LectureTopic26-PlantHormones

Plant Responses to Stimuli

• Organisms, particularly plants, have mechanisms to respond to environmental stimuli despite being sessile.

• Hormones play a key role in these responses.

I. Signal Transduction Pathways

• Signal transduction connects signal reception to response.

  • A. A potato left in darkness exhibits unhealthy, etiolation-induced shoots and underdeveloped roots.

  • B. Etiolation is a morphological adaptation for growing in low light.

  • C. Exposure to light initiates de-etiolation, allowing normal shoot and root development.

  • D. This process exemplifies cell-signal processing with stages of reception, transduction, and response.

II. Plant Hormones

• Plant hormones coordinate growth, development, and responses to stimuli.

  • A. Defined as chemical signals that modify/control physiological processes within a plant. Key hormones include:

    • Auxins

    • Abscisic Acid

    • Cytokinins

    • Ethylene

    • Gibberellins

  • B. Hormones are produced in low concentrations but have significant effects on growth and organ development.

  • C. Hormonal control influences cell division, elongation, and differentiation.

III. Discovery of Plant Hormones

• A. Tropism refers to an organ's curvature in response to stimuli (like phototropism and geotropism).

• B. Darwin and his son Francis established that the coleoptile tip is necessary for bending towards light.

• C. Boysen-Jensen in 1913 demonstrated that the response was due to a mobile chemical.

• D. In 1926, Went extracted auxin as the chemical messenger for phototropism through modified experiments.

IV. Auxin

• A. Auxin is any compound that promotes coleoptile elongation, primarily indoleacetic acid (IAA).

• B. Produced in shoot tips and transported downward in the stem.

  • C. Auxin stimulates plasma membrane proton pumps, lowering pH in the cell wall and activating expansins for cell wall loosening.

V. Auxin's Role in Development

• A. Polar transport of auxin influences pattern formation and growth; reduced flow stimulates lower branches.

• B. Auxin transport is involved in phyllotaxy, affecting leaf arrangement on stems.

• C. It directs leaf venation pattern through transportation dynamics.

• D. Auxin controls vascular cambium activity, influencing plant structure.

VI. Practical Uses for Auxins

• A. The auxin indolbutyric acid (IBA) encourages adventitious root growth and facilitates propagation via cuttings.

• B. Overuse of synthetic auxins, like 2,4-D, can lead to plant death and is used as an herbicide on certain plants.

VII. Cytokinins: Cell Division Promotion

• A. Cytokinins are produced in actively growing tissues such as roots, embryos, and fruits.

• B. They work alongside auxins to regulate cell division and differentiation processes.

• C. Cytokinins, auxin, and strigolactone interact to control apical dominance, affecting the development of axillary buds.

  • D. Removing the apical bud leads to bushier plant growth.

VIII. Gibberellins

• A. Gibberellins influence stem elongation, fruit growth, and seed germination.

  • B. Produced primarily in young roots, leaves, and stems; stimulate growth via cell elongation and division.

  • C. Both auxin and gibberellins are required for fruit maturation, leading to successful germination post-water intake.

IX. Abscisic Acid

• A. Crucial for seed dormancy, ensuring germination occurs under optimal conditions.

• B. Dormancy is lifted by environmental factors like rain or light

• C. Acts as a signal for plants to endure drought; accumulation causes rapid stomatal closure.

X. Ethylene

• A. Produced in response to environmental stresses (drought, flooding, mechanical stress, injury, infection).

  • 1. Induces a triple response enabling shoots to grow around obstacles (reduction in stem elongation, thickening, horizontal growth).

• B. Ethylene is associated with senescence, the programmed cell death process (apoptosis).

• C. Governs leaf abscission; a balance of auxin and ethylene triggers leaf drop.

• D. Ethylene governs fruit ripening, stimulating its process and encouraging further ethylene release for continued ripening.

XI. Key Concepts to Remember

• Determine the five major classes of plant hormones and their functions.

• Understand the triple response and the associated hormones.

• Define expansin and its role in plant growth.

• Explain etiolation and its triggers.

• Describe the sequence of stages in cell-signal processing.