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Rhizobium Definition
Rhizobium is a bacteria that resides in the soil and is responsible for fixing atmospheric nitrogen after finding a base in the root inside the soil.
What is Rhizobium?
Rhizobium is a bacterium that lives in symbiotic association with the root. It is usually present in the root nodules of the leguminous plant. It is responsible for fixing atmospheric nitrogen in the root nodules.
Since nitrogen cannot be directly fixed from the atmosphere it is initially converted into ammonia. This ammonia being organic is soluble in the soil.
In arid regions, Rhizobium is an important source of nitrogen. Ammonia is produced when dinitrogen is converted into ammonia. Ammonia is toxic by nature.
Role of Nitrogen Fixation for Soil
Nitrogen fixation is important for the soil fertility and productivity. The various behavioral factors of nitrogen-fixing systems are discussed, including drought stress, nutrient deficiency, salt stress, fertilizers, pesticides, etc.
Nutrient supply
- Nitrogen is a vital nutrient for plant growth.
- It is a key component of proteins, enzymes, and chlorophyll.
- Nitrogen-fixing bacteria like Rhizobium convert atmospheric nitrogen into forms such as ammonia.
- Plants absorb this ammonia through their roots.
Soil fertility improves
- Nitrogen-fixing bacteria enrich the soil with essential nutrients by converting nitrogen into a usable form.
- This reduces the need for chemical fertilizers.
- It also enhances the fertility of the soil, promoting healthier plant growth.
Supports soil micro bacteria
- The quality of soil improves due to nitrogen fixation.
- Nitrogen fixation also benefits other soil organisms by increasing the overall nutrient content.
- Thus creating a healthier and more balanced soil ecosystem.
Classification of Rhizobium Bacteria
Rhizobium bacteria belong to the domain of bacteria and are classified as follows:
- Domain: Bacteria
- Phylum: Proteobacteria
- Class: Alphaproteobacteria
- Order: Rhizobiales
- Family: Rhizobiaceae
- Genus: Rhizobium
There are several types of Rhizobiums, which can be classified based on the type of plant they are associated with as well as the rate at which they grow. Few species of Rhizobium bacteria include:
- Rhizobium leguminosarum
- Rhizobium alamii
- Rhizobium lantis
- Rhizobium japonicum
- Rhizobium trifolii
- Rhizobium phaseolii
- Rhizobium smilacinae
Characteristics of Rhizobium
There are various characteristics of rhizobium which is helpful for humans and also the environment. Let us discuss these characteristics in brief going further.
- Gram-negative: Rhizobium bacteria are gram-negative. They have a thin peptidoglycan layer. Thus, do not retain the crystal violet stain used in the Gram staining method.
- Rod-shaped: They are rod-shaped in structure like bacilli.
- Nitrogen fixation: It is the primary characteristic of rhizobium to fix nitrogen from the atmosphere and convert it into ammonia.
- Symbiotic relationship: Rhizobium has a symbiotic association with leguminous plants, such as peas, beans, and lentils. They infect the root hairs of these plants and produce nodules where nitrogen is fixed.
- Temperature sensitivity: Rhizobium bacteria perform optimally at a specified temperature range. Extreme temperatures may impair their ability to fix nitrogen.
- pH sensitivity: They prefer slightly acidic to neutral pH values for optimal growth and nitrogen fixation. While certain species can survive more alkaline or acidic conditions.
- Aerobic: Rhizobium bacteria are aerobic, meaning they require oxygen to survive and grow.
Diseases Caused by Rhizobium Bacteria
Rhizobium bacteria are generally not known to cause disease. Instead, they are helpful bacteria recognized for their symbiotic connection with leguminous plants, which aids in nitrogen fixation.
They promote plant growth and soil fertility rather than harming plants, animals, or humans. However, they are of two different types namely pathogenic and nonpathogenic. The pathogenic rhizobium bacteria includes the following-
- Rhizobium rhizogenes, commonly known as Agrobacterium rhizogenes, causes infectious hairy roots in dicotyledonous plants.
- Rhizobium radiobacter, also known as Agrobacterium tumefaciens, causes crown gall disease.
Other diseases caused by bacteria Rhizobiales is as follows –
- Agrobacterium: Causes crown gall disease in plants.
- Brucella: Causes brucellosis in animals and humans.
FAQs on Rhizobium
What is Rhizobium?
Rhizobium is a type of bacteria that forms symbiotic relationships with leguminous plants. The major role is to fix atmospheric nitrogen into a form usable by the plants.
Which plants form a symbiotic relationship with Rhizobium?
Rhizobium plant shares a symbiotic relationship with leguminous plants such as - peas, beans, lentils, clover, and alfalfa.
How does Rhizobium bacteria fix nitrogen?
Rhizobium attacks the root hairs of leguminous plants. This creates nodules in which the bacteria convert atmospheric nitrogen into ammonia using a process known as biological nitrogen fixation.
Can Rhizobium bacteria live independently in soil?
Yes, Rhizobium bacteria can live independently in soil. While they are primarily known for their symbiotic relationship with legumes, where they fix nitrogen in root nodules, many strains of Rhizobium are also capable of existing as free-living organisms in the soil environment.
What environmental conditions affect Rhizobium activity?
Rhizobium activity is influenced by soil pH, temperature, and oxygen levels. It prefers slightly acidic to neutral soils with low temperatures for nitrogen fixation.
