CHAPTER 39
PLANT RESPONSES TO
INTERNAL AND EXTERNAL
SIGNALS
1
Fig. 39-1
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
• 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
Fig. 39-2
(a) Before exposure to light
(b) After a week’s exposure to
natural daylight
• A potato’s response to light is an example of cell-signal
processing
• The stages are reception, transduction, and response
General Model for Signal Transduction
Pathways
7
Reception
• Internal and external signals are detected by receptors,
proteins that change in response to specific stimuli
Transduction
• Second messengers transfer and amplify signals from
receptors to proteins that cause responses
Reception-Transduction
10
Second Messengers
11
Reception-Transduction-Response
12
3. Response:
• Cellular response is primarily accomplished by two
mechanisms: (1) increasing or decreasing mRNA
production, or (2) activating existing enzyme molecules
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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
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Phototropism in Grass Seedlings
15
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
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F. W. Went’s Experiment
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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
Table 39-1
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
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Apical Dominance—Auxin/Cytokinin
Control
23
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)
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Effect of Gibberellin (on right)
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f. Ethylene (gas)
• Promotes fruit ripening
• Prepare for leaf abscission
• Initiates triple response (growth maneuver so a
shoot can avoid an obstacle)
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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
28
• Phytochromes exist in two photoreversible states, with
conversion of Pr to Pfr triggering many developmental
responses
• Phytochromes absorb mostly red light
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Fig. 39-UN3
Photoreversible states of phytochrome
Pfr
Pr
Red light
Responses
Far-red
light
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
Fig. 39-20
Noon
Midnight
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
33
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
34
• 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.
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36
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
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Fig. 39-26ab
(a) Unstimulated state
(b) Stimulated state
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
Concept 39.5: Plants
respond to attacks by
herbivores and pathogens
• Plants use defense systems to deter herbivory, prevent
infection, and combat pathogens
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
Fig. 39-28
4 Recruitment of
parasitoid wasps
that lay their eggs
within caterpillars
3 Synthesis and
release of
volatile attractants
1 Wounding
1 Chemical
in saliva
2 Signal transduction
pathway
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
• 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
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