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 whatever environmental change comes their way

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings For example, the bending of a grass seedling toward light – Begins with the plant sensing the direction, quantity, and color of the light Figure 39.1

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 39.1: Signal transduction pathways link signal reception to response Plants have cellular receptors – That they use to detect important 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 – Will produce shoots that do not appear healthy, and will lack elongated roots These are morphological adaptations for growing in darkness – Collectively referred to as etiolation Figure 39.2a (a) 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.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings After the potato is exposed to light – The plant undergoes profound changes called de- etiolation, in which shoots and roots grow normally Figure 39.2b (b) 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 The potato’s response to light – Is an example of cell-signal processing Figure 39.3 CELL WALL CYTOPLASM 1 Reception 2 Transduction 3 Response Receptor Relay molecules Activation of cellular responses Hormone or environmental stimulus 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 specific responses

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure Reception 2 Transduction 3 Response CYTOPLASM Plasma membrane Phytochrome activated by light Cell wall Light cGMP Second messenger produced Specific protein kinase 1 activated Transcription factor 1 NUCLEUS P P Transcription Translation De-etiolation (greening) response proteins Ca 2+ Ca 2+ channel opened Specific protein kinase 2 activated Transcription factor 2 An example of signal transduction in plants 1 The light signal is detected by the phytochrome receptor, which then activates at least two signal transduction pathways. 2 One pathway uses cGMP as a second messenger that activates a specific protein kinase.The other pathway involves an increase in cytoplasmic Ca 2+ that activates another specific protein kinase. 3 Both pathways lead to expression of genes for proteins that function in the de-etiolation (greening) response.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Response Ultimately, a signal transduction pathway – Leads to a regulation of one or more cellular activities In most cases – These responses to stimulation involve the increased activity of certain enzymes

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Transcriptional Regulation Transcription factors bind directly to specific regions of DNA – And control the transcription of specific genes

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Post-Translational Modification of Proteins Post-translational modification – Involves the activation of existing proteins involved 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 involved in photosynthesis directly – While others are involved in supplying the chemical precursors necessary 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 the different parts of an organism

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Discovery of Plant Hormones Any growth response – That results in curvatures of whole plant organs toward or away from a stimulus is called a tropism – Is often caused by hormones

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Charles Darwin and his son Francis – Conducted some of the earliest experiments on phototropism, a plant’s response to light, in the late 19th century

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 39.5 In 1880, Charles Darwin and his son Francis designed an experiment to determine what part of the coleoptile senses light. In 1913, Peter Boysen-Jensen conducted an experiment to determine how the signal for phototropism is transmitted. EXPERIMENT In the Darwins’ experiment, a phototropic response occurred only when light could reach the tip of coleoptile. Therefore, they concluded that only the tip senses light. Boysen-Jensen observed that a phototropic response occurred if the tip was separated by a permeable barrier (gelatin) but not if separated by an impermeable solid barrier (a mineral called mica). These results suggested that the signal is a light-activated mobile chemical. CONCLUSION RESULTS ControlDarwin and Darwin (1880) Boysen-Jensen (1913) Light Shaded side of coleoptile Illuminated side of coleoptile Light Tip removed Tip covered by opaque cap Tip covered by trans- parent cap Base covered by opaque shield Light 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 Went concluded that a coleoptile curved toward light because its dark side had a higher concentration of the growth-promoting chemical, which he named auxin. The coleoptile grew straight if the chemical was distributed evenly. If the chemical was distributed unevenly, the coleoptile curved away from the side with the block, as if growing toward light, even though it was grown in the dark. Excised tip placed on agar block Growth-promoting chemical diffuses into agar block Agar block with chemical stimulates growth Control (agar block lacking chemical) has no effect Control Offset blocks cause curvature RESULTS CONCLUSION In 1926, Frits Went’s experiment identified how a growth-promoting chemical causes a coleoptile to grow toward light. He placed coleoptiles in the dark and removed their tips, putting some tips on agar blocks that he predicted would absorb the chemical. On a control coleoptile, he placed a block that lacked the chemical. On others, he placed blocks containing the chemical, either centered on top of the coleoptile to distribute the chemical evenly or offset to increase the concentration on one side. EXPERIMENT Figure 39.6

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A Survey of Plant Hormones

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In general, hormones control plant growth and development – By affecting the division, elongation, and differentiation of cells Plant hormones are produced in very low concentrations – But a minute amount can have a profound effect on the growth and development of a plant organ

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Auxin The term auxin – Is used for any chemical substance that promotes cell elongation in different target tissues Auxin – Is involved in the formation and branching of roots 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 – Stimulate cell division Cytokinins – Are produced in actively growing tissues such as roots, embryos, and fruits – 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 – The ability of a terminal bud to suppress development of axillary buds Figure 39.9a Axillary buds

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings If the terminal bud is removed – Plants become bushier Figure 39.9b “Stump” after removal of apical bud Lateral branches

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 both leaves and stems In stems – Gibberellins 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

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gibberellins are used commercially – In the spraying of Thompson seedless grapes Figure 39.10

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings After water is imbibed, the release of gibberellins from the embryo – Signals the seeds to break dormancy and germinate Germination Figure The aleurone responds by synthesizing and secreting digestive enzymes that hydrolyze stored nutrients in the endosperm. One example is  -amylase, which hydrolyzes starch. (A similar enzyme in our saliva helps in digesting bread and other starchy foods.) Aleurone Endosperm Water Scutellum (cotyledon) GA  -amylase Radicle Sugar 1 After a seed imbibes water, the embryo releases gibberellin (GA) as a signal to the aleurone, the thin outer layer of the endosperm. 3 Sugars and other nutrients absorbed from the endosperm by the scutellum (cotyledon) are consumed during growth of the embryo into a seedling.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2 The aleurone responds by synthesizing and secreting digestive enzymes that hydrolyze stored nutrients in the endosperm. One example is  -amylase, which hydrolyzes starch. (A similar enzyme in our saliva helps in digesting bread and other starchy foods.) Aleurone Endosperm Water Scutellum (cotyledon) GA  -amylase Radicle Sugar 2 1 After a seed imbibes water, the embryo releases gibberellin (GA) as a signal to the aleurone, the thin outer layer of the endosperm. 3 Sugars and other nutrients absorbed from the endosperm by the scutellum (cotyledon) are consumed during growth of the embryo into a seedling.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Abscisic Acid Two of the many effects of abscisic acid (ABA) are – Seed dormancy – Drought tolerance

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Seed Dormancy Seed dormancy has great survival value – Because it ensures that the seed will germinate only when there are optimal conditions

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Precocious germination is observed in maize mutants – That lack a functional transcription factor required for ABA to induce expression of certain genes Figure 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 Figure Ethylene induces the triple response in pea seedlings, with increased ethylene concentration causing increased response. CONCLUSION Germinating pea seedlings were placed in the dark and exposed to varying ethylene concentrations. Their growth was compared with a control seedling not treated with ethylene. EXPERIMENT All the treated seedlings exhibited the triple response. Response was greater with increased concentration. RESULTS 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 Figure 39.14a ein mutant

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Apoptosis: Programmed Cell Death A burst of ethylene – Is associated with 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 Figure mm Protective layer Abscission layer Stem Petiole

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fruit Ripening A burst of ethylene production in the fruit – Triggers the ripening process

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 what plant biologists call photomorphogenesis

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Plants not only detect the presence of light – But also its direction, intensity, and wavelength (color) A graph called an action spectrum – Depicts the relative response of a process to different wavelengths of light

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Action spectra – Are useful in the study of any process that depends on light Figure Wavelength (nm) Light Time = 0 min. Time = 90 min Phototropic effectiveness relative to 436 nm Researchers exposed maize (Zea mays) coleoptiles to violet, blue, green, yellow, orange, and red light to test which wavelengths stimulate the phototropic bending toward light. EXPERIMENT The graph below shows phototropic effectiveness (curvature per photon) relative to effectiveness of light with a wavelength of 436 nm. The photo collages show coleoptiles before and after 90-minute exposure to side lighting of the indicated colors. Pronounced curvature occurred only with wavelengths below 500 nm and was greatest with blue light. RESULTS CONCLUSION The phototropic bending toward light is caused by a photoreceptor that is sensitive to blue and violet light, particularly blue light.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Biological Clocks and Circadian Rhythms Many plant processes – Oscillate during the day

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Many legumes – Lower their leaves in the evening and raise them in the morning Figure Noon Midnight

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cyclical responses to environmental stimuli are called circadian rhythms – And are approximately 24 hours long – 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 developmental processes, including flowering in many species – Requires a certain photoperiod

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Critical Night Length In the 1940s, researchers discovered that flowering and other responses to photoperiod – Are actually controlled by night length, not day length Figure During the 1940s, researchers conducted experiments in which periods of darkness were interrupted with brief exposure to light to test how the light and dark portions of a photoperiod affected flowering in “short-day” and “long-day” plants. EXPERIMENT RESULTS CONCLUSION The experiments indicated that flowering of each species was determined by a critical period of darkness (“critical night length”) for that species, not by a specific period of light. Therefore, “short-day” plants are more properly called “long-night” plants, and “long-day” plants are really “short-night” plants. 24 hours Darkness Flash of light Critical dark period Light (a) “Short-day” plants flowered only if a period of continuous darkness was longer than a critical dark period for that particular species (13 hours in this example). A period of darkness can be ended by a brief exposure to light. (b) “Long-day” plants flowered only if a period of continuous darkness was shorter than a critical dark period for that particular species (13 hours in this example).

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Action spectra and photoreversibility experiments – Show that phytochrome is the pigment that receives red light, which can interrupt the nighttime portion of the photoperiod Figure A unique characteristic of phytochrome is reversibility in response to red and far-red light. To test whether phytochrome is the pigment measuring interruption of dark periods, researchers observed how flashes of red light and far-red light affected flowering in “short-day” and “long-day” plants. EXPERIMENT RESULTS CONCLUSION A flash of red light shortened the dark period. A subsequent flash of far-red light canceled the red light’s effect. If a red flash followed a far-red flash, the effect of the far-red light was canceled. This reversibility indicated that it is phytochrome that measures the interruption of dark periods Hours Short-day (long-night) plant Long-day (short-night) plant R R FR R RR 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, and it may be a hormone or a change in relative concentrations of multiple hormones

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure To test whether there is a flowering hormone, researchers conducted an experiment in which a plant that had been induced to flower by photoperiod was grafted to a plant that had not been induced. EXPERIMENT RESULTS CONCLUSION Both plants flowered, indicating the transmission of a flower-inducing substance. In some cases, the transmission worked even if one was a short-day plant and the other was a long-day plant. Plant subjected to photoperiod that induces flowering Plant subjected to photoperiod that does not induce flowering Graft Time (several weeks)

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Meristem Transition and Flowering Whatever combination of environmental cues and internal signals is necessary for flowering to occur – The outcome is the transition of a bud’s meristem from a vegetative to a flowering state

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 their immobility – Plants must adjust to a wide 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 Figure 39.25a, b Statoliths 20  m (a) (b)

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mechanical Stimuli The term thigmomorphogenesis – Refers to the changes in form that result from mechanical perturbation

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Rubbing the stems of young plants a couple of times daily – Results in plants that are shorter than controls Figure 39.26

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Growth in response to touch – Is called thigmotropism – Occurs in vines and other climbing plants