Plant Responses

Plant Responses to Internal and External Signals

Concept 31.1: Plant Hormones

• Plant hormones, also known as phytohormones, are organic compounds that modify or control one or more specific physiological processes within a plant. These processes include growth, development, metabolism, and immunity. Hormones are effective in very low concentrations, often parts per million.

• They are effective in low concentrations.

• Hormones influence growth and development at various stages of the plant life cycle, from seed germination to flowering and fruit development.

• Each hormone can have multiple effects depending on the tissue, developmental stage, and environmental conditions.

• Multiple hormones can interact synergistically or antagonistically to affect a single process. For example, auxin and cytokinin interact in apical dominance and cell differentiation.

• Hormone response depends on concentration relative to other hormones, creating a balance that fine-tunes plant responses.

Discovery of Plant Hormones

• Tropism: A directional growth response where the curvature of organs occurs toward or away from a stimulus. Examples include phototropism (response to light) and gravitropism (response to gravity).

• Charles and Francis Darwin observed grass seedlings bending towards light if the coleoptile tip (protective sheath covering the emerging shoot) was present, indicating the tip's role in light detection.

Plant Hormones

• Major classes of plant hormones:

  • Auxin

  • Cytokinins

  • Gibberellins

  • Brassinosteroids

  • Abscisic acid

  • Ethylene

Auxin

• Promotes coleoptile elongation by stimulating cell expansion.

• Produced primarily in shoot tips (apical meristems), young leaves, and developing seeds, then transported polarly (unidirectionally) down the stem.

• Acid Growth Hypothesis:

  • Proton pumps (H+-ATPases) actively transport H+ ions out of the cytoplasm into the cell wall, lowering the pH.

  • Activates expansins, enzymes that disrupt hydrogen bonds between cellulose microfibrils in the cell wall, loosening cell wall fabric and allowing for cell expansion.

• Impacts gene expression by influencing the transcription of specific genes involved in cell growth and development. This action sustains growth over longer periods.

• Reduced auxin flow from the shoot encourages lower branch growth, preventing apical dominance and promoting a bushier growth habit.

• Auxin plays a role in phyllotaxy (leaf arrangement) by influencing the positioning of new leaves at the shoot apex.

Cytokinins

• Stimulate cytokinesis (cell division) and promote cell growth.

• Influence cell differentiation by affecting the fate of dividing cells, ensuring proper tissue development. They also play a role in apical dominance, and delay aging (senescence).

• Work with auxin to control cell division and differentiation in developing tissues. The ratio of auxin to cytokinin is crucial for determining whether cells divide, elongate, or differentiate.

• Produced in actively growing tissues such as roots, embryos, and fruits.

• Anti-aging effects: Inhibit protein breakdown, stimulate RNA and protein synthesis, and mobilize nutrients from aging tissues to actively growing ones, keeping tissues youthful.

Gibberellins

• Affect stem elongation, fruit growth, and seed germination by stimulating cell division and elongation.

• Stimulate stem and leaf growth by enhancing both cell elongation and cell division.

• Produced in young roots and leaves and transported to other parts of the plant.

• Induce bolting (rapid floral stalk growth) in long-day plants under the appropriate photoperiod.

• Required for fruit development in many plants (along with auxin), promoting fruit set and expansion.

• Used to increase the size of Thompson seedless grapes and improve their marketability.

• Germination: Release of gibberellins from the embryo signals seeds to germinate after water imbibition. Gibberellins activate enzymes that mobilize stored nutrients in the seed, providing energy for germination.

Brassinosteroids

• Chemically similar to cholesterol and animal sex hormones, these steroids are essential plant hormones.

• Induce cell elongation and division in stem segments and seedlings, promoting overall growth.

• Slow leaf abscission (leaf drop) and promote xylem differentiation, which is crucial for water transport.

Abscisic Acid (ABA)

• Slows plant growth, promotes seed dormancy and drought tolerance.

• Primary internal signal for drought resistance, helping plants conserve water during dry conditions.

• Accumulation causes stomata to close, reducing transpiration and preventing water loss.

• Seed dormancy ensures germination occurs only in optimal conditions, preventing premature germination in unfavorable environments.

• Dormancy is broken when ABA is removed by rain (leaching), inactivated by light, or broken down by cold temperatures.

• Low ABA levels can cause precocious germination (vivipary), where seeds germinate inside the fruit.

Ethylene

• Produced in response to stress (drought, flooding, mechanical pressure, insect damage, infection), as well as during fruit ripening and senescence.

• Effects include:

  • Response to mechanical stress: Triggers the triple response in seedlings to avoid obstacles.

  • Senescence: Promotes the breakdown of chlorophyll and other macromolecules in aging tissues.

  • Leaf abscission: Facilitates the detachment of leaves from the stem in autumn.

  • Fruit ripening: Stimulates the conversion of starches to sugars, softening of fruit tissues, and production of aromatic compounds.

• Triple Response:

  • Slowing of stem elongation to conserve energy.

  • Thickening of the stem to provide mechanical support.

  • Horizontal growth to avoid obstacles.

• Senescence: Programmed cell/organ death; associated with apoptosis (programmed cell death), which is essential for plant development and adaptation.

• Leaf Abscission: Controlled by the balance of auxin and ethylene. Auxin maintains leaf attachment, while ethylene promotes abscission.

• Fruit Ripening: Ethylene triggers ripening, which in turn produces more ethylene (positive feedback). This autocatalytic process ensures uniform ripening of fruits.

Concept 31.2: Responses to Light

• Light triggers photomorphogenesis (growth and development in response to light), enabling plants to optimize their growth and development based on light availability.

• Etiolation: Morphological adaptations for growing in darkness, including elongated, unhealthy shoots and a lack of elongated roots.

• De-etiolation: Changes after light exposure, resulting in normal shoot and root growth, including the development of chloroplasts and the synthesis of chlorophyll.

• Plants detect light direction, intensity, and wavelength using specialized photoreceptors.

• Action spectrum: Depicts the relative response of a process (e.g., photosynthesis) to different wavelengths of light. This helps identify which wavelengths are most effective for specific plant responses.

• Light Detection: Response can be mediated by the same or different photoreceptors. Different photoreceptors allow plants to respond to a wide range of light conditions.

  • There are two major classes of light receptors:

  • Blue-light photoreceptors

  • Phytochromes, which absorb mostly red light

Blue-Light Photoreceptors

• Control phototropism (directional growth towards light), stomatal opening (regulating gas exchange), and hypocotyl elongation (stem elongation in seedlings).

Phytochrome Photoreceptors

• Regulate responses to light throughout a plant’s life, from seed germination to flowering and senescence.

• Seed germination and shade avoidance are influenced by phytochrome activity.

• Red light increases germination, while far-red light inhibits it, allowing plants to respond to changes in light quality.

• Effects of red and far-red light are reversible, with the final exposure determining the response. This reversibility enables plants to sense and respond to changes in light conditions throughout the day.

• Phytochromes exist in two photoreversible states, Pr and Pfr:

  • Red light converts Pr (inactive form) to Pfr (active form).

  • Far-red light converts Pfr back to Pr.

  • Pr ${\longrightarrow}$, Red Light, ${\longrightarrow}$ Pfr.

  • Pfr ${\longrightarrow}$, Far-red Light, ${\longrightarrow}$ Pr.

• Conversion of Pr to Pfr triggers developmental responses to light by initiating signal transduction pathways that regulate gene expression and physiological processes.

• Responses to Pfr:

  • Seed germination

  • Inhibition of vertical growth & stimulation of branching

  • Setting internal clocks

  • Control of flowering

• Shade Avoidance:

  • Leaves in the canopy absorb red light, reducing the red/far-red ratio.

  • Shaded plants receive more far-red light, which converts Pfr to Pr.

  • Phytochrome ratio shifts in favor of Pr, stimulating vertical growth to compete for light.

Biological Clocks and Circadian Rhythms

• Many plant processes oscillate during the day, independent of environmental conditions, demonstrating endogenous control.

• Circadian rhythms are
  ~24-hour cycles governed by an internal clock, influencing processes such as gene expression, hormone production, and stomatal opening.

• The biological clock can be set (entrained) via negative feedback loops involving gene expression and protein degradation.

• Both phytochromes and blue-light photoreceptors can entrain circadian rhythms by providing light signals that synchronize the internal clock with the external environment.

• Phytochrome conversion marks sunrise and sunset, helping plants track the time of day and adjust their physiology accordingly.

Photoperiodism and Responses to Seasons

• Photoperiod (relative lengths of night and day) is used to detect the time of year and regulate seasonal responses such as flowering.

• Photoperiodism is a physiological response to photoperiod, allowing plants to coordinate their development with seasonal changes.

• Short-day plants flower when the light period is shorter than a critical length (i.e., long nights).

• Long-day plants flower when the light period is longer than a critical length (i.e., short nights).

• Day-neutral plants flowering is controlled by plant maturity, not photoperiod.

• Flowering and other responses to photoperiod are controlled by night length, not day length.

• Red light is most effective in interrupting nighttime, preventing short-day plants from flowering.

• A flash of red light followed by far-red light does not disrupt night length because the far-red light converts Pfr back to Pr, negating the effect of the red light.

• Photoperiod is detected by leaves, which cue buds to develop as flowers via a signaling molecule.

• Florigen is the signaling molecule that induces flowering. It is transported from leaves to the shoot apex.

• Florigen is likely governed by the FLOWERING LOCUS T (FT) gene. The FT protein interacts with transcription factors in the shoot apex to initiate flowering.

Concept 31.3: Responses to Other Stimuli

• Plants adjust to environmental circumstances via developmental and physiological mechanisms, optimizing their growth, survival, and reproduction.

Gravity

• Gravitropism: Response to gravity, ensuring that roots grow downward into the soil and shoots grow upward towards light.

• Roots show positive gravitropism; shoots show negative gravitropism.

• Plants may detect gravity by the settling of statoliths (dense cytoplasmic components, such as amyloplasts) in specialized cells called statocytes. The position of statoliths provides information about the plant's orientation with respect to gravity.

• Mechanical pulling may also aid in gravity detection by activating mechanosensitive channels in the plasma membrane.

Mechanical Stimuli

• Thigmomorphogenesis: Changes in form due to mechanical disturbance, such as wind or touch. This includes矮化 stems and thicker trunks.

• Thigmotropism: Directional growth in response to touch, allowing climbing plants to wrap around supports.

• Touch specialists: Plants with acute responses to mechanical stimuli (e.g., Mimosa pudica, the sensitive plant). Its leaves rapidly fold inward when touched.

• Rapid leaf movements result from action potentials, which are electrical signals that propagate through the plant and trigger changes in turgor pressure in specialized cells called pulvini at the base of the leaves.

Environmental Stresses

• Can be abiotic (nonliving) or biotic (living). Plants have evolved various mechanisms to cope with both types of stresses.

• Abiotic stresses: drought, flooding, salt stress, heat stress, and cold stress. Each stress requires specific adaptations to minimize damage and maintain growth.

• Biotic stresses: herbivores and pathogens. Plants defend themselves through physical barriers, chemical defenses, and induced resistance.

• Drought: Plants can reduce transpiration by closing stomata (reducing water loss), reducing leaf surface area (decreasing transpiration area), or shedding leaves (avoiding water loss).

• Flooding: Ethylene production creates air tubes (aerenchyma) in roots to provide oxygen to submerged tissues.

• Salt Stress: Plants produce compatible solutes (e.g., proline, glycine betaine) that accumulate in cells and help maintain osmotic balance, preventing water loss under saline conditions.

• Heat Stress: Plants produce heat-shock proteins, which act as molecular chaperones that stabilize other proteins and prevent them from denaturing under high temperatures.

• Cold Stress:

  • Adjust membrane fluidity by increasing the proportion of unsaturated fatty acids in membrane lipids, preventing membranes from solidifying at low temperatures.

  • Increase concentration of nontoxic solutes (e.g., sugars, amino acids) in cells, which lowers the freezing point and protects against ice crystal formation.

  • Antifreeze proteins prevent ice crystal formation by binding to ice crystals and inhibiting their growth, protecting plant tissues from damage.

Concept 31.4: Defenses Against Herbivores and Pathogens

• Plants have evolved defense systems to deter herbivory, prevent infection, and combat pathogens, enhancing their survival and reproductive success.

Defenses Against Herbivores

• Herbivory reduces plant size, hinders growth, and increases vulnerability to pathogens, impacting plant fitness.

• Plants counter herbivory with:

  • Physical defenses (thorns, trichomes, thick cuticles) that deter herbivores or make it difficult for them to feed.

  • Chemical defenses (distasteful/toxic compounds) such as alkaloids, terpenes, and phenolics that deter herbivores or poison them.

  • Behavioral defenses (recruitment of predatory animals) where plants emit volatile compounds that attract predators or parasitoids of herbivores.

• Release volatile chemicals to warn other plants or attract predatory insects, enhancing the overall defense of the plant community.

Defenses Against Pathogens

• Epidermis and periderm provide a physical barrier that prevents pathogen entry. The cuticle also acts as a barrier.

• Pathogens enter through wounds or natural openings, such as stomata or lenticels.

• Plants have two types of immune response:

  • PAMP-triggered immunity (PTI)

  • Effector-triggered immunity (ETI)

• PAMP-Triggered Immunity:

  • Involves isolating the pathogen from the site of infection, limiting its spread.

  • Pathogen-associated molecular patterns (PAMPS) are recognized by the plant's pattern recognition receptors (PRRs), triggering defense responses.

• Effector-Triggered Immunity:

  • Pathogens release effectors to evade PAMP-triggered immunity and suppress plant defenses.

  • Recognition of effector molecules by disease resistance (R) genes, leading to a strong defense response.

  • R proteins activate plant defenses by triggering signal transduction pathways, including the hypersensitive response and systemic acquired resistance.

• Defenses include:

  • Hypersensitive response (localized tissue death) that prevents the spread of the pathogen from the infection site.

  • Systemic acquired