plant responses to internal and external signals

Chapter 39: Plant Responses to Internal and External Signals

1. Stimuli and a Stationary Life

  • Reception of Environmental Signals:
    • Plants perceive signals from their surroundings and adapt their growth and development accordingly.
    • Example: The bending of a dodder seedling toward a host plant is a response to chemicals released by the host.

2. Factors that Plants Sense and Respond To

  • Plant Interaction with the Environment:
    • Plants actively sense and integrate environmental information rather than being passive.
    • Plant development is simple; however, the complexity of their cells and molecular biology parallels that of animals.
    • Animals generally move in response to environmental changes, while plants alter their growth or development.

3. Signal Transduction Pathways Overview

  • Function of Signal Transduction:
    • Signal transduction pathways connect the reception of a signal to a response.
    • Example of Etiolation:
      • Potatoes grown in darkness exhibit pale stems, unexpanded leaves, and short roots, a phenomenon called etiolation.
      • Upon exposure to light, potatoes transition to a process called de-etiolation.
      • Morphological Changes Post Light Exposure:
        • After light exposure, shoots and roots grow normally and develop chlorophyll.

4. Cell Signal Processing: Stages

  • Stages of Response:
    1. Reception:
      • Signals are detected by specialized receptors that change shape in response to stimuli.
      • Example of receptor: Phytochrome for detecting light during de-etiolation.
    2. Transduction:
      • Second messengers amplify signals to proteins that initiate responses:
        • Key Second Messengers: Calcium ions ($Ca^{2+}$), cyclic GMP ($cGMP$).
        • Phytochrome opens Ca$^{2+}$ channels, increasing intracellular Ca$^{2+}$ levels, and activates an enzyme to produce cGMP.
    3. Response:
      • Involves regulating one or more cellular activities, often through increased enzyme activity, by either transcriptional regulation or post-translational modification.

5. Post-translational Modification of Proteins

  • Mechanisms of Modification:
    • Post-translational modifications involve altering existing proteins in the damage response, commonly through phosphorylation.
    • Both cGMP and Ca$^{2+}$ work to directly activate protein kinases, forming a cascade linking initial stimuli to gene expression via transcription factor phosphorylation.
    • Protein phosphatases deactivate signaling pathways by dephosphorylating target proteins.

6. Transcriptional Regulation by Plant Hormones

  • Role of Transcription Factors:
    • Specific transcription factors bind to DNA and modulate transcription:
      • Activators enhance gene transcription.
      • Repressors diminish transcription.
  • De-etiolation Proteins Impact:
    • Activated enzymes play roles in photosynthesis, chlorophyll production, and hormone regulation.

7. Overview of Plant Hormones

  • General Properties of Plant Hormones:
    • Chemical signals modifying physiological processes in low concentrations while having substantial effects.
    • Their impact on plant responses depends on hormone concentration, presence, and interaction.

8. Major Plant Hormones

  • Types of Plant Hormones:
    • Auxin, Cytokinins, Gibberellins, Abscisic acid (ABA), Ethylene, Brassinosteroids, Jasmonates, Strigolactones.

9. Auxin Overview

  • Functionality of Auxin:
    • Tropism: Movement of plant organs toward or away from stimuli.
    • Historical Context:
      • Charles Darwin and Francis Darwin's work on phototropism led to the understanding that the tip of the plant transmits signals to growth zones.
      • Peter Boysen-Jensen established that a chemical, rather than electrical, signal was responsible for movement.
  • Experiments on Phototropism:
    • Control setups demonstrated the importance of the upper tip of the coleoptile in curvature responses to light.

10. Auxin’s Role in Elongation

  • Acid Growth Hypothesis:
    • Auxin activates proton pumps which lower cell wall pH and increase membrane potential.
    • Activation of expansins facilitates cell wall loosening, leading to water uptake and increased turgor, allowing cell elongation.

11. Influences of Auxin on Development

  • Polar Transport:
    • Auxin influences growth patterns, leaf arrangement, and vascular cambium activity.
    • Practical applications include the use of indolebutyric acid (IBA) for plant propagation and synthetic auxins in horticulture.

12. Cytokinins Overview

  • Cytokinin Functions:
    • Stimulates cell division and differentiation, produced primarily in actively growing tissues.
    • Works alongside auxin to regulate growth processes.

13. Gibberellins Overview

  • Impact on Growth:
    • Stimulates elongation and division in leaves and stems, inducing rapid growth in flowering stalks (bolting).
    • Plays a significant role in seed germination alongside auxins.

14. Abscisic Acid (ABA)

  • Functions of ABA:
    • Slows growth, promotes seed dormancy, and provides drought tolerance by facilitating reduced transpiration through stomatal closure.

15. Ethylene Overview

  • Responses to Stress:
    • Produced in response to stress (e.g., drought, injury) and governs processes like senescence and fruit ripening.
    • Changes in auxin-ethylene balances affect leaf abscission.

16. Recent Discoveries in Plant Hormones

  • Brassinosteroids:
    • Promote cell elongation and division, support xylem differentiation.
  • Jasmonates:
    • Involved in various physiological processes, including responses to wounding.

17. Light Responses in Plants

  • Photomorphogenesis:
    • Plants respond to light for growth and development, recognizing presence, intensity, and quality.
    • Importance of action spectra in studying light-mediated processes.

18. Blue-Light and Phytochrome Photoreceptors

  • Role of Blue-Light Photoreceptors:
    • Initiate responses like stomatal opening and hypocotyl elongation.
  • Phytochrome Functions:
    • Encompasses many light responses, including de-etiolation and shade avoidance.

19. Phytochromes and Seed Germination

  • Effects of Light on Seed Dormancy:
    • Red light enhances germination, whereas far-red light inhibits it.

20. Biological Clocks and Circadian Rhythms

  • Understanding Biological Cycles:
    • Circadian rhythms, governed by phytochrome transitions, help regulate daily physiological cycles.

21. Photoperiodism and Flowering Control

  • Photoperiodism Defined:
    • Physiological responses to day/night length, crucial in determining flowering time.

22. Critical Night Length Impact

  • Research Discoveries:
    • Night length impacts flowering, with critical periods determining responses in short and long day plants.

23. Plants’ Non-Light Responses

  • Responses to Gravity:
    • Gravitropism dictates root and shoot responses to gravity via statolith movement.

24. Mechanical Stimuli Impact

  • Thigmomorphogenesis:
    • Adaptations to mechanical disturbances, affecting growth patterns.

25. Environmental Stress Responses

  • Types of Stress:
    • Biotic stress (herbivores, pathogens) and abiotic stress (drought, flooding, salt, heat).
  • Drought Management:
    • Stomatal closure reduces water loss; ABA plays a critical role.

26. Defense Mechanisms Against Pathogens

  • Immune Responses:
    • Two immune responses: PAMP-triggered immunity and effector-triggered immunity.

27. Defenses Against Herbivores

  • Levels of Defense:
    • Defense strategies operate at the molecular, cellular, tissue, organ, and organismal levels.
    • Community-level defenses include attracting predators against herbivores.