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(6) Chapter 39
Plant Responses to Internal
and External Signals
植物對內在與外在訊息的反應
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Key Concepts
• Concept 39.1: Signal transduction pathways
link signal reception to response
• Concept 39.2: Plant hormones help coordinate
growth, development, and responses to stimuli
• Concept 39.3: Responses to light are critical
for plant success
• Concept 39.4: Plants respond to a wide variety
of stimuli other than light
• Concept 39.5: Plants defend themselves
against herbivores and pathogens
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Stimulus and response (刺激與回應)
•Overview: Stimuli and a Stationary Life (刺激與穩定)
•Plants, being rooted to the ground must respond to
whatever environmental change (環境變遷) comes
their way
•For example, the bending of a grass seedling (幼苗
的彎曲) toward light begins with the plant sensing
the direction, quantity, and color of the light
Figure 39.1
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Stimulus and response (刺激與回應)
• Concept 39.1: Signal transduction
pathways link signal reception to response
• Plants have cellular receptors (細胞接受器)
– That they use to detect important changes
in their environment
• For a stimulus to elicit a response (為了誘發
對一個刺激的回應)
– Certain cells must have an appropriate
receptor
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• A potato left growing in darkness (馬鈴薯在
黑暗中的生長) will produce shoots that do
not appear healthy, and will lack elongated
roots
• These are morphological adaptations (形態
適應) for growing in darkness collectively
referred to as etiolation (白化作用)
(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.
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Figure 39.2a
• After the potato is exposed to light
– The plant undergoes profound changes called
de-etiolation (去白化作用), in which shoots and
roots grow normally
(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 (光敏素).
Figure 39.2b
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• The potato’s response to light is an
example of cell-signal processing (細胞-訊
息加工過程)
Cell wall
1. 接收
1. Reception
Receptor
受體
(接受器)
Hormone or
environmental
stimulus 賀爾蒙或
環境刺激
Figure 39.3
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Cytoplasm
2. 傳遞
2. Transduction
Relay molecules
備用分子
(第二傳訊者)
3. 反應
3. Response
Activation
of cellular
responses
Plasma membrane
細胞反應
的活化
Reception (接收或接受)
• Internal and external signals are detected
by receptors (接受器偵測內在與外在訊息)
– Proteins that change in response to
specific stimuli
– Reception protein (受體蛋白)
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Transduction (傳遞/傳導)
• Second messengers (第二傳訊者)
– Transfer and amplify signals (轉移與擴大訊
息) from receptors to proteins that cause
specific responses
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Plant signal transduction (植物的訊息傳遞)
• An example of signal transduction in plants
2 Transduction
1 Reception
3 Response
CYTOPLASM
cGMP
Plasma
membrane
Second messenger
produced
Phytochrome
activated
by light
Cell
wall
Specific
protein
kinase 1
activated
2 One pathway uses cGMP
as a second messenger that
activates a specific protein
kinase.The other pathway
involves an increase in
cytoplasmic Ca2+ that
activates another specific
protein kinase.
Transcription
factor 1
NUCLEUS
P
Transcription
factor 2
P
Specific
protein
kinase 2
activated
Transcription
Light
Translation
1 The light signal is
detected by the
phytochrome receptor,
which then activates
at least two signal
transduction pathways.
Ca2+ channel
opened
Ca2+
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Figure 39.4
3 Both pathways
lead to expression
of genes for proteins
that function in the
de-etiolation
(greening) response.
De-etiolation
(greening)
response
proteins
Response (回應或反應)
• 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
– Protein kinases
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Transcriptional Regulation (轉錄調節)
• Transcription factors bind directly to
specific regions of DNA
– And control the transcription of specific
genes
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Post-Translational Modification of Proteins
• Post-translational modification (轉譯後修飾)
– Involves the activation of existing proteins
involved in the signal response真核細胞
細胞核
Post-transcription modification
轉錄
RNA加工
核醣體
轉譯
Post-translation modification
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蛋白質
De-Etioloation (“Greening”) Proteins
去白化(綠化)蛋白
• Many enzymes that function in certain
signal responses are involved in
photosynthesis directly
– While others are involved in supplying the
chemical precursors (化學前驅物)
necessary for chlorophyll production
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• Concept 39.2: Plant hormones help coordinate
growth, development, and responses to
stimuli
• Hormones
– Are chemical signals (化學訊息) that
coordinate the different parts of an organism
• Classification of plant hormones
– Auxin, Cytokinins, Gibberellins, Abscisic acids,
Ethylene, Brassinosteroids
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The Discovery of Plant Hormones
• Any growth response
– That results in curvatures of whole plant
organs toward or away from a stimulus is
called a tropism (向性)
– Is often caused by hormones
• Charles Darwin and his son Francis
– Conducted some of the earliest
experiments on phototropism (向光性), a
plant’s response to light, in the late 19th
century
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Photo-signal is a light-activated mobile chemical
Eperiment: In 1880, Charles Darwin and his son Francis designed an experiment to determine what
part of the coleoptile(芽鞘) senses light. In 1913, Peter Boysen-Jensen conducted an experiment to
determine how the signal for phototropism is transmitted.
Results:
芽鞘背光面
Control
Darwin and Darwin (1880)
Shaded
side of
coleoptile
Light
Light
Illuminated
side of
coleoptile
芽鞘照光面
Boysen-Jensen (1913)
Light
Tip
removed
Base covered
by opaque
shield
Tip
Tip covered covered
by opaque by transparent
cap
cap
Tip separated Tip separated
by gelatin
by mica
block
雲母
Conclusion: 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.
Figure 39.5
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The discovery of Auxin (植物生長素)
•
In 1926, Frits Went extracted the chemical messenger for
phototropism, auxin, by modifying earlier experiments
Experiment: 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.
Results: 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
Offset blocks
cause curvature
Control
Figure 39.6
Control
(agar
block
lacking
chemical)
has no
effect
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Conclusion: Went concluded that a
coleoptile curved toward light
because its dark side had a higher
concentration of the growthpromoting chemical, which he named
auxin.
A Survey of Plant Hormones
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A Survey of Plant Hormones
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A Survey of Plant Hormones (1)
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A Survey of Plant Hormones (2)
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• 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
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Auxin (植物生長素)
• The term auxin
– Is used for any chemical substance that
promotes cell elongation (細胞延伸) in
different target tissues
• Auxin transporters (植物生長素運輸蛋白)
– move the hormone out of the basal end of one
cell, and into the apical end of neighboring
cells
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Auxin transporters and polar transport of auxin
植物生長素運輸蛋白與其極性運輸
Experimrnt: 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.
Cell 1
100 m
Epidermis
Cortex
Phloem
Xylem
Pith
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.
Cell 2
Basal end
of cell
Figure 39.7
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25 m
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
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• Cell elongation in response to auxin
Cross-linking
cell wall
polysaccharides
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.
Cell wall
enzymes
Expansin
胞壁擴張酶
Cell wall
Microfibril
H+
2 The cell wall
becomes more
acidic.
H+
H+
H+
1 Auxin
increases the
activity of
proton pumps.
H+
4 The enzymatic cleaving
of the cross-linking
polysaccharides allows the
microfibrils to slide (滑動).
The extensibility (延展性) of
the cell wall is increased.
Turgor (膨壓) causes the cell
to expand.
H2O
Cell
Plasma
wall
membrane
H+
H+
H+
Nucleus
ATP
H+
Plasma membrane
Cytoplasm
Figure 39.8
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Cytoplasm
Vacuole
5 With the cellulose loosened,
the cell can elongate.
Physiologic functions of auxin in plant
• Auxin and 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
• Lateral and Adventitious Root Formation: Auxin is
involved in the formation and branching of roots
• Auxins as Herbicides: an overdose of auxins can
kill eudicots
• Other Effects of Auxin: Auxin affects secondary
growth by inducing cell division in the vascular
cambium and influencing differentiation of
secondary xylem
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Cytokinins (細胞分裂素)
• Cytokinins
– Stimulate cell division
• Control of Cell Division and Differentiation
– Cytokinins are produced in actively
growing tissues such as roots, embryos,
and fruits
– Work together with auxin
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Control of Apical Dominance (頂端優勢)
• Cytokinins, auxin, and other factors interact in the
control of apical dominance. The ability of a terminal
bud to suppress development of axillary buds (腋芽)
• If the terminal bud (頂芽) is removed, plants become
bushier (茂密)
“Stump” after
removal of
apical bud
Axillary buds
Figure 39.9
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Lateral branches
Anti-Aging Effects (抗老化效應)
• Cytokinins retard (阻礙/延遲) the aging of
some plant organs
– By inhibiting protein breakdown,
stimulating RNA and protein synthesis, and
mobilizing nutrients from surrounding
tissues
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Gibberellins (吉貝林素)
• Gibberellins have a variety of effects such
as:
– stem elongation
– fruit growth
– seed germination
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Stem Elongation (莖的延伸)
• Gibberellins stimulate growth of both
leaves and stems
• In stems
– Gibberellins stimulate cell elongation and
cell division
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Fruit Growth
• In many plants
– Both auxin and gibberellins must be
present for fruit to set
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• Gibberellins are used commercially
– In the spraying of Thompson seedless
grapes
Figure 39.10
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Germination—break seed dorminancy to germinate
• After water is imbibed, the release of gibberellins from
the embryo signals the seeds to break dormancy and
germinate (打破休眠促進萌芽)
1. After a seed imbibes
water, the embryo releases
gibberellin (GA) as a signal
to the aleurone, the thin
outer layer of the
endosperm.
Aleurone
糊粉層
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.)
Endosperm
內胚乳
3. Sugars and other nutrients
absorbed from the
endosperm by the scutellum
(cotyledon) are consumed
during growth of the embryo
into a seedling.
shoot
澱粉
分解酶
-amylase
Sugar
GA
seedling
GA
Water
Figure 39.11
Scutellum
(cotyledon)
子葉
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Radicle
胚根
root
Brassinosteroids (油菜類固醇)
• Brassinosteroids
– Are similar to the sex hormones of animals
– Induce cell elongation and division
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Abscisic Acid (離層酸)
• Two of the many effects of abscisic acid
(ABA) are
– Seed dormancy (種子休眠)
– Drought tolerance (耐乾旱)
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Seed Dormancy (種子休眠)
• Seed dormancy has great survival value
– Because it ensures that the seed will
germinate only when there are optimal
conditions
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•Precocious germination (早熟的萌芽) is observed in
maize mutants
– That lack a functional transcription factor (功能性
轉錄因子) required for ABA to induce expression of
certain genes
Coleoptile
芽鞘
Figure 39.12
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Drought Tolerance (耐乾旱)
• ABA is the primary internal signal
– That enables plants to withstand drought
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Ethylene (乙烯)
• Plants produce ethylene in response to
stresses (逆境) such as
– Drought (乾旱)
–
flooding (淹水)
– mechanical pressure (機械性壓破)
– injury (受傷)
– Infection (感染)
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The Triple Response to Mechanical Stress
• Ethylene induces the triple response (三重反應) which
allows a growing shoot to avoid obstacles (障礙)
Experiment: 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.
Results: All the treated seedlings exhibited the triple response. Response was
greater with increased concentration.
Conclusion: Ethylene induces the triple
response in pea seedlings, with
increased ethylene concentration
causing increased response.
Triple response to avoid obstacles
1. A slowing of stem elongation
2. A thickening of the stem which
0.10
0.00
0.40
0.20
0.80
makes it strong
Ethylene concentration (parts per million, ppm)3. A curvature that causes the stem
to start growing horizonically
Figure 39.13
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• Ethylene-insensitive mutants
– Fail to undergo the triple response after
exposure to ethylene
ein mutant
Figure 39.14a
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• Other types of mutants
– Undergo the triple response in air but do
not respond to inhibitors of ethylene
synthesis
ctr mutant
Figure 39.14b
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• A summary of ethylene signal transduction
mutants
Ethylene
Ethylene
synthesis
Control
Wild-type
Ethylene insensitive
(ein)
Ethylene
overproducing (eto)
Constitutive triple
response (ctr)
Figure 39.15
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added
inhibitor
Apoptosis: Programmed Cell Death
細胞凋亡：程式化細胞死亡
• A burst of ethylene (乙烯的爆發)
– Is associated with the programmed
destruction (程式化崩壞) of cells, organs, or
whole plants
• Yang’s cycle (楊氏循環；楊祥發博士)
– Biochemical pathway of ethylene
biosynthesis
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Leaf Abscission (葉的離層酸)
• A change in the balance of auxin and
ethylene controls leaf abscission
– The process that occurs in autumn when a
leaf falls
0.5 mm
保護層Protective layer Abscission layer 離層
Figure 39.16
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Stem
Petiole葉柄
Fruit Ripening (果實成熟)
• A burst of ethylene production in the fruit
– Triggers the ripening process
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Systems Biology and Hormone Interactions
• Interactions between hormones and their
signal transduction pathways
– Make it difficult to predict what effect a
genetic manipulation will have on a plant
• Systems biology seeks a comprehensive
understanding of plants
– That will permit successful modeling of
plant functions
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• Concept 39.3: 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 (光形態發生)
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• Plants not only detect the presence of light
– But also its direction, intensity, and
wavelength (color) (植物偵測光質、光量、光
度、光向)
• A graph called an action spectrum (作用光譜)
– Depicts the relative response of a process
to different wavelengths of light (光的波長)
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• Action spectra (作用光譜) are useful in the study
of any process that depends on light
Experiment: 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.
Conclusion: The phototropic bending
toward light is caused by a photoreceptor
that is sensitive to blue and violet light,
particularly blue light.
Phototropic effectiveness
relative to 436 nm
Results: 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.
1.0
0.8
0.6
0.4
0.2
0
400 450 500 550 600 650 700
Wavelength (nm)
Light
Time = 0 min.
Figure 39.17
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Time = 90 min.
• Research on action spectra and absorption
spectra of pigments
– Led to the identification of two major
classes of light receptors: blue-light
photoreceptors and phytochromes (藍光光
受體與光敏素)
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Blue-Light Photoreceptors (藍光光受體)
• Various blue-light photoreceptors controls
– hypocotyl elongation (下胚軸的延伸)
– stomatal opening (氣孔的開張)
– Phototropism (向光性)
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Phytochromes as Photoreceptors (光敏素是光受體)
• Phytochromes (光敏素)
– Regulate many of a plant’s responses to
light throughout its life
• Phytochromes and Seed Germination
– Studies of seed germination led to the
discovery of phytochromes
• In the 1930s, scientists at the U.S.
Department of Agriculture (USDA)
– Determined the action spectrum for lightinduced germination of lettuce seeds
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Red light effect on germination (紅光效應)
Experiment: 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.
Results: 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).
Dark (control)
Red
Dark
FarRed red Dark
Red Far- Red Dark Red Far- Red Farred
red
red
Figure 39.18
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Conclusion: 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.
• 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
=peptide+
chromophore
(發色團)
Kinase
Figure 39.19
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Photoreceptor activity. One domain,
which functions as the photoreceptor,
is covalently bonded to a nonprotein
pigment, or chromophore (發色團).
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.
• Phytochromes exist in two photoreversible
states with conversion of Pr to Pfr triggering
many developmental responses
Pr
Pfr
Red light
Responses:
seed germination,
control of
flowering, etc.
Synthesis
Far-red
light
Slow conversion
in darkness
(some plants)
Figure 39.20
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Enzymatic
destruction
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 (Pr / Pfr) shifts in
favor of Pr when a tree is shaded
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Biological Clocks and Circadian Rhythms
生物時鐘與生理節律
• Many plant processes
– Oscillate (擺動) during the day
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• Many legumes (豆科植物)
– Lower their leaves in the evening and raise
them in the morning
Figure 39.21
Noon
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Midnight
• Cyclical responses to environmental stimuli
are called circadian rhythms (日週期、生理
節律)
– And are approximately 24 hours long
– Can be entrained to exactly 24 hours by the
day/night cycle (日夜循環)
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The Effect of Light on the Biological Clock
光對生物時鐘的效應
• Phytochrome conversion (光敏素的轉換)
marks sunrise and sunset
– Providing the biological clock with
environmental cues (啟示/提示)
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Photoperiodism and Responses to Seasons
光週期現象與對季節變化的反應
• Photoperiod (光週期), the relative lengths of
night and day
– Is the environmental stimulus plants use
most often to detect the time of year
• Photoperiodism (光週期現象)
– Is a physiological response to photoperiod
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Photoperiodism and Control of Flowering
光週期現象與開花的調空
• Some developmental processes, including
flowering in many species
– Requires a certain photoperiod
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Critical Night Length
• In the 1940s, researchers discovered that
flowering and other responses to photoperiod are
actually controlled by night length, not day length
Experiment: 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.
Darkness
Experiment:
Flash of light
Figure 39.22
(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.
Critical dark
period
Conclusion: The
Light
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.
(b) “Long-day” plants flowered
Therefore, “short-day”
only if a period of continuous
plants are more properly
darkness was shorter than a
called “long-night” plants,
critical dark period for that
particular species (13 hours in
and “long-day” plants are
this example).
really “short-night” plants.
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• Action spectra and photoreversibility experiments show
that phytochrome is the pigment that receives red light,
which can interrupt the nighttime portion of the photoperiod
Experiment： A unique characteristic of phytochrome is reversibility in response to red and far-red
light. To test whether phytochrome is the pigment measuring interruption of dark periods,
researchers observed how flashes of red light and far-red light affected flowering in “short-day”
and “long-day” plants.
Result
24
20
R
FR
R
R
FR
R
FR
R
FR
R
Critical dark period
16
12
8
4
0
Short-day (long-night) plant
Long-day (short-night) plant
Figure 39.23
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Conclusion: A flash of red light shortened the
dark period. A subsequent flash of far-red
light canceled the red light’s effect. If a red
flash followed a far-red flash, the effect of the
far-red light was canceled. This reversibility
indicated that it is phytochrome that measures
the interruption of dark periods.
A Flowering Hormone? (開花賀爾蒙/開花激素)
• The flowering signal, not yet chemically
identified
– Is called florigen (開花賀爾蒙/開花激素), and
it may be a hormone or a change in relative
concentrations of multiple hormones
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Experiment: To test whether there is a flowering hormone, researchers conducted an
experiment in which a plant that had been induced to flower by photoperiod was grafted to
a plant that had not been induced.
Result
Plant subjected to photoperiod
Plant subjected to photoperiod
that does not induce flowering
that induces flowering
Graft
Time
(several
weeks)
Figure 39.24
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Conclusion: Both plants
flowered, indicating the
transmission of a flowerinducing substance. In some
cases, the transmission worked
even if one was a short-day plant
and the other was a long-day
plant.
Meristem Transition and Flowering
• Whatever combination of environmental cues
and internal signals is necessary for flowering
to occur
– The outcome is the transition of a bud’s
meristem from a vegetative to a flowering
state
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• Concept 39.4: Plants respond to a wide
variety of stimuli other than light
• Because of their immobility (不動性)
– Plants must adjust to a wide range of
environmental circumstances through
developmental and physiological
mechanisms
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Gravity (重力/地心引力)
• Response to gravity
– Is known as gravitropism (向地性)
• Roots show positive gravitropism
• Stems show negative gravitropism
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• Plants may detect gravity by the settling of
statoliths (平衡石)
– Specialized plastids containing dense
starch grains
Statoliths
Figure 39.25a, b
(a)
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(b)
20 m
Mechanical Stimuli (機械性刺激)
• The term thigmomorphogenesis (接觸形態發
生)
– Refers to the changes in form that result
from mechanical perturbation (機械性干擾)
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• Rubbing (觸摸) the stems of young plants a
couple of times daily (每天觸摸二次)
– Results in plants that are shorter than
controls
Figure 39.26
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• Growth in response to touch
– Is called thigmotropism (向觸性/接觸性)
– Occurs in vines (藤蔓/藤本值物) and other
climbing plants (攀爬植物)
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敏感植物葉的快速膨壓運動
• Rapid leaf movements in response to
mechanical stimulation are examples of
transmission of electrical impulses (電脈衝)
called action potentials (動作電位)
(a) Unstimulated
未受刺激
(b) Stimulated
受刺激後
Side of pulvinus with flaccid cells
葉枕側面具皺縮細胞(失水)
Leaflets after stimulation
受刺激後的小葉
Side of pulvinus with turgid cells
葉枕側面具膨脹細胞
Pulvinus (motor organ)
葉枕(運動器官)
Vein葉脈
(c) Motor organs 運動器官
0.5 m
Figure 39.27a–c
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Environmental Stresses (環境逆境)
• Environmental stresses
– Have a potentially adverse effect (相反的)
on a plant’s survival, growth, and
reproduction
– Can have a devastating impact on crop
yields in agriculture
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Drought (乾旱)
• During drought
– Plants respond to water deficit (水份缺少)
by reducing transpiration
– Deeper roots (深根) continue to grow
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Flooding (淹水逆境)
• Enzymatic destruction of cells
– Creates air tubes that help plants survive
oxygen deprivation during flooding
Vascular
cylinder
Air tubes
Epidermis
Figure 39.28a, b
100 m
(a) Control root (aerated)
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100 m
(b) Experimental root (nonaerated)
Salt Stress (鹽分逆境)
• Plants respond to salt stress by producing
solutes tolerated at high concentrations
– Keeping the water potential of cells more
negative than that of the soil solution
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Heat Stress (高溫逆境)
• Heat-shock proteins (熱休克蛋白)
– Help plants survive heat stress
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Cold Stress (低溫逆境)
• Altering lipid composition of membranes
– Is a response to cold stress
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• Concept 39.5: Plants defend themselves
against herbivores (草食性動物) and
pathogens (病源菌)
• Plants counter external threats (植物克服外
來威脅)
– With defense systems (防禦系統) that deter
herbivory and prevent infection or combat
pathogens
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Defenses Against Herbivores
(反抗草食性動物的防禦)
• Herbivory (草食性動物), animals eating
plants
– Is a stress that plants face in any
ecosystem
• Plants counter excessive herbivory
– With physical defenses (物理性防禦) such
as thorns (刺針)
– With chemical defenses (化學性防禦) such
as distasteful or toxic compounds (有毒物質)
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• Some plants even “recruit” (招引) predatory
animals (掠食性動物) that help defend the plant
against specific herbivores
4. Recruitment of 招引寄生蜂
parasitoid wasps
that lay their eggs 產卵於毛蟲
within caterpillars 體內
3. Synthesis and
release of volatile
attractants (合成
與釋放揮發性物質)
1. Wounding
2. Chemical
in saliva
2. Signal transduction
pathway(訊息傳遞途徑)
Figure 39.29
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Defenses Against Pathogens
(反抗病源的防禦)
• A plant’s first line of defense (第一道防線)
against infection
– Is the physical barrier (物理性障礙物) of the
plant’s “skin,” the epidermis and the
periderm (表皮與內皮)
• Once a pathogen invades (侵入) a plant
– The plant mounts a chemical attack (化學攻
擊) as a second line of defense (第二道防線)
that kills the pathogen and prevents its
spread
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• The second defense system (第二道防線)
– Is enhanced by the plant’s inherited ability
(遺傳能力) to recognize certain pathogens
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Gene-for-Gene Recognition
基因對基因的辨識
• A virulent pathogen (致病性病原菌)
– Is one that a plant has little specific
defense against
• An avirulent pathogen (非致病性病原菌)
– Is one that may harm but not kill the host
plant
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• Gene-for-gene recognition is a widespread
form of plant disease resistance
– That involves recognition of pathogenderived molecules by the protein products
of specific plant disease resistance (R)
genes
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• A pathogen is avirulent (非致病性) if it has a
specific Avr gene corresponding to a particular
R allele in the host plant
Signal molecule (ligand)
from Avr gene product
Receptor coded by R allele
R
Avr allele
Avirulent pathogen
Plant cell is resistant
(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.
Figure 39.30a
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• If the plant host lacks the R gene that
counteracts the pathogen’s Avr gene then the
pathogen can invade and kill the plant
No Avr allele;
virulent pathogen
Avr allele
Virulent pathogen
Virulent pathogen
Figure 39.30b
R
Plant cell becomes diseased
No R allele;
plant cell becomes diseased
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.
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Plant Responses to Pathogen Invasions
• A hypersensitive response (過敏反應) against an
avirulent pathogen seals off the infection and kills both
pathogen and host cells in the region of the infection
4. Before they die,
infected cells release a
chemical signal, probably
salicylic acid (水楊酸).
3. In 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.
2. This identification step
triggers a signal
transduction pathway (STP).
1. Specific resistance (專一性
抗性) is based on the binding
of ligands from the pathogen
to receptors in plant cells.
5. The signal is distributed
to the rest of the plant.
4 Signal
3
Hypersensitive
response
Signal transduction
2 pathway (STP)
Avirulent
pathogen
5
Signal 6
transduction
pathway (STP)
7 Acquired
resistance
1
R-Avr recognition
and hypersensitive
response(R-Avr之辨
識及過敏性反應)
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6. In cells remote from
the infection site, the
chemical initiates a signal
transduction pathway.
7. Systemic acquired
resistance (SAR) is
activated: the production of
molecules that help protect
the cell against a diversity
of pathogens for several
days.
Systemic acquired
Resistance
(後天性系統抗性)
Figure 39.31
Systemic Acquired Resistance (SAR)
(後天性系統抗性)
• 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 (水楊酸)
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings