Plant Responses to Internal and External Signals

Plant Responses to Internal and External Signals

Introduction to Plant Responses

• **Factors that Plants Sense and Respond To: **

  • Internal Chemical Signals

  • External Environmental Signals:

  • Light

  • Heat or Cold Stress

  • Touch by Wind or Objects

  • Drought or Flooding

  • Time (Daylength and Seasons)

  • Gravity

  • Wounding by Herbivores

  • Infection by Pathogens

Concept 31.1: Sensitivity to Environmental Stimuli

• Plants must integrate environmental information for competition and survival.

• Cell Signaling Steps:

  1. Reception: Receptor proteins detect external stimuli.

  2. Transduction: Initiation of signal transduction pathways.

  3. Response: Execution of specific behaviors based on the signal.

• Receptor Proteins:

  • Unique to plants and some similar to eukaryotes/cyanobacteria.

• Signal Transduction Molecules:

  • Shared with other eukaryotic cells but function differently in plants (e.g., cyclic nucleotides affecting ion channels in plants instead of protein kinases as in animals).

• Behavior Dynamics:

  • Animals respond with movement; plants modify growth and development.

  • Both can exhibit comparable responses based on resource availability (e.g., parasitic dodder moving to new hosts when nutrient quality declines).

Concept 31.2: Role of Plant Hormones

• Hormones: Signaling molecules produced in small quantities at one site and transported to another.

  • Each hormone can produce diverse effects based on:

  • Site of action

  • Concentration

  • Developmental stage

  • Interaction between multiple hormones influences growth processes.

The Discovery of Plant Hormones

• Tropisms: Curvatures of plant organs in response to stimuli.

  • Phototropism: Curvature towards or away from light.

  • Positive Phototropism: Shoots curve towards light.

  • Negative Phototropism: Roots curve away from light.

• Early Experiments on Phototropism:

  • In grass seedlings, curvature only occurs with the coleoptile tip intact.

  • Covering the coleoptile tip results in no phototropic curvature.

• Discovery of Auxin:

  • A signal transmitted from the coleoptile tip induces growth in location of curvature.

  • Experiments found that chemotrophic responses were contingent on allowing chemical movement through barriers.

• Extracting substances from the coleoptile tip showed that auxin leading to differential growth concentrations leads to the curvature.

Survey of Plant Hormones

• The five classic plant hormones:

  • Auxin

  • Cytokinins

  • Gibberellins

  • Abscisic acid (ABA)

  • Ethylene

Auxin Overview

• Auxin: Promotes elongation of coleoptiles, primarily produced in shoot tips.

  • Indoleacetic Acid (IAA): Major natural auxin in plants.

  • Polar Transport: Auxin movement down from shoot tip to base through transporter proteins concentrated in cells.

Role of Auxin in Cell Elongation

• Binds to nuclear receptors and stimulates elongation within specific concentration ranges.

• Acid Growth Hypothesis:

  • Auxin stimulates proton pumps leading to a decrease in pH, activating expansins, which loosen cell wall links.

• Increased water uptake associated with cell wall loosening elevates turgor pressure facilitating elongation.

• Auxin also stimulates rapid protein production in the zone of elongation.

  • Produces materials for cell wall development to support continuous growth.

Auxin and Plant Development

• Polar transport of auxin facilitates spatial organization in growing plants, controlling branching and leaf emergence from the shoot system.

• Apical Dominance: Apical bud is the primary auxin source, suppressing the growth of axillary buds.

  • Removal of the apical bud results in bushier growth due to reduced inhibition of axillary buds.

Practical Implications of Auxins

• Used in agriculture:

  • Induces fruit production, root growth in cuttings (e.g., using IBA), and selective herbicides (2,4-D) for weed management.

Cytokinins Overview

• Cytokinins: Stimulate cell division (cytokinesis), differentiation, and apical dominance, with Zeatin being the most common natural cytokinin.

Cell Division and Differentiation

• Produced in actively growing tissues and collaborate with auxin in promoting cell division in callus tissue.

• Cytokinins vs Auxin Ratio:

  • The balance between cytokinins and auxin ratios influences differentiation processes, promoting shoots or roots based on concentration changes.

Control of Apical Dominance by Cytokinins

• Controlled by sugar and hormone levels, where removal of the apical bud boosts axillary bud availability by shifting sugar dynamics and altering hormonal interactions.

Gibberellins Overview

• Effects: Promote stem elongation, fruit growth, and seed germination; over 100 types exist naturally in plants.

Stem Elongation

• Gibberellins stimulate growth primarily in young roots and leaves to enhance cell elongation and division.

• Induced bolting case (rapid flower stalk growth) illustrates their impact.

Fruit Growth

• Auxin and gibberellins work together for optimal fruit development.

• Common application in grape cultivation for size enhancement and internode elongation.

Germination Process

• Water absorption leads to gibberellin release, signaling seeds for germination.

• Treatment can bypass dormancy requirements, facilitating metabolic processes in seed germination.

Abscisic Acid (ABA) Overview

• Antagonistic to growth hormones, slowing growth, inducing seed dormancy, and drought responses.

Seed Dormancy and Drought Tolerance

• ABA concentrations rise during seed maturation, inhibiting early germination, and facilitating protective protein production.

• In drought conditions, ABA accumulation leads to stomatal closure to reduce water loss.

Ethylene Overview

• Produced under stressors (drought, flooding, damage, infections), and naturally during ripening and cell death processes.

Effects of Ethylene

• Induced physiological responses include:

  • Mechanical stress response

  • Senescence

  • Leaf abscission

  • Fruit ripening

The Triple Response to Mechanical Stress

• Ethylene prompts a triple response enabling shoots to navigate obstacles:

  1. Stem elongation slowing

  2. Stem thickening

  3. Horizontal growth until the obstacle is cleared

• Varied mutant models elucidate distinction in ethylene response mechanisms.

Ripening and Abscission Processes

• Ethylene's role in senescence and leaf drop through an increased ratio of ethylene to auxin; critical in ripening coordination across fruits.

Concept 31.3: Importance of Light in Plant Adaptation

• Light is crucial for growth and development, with photomorphogenesis describing plant responses to light signals.

Photomorphogenesis Characteristics

• Etiolation: Morphological responses in darkness (e.g., elongated stems, pale leaves).

• Exposure to light initiates de-etiolation, normalizing cellular structures for optimized growth.

• Plants detect light qualities such as:

  • Presence

  • Direction

  • Intensity

  • Wavelength

• Action Spectrum: Graph depicting responsiveness to different wavelengths, vital for understanding processes like phototropism.

Photoreceptors and Light Types

• Two main photoreceptor classes in plants:

  • Blue-light photoreceptors

  • Phytochromes (red light absorbents)

Phytochrome Photoreceptors

• Regulate diverse light-induced reactions, influencing germination and shade adaptation.

• Seeds often rely on specific light wavelengths for germination, as demonstrated with lettuce seeds.

• Phytochrome States:

  • Two forms: and

  • Light-induced interconversion influences numerous growth processes leading to successful adaptation (e.g., germination, branching dynamics).

Biological Clocks and Circadian Rhythms

• Many plant processes operate on a circadian rhythm of about 24 hours, including:

  • Stomata opening and closing

  • Photosynthetic enzyme production

  • Sleep movement in legumes

• Circadian Rhythms: Governed by internal clocks that can adjust based on environmental cues; impacted through phytochrome activity.

Responses to Environmental Stimuli

• Plants adapt to abiotic (drought, flooding) and biotic (herbivores, pathogens) stressors through physiological developments.

Drought Responses

• Key mechanisms include stomatal closure, reduced leaf area, and water conservation approaches.

Flooding Responses

• Formation of air tubes via ethylene pathways permits oxygen access to submerged roots.

Salt Stress

• Solutions include synthesis of osmoprotective solutes prevalent in halophytes or species adapted to saline conditions.

Heat and Cold Stress

• Heat-shock proteins aid in thermal resilience; cold adaptations may involve membrane fluidity enhancements and antifreeze protein synthesis.

Concept 31.5: Plant Defenses Against Herbivores and Pathogens

• Plants utilize complex defense strategies to deter both herbivorous and microbial attackers.

Defense Mechanisms

• Against Herbivores:

  • Physical (thorns, trichomes)

  • Chemical (toxic compounds)

  • Behavioral (attracting predators of herbivores)

• Upon Encountering Pathogens:

  • Physical barriers (epidermis, periderm) deter entry, while biochemical responses (PAMP-triggered immunity and effector-triggered immunity) mobilize defense.

Immune Responses

• PAMP-Triggered Immunity: Targets recognized PAMPs leading to heightened antimicrobial chemical production.

• Effector-Triggered Immunity: Allows plants to respond against evader pathogens through signal transduction and localized defense propagation.

• Hypersensitive Response: Localized cell death around infection areas helps confine pathogens.

• Systemic Acquired Resistance: Enables long-lasting defense response through systemic signaling molecules (e.g., methylsalicylic acid).

Summary of Plant Hormone Responses

Summary of Environmental Stress Responses

Conclusion

• Plant responses to various stimuli are critical for survival and adaptation, driven by complex hormonal interactions and modifications in growth and defense strategies against abiotic and biotic challenges.