BIO201 Plant Responses to the Environment 1 Slides

Plant Responses to Environmental Signals

Plant Hormones (Phytohormones) Overview

• Small organic molecules active at low concentrations.

• Respond to internal and external conditions.

• Transported between tissues or act where produced.

Major Plant Hormones and Their Functions

Auxin

• Characteristics and Functions:

  • Involved in cell elongation and apical dominance.

  • Promotes differentiation of xylem and phloem.

  • Helps define the body’s long axis (phototropism and gravitropism responses).

  • First plant hormone to be isolated and characterized.

  • Produced in shoot apical meristems and young leaves.

• Mechanism of Action:

  • Auxin leads to differential cell elongation by promoting elongation in cells on the side of the plant away from light (phototropism).

  • Triggered by the upward movement and concentration gradients of auxin.

• Experimental Evidence:

  • Darwin’s hypothesis (1897) on seedling light direction.

  • Boysen-Jensen (1913) showed that the signal for light response was a diffusing chemical.

  • Went (1925) identified auxin as the active growth substance.

  • Hypothesis for how auxin causes bending:

  1. Auxin stimulates proton pumps that acidify the cell wall.

  2. Activates expansins that increase wall loosening.

  3. Water influx leads to increased turgor pressure.

  4. Cells elongate due to pressure and loosening of the wall.

Cytokinins

• Characteristics and Functions:

  • Promote cell division in conjunction with auxin.

  • Induce chloroplast development.

  • Break dormancy in lateral buds.

  • Delay aging processes (senescence).

  • Produced mainly in root apical meristems.

Gibberellins (GAs)

• Characteristics and Functions:

  • Promote stem growth via cell elongation and division.

  • Stimulate seed germination.

  • Origins of synthesis remain somewhat uncertain.

  • Notably involved in beer production and fruit development without seed production.

Abscisic Acid (ABA)

• Characteristics and Functions:

  • Indicates environmental stress responses (e.g., drought).

  • Inhibits bud growth and seed germination.

  • Induces stomata closure to conserve water.

  • Acts similarly to cortisol in humans.

Ethylene

• Characteristics and Functions:

  • Exists as a gas and is produced in most plant tissues.

  • Involved in fruit ripening and leaf senescence.

  • Promotes leaf and flower abscission.

Auxin's Role in Leaf Abscission

• Auxin collaborates with ethylene in leaf senescence and abscission:

  • Healthy leaf has high auxin levels allowing normal function.

  • Low auxin levels initiate senescence leading to abscission.

  • An abscission zone forms at the leaf attachment to the stem, sensitive to ethylene once auxin declines.

Experimental Evidence on Auxin Transport

• Polar transport of auxin is predominantly driven by a pH gradient across the membrane (IAA forms).

• Auxin can affect gene transcription and translation, hinting at complex regulatory pathways.

Practical Applications of Plant Hormones

• Auxin widely used as a herbicide.

• Example: 2,4-Dichlorophenoxyacetic acid targets dicots while sparing monocots; used in lawn care and controversial herbicides like Agent Orange.

• Gibberellins promote germination and flavor development in malt production from barley.

Summary Table of Major Plant Growth Regulators

Auxin Involvement in Cambium Division

• Auxin stimulates the cambium for secondary growth.

• Procedures in plant regeneration utilizing auxin concentrations.

Conclusion

• Understanding plant hormones is crucial for applications in agriculture, horticulture, and understanding plant development under variable environmental conditions.