Introduction
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
1. What is Rhizobium and where is it typically found?
Rhizobium is a genus of Gram-negative soil bacteria known for its crucial role in the nitrogen cycle. These bacteria are most famously found living in a symbiotic relationship within specialized structures called root nodules on the roots of leguminous plants, such as peas, beans, and clover. They can also exist as free-living organisms in the soil, but their nitrogen-fixing ability is activated primarily inside the plant nodules.
2. How does Rhizobium perform nitrogen fixation in leguminous plants?
The process begins when the roots of a leguminous plant release chemical signals that attract Rhizobium bacteria. The bacteria, in turn, cause the plant's root hairs to curl and form an infection thread, allowing them to enter the root cells. Inside, the bacteria multiply and transform into bacteroids, forming a nodule. Within this nodule, the bacteria use an enzyme called nitrogenase to convert inert atmospheric nitrogen (N₂) into ammonia (NH₃), a form of nitrogen that the plant can readily absorb and use for growth.
3. Why is the symbiotic relationship with plants essential for Rhizobium to fix nitrogen?
The symbiotic relationship is crucial because the nitrogen-fixing enzyme, nitrogenase, is extremely sensitive to oxygen and becomes inactive in its presence. The host plant creates and maintains a low-oxygen environment inside the root nodules using a special oxygen-scavenging pigment called leghaemoglobin. This protects the enzyme, allowing it to function effectively. In return for this protected environment and a steady supply of carbohydrates from the plant's photosynthesis, the bacterium provides the plant with fixed nitrogen, making the relationship mutually essential.
4. What is the importance of Rhizobium as a biofertilizer in agriculture?
Rhizobium is highly valued as a biofertilizer because it naturally enriches the soil with nitrogen. By colonising the roots of leguminous crops, it provides a direct supply of usable nitrogen, which is a primary nutrient for plant growth. Using Rhizobium inoculants can significantly reduce the need for synthetic nitrogen fertilizers, which are costly, energy-intensive to produce, and can contribute to environmental pollution. This makes agriculture more sustainable and cost-effective.
5. Is Rhizobium a type of bacteria or a virus?
Rhizobium is unequivocally a bacterium, not a virus. Bacteria are single-celled, living microorganisms with their own cellular structure (cell wall, cytoplasm, ribosomes, etc.) capable of independent growth and reproduction. In contrast, viruses are non-living infectious particles that lack cellular structures and can only replicate by invading and hijacking the machinery of a living host cell.
6. What are the key structural features of a Rhizobium bacterium?
A typical Rhizobium bacterium is a rod-shaped (bacillus), motile organism. When free-living in the soil, it often possesses flagella for movement. Its cell wall is Gram-negative, meaning it has a thin peptidoglycan layer between two cell membranes. Internally, it contains a cytoplasm, ribosomes for protein synthesis, and a nucleoid region containing its DNA. It does not form spores but can transform into a non-motile, irregularly shaped form called a bacteroid once inside the plant root nodule.
7. How does symbiotic nitrogen fixation by Rhizobium differ from that of free-living nitrogen-fixing bacteria?
The main difference lies in their lifestyle and efficiency. Symbiotic bacteria like Rhizobium establish an intimate, mutually beneficial relationship with a host plant, living inside its root nodules where conditions are ideal for nitrogen fixation. In contrast, free-living bacteria like Azotobacter live independently in the soil and fix nitrogen for their own metabolic needs, enriching the soil only after they die and decompose. Symbiotic fixation is generally a far more efficient process for delivering nitrogen directly to a plant.
8. Are all Rhizobium species the same? Provide examples of species-specific relationships.
No, there is a high degree of host specificity among Rhizobium species, meaning certain species of Rhizobium can only form nodules on specific groups of leguminous plants. This ensures an effective symbiotic partnership. Some well-known examples include:
- Rhizobium leguminosarum: Forms nodules on pea and vetch plants.
- Rhizobium phaseoli: Associates specifically with bean plants (Phaseolus species).
- Bradyrhizobium japonicum: A slow-growing relative that forms nodules on soybeans.
- Sinorhizobium meliloti: Forms a symbiotic relationship with alfalfa and sweet clover.
9. Can Rhizobium species cause any harm or diseases in plants?
While the vast majority of Rhizobium species are beneficial symbionts, some closely related species are known plant pathogens. For instance, Rhizobium rhizogenes (formerly Agrobacterium rhizogenes) causes hairy root disease, and Rhizobium radiobacter (formerly Agrobacterium tumefaciens) is responsible for causing crown gall disease in a wide variety of dicotyledonous plants. These pathogenic species utilize a similar mechanism of gene transfer to infect plants for their own benefit, rather than to establish a symbiosis.
10. What would be the environmental impact if Rhizobium bacteria could no longer form nodules?
If Rhizobium lost its ability to form nodules, the natural process of nitrogen fixation in legumes would cease. This would have a severe agricultural and environmental impact. Leguminous crops would lose their natural advantage in nitrogen-poor soils, forcing a massive increase in the global use of synthetic nitrogen fertilizers. This would lead to higher farming costs, greater fossil fuel consumption for fertilizer production, and increased environmental damage, including water pollution (eutrophication) and the emission of nitrous oxide, a potent greenhouse gas.
