CH 39: Plant Responses to Internal and External Signals Study Notes

Signal Transduction Pathways in Plants

• Plants are dynamic organisms that sense and integrate many environmental signals. While plant development is often viewed as simple, their cellular and molecular biology is as complex as that of animal cells.

• Unlike animals, which respond to environmental changes via movement, plants respond by altering their growth and development.

• Etiolation: A potato or seedling grown in total darkness produces pale stems, unexpanded leaves, and short roots. These are morphological adaptations for growing in the dark.

• De-etiolation (Greening): Upon exposure to light, the plant undergoes changes where shoots and roots grow normally, and the plant begins to produce chlorophyll.

• The Three Stages of Cell Signal Processing:
      - Reception: Signals are detected by receptors, which are proteins that change shape in response to specific stimuli. In de-etiolation, the light-detecting receptor is phytochrome.
      - Transduction: Second messengers transfer and amplify signals from the receptor to proteins that cause the response. De-etiolation requires two second messengers: calcium ions ($Ca^{2+}$) and cyclic GMP ($cGMP$).
          - The phytochrome signal opens $Ca^{2+}$ channels, increasing $Ca^{2+}$ levels in the cytosol.
          - The signal also activates an enzyme that produces $cGMP$.
      - Response: The pathway leads to the regulation of cellular activities, usually through increased enzyme activity.
          - Post-translational Modification: This activates preexisting proteins, typically by the phosphorylation of specific amino acids. This alters the enzyme's hydrophobicity and activity. The second messengers directly activate protein kinases, often working in a cascade to phosphorylate transcription factors. Protein phosphatases serve to "switch off" the signal by dephosphorylating these proteins.
          - Transcriptional Regulation: Specific transcription factors bind to DNA to control gene expression. Activators increase transcription, while repressors decrease it.

• De-etiolation Proteins: The process activates enzymes that function in photosynthesis directly, supply chemical precursors for chlorophyll production, and regulate the levels of plant hormones involved in growth.

General Characteristics of Plant Hormones

• Definition: A hormone is a signaling molecule produced in low concentrations in one part of the body and transported to other parts, where it triggers responses in target cells by binding to specific receptors.

• Plant Growth Regulators: Also known as plant hormones, these are molecules controlling specific physiological processes. Unlike animal hormones, plant hormones can have multiple effects, and multiple hormones can influence a single process.

• Transport: Plants can transport macromolecules like proteins from cell to cell through plasmodesmata.

• Response Factors: Plant responses depend on the amount, concentration, and specific combination of hormones present.

Survey of Major Plant Hormones and Produciton Sites

• Auxin (Indoleacetic acid, IAA):
      - Location: Generated in shoot apical meristems and young leaves. Root apical meristems also produce it, but roots depend on the shoot for most of their supply. High levels are found in developing seeds and fruits.
      - Functions: Stimulates stem elongation (at low concentrations); promotes lateral and adventitious root formation; regulates fruit development; enhances apical dominance; functions in phototropism and gravitropism; promotes vascular differentiation; retards leaf abscission.

• Cytokinins:
      - Location: Synthesized primarily in roots and transported to other organs.
      - Functions: Regulate cell division in shoots and roots; modify apical dominance and promote lateral bud growth; promote nutrient movement into sink tissues; stimulate seed germination; delay leaf senescence.

• Gibberellins (GA):
      - Location: Produced in meristems of apical buds and roots, young leaves, and developing seeds.
      - Functions: Stimulate stem elongation, pollen development, pollen tube growth, fruit growth, and seed germination; regulate sex determination and the transition from juvenile to adult phases.

• Abscisic acid (ABA):
      - Location: Nearly all plant cells can synthesize ABA. It is found in every major organ and living tissue; it may be transported in phloem or xylem.
      - Functions: Inhibits growth; promotes stomatal closure during drought; promotes seed dormancy and inhibits early germination; promotes leaf senescence; promotes desiccation tolerance.

• Ethylene:
      - Location: A gaseous hormone produced by most plant parts, especially during senescence, leaf abscission, and fruit ripening. Synthesis is stimulated by wounding and stress.
      - Functions: Promotes ripening of many fruits, leaf abscission, and the triple response in seedlings; enhances senescence; promotes root and root hair formation; promotes flowering in the pineapple family.

• Brassinosteroids:
      - Location: Present in all plant tissues; act near the site of synthesis.
      - Functions: Promote cell expansion and division in shoots; promote root growth at low concentrations and inhibit it at high concentrations; promote xylem differentiation and inhibit phloem differentiation; promote seed germination and pollen tube elongation.

• Jasmonates:
      - Location: Derived from the fatty acid linolenic acid. Produced in several parts and travel in the phloem.
      - Functions: Regulate fruit ripening, floral development, pollen production, tendril coiling, root growth, seed germination, and nectar secretion; produced in response to herbivory and pathogens.

• Strigolactones:
      - Location: Carotenoid-derived signals produced in roots in response to low phosphate or high auxin flow.
      - Functions: Promote seed germination, control apical dominance, and attract mycorrhizal fungi to the root.

Detailed Mechanisms of Individual Hormones

• Auxin and Cell Elongation:
      - Tropism: Any response resulting in the curvature of organs toward or away from a stimulus. Phototropism is growth toward (positive) or away (negative) from light.
      - Discovery: Charles and Francis Darwin observed phototropism in grass seedlings only if the coleoptile tip was present. Peter Boysen-Jensen identified the mobile chemical substance.
      - Polar Transport: Auxin is produced in shoot tips and transported cell to cell down the stem by transporter proteins moving it from the basal end of one cell to the apical end of the next. Gravity does not change the direction of this transport.
      - Concentration: Stimulation of growth occurs between $10^{-8}$ and $10^{-4}\,M$. Higher concentrations inhibit elongation.
      - Acid Growth Hypothesis: Auxin stimulates proton pumps in the plasma membrane, moving $H^{+}$ into the cell wall, lowering the pH and increasing membrane potential. The reduced pH activates expansins, which loosen the cell wall. Osmotic uptake of water increases turgor pressure, allowing the cell to elongate.
      - Pattern Formation: Auxin influences phyllotaxy (leaf arrangement), leaf venation, and vascular cambium activity. Lower branch growth is stimulated by reduced auxin flow from the main shoot.
      - Practical Uses: Indolebutyric acid (IBA) is used for root cuttings. Synthetic auxins like 2,4-D are used as herbicides to kill eudicots (monocots can inactivate them). Spraying synthetic auxins on greenhouse tomatoes improves fruit development.

• Cytokinins:
      - Named after their role in cytokinesis (cell division). The most common natural cytokinin is zeatin (first found in Zea mays).
      - Interaction with Auxin: If auxin and cytokinin are equal, a mass of undifferentiated cells (callus) grows. High cytokinin levels lead to shoot buds; high auxin levels lead to root formation.
      - Apical Dominance: Controlled by the balance of sugar, cytokinins, auxin, and strigolactones. Removing the terminal bud decreases auxin and strigolactone, allowing axillary buds to grow.

• Gibberellins (GA):
      - Foolish Seedling Disease: Caused by a fungus producing GA, making rice seedlings grow too tall and spindly.
      - Stem Elongation: GA induces bolting (rapid growth of the floral stalk).
      - Fruit Growth: Auxin and GA are both required for fruit development. GA spray is used on Thompson seedless grapes to increase size and elongate internodes.
      - Germination: Water imbibition triggers GA release from the embryo, signaling the seed to break dormancy.

• Abscisic Acid (ABA):
      - Often acts as an antagonist to growth hormones.
      - Seed Dormancy: ABA levels must be low (or GA levels high) for germination. Precocious (early) germination occurs if ABA is low.
      - Drought Tolerance: ABA is the main signal for stomatal closure. It can be transported from water-stressed roots to leaves as an "early warning system."

• Ethylene:
      - Triple Response: Induced by mechanical stress (e.g., an obstacle). 1) Inhibition of stem elongation. 2) Stem thickening. 3) Horizontal growth.
          - ein mutants: Ethylene-insensitive; fail to undergo triple response.
          - eto mutants: Ethylene-overproducing; undergo response without obstacles.
          - ctr mutants: Constitutive triple-response; always exhibit the response regardless of ethylene presence.
      - Senescence and Abscission: Associated with a burst of ethylene and apoptosis. Leaf abscission occurs at the petiole base when ethylene prevails over auxin.
      - Fruit Ripening: Positive feedback loop (Ethylene triggers ripening, which triggers more ethylene). Ripening can be controlled by managing ethylene levels in green-picked fruit.

Plant Responses to Light (Photomorphogenesis)

• Action Spectrum: A graph depicting the relative response of a process (like germination) to different wavelengths of light. It identifies which photoreceptor is active.

• Blue-Light Photoreceptors: Pigments like cryptochromes (stem elongation inhibition) and phototropin (stomatal opening, chloroplast movement, phototropism).

• Phytochromes:
      - Pigments absorbing red and far-red light.
      - Photoreversible States: Phytochromes exist in two states: $P_r$ (absorbs red light) and $P_{fr}$ (absorbs far-red light).
          - Red light converts $P_r \rightarrow P_{fr}$ (triggers germination).
          - Far-red light converts $P_{fr} \rightarrow P_r$ (inhibits germination).
          - The final exposure determines the response.
      - Shade Avoidance: In a canopy, leaves absorb red light. Shaded plants receive more far-red light, shifting the ratio to $P_r$, which induces vertical growth.

Biological Clocks and Photoperiodism

• Circadian Rhythms: Internal 24-hour cycles (can be free-running from 21-27 hours). Governed by negative-feedback loops of "clock genes."

• Entrainment: Light (via phytochromes and blue-light receptors) resets the clock to 24 hours daily. Phytochromes mark sunrise and sunset.

• Photoperiodism: Physiological response to the relative lengths of day and night.
      - Short-day plants: Flower when light is shorter than a critical length (specifically, when night length exceeds a minimum threshold).
      - Long-day plants: Flower when light is longer than a certain period (when night length is less than a maximum threshold).
      - Day-neutral plants: Flower based on maturity.

• Critical Night Length: Experiments in the 1940s proved that night length, not day length, controls flowering. A flash of red light can disrupt the night length, but a subsequent flash of far-red light cancels the effect.

• Florigen: A signaling molecule produced in leaves that travels to buds to induce flowering. It is a protein encoded by the FLOWERING LOCUS T (FT) gene.

• Vernalization: A pretreatment with cold required by some plants to induce flowering.

Responses to Other External Stimuli

• Gravity (Gravitropism): Roots are positively gravitropic (down); shoots are negatively gravitropic (up). Detection involves the settling of statoliths (dense organelles/starch granules).

• Mechanical Stimuli:
      - Thigmomorphogenesis: Changes in form from mechanical disturbance (e.g., rubbing stems makes plants shorter).
      - Thigmotropism: Growth in response to touch (e.g., vines).
      - Rapid leaf movements: In Mimosa pudica, leaflets collapse due to action potentials (electrical impulses).

• Environmental Stresses:
      - Drought: Reduced transpiration via stomatal closure, reduced leaf area, or shedding leaves.
      - Flooding: Excess water leads to oxygen deprivation. Plants create air tubes (snorkels) via enzymatic destruction of root cortex cells. Mangroves have aerial roots.
      - Salt Stress: Toxic ions and low water potential. Plants produce compatible solutes to keep internal water potential more negative than the soil.
      - Heat Stress: Temperatures above $40^\circ C$ induce heat-shock proteins to prevent enzyme denaturation. Stomatal closure reduces cooling but prevents dehydration.
      - Cold Stress: Membranes become less fluid; plants increase unsaturated fatty acids. Freezing causes osmotic water loss. Plants produce antifreeze proteins to hinder ice crystal formation.

Plant Defenses Against Pathogens and Herbivores

• Pathogen Defense:
      - Barrier: Epidermis and periderm.
      - PAMP-triggered immunity: Recognition of pathogen-associated molecular patterns (PAMPs) leads to antimicrobial production and cell wall toughening.
      - Effector-triggered immunity: Pathogens release "effectors" to suppress PAMP-triggered immunity. Plants respond via disease resistance (R) genes. This triggers:
          - Hypersensitive Response (HR): Localized cell death and lignin formation to isolate the pathogen.
          - Systemic Acquired Resistance (SAR): Plant-wide alarm. Methylsalicylic acid is produced at the infection site, transported via phloem, and converted to salicylic acid to activate systemic defenses.

• Herbivore Defense Levels:
      - Molecular: Production of terpenoids, phenolics, and alkaloids to disrupt digestion or taste bad.
      - Cellular: Specialized trichomes (hairs) or irritants.
      - Tissue: Sclerenchyma tissue (toughness).
      - Organ: Spines, bristles, or mimicry (e.g., snowflake plant or fake insect eggs).
      - Organismal: Altering physiology (e.g., tobacco changing flowering time from night to morning to avoid hawk-moth larvae).
      - Population: Releasing chemicals to warn neighbors or using masting (synchronous mass seed production).
      - Community: Recruiting predators (e.g., plants releasing chemicals to attract parasitoid wasps that kill caterpillars).