Plant Responses Notes

Plant Responses to Internal and External Signals

Concept 31.1: Plant Hormones

  • Plant hormones are chemical signals that modify or control physiological processes.
  • They are effective in very low concentrations.
  • Most aspects of plant growth and development are under hormonal control.
  • Each hormone can have multiple effects.
  • Multiple hormones can work together to influence a single process.
  • Hormone response depends on the concentration of a specific hormone relative to other hormones.

The Discovery of Plant Hormones

  • Tropism: Curvature of organs toward or away from a stimulus.
  • Charles and Francis Darwin observed that grass seedlings bend toward light only if the coleoptile tip is present.

Survey of Plant Hormones

  • Major classes of plant hormones:
    • Auxin
    • Cytokinins
    • Gibberellins
    • Brassinosteroids
    • Abscisic acid
    • Ethylene
Auxin
  • Auxin promotes elongation of coleoptiles.
  • It is produced in shoot tips and transported down the stem.
  • Acid Growth Hypothesis: Proton pumps lower the pH in the cell wall, activating expansins that loosen the wall, allowing cell elongation.
  • Auxin also alters gene expression and stimulates sustained growth.
  • Reduced auxin flow from the shoot stimulates growth in lower branches.
  • Auxin transport plays a key role in phyllotaxy (arrangement of leaves on the stem).
Cytokinins
  • Stimulate cytokinesis (cell division).
  • Influence cell differentiation, apical dominance, and aging.
  • Work with auxin to control cell division and differentiation.
  • Produced in actively growing tissues like roots, embryos, and fruits.
  • Anti-aging effects: Inhibit protein breakdown, stimulate RNA and protein synthesis, and mobilize nutrients.
Gibberellins
  • Affect stem elongation, fruit growth, and seed germination.
  • Stimulate stem and leaf growth by enhancing cell elongation and division.
  • Produced in young roots and leaves.
  • Can induce bolting (rapid growth of the floral stalk).
  • Fruit Growth: Auxin and gibberellins are often both required for fruit development. Gibberellins can be sprayed on grapes to increase their size.
  • Germination: Gibberellins released from the embryo signal seeds to germinate after water is imbibed.
    • GAGA signals the aleurone layer to produce αα-amylase, which hydrolyzes starch in the endosperm, releasing sugars for the embryo.
Brassinosteroids
  • Chemically similar to cholesterol and animal sex hormones.
  • Induce cell elongation and division in stems and seedlings.
  • Slow leaf abscission and promote xylem differentiation.
Abscisic Acid (ABA)
  • Slows plant growth, promotes seed dormancy and drought tolerance.
  • Drought Tolerance: ABA is the primary internal signal that enables plants to withstand drought; it causes stomata to close rapidly.
  • Seed Dormancy: Ensures germination occurs in optimal conditions. Dormancy is broken when ABA is removed by rain or inactivated by light or cold.
  • Inactive or low levels of ABA can cause precocious (early) germination.
Ethylene
  • Produced in response to stress like drought, flooding, mechanical pressure, insect damage, and infection.
  • Effects include:
    • Response to mechanical stress
    • Senescence
    • Leaf abscission
    • Fruit ripening
  • Triple Response: Induced by ethylene, includes slowing of stem elongation, thickening of the stem, and horizontal growth.
  • Senescence: Programmed death of cells or organs. Ethylene is associated with apoptosis.
  • Leaf Abscission: A change in the balance of auxin and ethylene controls leaf abscission.
  • Fruit Ripening: A burst of ethylene production triggers the ripening process, and ripening triggers more ethylene production. Fruit producers can control ripening by managing ethylene levels.

Concept 31.2: Responses to Light

  • Light triggers photomorphogenesis (key events in plant growth and development).
  • Etiolation: Morphological adaptations for growing in darkness (e.g., unhealthy shoots, lack of elongated roots).
  • De-etiolation: Changes after exposure to light, leading to normal shoot and root growth.
  • Plants detect the presence, direction, intensity, and wavelength (color) of light.
  • Action Spectrum: Depicts the relative response of a process to different wavelengths and can be used to study light-dependent processes.
  • Different plant responses can be mediated by the same or different photoreceptors.
    • Blue-light photoreceptors: Control phototropism, stomatal opening, and hypocotyl elongation.
    • Phytochromes: Photoreceptors that absorb mostly red light and regulate many responses to light, including seed germination and shade avoidance.
  • Some seeds remain dormant until light conditions are optimal.
    • Red light promotes germination, while far-red light inhibits it. The final light exposure determines the response.
  • Phytochromes exist in two photoreversible states:
    • Pr (red-light absorbing)
    • Pfr (far-red-light absorbing)
    • Red light triggers the conversion of Pr to Pfr, while far-red light triggers Pfr to Pr.
  • Conversion of Pr to Pfr triggers many developmental responses.
  • Phytochromes and shade avoidance:
    • Leaves in the canopy absorb red light, so shaded plants receive more far-red light.
    • The phytochrome ratio shifts in favor of Pr, stimulating the “shade avoidance” response, which induces vertical growth.
Biological Clocks and Circadian Rhythms
  • Many plant processes oscillate during the day, independent of environmental conditions. These cycles are called circadian rhythms.
  • Circadian rhythms are cycles that are approximately 24 hours long and are governed by an internal “clock”.
  • They can be entrained to exactly 24 hours by the day/night cycle.
  • The clock may depend on the synthesis of a protein regulated through negative-feedback loops.
The Effect of Light on the Biological Clock
  • Both phytochromes and blue-light photoreceptors can entrain circadian rhythms.
  • Phytochrome conversion marks sunrise and sunset, providing environmental cues to the biological clock.
Photoperiodism and Responses to Seasons
  • Photoperiod (relative lengths of night and day) is the environmental stimulus plants use most often to detect the time of year.
  • Photoperiodism is a physiological response to photoperiod.
  • Flowering and photoperiod:
    • Short-day plants flower when a light period is shorter than a critical length.
    • Long-day plants flower when a light period is longer than a critical length.
    • Day-neutral plants' flowering is controlled by plant maturity, not photoperiod.
  • Flowering and other responses to photoperiod are actually controlled by night length, not day length.
  • Red light is most effective in interrupting the nighttime portion of the photoperiod.
    • A flash of red light followed by a flash of far-red light does not disrupt night length.
    • Florigen is the signaling molecule that induces flowering in both short-day and long-day plants. It is likely a protein governed by the FLOWERING LOCUS T (FT) gene.

Concept 31.3: Responses to Other Stimuli

  • Plants must adjust to a range of environmental circumstances through developmental and physiological mechanisms because they are immobile.
Gravity
  • Gravitropism is the response to gravity.
  • Roots show positive gravitropism (growing downward), and shoots show negative gravitropism (growing upward).
  • Plants may detect gravity by the settling of statoliths (dense cytoplasmic components).
  • Mechanical pulling on proteins that connect the protoplast to the cell wall may also aid in gravity detection.
  • Dense organelles and starch granules may also contribute to gravity detection.
Mechanical Stimuli
  • Thigmomorphogenesis refers to changes in form that result from mechanical disturbance.
  • Thigmotropism is directional growth in response to touch.
  • Some plants have acute responses to mechanical stimuli and are called “touch specialists”.
  • For example, tendrils of climbing plants coil around objects they touch for support.
  • The sensitive plant, Mimosa pudica, folds its leaflets and collapses in response to mechanical stimulation due to the transmission of electrical impulses called action potentials.
Environmental Stresses
  • Environmental stresses can have adverse effects on survival, growth, and reproduction.
  • Stresses can be abiotic (nonliving) or biotic (living).
    • Abiotic stresses include drought, flooding, salt stress, heat stress, and cold stress.
    • Biotic stresses include herbivores and pathogens.
    • Plants can reduce transpiration by closing stomata, reducing exposed leaf surface area, or shedding leaves.
  • Production of ethylene kills root cortex cells, creating air tubes that function as "snorkels" to provide oxygen to submerged roots.
  • Plants respond to salt stress by producing solutes tolerated at high concentrations to keep the water potential of cells more negative than that of the soil solution.
  • Transpiration cools leaves, but stomata close at high temperatures to reduce water loss.
  • Heat-shock proteins are produced at high temperatures to protect other proteins from denaturing.
  • Plants alter the lipid composition of membranes to maintain fluidity during cold conditions.
  • Freezing causes ice to form in a plant’s cell walls and intercellular spaces, leading to water loss from the cell and toxic solute concentrations in the cytoplasm.
  • Frost-tolerant plants increase the concentration of nontoxic solutes inside their cells to reduce water loss in response to freezing.
  • Antifreeze proteins that prevent the formation of ice crystals have evolved in many taxonomic groups, including plants. They have similar tertiary structures but differ in amino acid sequence and likely arose through convergent evolution.

Concept 31.4: Responses to Attacks

  • Plants have evolved defense systems to deter herbivory, prevent infection, and combat pathogens through natural selection.
Defenses Against Herbivores
  • Herbivory (animals eating plants) is a stress faced by plants in all ecosystems.
  • Damage by herbivores can reduce plant size, hinder growth and resource acquisition, and increase vulnerability to pathogen infection.
  • Plants counter excessive herbivory with:
    • Physical defenses (e.g., thorns and trichomes)
    • Chemical defenses (e.g., distasteful or toxic compounds)
    • Behavioral defenses (e.g., recruitment of predatory animals that feed on herbivores)
  • Plants damaged by insects can release volatile chemicals to warn other plants.
  • Some plants release volatile compounds to attract predatory insects.
  • Arabidopsis can be transgenically engineered to produce volatile compounds that attract predatory mites.
Defenses Against Pathogens
  • The epidermis and periderm present a physical barrier against pathogen entry. Pathogens may enter through wounds or natural openings.
  • Plants have two types of immune response:
    • PAMP-triggered immunity
    • Effector-triggered immunity
    • PAMP-triggered immunity involves a chemical attack that isolates and prevents the spread of the pathogen from the site of infection. It requires the recognition of pathogen-associated molecular patterns (PAMPS).
    • Effector-triggered immunity involves recognition of effector molecules by the protein products of specific plant disease resistance (R) genes.
  • R proteins activate plant defenses by triggering signal transduction pathways.
  • These defenses include the hypersensitive response and systemic acquired resistance.
  • Hypersensitive Response: Causes localized tissue death near the infection site and restricts the spread of the pathogen. The plant produces enzymes and chemicals that damage the pathogen’s cell wall, metabolism, or reproduction.
  • Systemic Acquired Resistance: Causes plant-wide expression of defense genes and provides a long-lasting response to a diversity of pathogens. Methylsalicylic acid travels from an infection site to remote areas of the plant, where it is converted to salicylic acid, which initiates pathogen resistance.