Unit 5: Plant Hormones & Nitrogen Metabolism

Plant hormones (Phytohormones) are signal molecules produced within the plant that occur in extremely low concentrations. This unit focuses on the biosynthesis of the four major growth regulators:

• Auxin: Primarily synthesized from the amino acid Tryptophan. Synthesis mainly occurs in rapidly growing tissues like apical buds and young leaves.
• Gibberellin: Synthesized via the terpenoid pathway in plastids, membranes, and the endoplasmic reticulum.
• Cytokinin: Primarily synthesized in roots and transported to shoots. They are derived from adenine.
• Abscisic Acid (ABA): Synthesized in chloroplasts and other plastids via the carotenoid (indirect) pathway.

Phytohormones must be transported to target cells where they trigger specific physiological responses.

• Transport: Auxins exhibit polar transport (from apex to base). Others like Gibberellins and Cytokinins move primarily through the xylem and phloem.
• Mode of Action: Hormones bind to specific protein receptors. This binding initiates a signal transduction cascade, leading to changes in gene expression or enzyme activity.

Each hormone has unique roles in plant development:

Biological nitrogen fixation is the process by which atmospheric nitrogen (N₂) is converted into ammonia (NH₃) by certain microorganisms.

• Symbiotic Fixation: Occurs in the root nodules of leguminous plants through an association with Rhizobium bacteria. Non-leguminous symbiotic systems (e.g., Frankia) also exist.
• Non-symbiotic Fixation: Carried out by free-living bacteria like Azotobacter and certain cyanobacteria.
• Biochemistry: The reaction is catalyzed by the Nitrogenase enzyme complex, which requires high ATP and a highly anaerobic environment to function.

Plants absorb nitrogen mostly in the form of nitrate (NO₃⁻), which must be converted into amino acids.

• Nitrate Assimilation: Nitrate is reduced to Nitrite (NO₂⁻) by Nitrate Reductase and then further reduced to Ammonia (NH₃) by Nitrite Reductase.
• Ammonia Assimilation: To avoid toxicity, ammonia is rapidly incorporated into organic molecules (amino acids) via the GS-GOGAT (Glutamine Synthetase - Glutamate Synthase) pathway.

6. Exam Focus: Tips and FAQs

Common Mistakes

• Mistake: Assuming nitrogenase can fix nitrogen in the presence of oxygen. Correction: Nitrogenase is extremely sensitive to oxygen; plants often use Leghaemoglobin to scavenge oxygen in nodules.
• Mistake: Confusing nitrate reduction and nitrogen fixation. Correction: Fixation converts atmospheric N₂ gas to NH₃, while nitrate reduction converts soil NO₃⁻ to NH₃.

Frequently Asked Questions

Q: What is the precursor for Auxin biosynthesis?
A: The amino acid Tryptophan is the most common precursor.

Q: Why is ammonia assimilation necessary?
A: Ammonia is toxic to plant cells at high concentrations because it can dissipate proton gradients; therefore, it must be quickly converted into Glutamate or Glutamine.