Signal Transduction and Plant Physiology

Key Concepts of Calcium Signalling in Plants

1. Role of Ca²⁺ in Polymerisation and Movement

• Ca²⁺ is a critical factor in regulating polymerization processes in cells.

• Cells manage Ca²⁺ gradients, crucial for maintaining cellular organization.

• Observations of pollen growth show a tip-directed Ca²⁺ gradient where red indicates high concentration and blue indicates low concentration.

2. Understanding Signal Transduction

• Signal transduction involves three critical components:

  1. A receptor that detects the stimulus.

  2. A transduction process (signal cascade) that communicates the signal.

  3. A response mechanism that executes the desired action based on the signal.

• A stimulus is usually detected externally, requiring a receptor that transforms the signal into a biologically relevant format through internal biochemical processes.

• Second messengers are essential for:

  • Amplifying the internal biochemical signal.

  • Transferring the signal to the response mechanism.

3. Guard Cells as a Model for Signalling

• stomata are hydraulic valves in the leaf epidermis that permit CO₂ entry for photosynthesis

• Guard cells regulate the size of stomata, balancing the need for CO₂ with the prevention of water loss.

• Guard cells are one of the best-characterized cellular models for signalling in eukaryotes.

4. Water Availability and Crop Production

• Water plays a vital role in global crop production.

• Significant increases in water usage for agriculture have been observed, particularly with rising global populations.

• Notably, the UK faces water scarcity issues, contrasting with Australia, which has ample water resources.

• The efficiency of stomatal regulation can directly impact water use and crop productivity, emphasizing the critical role of guard cells in plant physiology.

5. Stomatal Control Mechanisms

• Stomata can close and open based on various signals, including light and the presence of specific hormones like auxin and abscisic acid (ABA).

• Stomatal closure is overwhelmingly influenced by ABA, particularly during drought, as it overrides other stimuli.

• closing

  • Auxin (>10-5 M)

  • CO2 (>100 ppm)

  • Drought

  • Abscisic acid

  • some gases

• opening

  • Light

  • Auxin (<10-5 M)

  • CO2 (<100 ppm)

  • Cytokinins

• Ion transport in guard cells is regulated by several channel types, impacting potassium (K⁺) and chloride (Cl⁻) ions, leading to changes in turgor pressure that control stomatal aperture.

6. Charge and Ion Movement in Stomata

• ABA facilitates the movement of K⁺ and Cl⁻ ions, triggering stomatal closure through:

  • Secondary messenger rise in Ca²⁺, affecting ion channel regulation.

  • Controlled efflux of ions leading to reduced stomatal aperture.

7. Measurement and Mechanisms of Ion Channels

• Ion channels can be studied through techniques such as voltage clamping and patch clamping, allowing for analysis of their gating and function.

• These channels have selective filters and gated mechanisms for rapid ion movement, crucial for maintaining cellular ion homeostasis.

• ion channel proteins include

  • selectivity filter to separate ion species (K+, Cl-, Na+, etc.)

  • gate that opens/closes rapidly (< 1 msec)

8. Jaffe's Laws of Signalling

• Jaffe's Laws guide the understanding of signalling:

  1. The stimulus, second messenger, and response must relate in time and space

  2. Blocking the initial signal or messenger must inhibit downstream signalling and response.

  3. Introducing the second messenger without the primary stimulus must give the response downstream.

9. Calcium's Role in Signalling

• ABA increases cytosolic Ca²⁺ concentration, critical for regulating ion channels involved in stomatal closure.

• Studies using Ca²⁺ sensitive dyes (e.g., Fura2) show a rise in Ca²⁺ levels that precedes stomatal closure.

• Manipulating Ca²⁺ levels by various means can influence the response pathways in guard cells and suppress ABA action

10. Memory and Learning in Guard Cells

• Guard cells have the capability to 'remember' variations in Ca²⁺ levels, resulting in sustained effects on stomatal responses and photosynthesis.

• The protein SNARE (SYP121) has been identified as crucial for this memory function, suggesting a sophisticated level of regulatory mechanisms in plants.