink color.
Zone IV (Senescent zone)- Plant and bacterial cells degrade here. The breakdown of leghaemoglobin causes the greenish color at the nodule's base.
This type is commonly seen in peas, clover, and alfalfa.
Determinate Root Nodules-
These are typically spherical and lack a persistent meristem after initiation. Growth occurs through expansion, not division.
They do not show a developmental gradient.
Examples include soybean, bean, peanut, and lotus.
Root Nodule Formation Process
The formation of root nodules begins when the soil's nitrogen content is low. The process is guided by symbiotic communication between the plant and the bacteria through signaling. The steps of nodule formation are as follows:
Attraction of Rhizobia- Legume roots release flavonoids that attract rhizobia, which gather around the root hairs.
Nod Factors- Rhizobia secrete nod factors that cause the root hairs to curl around them.
Developmental Changes-
Nod factors trigger several changes, such as:Membrane depolarisation
Root hair curling
Cell division in the root cortex
Movement of calcium ions inside the cells
Formation of Infection Thread- Nod factors bind to receptors on the root hair plasma membrane, leading to the formation of an infection thread, which allows bacteria to enter the root epidermal cells.
Bacteria Spread- Rhizobia travel through the infection thread and enter the cortex cells. Each bacterium becomes surrounded by a plant-derived membrane called a symbiosome.
Nodule Formation- Chemicals produced by rhizobia stimulate nodule formation, which is triggered by a calcium-dependent signaling pathway that promotes cell division. Cytokinin also plays a significant role in the process.
Differentiation of Bacteria- Inside the nodule, the bacteria differentiate into bacteroids, which are responsible for nitrogen fixation. Vascular tissue also develops to facilitate nutrient exchange between the plant and the bacteria.
Factors Affecting Nodule Formation:
Both external and internal factors influence the formation of root nodules-
External Factors-
Heat, soil acidity, and nitrate levels can affect nodulation.
High soil nitrogen content inhibits nodule formation since the plant already has enough nitrogen and doesn't need more.
Nitrogen fixation is oxygen-sensitive, and leghaemoglobin in the nodules helps regulate oxygen diffusion.
Internal Factors-
Autoregulation occurs through leaf tissues to check infection and control nodule formation.
Ethylene also regulates nodulation. Excess ethylene can inhibit nodule formation.
Root nodules are important for sustainable agriculture, particularly in leguminous plants. Through their symbiotic relationship with nitrogen-fixing bacteria, they help enhance soil fertility and reduce the need for chemical fertilisers, benefiting both plants and the environment.
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FAQs on Root Nodule Formation in Leguminous Plants and Function
1. How Are the Root Nodules Formed?
Root nodules are specialised structures found in the roots of leguminous plants, formed through a symbiotic relationship with nitrogen-fixing bacteria like Rhizobium. These nodules help convert atmospheric nitrogen into a form that plants can use.
2. What Plants Have Nodules on Their Roots?
Root nodules are typically found in plants from the Fabaceae family, such as peas, beans, soybeans, alfalfa, and clover. Some non-leguminous plants, like Parasponia, alder, and bayberry, can also develop root nodules.
3. How do root nodules benefit plants?
Root nodules house nitrogen-fixing bacteria that convert atmospheric nitrogen into The plant absorbs this ammonia plant, helping it produce essential macromolecules such as amino acids and nucleotides, which are vital for growth.
4. What are the two types of root nodules?
There are two main types of root nodules:
Indeterminate Root Nodules: These have a persistent meristem and exhibit a developmental gradient with several distinct zones.
Determinate Root Nodules: These are spherical and grow through expansion rather than division, without a developmental gradient.
5. What is the process of root nodule formation?
Root nodule formation starts when soil nitrogen levels are low. The plant roots release flavonoids that attract rhizobia. The bacteria secrete nod factors, which trigger the curling of root hairs and the formation of infection threads. This leads to the bacteria entering the plant cells and forming nodules.
6. What role does leghaemoglobin play in root nodules?
Leghemoglobin is a heme pigment present in root nodules that helps regulate oxygen levels. It facilitates the diffusion of oxygen within the nodule, which is essential for the nitrogen fixation process, as it is oxygen-sensitive.
7. How does nitrogen fixation benefit the soil?
Nitrogen fixation by root nodules increases the nitrogen content of the soil. When legume plants decompose, they release nitrogen back into the soil, enriching it and making it available for other plants to use, thus reducing the need for chemical fertilisers.
8. What factors affect root nodule formation?
Several factors can influence nodule formation:
External Factors: Soil heat, acidity, and nitrogen levels can impact nodulation. High nitrogen content can inhibit nodule formation.
Internal Factors: Ethylene regulates nodulation internally, and excess ethylene can inhibit nodule formation. The plant also has defense mechanisms to check for infection.
9. Can non-leguminous plants form root nodules?
Yes, some non-leguminous plants, such as Parasponia, alder, and bayberry, can form root nodules, but they are less common than in leguminous plants.
10. How do root nodules help in sustainable agriculture?
Root nodules play a crucial role in sustainable farming by reducing the need for chemical fertilisers. The nitrogen-fixing bacteria in the nodules help improve soil fertility naturally, promoting eco-friendly farming practices.
