In-Depth Notes on Plant Responses to Internal and External Signals

Plants vs. Animals: Unlike animals that respond through movement, plants respond by altering growth and development.
Example: The dodder (Cuscuta) plant exhibits growth toward a host by detecting chemicals released by the host.

Definition: Plant hormones are chemical signals that modify or control physiological processes in plants.
- Work in low concentrations but can significantly influence plant growth.
- Regulate most aspects of plant growth and development.
- Individual hormones can have multiple effects; interactions between hormones can influence single processes.

Tropism: Curvature of plant organs in response to stimuli, e.g., phototropism (growth towards light).
Key Experiments:
- Darwins' work (1880): Demonstrated importance of coleoptile tip for phototropic response.
- Boysen-Jensen (1913): Established that the signal for phototropism is a chemical substance.
- Frits Went (1926): Extracted auxin, critical for phototropism, using agar blocks with coleoptile tips.

Auxin:
- Promotes elongation of coleoptiles.
- Mainly indoleacetic acid (IAA) produced in shoot tips.
- Mechanism: Promotes cell elongation via the acid growth hypothesis, which operationalizes proton pumps to facilitate cell wall loosening through expansins.
- Uses: Enhances fruit development, root growth, and helps regulate plant architecture (e.g., apical dominance).
- Practical Applications: Used in agriculture to stimulate growth; can be toxic at high levels (e.g., 2,4-D herbicide).
Cytokinins:
- Stimulates cytokinesis (cell division) and affects cell differentiation.
- Mechanism: Produced in growing tissues; ratios of cytokinins to auxins influence differentiation processes, including root and shoot formation.
Gibberellins (GA):
- Influence growth and development including stem elongation, fruit growth, and seed germination.
- Mechanism: Produced in young tissues; when applied to plants, they can enhance growth velocity and fruit size (e.g., Thompson Seedless grapes).
Abscisic Acid (ABA):
- Slows growth, promotes seed dormancy, and aids drought tolerance.
- Mechanisms: Induces stomatal closure during drought; role in seed dormancy through chemical signaling.
Ethylene:
- Involved in responses to stress (e.g. drought, flooding) and influences processes such as senescence, leaf abscission, and fruit ripening.
- Triple Response to Mechanical Stress: Ethylene mediates a growth pattern allowing plants to circumvent obstacles.

Etiolation and De-Etiolation: Morphological adaptations when plants grow in darkness (etiolation) and changes upon exposure to light (de-etiolation).
Photoreceptors: Plants have photoreceptors like phytochromes (mainly sensing red light) and blue-light photoreceptors for phototropism and other light responses.
Photoperiodism: Plants use changes in day length to time flowering. Short-day plants flower when nights exceed a critical length, while long-day plants flower when nights are shorter.

Gravitropism: Roots exhibit positive gravitropism (grow down) while shoots exhibit negative gravitropism (grow up) through detection mechanisms involving statoliths.
Drought Responses: Reducing water loss through stomatal closure, leaf shedding.
Flooding Adaptations: Production of ethylene contributing to air tube formation to aid submerged roots.
Salt and Temperature Stresses: Various physiological changes to prevent damage from extreme environmental conditions aimed at maintaining cellular homeostasis.

Herbivory and Responses: Physical defenses (thorns, trichomes), chemical deterrents, and recruitment of predators.
Pathogen Defense Mechanisms: Capacity for localized immune responses (PAMP-triggered immunity) and systemic responses involving salicylic acid (Systemic Acquired Resistance) to develop long-term resistance against threats.
Hypersensitive Response: Programmed cell death at infection site to limit pathogen spread.