Gravity, Wind, Touch

• Gravitropism

  • Move/grow in response to gravity - roots grow down
  • Root cap at root tips protect root meristem - if removed, root stops responding to gravity
  • Gravity-sensing cells containing starch granules - located at centre of root cap
  • Change in gravitation pull direction:
  • Auxin normally flows down middle of root toward perimeter + away from root cap
  • Root tipped, gravity-sensing cells redistribute auxin
  • Lower portion of root receives more auxin while upper portion receives less
  • High auxin levels slows cell elongation; upper side grows faster and root bends downward
• Statolith Hypothesis

  • Based on two interconnected ideas:
  • Amyloplasts - dense, starch-storing organelles
  • Gravity pulls amyloplasts to bottom of root cap cells
  • Position of amyloplasts activates sensory proteins (pressure sensors) in plasma membrane, initiating gravitropic response
• Venus Flytraps

  • Native to wetlands of North and South Carolina (USA)
  • Lives in nutrient poor soil (water-logged, acidic) with low nitrogen availability
  • Rapid, thigmonastic movement occur when a touch-receptor cell transducer a mechanical signal to an electrical signal - an action potential
• Venus flytraps (Part 2)

  • Action potentials move from cell-to-cell through plasmodesmata
  • When action potential reaches cells on outer surface of the leaf trap, cells change shape by swelling + push trap shut in 1/10 seconds
  • Trigger hairs must be touched in succession within 20 seconds of each other, or one hair touched twice quickly - prevents closure due to non-prey touch (raindrops)
• Thigmotropism (Part 1)

  • Movement in response to touch
  • Plant tendril contacts object and responds by wrapping around it (positive thigmotropism)
  • Provides support to plant stem + allows some plant to grow by climbing to access light
  • Helps roots grow around obstacles (negative thigmotropism)
  • Growth rate on side of stem being touched is slower than on side opposite touch - unequal distribution of auxin hormone
• Thigmotropism (Part 2)

  • Membrane depolarization pathway:
  • When touch sensed, Ca2+ channels open and Ca2+ flows into cell
  • This triggers voltage-gated Cl- and K+ channels to open, creating action potential that signals the perception of touch (mechanical signal (touch) converted to electrical signal)