15.09 Plant Hormones

Overview of Plant Hormones

• Definition: Plant hormones, or plant growth regulators, are chemical messengers coordinating plant growth and responses.
• Differences from Animal Hormones:
  • Not produced in specialized glands but in various plant tissues.
  • Can move cell-to-cell by diffusion or active transport or via phloem/xylem sap.
  • Often act locally, influencing nearby cells.

Major Plant Hormones

1.Auxins (e.g., IAA – Indole-3-acetic acid)

• Synthesis Location: Produced in shoot and root tips (meristems).
• Transport: Primarily by active transport from cell to cell; also in phloem sap.
• Functions:
  • Cell Elongation: Promotes growth by stimulating cells to pump protons into cell walls, acidifying them and loosening cellulose microfibril connections, allowing cell expansion.
  • Mechanism:
    • Auxin Binding: Binds to receptors on the cell membrane, activating ATPase proton pumps.
    • Acidification: Protons lower pH in cell walls, activating expansin proteins.
    • Cell Wall Loosening: Expansins disrupt bonds between cellulose and other cell wall components, allowing water intake and cell elongation.

2.Gibberellins

• Synthesis: Present in high concentrations in young leaves, seeds, and stems.
• Functions:
  • Stem Elongation: Stimulates cell elongation by activating XET enzymes in cell walls, which break bonds in hemicellulose, allowing cellulose microfibrils to separate.
  • Seed Germination:
    • Gibberellins in seeds trigger germination by stimulating amylase production in the aleurone layer, which breaks down starch in the endosperm into glucose, providing energy for embryo growth.
    • Gene Regulation: Gibberellins promote the breakdown of DELLA proteins, which otherwise inhibit transcription factors for amylase synthesis.

3.Abscisic Acid (ABA)

• Function: Controls plant responses to stress, such as drought, by closing stomata to conserve water.

Mechanism of Auxin-Induced Elongation

1. Auxin Binding: Auxin binds to receptors on the cell membrane.
2. Proton Pump Activation: Stimulates ATPase proton pumps to move H⁺ ions from cytoplasm into cell walls.
3. Acidification and Expansin Activation: Lower pH in cell walls activates expansins, which loosen the matrix surrounding cellulose microfibrils.
4. Water Absorption and Elongation: Loosened cell walls allow cells to absorb water, increasing internal pressure and causing elongation.

Mechanism of Gibberellin-Induced Seed Germination

1. Water Uptake: Triggers gibberellin synthesis in the embryo.
2. Gibberellin Release: Diffuses to the aleurone layer, stimulating amylase production.
3. Starch Breakdown: Amylase converts endosperm starch to maltose, which is then hydrolyzed to glucose.
4. Energy for Growth: Glucose fuels cellular respiration in the embryo, supporting growth.

Plant Hormone Transport and Signal Amplification

• Receptors: Located on the cell surface, cytoplasm, or nucleus; initiate a cascade of signals to amplify and transmit the hormone’s effects.
• Signal Pathways: Often involve ionic or chemical cascades within the cell, similar to signaling pathways in animals.

Key Concepts and Structures

• Expansins: Proteins in cell walls activated by low pH to loosen cellulose microfibril connections, aiding cell elongation.
• Endosperm: Nutrient-rich tissue in seeds, providing starch as an energy source during germination.
• Aleurone Layer: Protein-rich layer surrounding the endosperm; synthesizes amylase in response to gibberellins during germination.

Key Terms

• DELLA Proteins: Inhibitors in gibberellin signaling; their degradation promotes transcription of growth-related genes.
• Auxin: Plant hormone promoting cell elongation.
• Gibberellin: Hormone promoting stem elongation and seed germination.
• Abscisic Acid (ABA): Hormone involved in stress responses, such as stomatal closure during drought.
• Expansins: Proteins that loosen cell walls to allow cell elongation.
• XET Enzyme: Enzyme that breaks bonds in hemicellulose, aiding cell wall flexibility for elongation.