[Biology Class Notes] on Rhizobium Pdf for Exam

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

1. Rhizobium leguminosarum in pea plants

2. Rhizobium phaseoli in beans

3. Rhizobium japonicum in soybeans

4. 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.