Plant Environmental Responses and Defense Mechanisms

Environmental Responses and Defense Mechanisms

Tropism

  • Definition: Directional growth in response to a stimulus.
  • Positive Tropism: Growth towards the stimulus.
  • Negative Tropism: Growth away from the stimulus.

Phototropism

  • Definition: Directional growth in response to light.
  • Photomorphogenesis: Growth and development of plants in response to light.
  • Mechanism:
    • The shoot tip detects light and induces phototropism.
    • Phot1 & Phot2 are involved.
    • The hormone auxin from the shoot tip stimulates elongation on the shaded side of the stem.
    • Cholodny-Went Hypothesis: Lateral redistribution of auxin causes growth.
    • pH, proton concentration, and sugar influx are related to this process.

Gravitropism

  • Definition: Directional growth in response to gravity.
  • Mechanism:
    • Auxin and specialized amyloplasts called statoliths mediate gravitropism.
    • Statoliths (amyloplasts) fall to the bottom of cells, activating pressure-sensitive receptors, directing growth downward.
    • Auxin distribution in the root tip changes as a result of gravity, accumulating in the direction of gravity.
    • Higher auxin distribution in roots inhibits cell elongation, while lower auxin distribution promotes cell elongation.
    • When amyloplasts settle to the bottom of gravity-sensing cells, they physically contact the endoplasmic reticulum (ER), causing the release of calcium ions from inside the ER.
    • Calcium signaling causes auxin transport proteins (PIN proteins) to redistribute to the underside of the cell, leading to the polar transport of auxin to the bottom of the cell.
    • In roots, a high concentration of indoleacetic acid (IAA) inhibits cell elongation.
    • In shoots, a higher concentration of IAA stimulates cell expansion, causing the shoot to grow up.
    • After the shoot or root begins to grow vertically, the amyloplasts return to their normal position.

Hydrotropism

  • Definition: Directional root growth toward water.
  • Gravitropism is often dominant over hydrotropic responses.
  • Hydrotropism does not result from an auxin gradient resulting from directional auxin transport, but auxin still plays a role in signaling.

Thigmotropism, Thigmomorphogenesis, and Thigmonastic Movements

  • Thigmotropism: The movement of a plant subjected to constant directional pressure.
    • From the Greek words