12: Plant Response to Gravity, Wind, + Touch

• Gravitropism: move in response to gravity
• Root cap at root tips protect root meristem
• Root stops responding to gravity if root cap is removed
• Gravity sensing: root cap cells containing starch granules
• Statolith hypothesis:

  • Based on 2 interconnected ideas:

  1. Amyloplasts - dense, starch-storing organelles
  2. Gravity pulls amyloplasts to the bottom of root cap cells
• Amyloplasts activate sensory proteins (pressure sensors) in the plasma membrane, initating gravitropic responses
• When root-cap cells detect change indirection of gravitational pull:

  1. Auxin normally flows down middle of the root
  2. Root is moved into horizontal position
  3. Gravity-sensing cells redistribute more auxin to bottom portion of the root
  4. High auxin levels slows cell elongation; the upper side grows faster and root bends downward
• Auxin accumulates on shaded side.

  • In shoot cells, auxin stimulates elongation, bending towards light
  • In root cells, auxin slows elongation, bending away from light
• Plants respond to mechanical stress (wind + touch)

  • Produce short stiff stems
  • Directional growth moves plants toward or away from contact with an object
• Thigmotropism: movement in response to touch
• Positive thigmotropism: grows towards the object it's touching
• Negative thigmotropism: grows around object it's touching
• Growth rate on the side of shoot being touched is slower due to unequal distribution of auxin --> growth towards the object, attaching + curling around it
• Thigmotropism + the membrane depolarization pathway:

  • When touch is sensed, Ca2+ channels open and Ca2+ flows into the cell
  • This triggers voltage-gated Cl- and K+ channels to open, creating an action potential that signals the perception of touch
  • Mechanical signal (touch) is converted to an electrical signal
• Thigmotropism Ex. Venus Fly Trap

  • Rapid thigmonastic movement occurs when a touch-receptor cell transduces a mechanical signal to an electrical signal, an action potential
  • Action potentials move from cell-to-cell through plasmodesmata
  • When the action potential reaches cells on the outer surface of the leaf trap, pores open --> allowing water to enter --> cells swell + push the trap shut in 1/10 of a second
• Membrane Depolarization when Touch is Sensed by Hair Cells:

  Phase 0: rest
• Membrane Depolarization when Touch is Sensed by Hair Cells:

  Phase 1: depolarization from opening of Ca2+ channels + activated release of Ca2+ from internal stores
• Membrane Depolarization when Touch is Sensed by Hair Cells:

  Phase 2: Activation of outward K+ channels, slowing of initial rapid depolarization
• Membrane Depolarization when Touch is Sensed by Hair Cells:

  Phase 3: peak depolarization dominated by outward K+ channels
• Membrane Depolarization when Touch is Sensed by Hair Cells:

  Phase 4: initial repolarization (due to K+ that has moved out of the cell)
• Membrane Depolarization when Touch is Sensed by Hair Cells:

  Phase 5: final repolarization phase + hyperpolarization overshoot
• Membrane Depolarization when Touch is Sensed by Hair Cells:

  Phase 6: further hyperpolarization is prevented by voltage-activation of inward K+ channels, H+ coupled anion, + K+ transporters, restoring cellular K+ and Cl- levels