Plant Response to Signals
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Plant Responses to Internal and External Signals Chapter 31 Following the Big Ideas Energy and Homeostasis- Growth and timing responses are essential to plant energy acquisition and survival. Information and Signaling- Plant hormones typically work by affecting gene expression in some manner. Plant Response Stimuli & a Stationary Life animals respond to stimuli by changing behavior move toward positive stimuli move away from negative stimuli plants respond to stimuli by adjusting growth & development Plant hormones Chemical signals that coordinate different parts of an organism only tiny amounts are required produced by 1 part of body transported to another part binds to specific receptor triggers response in target cells & tissues The Discovery of Plant Hormones Any response resulting in curvature of organs toward or away from a stimulus is called a tropism In the late 1800s, Charles Darwin and his son Francis conducted experiments on phototropism, a plant’s response to light They observed that a grass seedling could bend toward light only if the tip of the coleoptile was present They postulated that a signal was transmitted from the tip to the elongating region Figure 31.2 Results Shaded side Control Inquiry: What part of a grass coleoptile senses light, and how is the signal transmitted? Light Boysen-Jensen Illuminated side Light Darwin and Darwin Gelatin (permeable) Light Tip removed Opaque cap Transparent cap Opaque shield over curvature Mica (impermeable) In 1913, Peter Boysen-Jensen demonstrated that the signal was a mobile chemical substance In 1926, Frits Went extracted the chemical messenger for phototropism, auxin, by modifying earlier experiments 2005-2006 Plant hormones auxins cytokinins gibberellins brassicosteroids abscisic acid ethylene YouTube: Bozeman Science Plant Control Table 31.1 Response to light: Positive Phototropism Physiological event that involves interactions between the environment and internal molecular signals Growth towards light Hormone: Auxin asymmetrical distribution of auxin cells on darker side elongate faster than cells on brighter side Auxin- in detail! The term auxin refers to any chemical that promotes elongation of coleoptiles Indoleacetic acid (IAA) is a common auxin in plants; in this lecture the term auxin refers specifically to IAA Auxin is produced in shoot tips and is transported down the stem Auxin transporter proteins move the hormone from the basal end of one cell into the apical end of the neighboring cell Figure 31.4 Results Cell 1 100 m Cell 2 Epidermis Cortex Phloem Xylem Pith 25 m Basal end of cell Response to light: Phototropism Changes in the light source lead to differential growth, resulting in maximum exposure of leaves to light for photosynthesis. IAA is a naturally occurring auxin The role of auxin in cell elongation: Polar transport of auxin stimulates proton pumps in the plasma membrane According to the acid growth hypothesis, the proton pumps lower the pH in the cell wall, activating expansins, enzymes that loosen the wall’s fabric With the cellulose loosened, the cell can elongate Figure 39.8 Cross-linking polysaccharides Cell wall–loosening enzymes Expansin CELL WALL Cellulose microfibril H2O H Plasma membrane H H H ATP H H H Cell wall H H Plasma membrane CYTOPLASM Nucleus Cytoplasm Vacuole 2005-2006 Apical dominance Auxin promotes apical dominance axillary buds do no grow while apical bud exerts control shoot root Auxin also alters gene expression and stimulates a sustained growth response Auxin’s role in plant development: Polar transport of auxin controls the spatial organization of the developing plant Reduced auxin flow from the shoot of a branch stimulates growth in lower branches Auxin transport plays a role in phyllotaxy, the arrangement of leaves on the stem Practical Uses of Auxin Tomato growers spray their plants with synthetic auxins to stimulate fruit growth An overdose of synthetic auxins can kill plants For example 2,4-D is used as an herbicide on eudicots Cytokinins Cytokinins are so named because they stimulate cytokinesis (cell division) Control of cell division and differentiation: Cytokinins work together with auxin to control cell division and differentiation Cytokinins are produced in actively growing tissues such as roots, embryos, and fruits Gibberellins Family of hormones over 100 different gibberellins identified Work in concert with auxins to promote cell growth Effects stem and leaf elongation fruit growth seed germination- After water is imbibed, release of gibberellins from the embryo signals seeds to germinate plump grapes in grocery stores have been treated with gibberellin hormones while on the vine Brassinosteroids Brassinosteroids are chemically similar to cholesterol and the sex hormones of animals They induce cell elongation and division in stem segments and seedlings They slow leaf abscission (leaf drop) and promote xylem differentiation Abscisic acid (ABA) Effects slows growth counteracts the breaking of dormancy during a winter thaw seed dormancy high concentrations of Abscisic acid germination only after ABA is inactivated down or leeched out survival value: seed will germinate only under optimal conditions light, temperature, moisture drought tolerance rapid stomate closing Ethylene Ethylene is a hormone gas released by plant cells Multiple effects response to mechanical stress triple response slow stem elongation thickening of stem curvature to stem growth leaf drop (like in Fall) Facilitates apoptosis Promotes fruit ripening Apoptosis & Leaf drop Ethylene Senescence: Senescence is the programmed death of cells or organs many events in plants involve apoptosis (programmed destruction of cells, organs, or whole plants) death of annual plant after flowering differentiation of xylem vessels loss of cytosol shedding of autumn leaves- Leaf abscission: A change in the balance of auxin and ethylene controls leaf abscission, the process that occurs in autumn when a leaf falls What is the evolutionary advantage of loss of leaves in autumn? Fruit ripening Adaptation hard, tart fruit protects developing seed from herbivores ripe, sweet, soft fruit attracts animals to disperse seed Ethylene triggers ripening process breakdown softening conversion of cell wall of starch to sugar sweetening positive feedback system ethylene triggers ripening ripening stimulates more ethylene production 2005-2006 Applications Truth in folk wisdom….. one bad apple spoils the whole bunch ripening apple releases ethylene to speed ripening of fruit nearby Ripen green bananas by bagging them with an apple Climate control storage of apples high CO2 storage = reduces ethylene production Responses to light and other cues are critical for plant success Light triggers many key events in plant growth and development, collectively known as photomorphogenesis A potato left growing in darkness produces shoots that look unhealthy, and it lacks elongated roots These are morphological adaptations for growing in darkness, collectively called etiolation After exposure to light, a potato undergoes changes called de-etiolation, in which shoots and roots grow normally Figure 31.11 (a) Before exposure to light (b) After a week’s exposure to natural daylight Light-induced de-etiolation (greening) of dark-grown potatoes Plants detect not only presence of light but also its direction, intensity, and wavelength (color) Different plant responses can be mediated by the same or different photoreceptors There are two major classes of light receptors: bluelight photoreceptors and phytochromes, photoreceptors that absorb mostly red light Various blue-light photoreceptors control phototropism (movement in response to light), stomatal opening, and hypocotyl elongation Phytochrome Photoreceptors respond to phytochromes which are pigments that regulate many of a plants responses to light Phytochromes exist in two photoreversible states, with conversion of Pr to Pfr triggering many developmental responses Red light at sunrise triggers the conversion of Pr to Pfr Far-red light at sunset triggers the conversion of Pfr to Pr The conversion to Pfr is faster than the conversion to Pr Sunlight increases the ratio of Pfr to Pr and triggers germination Figure 31.14 Red light Synthesis Pr Pfr Far-red light Slow conversion in darkness (some species) Responses to Pfr: • Seed germination • Inhibition of vertical growth and stimulation of branching • Setting internal clocks • Control of flowering Enzymatic destruction Phytochromes and shade avoidance: The phytochrome system also provides the plant with information about the quality of light Leaves in the canopy absorb red light Shaded plants receive more far-red than red light In the “shade avoidance” response, the phytochrome ratio shifts in favor of Pr when a tree is shaded This shift induces the vertical growth of the plant Circadian rhythms Circadian rhythms are cycles that are about 24 hours long and are governed by an internal “clock” Circadian rhythms can be entrained to exactly 24 hours by the day/night cycle The clock may depend on synthesis of a protein regulated through feedback control Phytochrome conversion marks sunrise and sunset, providing the biological clock with environmental cues The conversion from one form to the other allows the plant to keep track of time! Circadian rhythms Internal (endogenous) 24-hour cycles 4 O’clock Morning glory Noon Midnight Photoperiodism Photoperiod, the 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. Critical night length: In the 1940s, researchers discovered that flowering and other responses to photoperiod are actually controlled by night length, not day length Long day plants- flower only when the light period is longer than a certain number of hours (short nights) Short day plants - flower only when the days are shorter and the nights longer Day neutral plants- don’t care one way or the other! Flowering Response- Photoperiodism Physiological event that involves the interaction between environmental stimuli and internal molecular signals Triggered by photoperiod- relative lengths of day & night night length—“critical period”— is trigger Plant is sensitive to red light exposure Florigen is a hypothetical hormone that promotes flowering Helps plants prepare for winter What is the evolutionary advantage of photoperiodism? Short-day plants Long-day plants Synchronizes plant responses to season 2005-2006 Response to gravity How does a sprouting shoot “know” to grow towards the surface from underground? environmental cues? roots = positive gravitropism shoots = negative gravitropism settling of statoliths (dense starch grains) may detect gravity 2005-2006 Response to touchThigmotropism Thigmotropism Mimosa (Sensitive plant) closes leaves in response to touch Caused by changes in osmotic pressure = rapid loss of K+ = rapid loss of H2O = loss of turgor in cells Environmental Stresses Environmental stresses have a potentially adverse effect 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 Abiotic Stresses During drought, plants reduce transpiration by closing stomata, reducing exposed surface area, or even shedding their leaves During flooding enzymatic destruction of root cortex cells creates air tubes that help plants survive oxygen deprivation Salt can lower the water potential of the soil solution and reduce water uptake Plants respond to salt stress by producing solutes tolerated at high concentrations This process keeps the water potential of cells more negative than that of the soil solution Abiotic Stresses Excessive heat can denature a plant’s enzymes Heat-shock proteins, which help protect other proteins from heat stress, are produced at high temperatures Cold temperatures decrease membrane fluidity Altering lipid composition of membranes is a response to cold stress Freezing causes ice to form in a plant’s cell walls and intercellular spaces Water leaves the cell in response to freezing, leading to toxic solute concentrations in the cytoplasm Plant DefensesBiotic Stresses 2005-2006 Defenses against herbivores include thorns, hairy leaves, toxins, bad odors or tastes… others? When herbivores eat the leaves of a plant that has hairy leaves the new leaves will have a greater density of hairs. Plant defenses Defenses against herbivores Parasitoid wasp larvae emerging from a caterpillar coevolution Defenses Against Pathogens A plant’s first line of defense against infection is the barrier presented by the epidermis and periderm If a pathogen penetrates the dermal tissue, the second line of defense is a chemical attack that kills the pathogen and prevents its spread This second defense system is enhanced by the plant’s ability to recognize certain pathogens Host-Pathogen Coevolution A virulent pathogen is one that a plant has little specific defense against An a virulent pathogen is one that may harm but does not kill the host plant Gene-for-gene recognition involves recognition of effector molecules by the protein products of specific plant disease resistance (R) genes An R protein recognizes a corresponding molecule made by the pathogen’s Avr gene R proteins activate plant defenses by triggering signal transduction pathways These defenses include the hypersensitive response and systemic acquired resistance The Hypersensitive Response The hypersensitive response Causes localized cell and tissue death near the infection site Induces production of phytoalexins and PR proteins, which attack the specific pathogen Stimulates changes in the cell wall that confine the pathogen Systemic Acquired Resistance Systemic acquired resistance Causes plant-wide expression of defense genes Protects against a diversity of pathogens Provides a long-lasting response Methylsalicylic acid travels from an infection site to remote areas of the plant where it is converted to salicylic acid, which initiates pathogen resistance Figure 31.24 Infected tobacco leaf with lesions 4 2 Signal 5 Hypersensitive 3 response Signal transduction pathway 6 Signal transduction pathway 7 Acquired resistance 1 R protein Avirulent pathogen Avr effector protein R-Avr recognition and hypersensitive response Systemic acquired resistance Connecting the Concepts With the Big Ideas Energy and Homeostasis- Phototropism facilitates plant response to light changes using molecular signals that maximize photosynthetic surface area. Photoperiodism involves phytochrome’s control of flowering and seasonal changes. Circadian rhythms, including stomata openings, allow plants to adjust to environmental conditions. Connecting the Concepts With the Big Ideas Information and Signaling- Cytokines trigger mitosis and cytokinesis by regulating gene expression. Increase in ethylene levels induce enzyme production that promotes fruit ripening. Gibberellins promote signal transmissions that affect specific genes, triggering seed germination.
