Chapter 39: Plant Responses to Internal and External Signals

Topics Covered

  1. Stages of signal transduction
  2. Characteristics of hormones
  3. The five major plant hormones and their location and effects
  4. Phototropism and experiments

39.1 Signal Transduction Pathways Link Signal Reception to Response

  • A potato left growing in darkness exhibits unhealthy morphological traits, such as elongated roots. This phenomenon of adaptation to dark conditions is termed etiolation.
  • Upon exposure to light, the potato undergoes changes known as de-etiolation (or "greening"), which facilitates normal growth of shoots and roots.
    • (a) Morphological traits before exposure to light.
    • (b) Morphological changes after one week's exposure to natural daylight.

Signal Reception and Response

  • The response of a potato to light exemplifies a process of cell signaling consisting of three stages: reception, transduction, and response.
    • Stages involved in signal transduction:
    1. Reception
    2. Transduction
    3. Response

1. Reception

  • Reception occurs when a hormone or external stimulus binds to a protein receptor on the cell membrane.

2. Transduction

  • Transduction involves second messengers that transfer and amplify signals from receptors to intracellular proteins, eliciting cellular responses.
    • Key second messengers in the process of de-etiolation include:
    • Ca^{2+} ions
    • cyclic GMP (cGMP)
  • The phytochrome receptor responds to light by:
    • Opening Ca^{2+} channels, thus raising cytosolic Ca^{2+} levels.
    • Activating an enzyme responsible for cGMP production.

3. Response

  • The overall response involves several changes:
    • Post-Translational Modification of preexisting proteins, generally involving the (de)phosphorylation of designated amino acids.
    • Protein kinases act directly upon activation by cGMP and Ca^{2+}.
    • Phosphatases function to deactivate the signal transduction pathways.
    • Transcriptional Regulation
    • Transcription factors can either activate or repress gene expression by binding to specific DNA regions.
    • De-Etiolation (“Greening”) Proteins
    • Enzymes activated during de-etiolation play vital roles in conducting photosynthesis, chlorophyll production, and managing plant hormone levels that influence growth.

39.2 Plants Use Chemicals to Communicate

  • Unlike animals, plants facilitate the intercellular transport of macromolecules, such as chemicals and hormones, via plasmodesmata.

General Characteristics of Hormones

  • Hormone: A signaling molecule generated in one part of the plant and transported to affect processes in other parts.
  • Characteristics of plant hormones include:
    1. They act as chemical signals that can modify or regulate various physiological processes.
    2. They are present in very low concentrations yet can induce significant effects on growth and development.
    3. Each hormone can exert multiple effects, and multiple hormones might influence a single physiological process.
    4. Responses are sensitive to both the concentration of specific hormones and their combinations.

A Survey of Plant Hormones

Table 39.1 Overview of Plant Hormones

  • Auxin (IAA)

    • Produced in: Shoot apical meristems, young leaves, root apical meristems, developing seeds and fruits.
    • Major Functions: Stem elongation, lateral root formation, fruit development, apical dominance, phototropism, vascular differentiation, leaf abscission regulation.

  • Cytokinins

    • Produced in: Roots and transported to other organs, minor sites include meristems of apical buds and roots, young leaves, and developing seeds.
    • Major Functions: Cell division in shoots and roots, apical dominance modulation, nutrient mobilization, seed germination stimulation, leaf senescence inhibition.

  • Gibberellins (GA)

    • Produced in: Meristems of shoots and roots.
    • Major Functions: Stem elongation, fruit and seed development, influencing pollen tube growth and sex determination.

  • Abscisic Acid (ABA)

    • Produced in: Almost all plant cells, detectable in major organs and living tissues; may be transported via phloem or xylem.
    • Major Functions: Growth inhibition, stomatal closure during drought, seed dormancy promotion.

  • Ethylene

    • Produced in: Most plant parts, with high concentrations during senescence, leaf abscission, and certain fruit ripening.
    • Major Functions: Promotes fruit ripening, leaf abscission, and promotes the triple response in seedlings.

The Role of Auxin in Plant Responses

  • Tropism: Any directional growth response of a plant organ towards or away from a stimulus.
    • Phototropism experiments conducted by Charles Darwin and Francis Darwin demonstrated that a grass seedling bends towards light only when the apical tip is present, indicating a signal from the tip to the elongating region. Peter Boysen-Jensen's later experiments indicated that this signal is a mobile chemical substance—auxin.
Control of Auxin Distribution
  • The role of auxin in cell elongation is mediated by loosening the cell wall through the activity of proton pumps, leading to an increase in cell elongation in the presence of auxins.

Auxin’s Role in Development

  • Reduced auxin levels from the shoot may stimulate growth in lower branches.
  • It also influences the arrangement of leaves (phyllotaxy) and regulates leaf venation and vascular cambium activity.

Practical Applications of Auxins

  • Auxins are used in the vegetative propagation of plants via cuttings as they stimulate the formation of adventitious roots.
  • In excessive amounts, synthetic auxins (e.g., 2,4-D) can act as herbicides, killing unwanted plants.

Cytokinins and Their Effects

  • Cytokinins stimulate cytokinesis (cell division) and are produced in tissues undergoing active growth (e.g., roots, embryos).
  • They cooperate with auxin in regulating cell division and differentiation and influence apical dominance through competition with auxin.
  • Cytokinins can also slow the aging process of certain plant organs by reducing protein degradation, stimulating RNA and protein synthesis, and mobilizing surrounding nutrients.

Gibberellins

  • Gibberellins stimulate significant growth in stems and leave elongation and are necessary for fruit development.
  • Gibberellins are important for germination, unleashing dormant seeds through gibberellin release upon water imbibition.

Abscisic Acid and Its Role

  • Abscisic Acid (ABA) regulates growth by inducing seed dormancy, making it crucial for optimal germination conditions.
  • ABA facilitates drought tolerance by accelerating the closure of stomata under water stresses.

Ethylene

  • Ethylene is produced in response to various plant stresses and is fundamental for enabling the triple response to mechanical stress: 1. Slows stem elongation; 2. Thickens stem; 3. Induces horizontal growth.
    • Ethylene also plays a significant role in leaf abscission and fruit ripening, often culminating in a cascade of additional ethylene production post-ripening.