1. PLANT RESPONSES TO INTERNAL AND EXTERNAL SIGNALS
CHAPTER 39
PLANT RESPONSES TO INTERNAL AND EXTERNAL SIGNALS

2. Fig. 39-1
Figure 39.1 Can flowers tell you the time of day?

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. (a) Before exposure to light (b) After a week’s exposure to
Fig. 39-2
Figure 39.2 Light-induced de-etiolation (greening) of dark-grown potatoes
(a) Before exposure to light
(b) After a week’s exposure to
natural daylight

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. A trophism is 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. Abscisic acid
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. Photomorphogenesis is 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. Phytochromes are 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. Photoreversible states of phytochrome
Fig. 39-UN3
Photoreversible states of phytochrome Pr
Pfr
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
Figure Sleep movements of a bean plant (Phaseolus vulgaris)
Noon
Midnight

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. Photoperiodism is 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 plants can flower in days of any length.

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

38. (a) Unstimulated state (b) Stimulated state
Fig ab
Figure Rapid turgor movements by the sensitive plant (Mimosa pudica)
(a) Unstimulated state
(b) Stimulated 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. 4 3 1 1 2 Recruitment of parasitoid wasps that lay their eggs
Fig 4
Recruitment of
parasitoid wasps
that lay their eggs
within caterpillars 3
Synthesis and
release of
volatile attractants
Figure A maize leaf “recruiting” a parasitoid wasp as a defensive response to an armyworm caterpillar, an herbivore 1
Wounding 1
Chemical
in saliva 2
Signal transduction
pathway

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 avirulent pathogen is one that may harm but does not kill the host plant

45. 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