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 need the plants as host to fix Nitrogen. Here, bacteria is 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 and the bacteria is thus symbiont in nature.

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 the 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 symbiotic as well as asymbiotic depending upon different microorganisms. 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

Slow growing species like Bradyrhizobium and fast growing species like Rhizobium are known.

Function of Rhizobium

  • The basic function of rhizobium is fixing atmospheric Nitrogen for the plants to provide them with nitrogenous compounds and establishes a symbiotic relationship with the plants as explained above.

  • In addition to this, Rhizobium helps in enhancing the soil productivity and fertility making the right environment for the plants to flourish.

  • Also, Rhizobium bacteria takes 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 (Rhizobium) and is applied to plant surfaces, seeds or soil. Here, the Rhizobium bacteria colonizes the rhizosphere or the interior of the plant to promote growth by enhancing the supply or nutrients 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. Below is the diagram for the rhizobium bacteria structure. 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. 

  • Latin meaning of Rhizobium is “root living”.

  • Some of the various species belonging to Rhizobium genus are as follows:

Classification of Rhizobium 

Rhizobium alamii

Rhizobium indigoferae

Rhizobium leguminosarum

Rhizobium freirei

Rhizobium fabae

Rhizobium mesosinicum

Rhizobium alkalisoli

Rhizobium huautlense

Rhizobium vallis

Rhizobium galegae

Rhizobium etli

Rhizobium tubonense

Rhizobium azibense

Rhizobium herbae

Rhizobium sullae

Rhizobium gallicum

Rhizobium endophyticum

Rhizobium jaguaris

Rhizobium calliandrae

Rhizobium halophytocola

Rhizobium lusitanum

Rhizobium cauense

Rhizobium daejeonense

Rhizobium sophorae

Rhizobium giarginii

Rhizobium hainanense

Rhizobium pisi

Rhizobium grahamii

Rhizobium cellulosilyticum

Rhizobium oryzae

FAQ (Frequently Asked Questions)

1. What is the function of rhizobium bacteria?

The role of rhizobium in nitrogen fixation is such that being a bacteria found in soil, it helps in fixing molecular nitrogen from the atmosphere in leguminous plants. It is attached to the roots of the plants to produce nodules. These nodules then fix atmospheric nitrogen and convert it into ammonia which is in the usable form for the growth and development of plants.

2. Where do rhizobium bacteria live?

Rhizobium bacteria are special bacteria that live in the soil or in the root nodules of leguminous plants. They form a symbiotic association in the root nodules to obtain nutrients from plants and engage themselves in producing nitrogen with the process of nitrogen fixation which is helpful for the plants as well.

3. Does Rhizobium cause diseases?

There are various diseases in plants known to be caused by Rhizobium species. For example, infectious hairy root disease is caused in dicotyledonous plants by Rhizobium rhizogenes. Another example is the crown gall disease caused by Rhizobium radiobacter.