Plant Responses to Internal and External Signals (Chapter 39) – Study Guide

Plant Responses to Internal and External Signals

Overview

This chapter explores how plants perceive and respond to a variety of internal and external signals. Key topics include plant hormones, signal transduction pathways, and the physiological and developmental processes regulated by these signals.

Plant Hormones: Types and Functions

• Definition: Plant hormones (phytohormones) are chemical messengers that regulate plant growth, development, and responses to stimuli.

• Main Hormones: Auxins, gibberellins, cytokinins, abscisic acid (ABA), ethylene, brassinosteroids, jasmonates, and strigolactones.

• Functions:

  • Auxins: Promote cell elongation, apical dominance, and root initiation.

  • Gibberellins: Stimulate stem elongation, seed germination, and flowering.

  • Cytokinins: Promote cell division and delay leaf senescence.

  • Abscisic Acid (ABA): Induces seed dormancy and helps plants respond to stress (e.g., drought).

  • Ethylene: Regulates fruit ripening and responses to mechanical stress.

• Example: Application of auxin to a plant stem can cause it to bend toward light (phototropism).

Signal Transduction in Plants

• Definition: Signal transduction is the process by which a plant cell converts a signal (such as a hormone or environmental cue) into a specific cellular response.

• Steps:

  1. Reception: Signal molecule binds to a receptor.

  2. Transduction: Relay molecules amplify and transmit the signal.

  3. Response: Activation of cellular processes (e.g., gene expression, enzyme activity).

• Example: Light perception by photoreceptors triggers a cascade leading to phototropic growth.

Photoreceptors and Light Responses

• Photoreceptors: Proteins that absorb light and initiate plant responses.

• Main Types: Phytochromes (red/far-red light), cryptochromes, and phototropins (blue light).

• Processes Regulated: Seed germination, shade avoidance, flowering, and circadian rhythms.

• Example: Phytochrome-mediated seed germination in response to red light.

Tropisms and Nastic Movements

• Tropism: Directional growth response toward or away from a stimulus.

• Types:

  • Phototropism: Growth toward light (mediated by auxin).

  • Gravitropism: Growth in response to gravity (roots show positive gravitropism, shoots show negative).

  • Thigmotropism: Growth in response to touch (e.g., tendrils wrapping around support).

• Nastic Movements: Non-directional responses to stimuli (e.g., closing of Venus flytrap).

Plant Responses to Biotic and Abiotic Stress

• Abiotic Stress: Drought, extreme temperatures, salinity, etc.

• Biotic Stress: Pathogen and herbivore attack.

• Hormonal Responses: ABA mediates drought response; jasmonates and salicylic acid mediate defense against herbivores and pathogens.

• Example: ABA triggers stomatal closure to reduce water loss during drought.

Seed Dormancy and Germination

• Seed Dormancy: A period during which seeds do not germinate, even under favorable conditions; often regulated by ABA.

• Germination: Initiated by water uptake and often promoted by gibberellins, which stimulate enzyme production for seedling growth.

• Example: Application of gibberellins can break dormancy in some seeds.

Plant Growth and Developmental Processes

• Cell Elongation: Driven by auxin-induced loosening of the cell wall.

• Cell Division: Promoted by cytokinins, especially in meristematic tissues.

• Senescence: The aging process in plants, delayed by cytokinins and accelerated by ethylene.

Summary Table: Major Plant Hormones and Their Functions

Hormone	Main Functions	Example
Auxin	Cell elongation, apical dominance, root initiation	Phototropism, root formation
Gibberellin	Stem elongation, seed germination, flowering	Breaking seed dormancy
Cytokinin	Cell division, delay of senescence	Promoting shoot growth in tissue culture
Abscisic Acid (ABA)	Seed dormancy, stress responses (e.g., drought)	Stomatal closure during drought
Ethylene	Fruit ripening, leaf abscission, response to mechanical stress	Ripening of bananas

Key Equations and Concepts

• Water Potential ($\Psi$): Determines the direction of water movement in plants. $\Psi = \Psi_s + \Psi_p$ Where $\Psi_s$ is solute potential and $\Psi_p$ is pressure potential.

• Photoperiodism: The physiological reaction of organisms to the length of day or night, important in flowering.

Additional info:

• For exam preparation, focus on understanding the mechanisms of hormone action, the integration of multiple signals, and the adaptive significance of plant responses to their environment.

• Be able to apply concepts to novel scenarios, such as predicting plant responses to environmental changes or genetic mutations affecting hormone pathways.