15.08 Control and Coordination in Plants

Plant Communication and Response Mechanisms

• Plants coordinate responses to both internal and external factors, including:
  • Environmental Stimuli: Gravity, light, water availability.
  • Rapid Changes: Carbon dioxide levels, water loss, animal grazing, and infections.
  • Turgor Responses: Quick responses via changes in cell turgidity, such as stomatal opening in response to humidity and CO₂ levels.

Electrical Communication in Plants

• Electrochemical Gradients: Plant cells, like animal cells, have electrochemical gradients and resting potentials across their cell membranes.
• Action Potentials: Triggered by membrane depolarization, often through the outflow of Cl⁻ ions rather than Na⁺ influx (common in animals).
• Transmission: Action potentials in plants travel from cell to cell via plasmodesmata (membrane-lined channels connecting cells), generally slower and longer-lasting than in animal neurons.

Examples of Electrical Responses in Plants

1.Mimosa pudica (Sensitive Plant):

• Response: Folds its leaves upon touch.
• Mechanism: Action potentials similar to those in animals propagate, causing rapid turgor changes that fold the leaves.

Mechanism of Closure

1. Stimulus Detection:
   • Mechanical touch, vibration, or heat is sensed by specialized mechanoreceptor cells in the leaf.
2. Action Potential Generation:
   • What happens?
     • Touch causes a rapid change in the electrical charge across cell membranes (depolarization).
     • This electrical signal, known as an action potential, is similar to nerve impulses in animals.
   • How it happens?
     • Mechanosensitive ion channels open, allowing positive ions (e.g., Ca²⁺, Na⁺) to flow into the cells.
     • This creates a wave of electrical depolarization that travels along the stem and leaf tissues.
3. Turgor Pressure Loss:
   • What happens?
     • The electrical signal triggers the opening of potassium ion (K⁺) channels in pulvinus cells (at the base of the leaflets and petiole).
     • Potassium ions (K⁺) move out of the cells into the extracellular space.
     • Water follows the ions via osmosis, reducing cell turgor (pressure exerted by water inside the cells).
   • How it happens?
     • Turgid cells (firm and swollen) lose water, causing them to shrink and collapse.
     • Loss of turgor in lower cells of the pulvinus results in the rapid folding of the leaflets.
4. Leaf Movement:
   • The shrinking of cells on one side of the pulvinus and the expansion of cells on the opposite side causes the leaflets to fold inward and droop.

Recovery Mechanism:

• Potassium ions (K⁺) are actively transported back into the cells.
• Water re-enters the cells, restoring turgor pressure.
• The leaflets reopen over minutes to hours, depending on environmental conditions.

2.Venus Fly Trap (Dionaea muscipula):

• Adaptation: Carnivorous, obtaining nitrogen by digesting insects.
• Trap Structure:
  • Each leaf lobe contains three sensory hairs that trigger the trap, and the edges have stiff hairs that interlock upon closing.
  • Nectar-secreting glands attract insects.
• Stimulus and Response Mechanism:
  • Trigger Hairs: When touched, Ca²⁺ channels open, creating a receptor potential.
  • Threshold: Requires two hairs to be triggered within 20-35 seconds or repeated stimulation of the same hair to close.
  • Trap Closure: Takes <0.3 seconds, possibly from a release of elastic tension in cell walls rather than simple water movement.
• Complete Seal and Digestion:
  • Continued Stimulation: The prey’s movement further deflects hairs, ensuring trap closure.
  • Digestive Enzyme Release: Further Ca²⁺ influx triggers exocytosis of enzymes for prey digestion.
  • Duration: Trap remains closed for up to a week; digested nutrients are absorbed, and the trap resets.

Mechanism of Trap Closure

1. Stimulus Detection:
   • What happens?
     • The trap has trigger hairs on the inner surface of each lobe.
     • When a prey (insect) touches the hairs twice within 20 seconds, it generates an electrical signal.
   • How it happens?
     • The movement bends mechanosensitive hairs, opening ion channels in the cells.
     • This causes an influx of calcium ions (Ca²⁺), creating a localized depolarization.
2. Action Potential Generation:
   • What happens?
     • The depolarization from the trigger hairs propagates as an action potential across the lobes.
   • How it happens?
     • The electrical signal spreads through specialized cells in the trap lobes and hinge area.
3. Turgor Pressure and Rapid Movement:
   • What happens?
     • The action potential triggers a change in turgor pressure in the cells of the trap lobes and hinge region.
     • Cells in the outer layer of the lobes rapidly lose water, while cells in the inner layer gain water, causing a sudden shape change.
   • How it happens?
     • Ion movements (e.g., K⁺) out of and into the cells lead to water redistribution via osmosis, resulting in the inward snapping motion.
4. Trap Closure:
   • The lobes snap shut in less than 0.3 seconds, creating a cage-like structure to capture prey.
5. Sealing and Digestion:
   • If the prey struggles, it stimulates the trigger hairs further, causing the trap to seal tightly.
   • Digestive enzymes are secreted, breaking down the prey into absorbable nutrients.

Recovery Mechanism:

• After digestion (5–12 days), the trap reabsorbs water into its cells.
• The lobes reopen, ready to capture new prey.

Function:

• Venus flytrap uses this mechanism to obtain nutrients (e.g., nitrogen) in nutrient-poor environments, such as bogs.

Adaptive Mechanisms to Conserve Energy

• Avoiding False Triggers:
  • Single hair stimulation does not activate the trap, preventing unnecessary closure from rain or debris.
  • The gaps between interlocking hairs allow very small prey to escape, conserving energy by avoiding digestion of tiny, low-nutrient meals.

Key Terms

• Turgor Response: Rapid plant response through changes in cell water pressure (turgor).
• Electrochemical Gradient: Difference in ion concentration across a membrane, crucial for generating action potentials.
• Plasmodesmata: Channels between plant cells enabling direct cell-to-cell communication.
• Mimosa pudica: Sensitive plant responding to touch by leaf folding.
• Venus Fly Trap: Carnivorous plant with specialized leaves for insect trapping and digestion.