Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Chapter 39 Plant Responses to Internal and External Signals
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Overview: Stimuli and a Stationary Life Plants, being rooted to the ground, must respond to environmental changes that come their way For example, the bending of a seedling toward light begins with sensing the direction, quantity, and color of the light
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Concept 39.1: Signal transduction pathways link signal reception to response Plants have cellular receptors that detect changes in their environment For a stimulus to elicit a response, certain cells must have an appropriate receptor
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A potato left growing in darkness produces shoots that look unhealthy and lacks elongated roots These are morphological adaptations for growing in darkness, collectively called etiolation
LE 39-2 Before exposure to light. A dark-grown potato has tall, spindly stems and nonexpanded leaves— morphological adaptations that enable the shoots to penetrate the soil. The roots are short, but there is little need for water absorption because little water is lost by the shoots. After a week’s exposure to natural daylight. The potato plant begins to resemble a typical plant with broad green leaves, short sturdy stems, and long roots. This transformation begins with the reception of light by a specific pigment, phytochrome.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings After exposure to light, a potato undergoes changes called de-etiolation, in which shoots and roots grow normally
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A potato’s response to light is an example of cell- signal processing The stages are reception, transduction, and response
LE 39-3 CELL WALL CYTOPLASM Reception Transduction Response Receptor Hormone or environmental stimulus Activation of cellular responses Relay molecules Plasma membrane
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Reception Internal and external signals are detected by receptors, proteins that change in response to specific stimuli
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Transduction Second messengers transfer and amplify signals from receptors to proteins that cause responses
LE 39-4_3 Cell wall CYTOPLASM Reception Transduction Response Second messenger produced Light Plasma membrane Specific protein kinase 1 activated cGMP Phytochrome activated by light NUCLEUS Specific protein kinase 2 activated Ca 2+ channel opened Ca 2+ Transcription factor 1 Transcription factor 2 Transcription Translation De-etiolation (greening) response proteins
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Response A signal transduction pathway leads to regulation of one or more cellular activities In most cases, these responses to stimulation involve increased activity of enzymes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Transcriptional Regulation Transcription factors bind directly to specific regions of DNA and control transcription of genes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Post-Translational Modification of Proteins Post-translational modification involves activation of existing proteins in the signal response
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings De-Etioloation (“Greening”) Proteins Many enzymes that function in certain signal responses are directly involved in photosynthesis Other enzymes are involved in supplying chemical precursors for chlorophyll production
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 39.2: Plant hormones help coordinate growth, development, and responses to stimuli Hormones are chemical signals that coordinate different parts of an organism
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Discovery of Plant Hormones Any response resulting in curvature of organs toward or away from a stimulus is called a tropism Tropisms are often caused by hormones
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In the late 1800s, Charles Darwin and his son Francis conducted experiments on phototropism, a plant’s response to light They observed that a 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 Video: Phototropism Video: Phototropism
LE 39-5a Light Control Illuminated side of coleoptile Shaded side of coleoptile
LE 39-5b Light Darwin and Darwin (1880) Tip covered by trans- parent cap Base covered by opaque shield Tip covered by opaque cap Tip removed
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In 1913, Peter Boysen-Jensen demonstrated that the signal was a mobile chemical substance
LE 39-5c Light Boysen-Jensen (1913) Tip separated by gelatin block Tip separated by mica
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In 1926, Frits Went extracted the chemical messenger for phototropism, auxin, by modifying earlier experiments
LE 39-6 Excised tip placed on agar block Growth-promoting chemical diffuses into agar block Agar block with chemical stimulates growth Offset blocks cause curvature Control (agar block lacking chemical) has no effect Control
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A Survey of Plant Hormones In general, hormones control plant growth and development by affecting the division, elongation, and differentiation of cells Plant hormones are produced in very low concentration, but a minute amount can greatly affect growth and development of a plant organ
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Auxin The term auxin refers to any chemical that promotes cell elongation in target tissues Auxin transporters move the hormone from the basal end of one cell into the apical end of the neighboring cell
LE 39-7 Epidermis Basal end of cell Cortex Phloem Xylem Pith Cell µm Cell 2 25 µm
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Role of Auxin in Cell Elongation According to the acid growth hypothesis, auxin stimulates proton pumps in the plasma membrane 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
LE 39-8a Cross-linking cell wall polysaccharides Cell wall enzymes Microfibril Expansin CELL WALL Plasma membrane CYTOPLASM ATP
LE 39-8b Plasma membrane Cytoplasm Nucleus Vacuole H2OH2O Cell wall
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Lateral and Adventitious Root Formation Auxin is involved in root formation and branching
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Auxins as Herbicides An overdose of auxins can kill eudicots
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Other Effects of Auxin Auxin affects secondary growth by inducing cell division in the vascular cambium and influencing differentiation of secondary xylem
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cytokinins Cytokinins are so named because they stimulate cytokinesis (cell division)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Control of Cell Division and Differentiation Cytokinins are produced in actively growing tissues such as roots, embryos, and fruits Cytokinins work together with auxin
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Control of Apical Dominance Cytokinins, auxin, and other factors interact in the control of apical dominance, a terminal bud’s ability to suppress development of axillary buds
LE 39-9 Intact plant Plant with apical bud removed Lateral branches “Stump” after removal of apical bud Axillary buds
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings If the terminal bud is removed, plants become bushier
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Anti-Aging Effects Cytokinins retard the aging of some plant organs by inhibiting protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients from surrounding tissues
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gibberellins Gibberellins have a variety of effects, such as stem elongation, fruit growth, and seed germination
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Stem Elongation Gibberellins stimulate growth of leaves and stems In stems, they stimulate cell elongation and cell division
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fruit Growth In many plants, both auxin and gibberellins must be present for fruit to set Gibberellins are used in spraying of Thompson seedless grapes
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Germination After water is imbibed, release of gibberellins from the embryo signals seeds to germinate
LE Aleurone Endosperm Water GA Scutellum (cotyledon) Radicle -amylase Sugar
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Brassinosteroids Brassinosteroids are similar to the sex hormones of animals They induce cell elongation and division
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Abscisic Acid Two of the many effects of abscisic acid (ABA): – Seed dormancy – Drought tolerance
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Seed Dormancy Seed dormancy ensures that the seed will germinate only in optimal conditions Precocious germination is observed in maize mutants that lack a transcription factor required for ABA to induce expression of certain genes
LE Coleoptile
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Drought Tolerance ABA is the primary internal signal that enables plants to withstand drought
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Ethylene Plants produce ethylene in response to stresses such as drought, flooding, mechanical pressure, injury, and infection
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Triple Response to Mechanical Stress Ethylene induces the triple response, which allows a growing shoot to avoid obstacles The triple response consists of a slowing of stem elongation, a thickening of the stem, and horizontal growth
LE Ethylene concentration (parts per million)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Ethylene-insensitive mutants fail to undergo the triple response after exposure to ethylene
LE ein mutant. An ethylene-insensitive (ein) mutant fails to undergo the triple response in the presence of ethylene. ein mutant ctr mutant ctr mutant. A constitutive triple-response (ctr) mutant undergoes the triple response even in the absence of ethylene.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Other mutants undergo the triple response in air but do not respond to inhibitors of ethylene synthesis
LE Ethylene insensitive (ein) Constitutive triple response (ctr) Ethylene overproducing (eto) Wild-type Control Ethylene synthesis inhibitor Ethylene added
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Apoptosis: Programmed Cell Death A burst of ethylene is associated with apoptosis, the programmed destruction of cells, organs, or whole plants
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Leaf Abscission A change in the balance of auxin and ethylene controls leaf abscission, the process that occurs in autumn when a leaf falls
LE Protective layer StemPetiole Abscission layer 0.5 mm
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fruit Ripening A burst of ethylene production in a fruit triggers the ripening process
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Systems Biology and Hormone Interactions Interactions between hormones and signal transduction pathways make it hard to predict how genetic manipulation will affect a plant Systems biology seeks a comprehensive understanding that permits modeling of plant functions
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 39.3: Responses to light are critical for plant success Light cues many key events in plant growth and development Effects of light on plant morphology are called photomorphogenesis
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Plants detect not only presence of light but also its direction, intensity, and wavelength (color) A graph called an action spectrum depicts relative response of a process to different wavelengths Action spectra are useful in studying any process that depends on light
LE Wavelength (nm) Light Time = 0 min. Time = 90 min Phototropic effectiveness relative to 436 nm
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings There are two major classes of light receptors: blue-light photoreceptors and phytochromes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Blue-Light Photoreceptors Various blue-light photoreceptors control hypocotyl elongation, stomatal opening, and phototropism
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Phytochromes as Photoreceptors Phytochromes regulate many of a plant’s responses to light throughout its life
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Phytochromes and Seed Germination Studies of seed germination led to the discovery of phytochromes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In the 1930s, scientists at the U.S. Department of Agriculture determined the action spectrum for light-induced germination of lettuce seeds Dark (control) Dark
LE Red Far-red Red Dark Far-red Red Dark Red Far-red Red Far-red Dark (control)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The photoreceptor responsible for the opposing effects of red and far-red light is a phytochrome
LE A phytochrome consists of two identical proteins joined to form one functional molecule. Each of these proteins has two domains. Chromophore Photoreceptor activity Kinase activity
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Phytochromes exist in two photoreversible states, with conversion of P r to P fr triggering many developmental responses
LE Synthesis PrPr P fr Red light Far-red light Slow conversion in darkness (some plants) Enzymatic destruction Responses: seed germination, control of flowering, etc.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Phytochromes and Shade Avoidance The phytochrome system also provides the plant with information about the quality of light In the “shade avoidance” response, the phytochrome ratio shifts in favor of P r when a tree is shaded
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Biological Clocks and Circadian Rhythms Many plant processes oscillate during the day Many legumes lower their leaves in the evening and raise them in the morning
LE Midnight Noon
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cyclical responses to environmental stimuli are called circadian rhythms and are about 24 hours long Circadian rhythms can be entrained to exactly 24 hours by the day/night cycle
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Effect of Light on the Biological Clock Phytochrome conversion marks sunrise and sunset, providing the biological clock with environmental cues
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Photoperiodism and Responses to Seasons 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Photoperiodism and Control of Flowering Some processes, including flowering in many species, require a certain photoperiod Plants that flower when a light period is shorter than a critical length are called short-day plants Plants that flower when a light period is longer than a certain number of hours are called long- day plants In the 1940s, researchers discovered that flowering and other responses to photoperiod are actually controlled by night length, not day length
LE Light “Short-day” plants “Long-day” plants Darkness Flash of light Critical dark period 24 hours
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Action spectra and photoreversibility experiments show that phytochrome is the pigment that receives red light Red light can interrupt the nighttime portion of the photoperiod
LE Short-day (long-night) plant Long-day (short-night) plant Hours FR R R R R R R Critical dark period
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A Flowering Hormone? The flowering signal, not yet chemically identified, is called florigen Florigen may be a hormone or a change in relative concentrations of multiple hormones
LE Graft Time (several weeks)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Meristem Transition and Flowering For a bud to form a flower instead of a vegetative shoot, meristem identity genes must first be switched on Researchers seek to identify the signal transduction pathways that link cues such as photoperiod and hormonal changes to the gene expression required for flowering
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 39.4: Plants respond to a wide variety of stimuli other than light Because of immobility, plants must adjust to a range of environmental circumstances through developmental and physiological mechanisms
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gravity Response to gravity is known as gravitropism Roots show positive gravitropism Stems show negative gravitropism
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Plants may detect gravity by the settling of statoliths, specialized plastids containing dense starch grains Video: Gravitropism Video: Gravitropism
LE µm Statoliths
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mechanical Stimuli The term thigmomorphogenesis refers to changes in form that result from mechanical perturbation Rubbing stems of young plants a couple of times daily results in plants that are shorter than controls
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Thigmotropism is growth in response to touch It occurs in vines and other climbing plants
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Rapid leaf movements in response to mechanical stimulation are examples of transmission of electrical impulses called action potentials Video: Mimosa Leaf Video: Mimosa Leaf
LE Leaflets after stimulation Unstimulated Pulvinus (motor organ) Motor organs Side of pulvinus with flaccid cells 0.5 mm Stimulated Side of pulvinus with turgid cells Vein
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Environmental Stresses Environmental stresses have a potentially adverse effect on survival, growth, and reproduction They can have a devastating impact on crop yields in agriculture
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Drought During drought, plants respond to water deficit by reducing transpiration Deeper roots continue to grow
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Flooding Enzymatic destruction of cells creates air tubes that help plants survive oxygen deprivation during flooding
LE Epidermis Air tubes Vascular cylinder Control root (aerated) Experimental root (nonaerated) 100 µm
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Salt Stress 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Heat Stress Heat-shock proteins help plants survive heat stress
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cold Stress Altering lipid composition of membranes is a response to cold stress
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 39.5: Plants defend themselves against herbivores and pathogens Plants counter external threats with defense systems that deter herbivory and prevent infection or combat pathogens
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Defenses Against Herbivores Herbivory, animals eating plants, is a stress that plants face in any ecosystem Plants counter excessive herbivory with physical defenses such as thorns and chemical defenses such as distasteful or toxic compounds
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Some plants even “recruit” predatory animals that help defend against specific herbivores
LE Recruitment of parasitoid wasps that lay their eggs within caterpillars Synthesis and release of volatile attractants Signal transduction pathway Chemical in saliva Wounding
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Defenses Against Pathogens A plant’s first line of defense against infection is its “skin,” the epidermis or 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 inherited ability to recognize certain pathogens
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gene-for-Gene Recognition A virulent pathogen is one that a plant has little specific defense against An avirulent pathogen is one that may harm but not kill the host plant Gene-for-gene recognition involves recognition of pathogen-derived molecules by protein products of specific plant disease resistance (R) genes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A pathogen is avirulent if it has a specific Avr gene corresponding to an R allele in the host plant
LE 39-30a Signal molecule (ligand) from Avr gene product If an Avr allele in the pathogen corresponds to an R allele in the host plant, the host plant will have resistance, making the pathogen avirulent. Receptor coded by R allele Plant cell is resistant Avirulent pathogen Avr allele R
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings If the plant host lacks the R gene that counteracts the pathogen’s Avr gene, then the pathogen can invade and kill the plant
LE 39-30b R No Avr allele; virulent pathogen R allele; plant cell becomes diseased If there is no gene-for-gene recognition because of one of the above three conditions, the pathogen will be virulent, causing disease to develop. Avr allele; virulent pathogen No R allele; plant cell becomes diseased Avr allele No R allele; plant cell becomes diseased No Avr allele; virulent pathogen
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Plant Responses to Pathogen Invasions A hypersensitive response against an avirulent pathogen seals off the infection and kills both pathogen and host cells in the region of the infection
LE R-Avr recognition and hypersensitive response Avirulent pathogen Systemic acquired resistance Signal transduction pathway Hypersensitive response Acquired resistance Signal transduction pathway
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Systemic Acquired Resistance Systemic acquired resistance (SAR) is a set of generalized defense responses in organs distant from the original site of infection Salicylic acid is a good candidate for one of the hormones that activates SAR