Plant Hormones and Their Roles

Overview

• Hormones are chemical messengers synthesized at one location, delivering messages to different locations in plants.
• Five main types of hormones influence plant growth: Auxin, Gibberellin, Cytokinin, Ethylene, and Abscisic Acid.

Auxin

• Enables plants to bend towards light (Phototropism).
• Phototropism: the bending of an organism towards an external stimulus.
• Synthesized at stem tips and travels downward.
• Removal of the stem tip results in loss of the plant's ability to grow towards light.
• Facilitates bending by moving to the shaded part of the stem.

Gibberellin

• Important for several stages of plant development.
• Required for plant germination; upregulated during this process.
• Promotes elongation of internodes (stem between nodes, where leaves grow).

Cytokinin

• Counters senescence in plants.
• Important for forming new plant organs (roots vs. shoots).
• Produced in the root apical meristem (root tips).
• Passively carried up through the plant's xylem.
• The auxin-to-cytokinin ratio determines organ formation:
  • More auxin promotes root formation.
  • More cytokinin promotes shoot formation.
• Works with ethylene to cause abscission (shedding) of leaves, flowers, and fruits.

Ethylene

• Known as the ripening hormone.
• Example: A ripe banana can speed up the ripening of unripe bananas when placed together.
• Gaseous with low water solubility, diffuses out of cells.
• Produced in rapidly growing and dividing cells.
• Inhibits leaf expansion in germinating plants, allowing shoots to push through soil.

Abscisic Acid

• Signals dehydration in plants.
• Synthesized in the chloroplasts of leaves.
• In water-stressed plants, it travels through xylem and phloem, causing stomata to close and reduce water loss.

Conclusion

• These hormones are just a few of many affecting plant growth.
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