Useful Microorganisms - Rhizobium

Rhizobium is a genus of symbiotic bacteria that forms a mutualistic relationship with leguminous plants. These bacteria live in specialised structures called root nodules, which they form on the roots of plants like beans, peas, and groundnuts. Rhizobium bacteria play a crucial role in fixing atmospheric nitrogen into ammonia, a form of nitrogen that plants can readily use for growth. In return, the plant provides Rhizobium with carbohydrates and a protective environment. This relationship not only benefits the host plant but also enriches the soil by increasing its nitrogen content, making Rhizobium a key player in sustainable agriculture.

• Rhizobium bacteria live in root nodules of leguminous plants and provide nitrates, nitrites, and amino acids to the plant. In return, they receive energy in the form of carbohydrates from the plant, forming a symbiotic relationship.
• Rhizobium fixes atmospheric nitrogen into nitrogenous compounds like nitrates and nitrites, which are essential for the plant's growth.
• Leguminous plants, such as beans, peas, and soybeans, act as hosts for Rhizobium, benefiting from the nitrogen compounds, which make beans and pulses rich in proteins.
• After harvesting leguminous crops, the leftover roots and plant parts are left in the soil to maintain the Rhizobium population and enrich the soil with nitrogen.
• The use of rhizobium reduces the need for chemical fertilisers, minimising their harmful effects on the environment and lowering farmers' expenses on fertilisers.
• Seeds are often coated with a rhizobial solution or powder before sowing, a process called rhizobial inoculation, which allows Rhizobium to enter plantlets and provide nitrogen even to non-leguminous crops, like cereals. This practice enhances sustainable agriculture by improving soil fertility and reducing dependency on synthetic fertilisers.

1. Aim: To observe Rhizobium bacteria in the root nodules of leguminous plants using a compound microscope.

2. Requirements: plantlet of fenugreek, groundnut, or any legume, 3-5% hydrogen peroxide solution, 70% ethyl alcohol, sterile water, safranin stain, glass slide and coverslip, compound microscope, scalpel or blade.

3. Procedure

• Sterilise the plantlet with 3-5% hydrogen peroxide.
• Immerse the plantlet in 70% ethyl alcohol for 4-5 minutes.
• Rinse the roots thoroughly with sterile water.
• Cut thin sections of the root nodules using a scalpel.
• Stain the sections in a solution of safranin for 2-3 minutes.
• Place the stained section on a glass slide, cover it with a coverslip, and observe under the microscope.

Root Nodules of Soybean Plant

4. Conclusion: Under the microscope, pinkish rod-shaped organisms (Rhizobium bacteria) were observed in the root nodules. These bacteria are beneficial to leguminous plants as they fix atmospheric nitrogen into a usable form, aiding in plant growth and soil fertility.