Process of Nodule Formation in Legumes

Process of Nodule Formation

• Legumes enrich soil fertility through symbiotic nitrogen fixation with Rhizobium, a practice recognized since ancient times.
• Nodulation occurs when free-living Rhizobia infect host legumes via an infection thread or at lateral root emergence points.
• Rhizobia transform into bacteroids, where nitrogen fixation takes place, within the nodules formed on the host roots.
• Root cap cells secrete polysaccharides to create mucigel, which entraps high numbers of Rhizobia.
• Initial plant response to Rhizobium is the curling and branching of root hairs, facilitating infection.
• Rhizobium secretes localized auxin to soften cell walls at root hair surfaces, promoting bacteria entry.
• The infection thread extends as the root hair grows, eventually trapping Rhizobia within deformed root hair cells.
• The encapsulation of Rhizobia induces additional cell wall synthesis, leading to nodule formation that serves as a protective structure.

Nitrogen Fixation and Nitrogenase Activity

• Nitrogen fixation is an anaerobic process where atmospheric nitrogen (N2) is converted into ammonia (NH3) through the action of nitrogenase.
• Nitrogenase consists of two components: one containing iron (Fe) and molybdenum (Mo), while the second component lacks molybdenum.
• Leghaemoglobin is crucial as it controls oxygen supply around the bacteroids, protecting nitrogenase from oxygen exposure, which can be detrimental to its activity.
• The first nitrogen fixation intermediate is ammonia, which plants can absorb and utilize.

Forms and Methods of Nitrogen Fixation

• Nitrogen fixation transforms molecular nitrogen into nitrogenous compounds, making them accessible to plants.
• Natural processes account for 10% of nitrogen fixation, while 90% occurs through biological means.
• Nitrogen fixation can be divided into:
  • Physical Nitrogen Fixation:
    • Includes both natural and industrial methods.
  • Biological Nitrogen Fixation:
    • Primarily carried out by microbes such as Rhizobium in symbiotic relationships with legumes.