Plant Sensory Systems Detailed Notes

Plant Sensory Systems

Instructor: Dr. Lorena Torres Martínez

Objectives

  • Describe plant responses to light
  • Explain how plants respond to stimuli
  • Explain plant defenses against herbivores and pathogens

Key Concepts

Active Nature of Plants

  • Plants are not inert; they actively sense environmental changes.
  • Unlike animals that respond with movement, plants alter their growth and development in response to stimuli.
  • Plant genomes can be significantly larger than those of animals (up to three times).

Signal Transduction Pathways

  • Cell signaling involves a series of stages that include: reception, transduction, and response.

Reception of Signals

Steps in Signal Processing

  1. Reception: Signals are received by receptors like phytochromes, which are activated by light.
  2. Transduction: Signals are processed in the cytoplasm, involving molecules like Ca2+ and cGMP, leading to the activation of protein kinases.
  3. Response: Final actions occur within the nucleus, resulting in changes such as gene expression for growth responses like de-etiolation (greening).

Tropisms

  • Definition: Growth responses involving the bending or curving of plant parts towards or away from stimuli.
Types of Tropisms
  1. Phototropism: Response to light.
  2. Gravitropism: Response to gravity.
  3. Thigmotropism: Response to mechanical stimuli.

Responses to Light

  • Photomorphogenesis: Light-triggered developmental processes.
  • Phototropisms: Directional growth responses to light, crucial for seed germination.
  • Pigments: Molecules that absorb light, some of which are involved in photosynthesis while others help detect light.

Blue-light Receptors

  • Cryptochromes: Inhibit stem elongation in response to blue light.
  • Phototropin: Mediates stomatal opening and chloroplast movement.

Phytochromes

  • Exist in two reversible forms affected by red and far-red light.
  • Pr (inactive form): Absorbs red light.
  • Pfr (active form): Absorbs far-red light and triggers various responses like seed germination and branching.

Mechanism of Phytochrome Action

  • The amount of Pfr is regulated by degradation; ubiquitin tags Pfr for transport to the proteasome for recycling.
  • When Pr is converted to Pfr, it facilitates transcription of light response genes, affecting growth and development.
  • Pfr is essential for detecting plant crowding based on light reflected from neighboring plants.

Gravitropism

  • This is the plant's response to Earth's gravitational field.
  • Positive gravitropism: Roots grow downward.
  • Negative gravitropism: Shoots grow upward.
Mechanism of Gravity Perception
  • In shoots, endodermal cells surrounding vascular tissue sense gravity.
  • In roots, gravity is perceived in the root cap, leading to differential cell elongation.

Statoliths and Amyloplasts

  • Amyloplasts (statoliths) are starch-containing plastids that help perceive gravity.
  • They settle on the lower side of root cap cells when the plant's orientation changes, influencing growth patterns.

Touch Responses

Thigmomorphogenesis

  • Permanent form changes in response to mechanical stress.

Thigmotropism

  • Directional growth in response to contact (e.g., climbing plants).

Thigmonastic Responses

  • Quick movements independent of stimulus direction, often caused by turgor changes in motor cells.

Nastic Movements

  • Movements that occur in response to stimuli but are not dependent on the stimulus's direction.
  • Nyctinastic Movements: Night closure of leaves, usually caused by turgor changes in pulvinus cells.

Conclusion

  • Plant sensory systems highlight the complexity and responsiveness of plants, making them active participants in their environments. Understanding these systems is crucial for recognizing how plants interact with light, gravity, and touch.