Overview of Plant Hormones and Their Functions

Ethylene

• Introduction to Ethylene

  • Noticed in Europe (e.g., Germany) where street lamps caused leaves to fall off trees.

  • First discovered by botanist Neljebov; he was only 17 years old.

  • Conducted experiments showing ethylene gas emitted from lamps caused the leaf drop.

• Synthesis of Ethylene

  • Ethylene is produced from the amino acid methionine through the enzymatic activity of ACC synthase.

  • ACC (1-aminocyclopropane-1-carboxylic acid) is the precursor to ethylene, which is produced in response to environmental stress (wounding, flooding) and can be stimulated by auxins.

  • Ethylene promotes various physiological responses when stress is detected.

• The Triple Response in Pea Seedlings

  • Ethylene concentrations increase in parts per million lead to specific growth responses in pea seedlings:

  • Horizontal growth instead of vertical elongation.

  • Increase in radial expansion (thicker stems).

  • Decrease in longitudinal elongation (shorter height).

  • This growth pattern is referred to as the "triple response."

• Stress Response and Fruit Ripening

  • Ethylene acts as a stress hormone; influences plants like rice to elongate their shoots under flood conditions.

  • Major role in fruit ripening for climacteric fruits (e.g., bananas, tomatoes) characterized by a spike in cellular respiration, indicated by CO₂ levels.

• Comparison of Climacteric vs. Non-Climacteric Fruits

  • Climacteric Fruits: Ripen quickly, have a peak in cellular respiration and ethylene (e.g., bananas, tomatoes).

  • Non-Climacteric Fruits: Ripen slowly without a climacteric spike (e.g., citrus, grapes, strawberries).

• Effects of Ethylene in Industry

  • Industry attempts to track and manipulate ethylene levels to prolong the ripening of fruits using:

  • Potassium permanganate (pink dye) for ethylene reduction, but staining is a concern.

  • Debbie Meyer Green Bags claim to trap ethylene using zeolite, but effectiveness is debated.

• Gene Mutations and Ethylene Response

  • Investigated mutations affecting ethylene receptor ETR1; without this receptor, the plant does not respond to ethylene signals.

  • Ethylene signals post-pollination to conserve energy by not maintaining unnecessary flowers.

  • Ethylene inhibitors, like silver thiosulfate, can delay flower senescence.

• Ethylene and Leaf Drop

  • Studies on mutations show ethylene regulates leaf drop in response to pollutants like ozone.

  • Ethylene, a simple hydrocarbon gas, diffuses through plants, particularly affecting fruit ripening, leaf drop, fruit drop, and flower senescence.

Abscisic Acid (ABA)

• Discovery and Function

  • Discovered in 1949 from leaf buds associated with dormancy.

  • Initially misnamed "abscisic" by Adecott in the 1960s; associated with leaf drop but primarily regulates dormancy in seeds and buds.

• Synthesis and Effects

  • ABA synthesized through carotenoid pathway and affects dormancy when environmental conditions warrant it (e.g., drought).

  • Mutation studies created viviparix mutants; lack of ABA leads to premature germination on the cob of maize.

• Signaling Pathway of ABA

  • ABA synthesized in the roots travels through vascular tissues to leaves.

  • ABA binding triggers a signaling cascade in guard cells, leading to increased calcium influx, causing potassium and other solutes to exit, ultimately resulting in stomatal closure due to loss of turgor pressure.

Gibberellins (GA)

• Discovery and Origin

  • Discovered from a fungus (Gibberella) causing "foolish seedling disease" in rice during the 1920s.

  • Over 100 gibberellins identified; GA3 is commercially important as gibberellic acid.

• Role in Plant Growth

  • Gibberellins promote stem elongation, break seed dormancy, and initiate flowering. Other effects include bolting in rosette plants and larger fruit set.

• Germination Pathway

  • Involved in breaking seed dormancy by stimulating enzyme release from the aleurone layer in seeds, crucial for nutrient mobilization needed for germination.

Concluding Overview of Plant Hormones

• Ethylene, ABA, and gibberellins play crucial roles in plant biology, regulating growth, development, stress responses, and dormancy. Experimentation and research continue to clarify their functions and commercial applications.

• Emphasis on understanding signaling pathways for each hormone crucial for effective application in both agriculture and research contexts.

Upcoming Topics in Chapter 28

• Introduction to plant responses to blue light, phototropism, and stomatal opening.

• Examination of gravitropism, circadian rhythms, photoperiodism, and flowering responses through specific light cues.

• A deeper understanding of photoreceptors and their role in regulating plant behavior under varying light conditions.