CHAPTER 39

1. CHAPTER 39 PLANT RESPONSES TO INTERNAL AND EXTERNAL SIGNALS

2. Fig. 39-1

3. Concept 39.1: Signal transduction pathways • Plants have cellular receptors that detect changes in their environment • For a stimulus to elicit a response, certain cells must have an appropriate receptor • Stimulation of the receptor initiates a specific signal transduction pathway

4. These are morphological adaptations for growing in darkness, collectively called etiolation • After exposure to light, a potato undergoes changes called de-etiolation, in which shoots and roots grow normally

5. Fig. 39-2 (b) After a week’s exposure to natural daylight (a) Before exposure to light

6. A potato’s response to light is an example of cell-signal processing • The stages are reception, transduction, and response

7. General Model for Signal Transduction Pathways

8. Reception • Internal and external signals are detected by receptors, proteins that change in response to specific stimuli

9. Transduction • Second messengers transfer and amplify signals from receptors to proteins that cause responses

10. Reception-Transduction

11. Second Messengers

12. Reception-Transduction-Response

13. 3. Response: • Cellular response is primarily accomplished by two mechanisms: (1) increasing or decreasing mRNA production, or (2) activating existing enzyme molecules

14. II. Concept 39.2: Plant Hormones A. Hormones are defined as chemical messengers that coordinate different parts of a multicellular organism • They are produced by one part of the body and transported to another B. Atrophismis a plant growth response from hormones that results in the plant growing either toward (positive) or away (negative) from a stimulus • Phototrophism is the growth of a shoot in a certain direction in response to light. • Positive phototropism is the growth of a plant toward light • Negative phototropism is the growth of a plant away from light

15. Phototropism in Grass Seedlings

16. C. Discovery of Auxin 1. Charles and Francis Darwin (1881) • Concluded that coleoptile tips were responsible for sensing light and producing a substance that was transported to elongating region 2. Peter Boysen-Jensen • Demonstrated that the substance for elongation was mobile 3. F. W. Went (1926) • Named substance for elongation—auxin

19. F. W. Went’s Experiment

20. 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

21. Table 39-1

22. D. Actions of Plant Hormones 1. Six Classes of Plant Hormones: a. Auxin (natural auxin—IAA–indoleacetic acid) • Stimulates cell elongation • Promotes root formation • Regulates fruit development • Enhances apical dominance b. Cytokinins • regulate cell division • Anti-aging effects (keeps cut flowers fresh) • Slow apoptosis

23. Apical Dominance—Auxin/Cytokinin Control

24. c. Gibberellins • Promote stem elongation • Promote seed germination • Contributes to fruit growth d. Brassinosteroids • Promote cell elongation and division • Promotes xylem differentiation • Slow leaf abscission e. Abscisicacid • Promotes seed dormancy until optimum conditions • Drought tolerance (closes stomata during water stress)

25. Effect of Gibberellin (on right)

27. f. Ethylene (gas) • Promotes fruit ripening • Prepare for leaf abscission • Initiates triple response (growth maneuver so a shoot can avoid an obstacle)

28. III. Concept 39.3: Plant Responses to Light A. Photomorphogenesisis the term used to describe the effects of light on plant morphology B. There are two major classes of light receptors: 1. Blue-light photoreceptors initiate a number of plant responses to light including phototropisms and the light-induced opening of the stomata 2. Phytochromesare pigments that regulate many of a plant’s responses to light throughout its life • Responses include seed germination and shade avoidance

29. Phytochromes exist in two photoreversible states, with conversion of Pr to Pfr triggering many developmental responses • Phytochromes absorb mostly red light

30. Fig. 39-UN3 Photoreversible states of phytochrome Pfr Pr Red light Responses Far-red light

31. 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, even when kept under constant light or dark conditions

32. Fig. 39-20 Midnight Noon

33. C. Circadian rhythms are physiological cycles that have a frequency of about 24 hours and that are not paced by a known environmental clock. • In plants, the surge of Pfr at dawn resets the biological clock. • The combination of a phytochrome system and a biological clock allow the plant to accurately assess the amount of daylight or darkness and hence the time of the year

34. D. Photoperiodismis defined as a physiological response to a photoperiod (the relative lengths of night and day). • Important in plant life cycles such as flowering • It is night length—not day length—that controls flowering and certain other response to photoperiod. • Short-day plants require a period of continuous darkness longer than a critical period (length of day) in order to flower. These plants usually flower in late summer, fall, and winter

35. Long-day plants flower only if a period of continuous darkness was shorter than a critical period. They often flower in late spring or early summer. They are actually short-night plants. • Day-neutral plantscan flower in days of any length.

37. E. Responses to Other Environmental Stimuli 1. Gravitropismis a plant’s response to gravity • Roots show positive gravitropism • Shoots show negative gravitropism • Auxins play a key role in gravitropism 2. Thigmotropismis directional growth as a response to touch • Ex: tendrils

38. Fig. 39-26ab (b) Stimulated state (a) Unstimulated state

39. Environmental Stresses • Environmental stresses have a potentially adverse effect on survival, growth, and reproduction • Stresses can be abiotic (nonliving) or biotic (living) • Abiotic stresses include drought, flooding, salt stress, heat stress, and cold stress

40. Concept 39.5: Plants respond to attacks by herbivores and pathogens • Plants use defense systems to deter herbivory, prevent infection, and combat pathogens

41. Defenses Against Herbivores • defenses such as thorns and chemical defenses such as distasteful or toxic compounds • Some plants even “recruit” predatory animals that help defend against specific herbivores

42. Fig. 39-28 Recruitment of parasitoid wasps that lay their eggs within caterpillars 4 3 Synthesis and release of volatile attractants Chemical in saliva 1 Wounding 1 Signal transduction pathway 2

43. Defenses Against Pathogens • A plant’s first line of defense against infection is the epidermis and periderm • If a pathogen penetrates the dermal tissue, the second line of defense is a chemical attack that kills the pathogen and prevents its spread

44. A virulent pathogen is one that a plant has little specific defense against • An avirulentpathogen is one that may harm but does not kill the host plant

45. The Hypersensitive Response • The hypersensitive response • Causes cell and tissue death near the infection site • Induces production proteins, which attack the pathogen • Stimulates changes in the cell wall that confine the pathogen