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Chapter 39
Plant Responses to Internal
and External Signals
Teaching Objectives
Signal Transduction and Plant Responses
1. Compare the growth of a plant in darkness (etiolation) to the
characteristics of greening (de-etiolation).
2. Describe the signal pathways associated with de-etiolation.
3. Describe the role of second messengers in the process of
de-etiolation.
4. Describe the two main mechanisms by which a signaling
pathway can activate an enzyme.
5. Explain, using several examples, what researchers have
learned about the activity of plant hormones by study of
mutant plants.
Plant Responses to Hormones
6. For the following scientists, describe their hypothesis,
experiments, and conclusions about the mechanism of
phototropism:
a. Charles and Francis Darwin
b. Peter Boysen-Jensen
c. Frits Went
7. List six classes of plant hormones, describe their major
functions, and note where they are produced in the plant.
8. Explain how a hormone may cause its effect on plant growth
and development.
9. Describe a possible mechanism for the polar transport of
auxin.
10. According to the acid growth hypothesis, explain how auxin
can initiate cell elongation.
11. Explain why 2,4-D is widely used as a weed killer.
12. Explain how the ratio of cytokinin to auxin affects cell
division and cell differentiation.
13. Describe the evidence that suggests that factors other than
auxin from the terminal bud may control apical dominance.
14. Describe how stem elongation and fruit growth depend on a
synergism between auxin and gibberellins.
15. Explain the probable mechanism by which gibberellins
trigger seed germination.
16. Describe the functions of brassinosteroids in plants.
17. Describe how abscisic acid (ABA) helps prepare a plant for
winter.
18. Describe the effects of ABA on seed dormancy and drought
stress.
19. Describe the role of ethylene in the triple response to
mechanical stress, apoptosis, leaf abscission, and fruit
ripening.
Plant Responses to Light
20. Define photomorphogenesis and note which colors are most
important to this process.
21. Compare the roles of blue-light photoreceptors and
phytochromes.
22. Describe the phenomenon of chromophore photoreversibility
and explain its role in light-induced germination of lettuce
seeds.
23. Define circadian rhythm and explain what happens when an
organism is artificially maintained in a constant environment.
24. List some common factors that entrain biological clocks.
25. Define photoperiodism.
26. Distinguish among short-day, long-day, and day-neutral
plants. Explain why these names are misleading.
27. Explain what factors other than night length may control
flowering and what is necessary for flowering to occur.
Plant Responses to Environmental Stimuli Other than
Light
28. Describe how plants apparently tell up from down. Explain
why roots display positive gravitropism and shoots exhibit
negative gravitropism.
29. Distinguish between thigmotropism and
thigmomorphogenesis.
30. Describe how motor organs can cause rapid leaf
movements.
31. Provide a plausible explanation for how a stimulus that
causes rapid leaf movement can be transmitted through the
plant.
32. Describe the challenges posed by, and the responses of
plants to, the following environmental stresses: drought,
flooding, salt stress, heat stress, and cold stress.
Plant Defense: Responses to Herbivores and
Pathogens
33. Explain how plants deter herbivores with physical and
chemical defenses.
34. Describe the multiple ways that plants defend against
pathogens.
Student Misconceptions
1.
Many students think of plants as essentially passive and helpless in the
face of environmental challenges. Despite being rooted in the soil, plants are
neither passive nor defenseless. Like animals, plants detect environmental
changes with cellular receptors. As in animals, plant receptors initiate signal
transduction pathways. Unlike animals, few plants respond by movement.
Instead, plants respond to environmental stimuli by modifying their morphology
and/or physiology.
2.
Regulation of circadian rhythms through a protein transcription factor that
self-inhibits its own production provides an excellent and interesting example of
feedback control and may help to clarify this concept for students.
3.
Students are aware that plants have physical and chemical defenses
against herbivores. However, they may be surprised to learn that some plants
broadcast volatile chemicals to recruit parasitoids to attack insect herbivores or
to warn other plants of herbivorous attack and give them time to mount their own
chemical defense.
Chapter Guide to Teaching Resources
Overview: Stimuli and a stationary life
Concept 39.1 Signal transduction pathways link signal reception to
response
Transparencies
Figure 39.3 Review of a general model for signal transduction
pathways
Figure 39.4 An example of signal transduction in plants: The
role of phytochrome in the de-etiolation (greening)
response (layer 1)
Figure 39.4 An example of signal transduction in plants: The
role of phytochrome in the de-etiolation (greening)
response (layer 2)
Figure 39.4 An example of signal transduction in plants: The
role of phytochrome in the de-etiolation (greening)
response (layer 3)
Concept 39.2 Plant hormones help coordinate
growth, development, and responses to stimuli
Transparencies
Figure 39.5 What part of a coleoptile senses light, and how is
the signal transmitted?
Figure 39.6 Does asymmetric distribution of a growthpromoting chemical cause a coleoptile to grow
toward the light?
Table 39.1 An overview of plant hormones
Figure 39.8 Cell elongation in response to auxin: the acid
growth hypothesis
Figure 39.11 Gibberellins mobilize nutrients during the
germination of grain seeds
Figure 39.13 How does ethylene concentration affect the triple
response in seedlings?
Figure 39.15 Ethylene signal transduction mutants can be
distinguished by their different responses to
experimental treatments
Instructor and Student Media Resources
Video: Phototropism
Activity: Leaf abscission
Investigation: What plant hormones affect organ formation?
Concept 39.3 Responses to light are critical for
plant success
Transparencies
Figure 39.17 What wavelengths stimulate phototropic bending
toward light?
Figure 39.19 Structure of a phytochrome
Figure 39.20 Phytochrome: A molecular switching mechanism
Figure 39.22 How does interrupting the dark period with a brief
exposure to light affect flowering?
Figure 39.23 Is phytochrome the pigment that measures the
interruption of dark periods in photoperiodic
response?
Figure 39.24 Is there a flowering hormone?
Student Media Resource
Activity: Flowering lab
Concept 39.4 Plants respond to a wide variety of
stimuli other than light
Instructor and Student Media Resources
Video: Gravitropism
Video: Mimosa leaf
Concept 39.5 Plants defend themselves against herbivores and pathogens
Transparencies
Figure 39.29 A maize leaf “recruits” a parasitoid wasp as a
defensive response to an herbivore, an army-worm
caterpillar
Figure 39.30 Gene-for-gene resistance of plants to pathogens: The receptorligand model
Figure 39.31 Defense responses against an avirulent pathogen
For additional resources such as digital images and lecture
outlines, go to the Campbell Media Manager or the Instructor
Resources section of www.campbellbiology.com.
Key Terms
abiotic
abscisic acid (ABA)
action potential
action spectrum
apoptosis
auxin
avirulent
biotic
blue-light photoreceptors
brassinosteroids
circadian rhythm
cytokinins
day-neutral plant
de-etiolation
elicitor
ethylene
etiolation
expansins
florigen
gene-for-gene recognition
gibberellins
gravitropism
heat-shock protein
hormone
hypersensitive response (HR)
jasmonic acid
long-day plant
oligosaccharin
photomorphogenesis
photoperiodism
phototropism
phytoalexin
phytochromes
PR protein
salicylic acid
second messenger
short-day plant
statolith
systemic acquired resistance (SAR)
thigmomorphogenesis
thigmotropism
triple response
tropism
vernalization
virulent
Word Roots
aux- 5 grow, enlarge (auxins: a class of plant hormones, including indoleacetic acid,
having a variety of effects, such as phototropic response through the stimulation of
cell elongation, stimulation of secondary growth, and the development of leaf traces
and fruit)
circ- 5 a circle (circadian rhythm: a physiological cycle of about 24 hours,
present in all eukaryotic organisms, that persists even in the absence of external
cues)
crypto- 5 hidden; -chromo 5 color (cryptochrome: the name given to the
unidentified blue-light photoreceptor)
cyto- 5 cell; -kine 5 moving (cytokinins: a class of related plant hormones that
retard aging and act in concert with auxins to stimulate cell division, influence the
pathway of differentiation, and control apical dominance)
gibb- 5 humped (gibberellins: a class of related plant hormones that stimulate
growth in the stem and leaves, trigger the germination of seeds and breaking of
bud dormancy, and stimulate fruit development with auxin)
hyper- 5 excessive (hypersensitive response: a vigorous, localized defense
response to a pathogen that is avirulent based on an R-Avr match)
photo- 5 light; -trop 5 turn, change (phototropism: growth of a plant shoot toward
or away from light)
phyto- 5 a plant; -alexi 5 to ward off (phytoalexin: an antibiotic, produced by
plants, that destroys microorganisms or inhibits their growth)
stato- 5 standing, placed; -lith 5 a stone (statolith: specialized plastids that help
a plant distinguish up from down)
thigmo- 5 a touch; morpho- 5 form; -genesis 5 origin (thigmomorphogenesis: a
response in plants to chronic mechanical stimulation, resulting from increased
ethylene production; an example is thickening stems in response to strong
winds)
zea- 5 a grain; -xantho 5 yellow (zeaxanthin: a blue-light photoreceptor involved
in stomatal opening)
Instructor’s Guide for Campbell/Reece Biology, Seventh EditionChapter 39 Plant Responses to Internal and External
Signals
Instructor’s Guide for Campbell/Reece Biology, Seventh EditionChapter 39 Plant Responses to Internal and
External Signals
Instructor’s Guide for Campbell/Reece Biology, Seventh EditionChapter 39 Plant Responses to
Internal and External Signals