1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Chapter 39 Plant Responses to Internal and External Signals

2. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 39.1: Signal transduction pathways  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

3. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Growing in Darkness  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.

4. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Growing in Light  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.

5. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Response to Light  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

6. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Reception-Transduction-Response  Internal and external signals are detected by receptors Receptors are proteins that change in response to specific stimuli  During transduction the message is transmitted to effectors or second messengers Second messengers transfer and amplify signals from receptors to proteins that cause specific responses

7. 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 Signal Transduction in Plants  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.

8. 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  Transcriptional Regulation: Transcription factors bind directly to specific regions of DNA and control the transcription of specific genes  Post-translational modification involves the activation of existing proteins involved in the signal response

9. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 39.2: Plant Hormones  Plant hormones help coordinate growth, development, and responses to stimuli  Hormones are chemical signals that coordinate the different parts of an organism  Tropisms are any growth response that results in curvatures of whole plant organs toward or away from a stimulus is often caused by hormones Ex. Plant Hormones

10. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Discovery of Plant Hormones 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

11. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Went’s Experiment  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

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

13. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Characteristics 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 concentrations But a minute amount can have a profound effect on the growth and development of a plant organ

14. 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 (lateral and adventitious root formation)  Auxins as Herbicides: an overdose of auxins can kill eudicots  Production sites:

15. 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

16. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Auxin transporters  Auxin transporters m ove the hormone out of the basal end of one cell, and into the apical end of neighboring cells Figure 39.7 EXPERIMENT Cell 1 Cell 2 100  m Epidermis Cortex Phloem Xylem Pith Basal end of cell 25  m To investigate how auxin is transported unidirectionally, researchers designed an experiment to identify the location of the auxin transport protein. They used a greenish-yellow fluorescent molecule to label antibodies that bind to the auxin transport protein. They applied the antibodies to longitudinally sectioned Arabidopsis stems. RESULTS The left micrograph shows that the auxin transport protein is not found in all tissues of the stem, but only in the xylem parenchyma. In the right micrograph, a higher magnification reveals that the auxin transport protein is primarily localized to the basal end of the cells. CONCLUSION The results support the hypothesis that concentration of the auxin transport protein at the basal ends of cells is responsible for polar transport of auxin.

17. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Role of Auxin in Cell Elongation  According to a model called the acid growth hypothesis Proton pumps play a major role in the growth response of cells to auxin

18. 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.

19. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cytokinins  Cytokinins stimulate cell division Cytokinins are produced in actively growing tissues such as roots, embryos, and fruits Work together with auxin Control Cell Division and Differentiation

20. 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 Apical dominance the ability of a terminal bud to suppress development of axillary buds Figure 39.9a Axillary buds

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

22. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Anti-Aging Effects  Cytokinins retard the aging of some plant organs They inhibit protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients from surrounding tissues

23. 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  Gibberellins stimulate growth of both leaves and stems In stems, gibberellins stimulate cell elongation and cell division In many plants both auxin and gibberellins must be present for fruit to set (fruit growth)

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

25. 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 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.

26. 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  Seed dormancy has great survival value Because it ensures that the seed will germinate only when there are optimal conditions  ABA is the primary internal signal that enables plants to withstand drought (drought tolerance)

27. 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  Apoptosis: Programmed Cell Death A burst of ethylene is associated with the programmed destruction of cells, organs, or whole plants  Fruit ripening: A burst of ethylene production in the fruit triggers the ripening process

28. 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 39.13 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 0.00 0.10 0.20 0.40 0.80 Ethylene concentration (parts per million)

29. 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

30. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 39.3: Responses to Light  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

31. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Plant Response to Light  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

32. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Action Spectra  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.

33. 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 Studies of seed germination led to the discovery of phytochromes

34. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Phytochrome & Seed Germination 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 Phytochrome Photoreceptors  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 States of Phytochrome  Phytochromes exist in two photoreversible states with conversion of P r to P fr triggering many developmental responses Phytochrome Signaling 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  Many legumes lower their leaves in the evening and raise them in the morning Figure 39.21 Noon Midnight

39. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Biological Clocks and Circadian Rhythms  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

40. 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

41. 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

42. 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

43. 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).

44. 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 Flowering

45. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 39.5: Plant Defenses  Plants defend themselves against herbivores and pathogens  Plants counter external threats with defense systems that deter herbivory and prevent infection or combat pathogens Plant Pathogens

46. 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 With chemical defenses such as distasteful or toxic compounds

47. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Recruitment of parasitoid wasps that lay their eggs within caterpillars 4 3 Synthesis and release of volatile attractants 1 Chemical in saliva 1 Wounding 2 Signal transduction pathway Recruitment  Some plants even “recruit” predatory animals that help defend the plant against specific herbivores Figure 39.29

48. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Defenses Against Pathogens  A plant’s first line of defense against infection is the physical barrier of the plant’s “skin,” the epidermis and the periderm  Once a pathogen invades a plant the plant mounts a chemical attack as a second line of defense that kills the pathogen and prevents its spread  The second defense system is enhanced by the plant’s inherited ability to recognize certain pathogens

49. 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 is a widespread form of plant disease resistance

50. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 39.30a Receptor coded by R allele (a) 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. R alleles probably code for receptors in the plasma membranes of host plant cells. Avr alleles produce compounds that can act as ligands, binding to receptors in host plant cells. Avirulent Pathogen  A pathogen is avirulent if it has a specific Avr gene corresponding to a particular R allele in the host plant Signal molecule (ligand) from Avr gene product Avr allele Plant cell is resistant Avirulent pathogen R

51. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Virulent Pathogens  If the plant host lacks the R gene that counteracts the pathogen’s Avr gene then the pathogen can invade and kill the plant Figure 39.30b No Avr allele; virulent pathogen Plant cell becomes diseased Avr allele No R allele; plant cell becomes diseased Virulent pathogen No R allele; plant cell becomes diseased (b) 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. R

52. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 3In a hypersensitive response (HR), plant cells produce anti- microbial molecules, seal off infected areas by modifying their walls, and then destroy themselves. This localized response produces lesions and protects other parts of an infected leaf. 4 Before they die, infected cells release a chemical signal, probably salicylic acid. 6 In cells remote from the infection site, the chemical initiates a signal transduction pathway. 5 The signal is distributed to the rest of the plant. 2 This identification step triggers a signal transduction pathway. 1 Specific resistance is based on the binding of ligands from the pathogen to receptors in plant cells. 7Systemic acquired resistance is activated: the production of molecules that help protect the cell against a diversity of pathogens for several days. Signal 7 6 5 4 3 2 1 Avirulent pathogen Signal transduction pathway Hypersensitive response Signal transduction pathway Acquired resistance R-Avr recognition and hypersensitive response Systemic acquired resistance Figure 39.31 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

53. 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 Is triggered by the signal molecule salicylic acid