Plant Responses to Internal and External Signals — Hormones, Signal Transduction, and Adaptation Explained | Chapter 39 of Campbell Biology

Plants are dynamic organisms capable of perceiving and responding to an array of internal and external signals—light, gravity, temperature, touch, water, and threats from herbivores and pathogens. Chapter 39 of Biology reveals the intricate signaling pathways and hormonal controls that allow plants to grow, adapt, and survive in constantly changing environments. This chapter guides you through the fundamental mechanisms of signal transduction, hormone function, photoreceptor activity, and plant responses to biotic and abiotic stress.

Watch the full podcast summary below, then continue reading for key concepts, glossary terms, and real-world examples of plant adaptation and defense.

Introduction: How Plants Sense and Respond

Plants rely on a sophisticated system of receptors, hormones, and signal transduction pathways to detect environmental cues and coordinate growth, development, and defense. Unlike animals, plants cannot move away from threats, so they rely on internal communication and flexible growth strategies to thrive.

Plant Hormones and Signal Transduction

• Auxin (IAA): Promotes stem elongation, root initiation, and directional growth (phototropism, gravitropism).
• Cytokinins: Stimulate cell division and shoot growth, often acting antagonistically to auxin.
• Gibberellins: Promote stem elongation, seed germination, and fruit development.
• Abscisic acid (ABA): Inhibits growth, induces seed dormancy, and closes stomata during drought.
• Ethylene: Gaseous hormone controlling fruit ripening, leaf abscission, and response to mechanical stress.
• Brassinosteroids: Steroid hormones stimulating cell expansion and division.
• Jasmonates: Regulate plant defense against herbivores and pathogens.

Hormones act through complex signal transduction pathways, where receptors perceive signals, transduce them via secondary messengers, and trigger specific gene expression or cellular responses.

Photoreceptors, Biological Clocks, and Plant Movements

• Phytochromes & Cryptochromes: Light receptors regulating seed germination, shade avoidance, and photoperiodic flowering.
• De-etiolation (Greening): Rapid plant development after exposure to light.
• Photoperiodism: Regulation of flowering and other processes based on day length, involving the plant's biological clock and circadian rhythms.
• Gravitropism & Thigmotropism: Growth in response to gravity and touch; roots grow downward, shoots upward, and tendrils coil around supports.
• Thigmomorphogenesis: Growth changes due to mechanical perturbation like wind or touch.

Stress Responses and Defense Mechanisms

• Abiotic Stress: Plants adjust water potential, close stomata, and synthesize protective proteins in response to drought, flooding, or temperature extremes.
• Biotic Stress: Plants activate defense pathways (systemic acquired resistance, jasmonate signaling) against pathogens and herbivores.
• Ethylene: Plays a central role in wound response and fruit ripening.

Key Glossary Terms

• Abscisic acid (ABA): Growth inhibitor, regulates stomatal closure, seed dormancy
• Auxin (IAA): Promotes elongation and root formation
• Brassinosteroids: Steroid hormones for cell expansion/division
• Cytokinins: Promote cell division, shoot growth
• De-etiolation: Plant "greening" after light exposure
• Ethylene: Fruit ripening, leaf abscission, stress responses
• Gravitropism: Growth in response to gravity
• Jasmonates: Hormones for defense against herbivores/pathogens
• Photoperiodism: Biological response to seasonal changes in day length
• Phytochromes: Red/far-red light receptors regulating development
• Signal Transduction Pathway: Sequence from signal detection to response
• Systemic Acquired Resistance (SAR): Plant-wide immune response to pathogens
• Thigmomorphogenesis/Thigmotropism: Growth responses to touch/mechanical stimuli
• Water Potential: Determines water movement in plants
• Zygote: Fertilized egg cell

Conclusion: The Adaptability and Resilience of Plants

Plants are constantly challenged by their environment, but through intricate signaling networks, hormonal controls, and responsive behaviors, they adapt and survive. Mastery of these mechanisms is vital for plant science, agriculture, and understanding the evolution of life on Earth.

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