Nitrogen Metabolism

Nitrogen Metabolism

Nitrogen metabolism in plants involves various processes that are crucial for the uptake, assimilation, and utilization of nitrogen. Nitrogen metabolism include: 

  • Nitrogen Uptake:

- Plants absorb nitrogen primarily in the form of nitrate (NO3-) or ammonium (NH4+) ions from the soil through their roots. 

  • Assimilation:

- Nitrogen assimilation involves the conversion of inorganic nitrogen compounds into organic forms, such as amino acids and proteins, which are essential for plant growth and development.

- In plants, nitrate is reduced to ammonium through the nitrate reductase and nitrite reductase enzymes, and then further assimilated into organic compounds via the glutamine synthetase-glutamate synthase pathway. 

  • Nitrogen Fixation:

- Some plants have the ability to convert atmospheric nitrogen (N2) into ammonia (NH3) through a process called nitrogen fixation.

- Nitrogen-fixing bacteria, such as Rhizobium species in legume root nodules, form symbiotic relationships with plants to provide them with fixed nitrogen. 

  • Nitrogen Remobilization:

- Plants have mechanisms to remobilize nitrogen from senescent or older tissues to younger, actively growing tissues, ensuring efficient nitrogen utilization. 

  • Regulation and Feedback Control:

- Nitrogen metabolism in plants is tightly regulated to maintain nitrogen homeostasis and prevent nitrogen toxicity or deficiency.

- Feedback mechanisms regulate enzyme activity and gene expression in response to nitrogen availability.


Nitrogen Cycle 

Nitrogen, a vital element for living organisms, is a key component of amino acids, proteins, hormones, chlorophylls, and many vitamins. Plants and microbes compete for limited nitrogen in the soil, making it a crucial limiting nutrient in both natural and agricultural ecosystems. Conversion of atmospheric nitrogen (N2) into ammonia (NH3) or other nitrogen compounds by nitrogen-fixing bacteria. 

  • Nitrogen Forms:

- Nitrogen exists primarily as N2, with two nitrogen atoms joined by a triple covalent bond (N ≡ N).

- Atmospheric Nitrogen (N2): The atmosphere is the largest reservoir of nitrogen, making up about 78% of the Earth's atmosphere. Atmospheric nitrogen is primarily in the form of inert N2 gas, which cannot be directly used by most organisms.

- Soil Nitrogen: Atmospheric nitrogen is converted into forms that can be utilized by plants through biological and chemical processes in the soil. Lightning and ultraviolet radiation provide enough energy to convert atmospheric nitrogen into nitrogen oxides (NO, NO2, N2O), which then dissolve in rainwater and enter the soil. Additionally, nitrogen-fixing bacteria in the soil, such as Rhizobium, convert atmospheric nitrogen into ammonia (NH3) through nitrogen fixation.

- Biomass: Nitrogen is taken up by plants from the soil in the form of nitrate (NO3-) or ammonium (NH4+). Plants assimilate nitrogen into their biomass through processes such as photosynthesis and protein synthesis. Animals obtain nitrogen by consuming plants or other animals that have consumed plants.

Conversion of N2 to ammonia (NH3) is called nitrogen fixation, occurring naturally through processes like lightning, ultraviolet radiation, and industrial activities. 

  • Assimilation:

- Plants take up nitrate or ammonium from the soil and incorporate nitrogen into their biomass through processes like protein synthesis. 

  • Ammonification:

- Decomposition of organic nitrogen from dead plants and animals produces ammonia (NH3) through ammonification.

- Soil bacteria further convert ammonia into nitrate (NO3-) through oxidation. 

  • Nitrification:

- Ammonia is oxidized to nitrite (NO2-) by bacteria like Nitrosomonas and/or Nitrococcus.

- These nitrifying bacteria are called chemoautotrophs.

- Nitrite is then oxidized to nitrate (NO3-) with the help of Nitrobacter bacteria, in a process known as nitrification. 

  • Plant Uptake and Utilization:

- Plants absorb nitrate from the soil and transport it to leaves.

- In leaves, nitrate is reduced to ammonia, which forms the amine group of amino acids. 

  • Denitrification:

- Some soil bacteria, like Pseudomonas and Thiobacillus, carry out denitrification, converting nitrate back into nitrogen gas (N2), which returns to the atmosphere. 



Biological Nitrogen Fixation 

1. Utilization of Atmospheric Nitrogen:

  • Only certain prokaryotic species can utilize atmospheric nitrogen (N2) abundantly available in the air.
  • Reduction of nitrogen to ammonia (NH3) by living organisms is known as biological nitrogen fixation. 

2. Nitrogenase Enzyme:

  •  Biological nitrogen fixation is facilitated by the enzyme nitrogenase, which is exclusively present in prokaryotes.
  • Nitrogenase catalyzes the conversion of nitrogen (N2) to ammonia (NH3). 

3. Types of Nitrogen-Fixing Microbes:

  • Free-Living: Some nitrogen-fixing microbes exist independently and include aerobic species like Azotobacter and Beijernickia, anaerobic species like Rhodospirillum, and free-living Bacillus.
  • Symbiotic: Certain nitrogen-fixing microbes form symbiotic relationships with plants. Examples include cyanobacteria such as Anabaena and Nostoc. 

Symbiotic Biological Nitrogen Fixation

1. Types of Associations:

  • Various symbiotic biological nitrogen-fixing associations exist, with the legume-bacteria relationship being the most prominent.
  • Rhizobium species form symbiotic relationships with the roots of legumes, while Frankia associates with non-leguminous plants like Alnus. 

2. Legume-Bacteria Relationship:

  • Rod-shaped Rhizobium bacteria establish symbiotic relationships with legume roots, forming nodules as small outgrowths.
  • Legumes such as alfalfa, sweet clover, lentils, peas, and beans commonly exhibit this association. 

3. Nitrogen Fixation:

  • Both Rhizobium and Frankia bacteria, although free-living in soil, can fix atmospheric nitrogen when in symbiosis with host plants.
  • This process enables the conversion of atmospheric nitrogen into a form usable by plants, enhancing nitrogen availability in the soil. 

Nodule Formation and Nitrogen Fixation in Legumes



1. Initial Interactions:

  • Rhizobia bacteria multiply and colonize the root surroundings, attaching to epidermal and root hair cells.
  •  Root hairs curl, facilitating bacterial invasion, and an infection thread is formed to carry bacteria into the root cortex. 

2. Nodule Initiation:

  •  Bacteria released from the infection thread initiate nodule formation in the root cortex.
  • Differentiation of specialized nitrogen-fixing cells occurs upon bacterial release into the root cells. 

3. Vascular Connection Establishment:

  • Nodule formation establishes a direct vascular connection with the host plant for nutrient exchange. 

4. Biochemical Components in Nodules:

  • Nodules contain essential biochemical components, including the enzyme nitrogenase and leghaemoglobin.
  • Nitrogenase, a Mo-Fe protein, catalyzes the conversion of atmospheric nitrogen to ammonia, the primary product of nitrogen fixation. 

5. Nitrogenase Sensitivity and Protection:

  • Nitrogenase enzyme is highly sensitive to molecular oxygen and requires anaerobic conditions for activity.
  • Nodules have adaptations to protect nitrogenase from oxygen, including the presence of leg-haemoglobin, an oxygen scavenger. 

6. Energy Requirement for Ammonia Synthesis:

  •  Ammonia synthesis by nitrogenase demands a high input of energy, obtained from the respiration of host cells.
  • Each ammonia molecule produced requires 8 ATP molecules.




Fate of Ammonia in Plants

At physiological pH, ammonia (NH3) protonates to form ammonium (NH4+) ions. While plants can assimilate both nitrate and ammonium ions, excessive accumulation of ammonium ions can be toxic to plants.


1. Utilization of NH4 in Plants:

  • Reductive Amination: In reductive amination, ammonia reacts with α-ketoglutaric acid to form glutamic acid, a vital amino acid in plants.  


  •  Transamination: Transamination involves transferring an amino group from one amino acid to a keto group of a keto acid. Glutamic acid serves as the primary amino acid for transferring NH2 groups, facilitating the formation of other amino acids. Transaminase enzymes catalyze these reactions.



2. Formation of Amides:

  •  Asparagine and glutamine, crucial amides in plants, are structural components of proteins. 
  • They are synthesized from aspartic acid and glutamic acid, respectively, through the addition of another amino group. 
  • The hydroxyl part of the acid is replaced by another NH2- radical. 
  •  Amides, containing more nitrogen than amino acids, are transported to various plant parts via xylem vessels. 

3. Export of Fixed Nitrogen:

  • Some plants, like soybeans, export fixed nitrogen as ureides along with the transpiration stream. 
  • Ureides have a high nitrogen to carbon ratio, facilitating efficient nitrogen transport within the plant.