1. 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

2. 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

3. 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

4. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 39.4 1 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.

5. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Plant hormones help coordinate growth, development, and responses to stimuli Hormones – Are chemical signals that coordinate the different parts of an organism

6. 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 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

7. 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

8. 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

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

10. 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

11. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Auxin [IAA] indoleacetic acid is naturally found 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 Increases plasticity of cells leading to growth (Acid growth hypothesis). When light illuminates one side of a shoot, auxin diffuses to side opposite of light, causing elongation of those cells which causes the shoot to bend toward the light!

12. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Expansin CELL WALL Cell wall enzymes Cross-linking cell wall polysaccharides Microfibril H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ ATP Plasma membrane Plasma membrane Cell wall Nucleus Vacuole Cytoplasm H2OH2O Cell elongation in response to auxin Figure 39.8 1 Auxin increases the activity of proton pumps. 4 The enzymatic cleaving of the cross-linking polysaccharides allows the microfibrils to slide. The extensibility of the cell wall is increased. Turgor causes the cell to expand. 2 The cell wall becomes more acidic. 5 With the cellulose loosened, the cell can elongate. 3 Wedge-shaped expansins, activated by low pH, separate cellulose microfibrils from cross-linking polysaccharides. The exposed cross-linking polysaccharides are now more accessible to cell wall enzymes.

13. 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 Removal of the terminal bud stimulates lateral growth from lateral/axillary meristems at node (where leaves attach to the stem). Auxin inhibits lateral meristems from growing so the removal of the apical meristem would allow the plant to grow laterally (gets bushy).

14. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cytokinins – Stimulate cell division – Are produced in actively growing tissues such as roots, embryos, and fruits – Work together with auxin

15. 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 – Stimulates the growth of lateral buds-release of apical dominance Axillary buds

16. 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

17. 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 – Primarily produced in roots and travels to the shoots in the xylem sap. – Promotes growth of lateral buds (axillary bud growth) – Signals break of dormancy in seeds (i.e. Germination) and winter buds.

18. 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 commercially – In the spraying of Thompson seedless grapes

19. 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 What happens during Germination? Figure 39.11 2 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 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.

20. 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.

21. 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 – Produced from methionine in biochemical pathway involving enzyme 1-amino- cyclopropane-1-carboxylic acid synthase (ACC-synthase)

22. 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 – Stimulates leaf and fruit abscission.

23. 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 39.16 0.5 mm Protective layer Abscission layer Stem Petiole

24. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fruit Ripening A burst of ethylene production in the fruit – Triggers the ripening process – ACC required to produce ethylene. Altering of ACC gene prevents ethylene production  prolonging shelf life of fruits Eg. Flavr Savr Tomato the first GM food on the market in 1994

25. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

26. 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

27. 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

28. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Action spectra – Are useful in the study of any process that depends on light Figure 39.17 Wavelength (nm) 1.0 0.8 0.6 0.2 0 450500550600650700 Light Time = 0 min. Time = 90 min. 0.4 400 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.

29. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Research on action spectra and absorption spectra of pigments – Led to the identification of two major classes of light receptors: blue-light photoreceptors and phytochromes

30. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Blue-Light Photoreceptors Various blue-light photoreceptors – Control hypocotyl elongation, stomatal opening, and phototropism

31. 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

32. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Phytochromes and Seed Germination Studies of seed germination – Led to the discovery of phytochromes

33. 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

34. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 39.18 Dark (control) Dark Red Far-red Red Far-red Red DarkRed Far-red Red Far-red CONCLUSION EXPERIMENT RESULTS During the 1930s, USDA scientists briefly exposed batches of lettuce seeds to red light or far-red light to test the effects on germination. After the light exposure, the seeds were placed in the dark, and the results were compared with control seeds that were not exposed to light. The bar below each photo indicates the sequence of red-light exposure, far-red light exposure, and darkness. The germination rate increased greatly in groups of seeds that were last exposed to red light (left). Germination was inhibited in groups of seeds that were last exposed to far-red light (right). Red light stimulated germination, and far-red light inhibited germination. The final exposure was the determining factor. The effects of red and far-red light were reversible.

35. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A phytochrome – Is the photoreceptor responsible for the opposing effects of red and far-red light A phytochrome consists of two identical proteins joined to form one functional molecule. Each of these proteins has two domains. Chromophore Photoreceptor activity. One domain, which functions as the photoreceptor, is covalently bonded to a nonprotein pigment, or chromophore. Kinase activity. The other domain has protein kinase activity. The photoreceptor domains interact with the kinase domains to link light reception to cellular responses triggered by the kinase. Figure 39.19

36. 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 Figure 39.20 Synthesis Far-red light Red light Slow conversion in darkness (some plants) Responses: seed germination, control of flowering, etc. Enzymatic destruction P fr PrPr

37. 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 of a tree – The phytochrome ratio shifts in favor of P r when a tree is shaded

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

39. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Many legumes – Lower their leaves in the evening and raise them in the morning Figure 39.21 Noon Midnight

40. 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

41. 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

42. 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

43. 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

44. 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 39.22 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).

45. 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 39.23 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. 24 20 16 12 8 4 0 Hours Short-day (long-night) plant Long-day (short-night) plant R R FR R RR R Critical dark period

46. 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

47. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 39.24 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)

48. 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