Rhizobium bacteria are known for their symbiotic relationship with leguminous plants. This specific association occurs due to the presence of specialized structures called root nodules on legume plant roots. There are a few reasons why Rhizobium bacteria appear primarily on leguminous plants and not on other plants:
Nodule formation: Leguminous plants have the unique ability to form nodules on their roots. These nodules provide a favorable environment for Rhizobium bacteria to establish a symbiotic relationship. The process begins when the plant releases specific chemical signals, known as flavonoids, into the soil. These flavonoids attract compatible Rhizobium bacteria and initiate the nodulation process.
Specificity of recognition: Rhizobium bacteria possess surface molecules called Nod factors, which are recognized by the plant's receptors. The compatibility between the Nod factors of Rhizobium and the receptors of leguminous plants is highly specific. This recognition mechanism ensures that only compatible Rhizobium strains are allowed to form symbiotic associations with legume plants.
Nitrogen fixation: One of the main benefits of the Rhizobium-legume symbiosis is nitrogen fixation. Rhizobium bacteria have the ability to convert atmospheric nitrogen (N₂) into a form that plants can utilize, called ammonia (NH₃). Leguminous plants often grow in nitrogen-deficient soils, and the nitrogen fixation ability of Rhizobium provides them with a significant advantage. Other plant species may have different mechanisms for obtaining nitrogen and may not require or benefit from the symbiosis with Rhizobium bacteria.
Coevolution: Leguminous plants and Rhizobium bacteria have coevolved over millions of years, developing a mutually beneficial relationship. This coevolution has led to the specialization of Rhizobium bacteria for leguminous plants and vice versa, resulting in a strong specificity and compatibility between the two.
It's worth noting that while Rhizobium bacteria are commonly associated with leguminous plants, other nitrogen-fixing bacteria, such as Azotobacter and Frankia, can form symbiotic relationships with non-leguminous plants. These bacteria employ different mechanisms to establish nitrogen-fixing associations based on the specific plant species they interact with.