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BIOLOGY
CONCEPTS & CONNECTIONS
Fourth Edition
Neil A. Campbell • Jane B. Reece • Lawrence G. Mitchell • Martha R. Taylor
CHAPTER 33
Control Systems in Plants
From PowerPoint® Lectures for Biology: Concepts & Connections
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
PLANT HORMONES
33.1 Experiments on how plants turn toward light
led to the discovery of a plant hormone
• Hormones coordinate the
activities of plant cells
and tissues
• The study of plant
hormones began with
observations of plants
bending toward light
– This phenomenon is
called phototropism
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 33.1A
• Phototropism results from faster cell growth on
the shaded side of the shoot than on the
illuminated side
Shaded
side of
shoot
Light
Illuminated
side of
shoot
Figure 33.1B
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Experiments carried out by Darwin and others
showed that the tip of a grass seedling detects
light and transmits a signal down to the
growing region of the shoot
Light
Control
Figure 33.1C
Tip
removed
Tip covered
by opaque
cap
Tip
covered
by transparent cap
DARWIN AND DARWIN (1880)
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Base
covered
by opaque
shield
Tip
separated
by gelatin
block
Tip
separated
by mica
BOYSEN-JENSEN (1913)
• It was discovered in the 1920s that a hormone
was responsible for the signaling Darwin
observed
– This hormone was dubbed auxin
– Auxin plays an important role in
phototropism
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Shoot tip placed on agar block.
Chemical (later called auxin)
diffuses from shoot tip into agar.
Agar
Control
Block with
chemical
stimulates
growth.
Offset blocks with
chemical stimulate
curved growth.
Other controls:
Blocks with no
chemical have
no effect.
NO LIGHT
Figure 33.1D
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33.2 Five major types of hormones regulate plant
growth and development
• Hormones regulate plant growth and
development by affecting
– cell division
– cell elongation
– cell differentiation
• Only small amounts of hormones are
necessary to trigger the signal-transduction
pathways that regulate plant growth and
development
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Table 33.2
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Plants produce auxin (IAA) in the apical
meristems at the tips of shoots
– At different concentrations, auxin stimulates or
inhibits the elongation of shoots and roots
STEMS
ROOTS
0.9 g/L
Figure 33.3B
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• The effect of auxin on pea plants
Figure 33.3A
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• Auxin may initiate elongation by weakening
cell walls
Auxin
stimulates
Plasma
membrane
Cell
wall
CELL WALL
H+
H+
Activates
Vacuole
H2O
CELL
ELONGATION
H+ pump
(protein)
Enzyme
CYTOPLASM
Cellulose
molecule
Cellulose loosens; cell can elongate
Figure 33.3C
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• Auxin stimulates cell division and the
development of vascular tissues in vascular
cambium
– This promotes growth in stem diameter
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33.4 Cytokinins stimulate cell division
• Cytokinins are hormones that promote cell
division
– They are produced in actively growing roots,
embryos, and fruits
• The antagonistic interaction of auxin and
cytokinin may be one way a plant coordinates
the growth of its root and shoot systems
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• Cytokinins from roots may balance the effects
of auxin from apical meristems, causing lower
buds to develop into branches
– The basil plant on the right has had its terminal
bud removed
– The inhibitory effect of
auxin on axillary buds
was thus eliminated
– Cytokinins from the
roots activated the
axillary buds, making
the plant grow more
branches
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Terminal bud
terminal bud removed
Figure 33.4
33.5 Gibberellins affect stem elongation and have
numerous other effects
• Gibberellins stimulate cell elongation and cell
division in stems and leaves
– Foolish seedling
disease occurs
when rice
plants infected
with the
Gibberella
fungus get an
overdose of
gibberellin
Figure 33.5A
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• Gibberellins, in combination with auxin, can
influence fruit development
– Gibberellins can make grapes grow larger and
farther apart in a cluster
– The grapes
at right were
treated with
gibberellin,
while those
at left were
not
Figure 33.5B
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• Gibberellin-auxin sprays can make apples,
currants, and eggplants develop without
fertilization
• Gibberellins released from embryos function
in some of the early events of seed
germination
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
33.6 Abscisic acid inhibits many plant processes
• Abscisic acid (ABA) inhibits the germination of
seeds
• The ratio of ABA to gibberellins often
determines whether a seed will remain
dormant or will germinate
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• Seeds of many plants
remain dormant until
their ABA is inactivated
or washed away
– These flowers grew
from seeds that
germinated after a
rainstorm in the
Mojave Desert
Figure 33.6
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• ABA also acts as a “stress hormone”
– It causes stomata to close when a plant is
dehydrated
– Thus the rate of transpiration is decreased and
further water loss prevented
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33.7 Ethylene triggers fruit ripening and other
aging processes
• Ethylene is a gaseous hormone that triggers
fruit ripening
• Ethylene is given off as cells age
• These bananas
were exposed
to different
amounts of
ethylene over
the same time
period
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Figure 33.7A
• Fruit growers use ethylene to control ripening
– Apple farmers take measures to retard the
ripening action of natural ethylene
– Tomato farmers pick unripe fruit and then
pipe ethylene into storage bins to promote
ripening
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• The shorter days of
autumn trigger a
changing ratio of
auxin to ethylene
Leaf
stalk
Stem
(twig)
– This is the likely
cause of the
changes seen in
deciduous trees —
color changes,
drying, and the
loss of leaves
Protective
layer
Figure 33.7B
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Stem
Abscission
layer
Leaf stalk
33.8 Connection: Plant hormones have many
agricultural uses
• Plant hormones have a variety of agricultural
uses
– Farmers use auxin to delay or promote fruit
drop
– Auxin and gibberellins are used to produce
seedless fruits
– A synthetic auxin (2,4-D) is used to kill weeds
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• There are many questions and concerns about
the safety of using such chemicals in food
production
Figure 33.8
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GROWTH RESPONSES AND BIOLOGICAL
RHYTHMS IN PLANTS
33.9 Tropisms orient plant growth toward or away
from environmental stimuli
• Plants sense and respond to environmental
changes in a variety of ways
• Tropisms are growth responses that change
the shape of a plant or make it grow toward or
away from a stimulus
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• Phototropism is the bending toward light
– It may result
from auxin
moving from
the illuminated
side to the
shaded side of
a stem
Figure 33.1A
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• Gravitropism is a response to gravity
– It may be caused
by the settling of
special organelles
on the low sides
of shoots and
roots
– This may trigger a
change in the
distribution of
hormones
Figure 33.9A
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• Gravitropism is an important adaptation
– It ensures that the shoot will grow upward
toward light and the roots will grow down into
the soil, no matter how the seed lands in the
soil
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• Thigmotropism is a response to touch
– It is responsible for the coiling of tendrils and
vines around objects
– It enables plants
to use other
objects for
support while
growing
toward sunlight
Figure 33.9B
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33.10 Plants have internal clocks
• Plants possess an internal biological clock that
controls daily cycles
• These cycles are called
circadian rhythms
– Even in the absence of
environmental cues, they
persist with periods of
about 24 hours
– But such cues are needed
to keep them synchronized
with day and night
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Figure 33.10
33.11 Plants mark the seasons by measuring
photoperiod
• Plants mark the seasons by measuring
photoperiod
– the relative lengths of day and night
• The timing of flowering is one of the seasonal
responses to photoperiod
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• Short-day plants flower when nights are longer
than a certain critical length
Critical night length
• Long-day plants flower when nights are
shorter than a certain critical length
Time (hr)
Darkness
Flash of
light
Light
Short-day (long-night) plants
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Long-day (short-night) plants
Figure 33.11
33.12 Phytochrome is a light detector that may
help set the biological clock
• A light-absorbing protein called phytochrome
may help plants set their biological clock and
monitor photoperiod
• Phytochromes were discovered during studies
on how different wavelengths of light affect
seed germination
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– Far-red light (730
nm) both inhibits
germination and
reverses the effect
of red light
R
FR
R
R
FR
R
FR
R
FR
R
Critical night length
Short-day (long-night) plant
Time (hr)
– Red light (660
nm) was found
to be most
effective at
increasing
germination
Long-day (short-night) plant
Figure 33.12A
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• Researchers have found that phytochrome
reverts back and forth between two forms that
differ only slightly in structure
• One form absorbs red light and the other
absorbs far-red light
– When red-absorbing
Pr absorbs red light,
it is quickly converted
to Pfr
– When Pfr absorbs
far-red light, it is
converted back to Pr
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Red light
Pr
Pfr
Far-red light
Slow conversion
in darkness
Figure 33.12B
• Plants also have a group of blue-light
photoreceptors
– These control light-sensitive plant responses,
such as phototropism and the opening of
stomata at daybreak
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33.13 Talking About Science: Joanne Chory studies
the effects of light and hormones in the model
plant Arabidopsis
• Biologist and plant researcher Joanne Chory
studies the popular model organism
Arabidopsis
• Arabidopsis is a small,
wild mustard whose
complete genome was
sequenced in 2000
• Her research has had
many agricultural
applications
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Figure 33.13
PLANT DEFENSES
33.14 Defenses against herbivores and infectious
microbes have evolved in plants
• Plants use both physical and chemical means
to defend themselves against herbivores and
pathogens
• Some plants produce an unusual amino acid
called cananvanine
– If an insect eats a plant containing cananvanine,
the molecule is incorporated into the insect’s
proteins in place of arginine, resulting in death
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• Plants may recruit predatory wasps to kill
caterpillars that feed on them
4
5
Recruitment of wasp
Wasp
lays
eggs
3
Synthesis
of chemical
attractants
1
Damage to plant
and chemical in
caterpillar saliva
2
PLANT
CELL
Signaltransduction
pathway
Figure 33.14A
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• Avirulent plant pathogens interact with host
plants in a specific way that stimulates both
local and systemic defenses in the plant
• Local defenses include
– microbe-killing chemicals
– sealing off of the infected area
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• Hormones trigger generalized defense
responses in other organs (systemic acquired
resistance)
– These provide protection against a diversity of
pathogens for days
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5
Signaltransduction
pathway
3
1
Binding of
pathogen’s signal
molecule to
plant’s receptor
molecule
Avirulent
pathogen
Enhanced
local
response
2
Signaltransduction
pathway
R-Avr recognition leading to a
strong local response
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6
4
Additional
defensive
chemicals
Hormones
Systemic acquired
resistance
Figure 33.14B