What is Rhizobium Definition Structure and Symbiotic Function
Rhizobium is a Gram negative bacterium that is motile and in the form of non-sporulating rods found in the soil that fixes atmospheric nitrogen. It is found mostly in the root nodules where it establishes a symbiotic relationship with the roots of leguminous plants and parasponia.
What is Rhizobium Bacteria?
The Rhizobium bacteria basically colonize plant cells within the root nodules and there, they convert atmospheric nitrogen into ammonia. It is done with the help of an enzyme called nitrogenase where the bacteria helps the plants to receive organic nitrogenous compounds such as ureides and glutamine. The Rhizobium bacteria cannot fix atmospheric nitrogen on their own, they only develop the ability to fix nitrogen as a symbiont. Here, bacteria are also benefited by the plants as they perform photosynthesis and prepare organic compounds that are provided to the bacteria as well. This way, a mutually beneficial relationship is established between the plants and the rhizobia. Chemical attractants are secreted by the roots of a legume and bacteria that release nod factors result in curling of the root hairs. It causes the degradation of cell wall and formation of an infection thread.
Nitrogenase Enzymes
Nitrogenase is an enzyme produced by certain bacteria like Rhizobium and Cyanobacteria and is responsible for reducing nitrogen to ammonia. It is very active in anaerobic conditions and is made up of two protein subunits called non-heme iron protein and iron-molybdenum protein.
Rhizobium Nitrogen Fixation
It is also known as biological nitrogen fixation where atmospheric or molecular nitrogen is converted into ammonia by an enzyme named nitrogenase. It converts free nitrogen into nitrogenous salts and helps in making it available for the absorption of plants. The biochemical reaction involved in nitrogen fixation is as follows-
N 2 + 8 H+ + 8 e− → 2 N H 3 + H 2
The reduction of N2 into NH3 requires 6 protons and 6 electrons where 12 molecules of ATP are also involved. The role of nitrogenous compounds in plants is huge as Nitrogen is the constituent element of chlorophyll, cytochromes, alkaloids, and many vitamins. It plays an important role in different processes like metabolism, reproduction, growth and heredity. Nitrogen is present around 78% in the atmosphere and other forms of nitrogenous compounds include nitrates, nitrites and ammonia.
Rhizobium nitrogen fixation is an essential process that takes place biologically and it is the initial stage in the nitrogen cycle. The bacterial species involved in fixing nitrogen include Azotobacter, Cyanobacteria, Anabaena, Nostoc and Rhizobium. Nitrogen fixation also takes place non-biologically where microorganisms are not involved and can be found in the rainy season during lightning.
Biological nitrogen fixation can be of various types-
Free living nitrogen fixing bacteria
Free living nitrogen fixing cyanobacteria
Symbiotic nitrogen-fixing bacteria
Free living nitrogen fixing cyanobacteria
Rhizobium nitrogen fixation is symbiotic in nature that results in nodule formation in leguminous plants.
Examples of symbiotic nitrogen fixation Rhizobium bacteria
Rhizobium leguminosarum in pea plants
Rhizobium phaseoli in beans
Rhizobium japonicum in soybeans
Rhizobium lupini in Lupins
Function of Rhizobium
The basic function of Rhizobium is fixing atmospheric nitrogen for the plants to provide them with nitrogenous compounds and establish a symbiotic relationship with the plants as explained above. In addition to this, Rhizobium helps in enhancing soil productivity and fertility making the right environment for the plants to flourish.
Also, Rhizobium bacteria take care of behavioural factors including nutrient deficiency, drought stress, salt stress and harmful effects of pesticides and fertilizers that may be unhealthy for the plants in some manner.
Uses of Rhizobium
Rhizobium biofertilizer is a substance that contains living microorganisms and is applied to plant surfaces, seeds or soil. Here, the Rhizobium bacteria colonize the rhizosphere or the interior of the plant to promote growth by enhancing the supply or nutrient availability to the host plant. Rhizobium uses the host plant to fix atmospheric nitrogen and convert it into useful organic compounds, benefiting both, the bacteria and the plant.
Structure of Rhizobium
Rhizobium bacteria structure is rod-like as it belongs to the Bacillus group. It differs from other spherical or spiral bacteria. It comprises two cell membranes in its cell wall. Rhizobium bacteria have different cell organelles such as nucleoid (DNA), ribosomes, mesosome, cytoplasm and capsule. Most of the Rhizobia living outside a plant have flagella in them and many of these living inside do not have flagella attached to their cell. The genus Rhizobium is engaged in the creation of spores and the process is called sporogenesis, it is a kind of asexual reproduction.
Rhizobium belongs to Alphaproteobacteria Class, Rhizobiales Order and Rhizobiacea family.
The Latin meaning of Rhizobium is “root living”.
Some of the various species belonging to Rhizobium are as follows-
Rhizobium is generally found in the soil and takes part in nodule formation after infecting the root of the leguminous plants. Thus, they help in the fixation of nitrogen from the atmosphere and plays an important role in the growth and development of plants.
FAQs on Rhizobium and Its Role in Biological Nitrogen Fixation
1. What is Rhizobium?
Rhizobium is a genus of nitrogen-fixing bacteria that lives symbiotically in the root nodules of leguminous plants. It converts atmospheric nitrogen (N₂) into a usable form like ammonia (NH₃) for the plant.
- Belongs to the group of Gram-negative soil bacteria.
- Forms a mutualistic association with plants such as pea, bean, and clover.
- Plays a major role in the nitrogen cycle and soil fertility.
2. What is the function of Rhizobium in plants?
The main function of Rhizobium is to fix atmospheric nitrogen into ammonia that plants can use for growth. This process supports protein and nucleic acid synthesis in plants.
- Converts N₂ → NH₃ through biological nitrogen fixation.
- Supplies nitrogen for making amino acids and proteins.
- In return, receives carbohydrates and shelter from the host plant.
3. How does Rhizobium fix nitrogen?
Rhizobium fixes nitrogen using the enzyme nitrogenase inside root nodules under low-oxygen conditions. The process is energy-intensive and requires ATP supplied by the plant.
- Infection of root hairs and formation of root nodules.
- Production of the enzyme nitrogenase.
- Conversion of atmospheric nitrogen into ammonia.
- Presence of leghaemoglobin to maintain low oxygen levels.
4. Where is Rhizobium found?
Rhizobium is found in the soil and inside the root nodules of leguminous plants. It survives freely in soil but becomes symbiotic after infecting suitable host roots.
- Common in agricultural soils.
- Associates with plants like pea, gram, soybean, and lentil.
- Forms nodules specifically on legume roots.
5. What are root nodules in legumes?
Root nodules are small swellings on legume roots that contain nitrogen-fixing Rhizobium bacteria. They are the sites where biological nitrogen fixation occurs.
- Formed due to interaction between plant roots and Rhizobium.
- Contain bacteroids, the active nitrogen-fixing form of the bacteria.
- Rich in leghaemoglobin, giving a pink color inside.
6. What is the relationship between Rhizobium and leguminous plants?
The relationship between Rhizobium and leguminous plants is a mutualistic symbiosis. Both organisms benefit from this association.
- Rhizobium receives food (carbohydrates) and protection.
- The plant receives fixed nitrogen in the form of ammonia.
- This partnership improves soil nitrogen content naturally.
7. What is the role of leghaemoglobin in Rhizobium?
Leghaemoglobin regulates oxygen concentration in root nodules to protect the enzyme nitrogenase. It maintains a low-oxygen environment necessary for nitrogen fixation.
- Acts as an oxygen carrier similar to hemoglobin.
- Prevents oxygen from inactivating nitrogenase.
- Gives nodules their characteristic pink color.
8. Is Rhizobium a free-living or symbiotic bacterium?
Rhizobium is both a free-living soil bacterium and a symbiotic nitrogen fixer inside legume root nodules. It switches to a symbiotic form when it infects a suitable host plant.
- Free-living form exists in soil.
- Symbiotic form develops inside root nodules.
- Active nitrogen fixation occurs only in the symbiotic state.
9. What is the importance of Rhizobium in agriculture?
Rhizobium is important in agriculture because it naturally enriches soil nitrogen through biological nitrogen fixation. This reduces the need for chemical nitrogen fertilizers.
- Improves soil fertility.
- Enhances crop yield in legumes.
- Used as a biofertilizer in sustainable farming.
10. What is the difference between Rhizobium and other nitrogen-fixing bacteria?
Rhizobium differs from many other nitrogen-fixing bacteria because it forms a specific symbiotic association with leguminous plants. Other nitrogen fixers may be free-living or associate with non-legumes.
- Rhizobium: Symbiotic, forms root nodules in legumes.
- Azotobacter: Free-living nitrogen fixer in soil.
- Anabaena: Photosynthetic cyanobacterium that fixes nitrogen in aquatic environments.
