Chemical Signals in Plants Module 12

Plant Sensory Systems and Environmental Response

Introduction

  • Plants sense and respond to various environmental factors.
  • This includes signal processing, directional and nondirectional movements, hormonal regulation, and defense mechanisms.

Signal Processing in Plants (Section 37.1)

  • External Stimulus: A sensory cell perceives an external stimulus and transduces it into an internal signal.
  • Cell-Cell Signal: The sensory cell releases a cell-cell signal that travels throughout the plant.
  • Target Cells: Target cells receive the cell-cell signal and change their activity to produce an appropriate response.
  • Sensory cells receive an external signal and convert it to an intracellular signal.
  • These sensory cells then transmit the signal to target cells.
  • The target cells respond by altering their activity to produce an appropriate response.

Light Responses: Phototropism (Section 37.1)

  • Shoots bend toward full-spectrum light and specifically toward blue light.

Phototropins as Blue-Light Receptors (Section 37.1)

  • Phototropins are photoreceptors that detect blue light and initiate phototropic responses.
  • Researchers identified the PHOT1 gene in Arabidopsis, which codes for a blue-light receptor.
  • The PHOT1 protein becomes phosphorylated after exposure to blue light.

Phototropic Response (Section 37.1)

  • Experiments by Darwin and Darwin (1880) and Boysen-Jensen (1913) explored where phototropins are found in plants.
  • The light responsible for triggering phototropism is sensed at the coleoptile tip.
  • Sensing Tissue: Cells at the coleoptile tip sense light, which is the stimulus.
  • Hormonal Signal: A hormone travels from the tip down the coleoptile.
  • Responding Tissue: Cells lower in the coleoptile respond to the hormone, resulting in bending.
  • The phototropic signal is a chemical; it diffuses through permeable agar but not through impermeable mica.
  • The hormone can cause bending in darkness if allowed to diffuse into an agar block.
  • The hormone causes bending by elongating cells on the shaded side.

Light Responses: Photomorphogenesis (Section 37.3)

  • Photomorphogenesis involves changes in plant development.
  • Phytochrome Pigment: This pigment absorbs both red and far-red light and exists in two shapes, demonstrating photoreversibility.
    • Pr (phytochrome red) absorbs red light (~660 nm).
    • Pfr (phytochrome far-red) absorbs far-red light (~735 nm).
  • Far-red wavelengths are not strongly absorbed by photosynthetic pigments and can pass through leaves, indicating shade light.

The Red/Far-Red Switch (Section 37.3)

  • In lettuce seeds, red and far-red light act like an on-off switch for seed germination.
  • Discoveries:
    • Red light promotes germination.
    • Far-red light inhibits germination.
    • The last wavelength sensed by the seed determines if germination will occur.

Gravitropic Response (Section 37.4)

  • Plants exhibit gravitropic responses in both shoots and roots.
  • Statolith Hypothesis:
    • The root cap senses gravity.
    • Amyloplasts (starch storage organelles) sink to the bottom of the cell.
    • Pressure receptors (sensory proteins) are activated.

Gravity

Cell in root tip (or shoot)

Amyloplasts are pulled to bottom of cells by gravity

Activated pressure receptors

Thigmotropism (Section 37.5)

  • Thigmotropism is the response to physical contact.
  • The response can be slow or rapid.
  • Examples include:
    • Mimosa pudica
    • Growth around objects by vines and climbing plants
    • Growth in response to wind direction (e.g., coastal trees, mountain tops).

Plant Hormones (Section 37.6)

  • Plant hormones coordinate growth, development, and responses to stimuli.
  • They produce change by:
    • Altering gene expression (turning genes on or off).
    • Modifying transcription of DNA.
    • Changing cell division.
    • Transforming cell growth.
  • Each hormone has multiple effects based on:
    • Site of action
    • Concentration
    • Plant’s developmental age

Auxins (Section 37.6)

  • Auxins are produced in apical meristems.
  • They have a variety of effects:
    • Phototropic and gravitropic responses
    • Apical dominance
  • Apical dominance is the restriction of primary growth to the main stem, where lateral shoots are dormant.
    • Removing the apical meristem can release lateral shoots from dormancy.

Ethylene (Section 37.6)

  • Ethylene is the only gaseous plant hormone.
  • It is involved in plant senescence:
    • Fruit ripening
    • Flowers fading
    • Leaf abscission
  • The process of leaf abscission involves:
    1. High auxin levels in a healthy leaf.
    2. Low auxin levels trigger leaf senescence.
    3. The leaf detaches at the abscission zone, where a protective layer has formed to seal the stem.

High auxin

Low auxin

  • Ethylene is used commercially.