Rhizobium is a gram-negative, soil-dwelling bacteria that fix nitrogen in the soil. It forms endosymbiotic relations with leguminous plants to form root nodules where they convert atmospheric nitrogen into a more useful biological form, ammonia, with the help of the enzyme nitrogenase.
Barter System
The Rhizobium bacteria and the plant that it forms symbiotic relations with follows a barter system, meaning give and take. The bacteria convert atmospheric nitrogen into a more usable and absorbable form of ammonia for the plants. In turn, the plants supply the bacteria with sugars and other organic compounds synthesised by photosynthesis.
Rhizobia
Rhizobia are a paraphyletic group (a group that consists of organisms with one common ancestor and all its descendants) that possesses several species of Rhizobium, Mesorhizobium, Phyllobacterium, etc.
Root Nodule Formation
- Rhizobium bacteria can live freely in the soil and does not necessarily need the host plant for their existence. However, when leguminous plants fall short of nitrogen, they release flavonoids into the soil, which signals the bacteria for an endosymbiotic relationship.
- This symbiosis activates nod genes in the bacteria, which releases nod factors for the initiation of root nodule formation.
- As soon as the roots sense the nod factors, cell division is triggered in the root cells for root nodule formation. The root hair starts to curl around the bacteria, forming a microcolony.
- The bacteria then form an infection thread and enter the developing root nodule through the epidermis cell and into the root cortex.
- As the bacterial colony is filled in the cytoplasm of the root cells, a pink pigment called leghaemoglobin is formed, which is an O2 It also protects the nitrogenase enzyme from inactivation and allows oxidative ATP generation, which is important for nitrogen fixation.
- The nodules form a vascular connection between the plants and bacteria for the exchange of sugars and organic compounds.
Nitrogen Fixation
Nitrogen fixation is the process of conversion of atmospheric nitrogen to a simpler form of ammonia and nitrates. It is the starting step of the nitrogen cycle. The Nitrogenase enzyme helps in the conversion of nitrogen (N2) to ammonia (NH3). There is a huge requirement of nitrogen in the soil as it is deficient in the compound naturally. Nitrogen fixation by bacteria in legumes is even used in crop rotation to fulfil the requirement of nitrogen in the soil. When the legumes die, the root nodules release the nitrogen into the soil that helps the plant in its growth and development. Rhizobium is also released from the nodules that can now infect new plants.
Read about nitrogen cycle and fixation in detail at BYJU’S Biology.
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