Transcript Chapter 26 Plants: Control and Growth Response

Biology
Sylvia S. Mader
Michael Windelspecht
Chapter 26
Flowering Plants:
Control of
Growth Responses
Lecture Outline
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Outline
• 26.1 Plant Hormones
• 26.2 Plant Responses
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26.1 Plant Hormones
• Flowering plants respond to environmental
stimuli
– Stimuli include light, gravity, carbon dioxide
levels, pathogen infection, drought, and touch
– Response to stimuli leads to the survival of the
species.
• The responses can be:
– Short term
• Stomata open and close in response to light levels.
– Long term
• The response to gravity causes downward growth of the
root and the upward growth of the stem.
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Plant Hormones
• Response of plants to environmental stimuli
involves signal transduction
– The binding of a molecular “signal” that initiates
and amplifies a response.
– Signal transduction involves the following:
– Receptors – proteins activated by a specific signal
– Transduction pathway – a series of relay proteins
or enzymes that amplify and transform the signal
to one understood by the machinery of the cell
– Cellular response – the result of the transduction
pathway
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Plant Hormones
• Hormones
– Enable plant cells to communicate
– Are synthesized in one part of the plant
– Travel within phloem or from cell to cell in
response to the appropriate stimulus
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Signal Transduction in Plants
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defense
hormones
hormone-binding site
3
blue light
signal
2
Receptor: Molecule in
the plasma membrane,
cytoplasm, or nucleus
that receives signal and
becomes activated.
Response: Most often
a change in gene expression
or a cellular process affects
plant growth and development.
Transduction pathway: A series
of relay proteins that amplify and
convert the original signal into one
that affects cellular machinery .
activated
phototropin
auxin
relay
proteins
Defense
responses
auxin carrier
Responses
include bending
of stem
1
activated
auxin receptor
Cytoplasm
Gene
expression
changes
Nucleus
Responses
include growth
of roots
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Plant Hormones
• Auxins
– Produced in shoot apical meristem
– Found in young leaves, flowers, and fruits
• Effects of auxin on growth and development:
– Apically produced auxin prevents the growth of
axillary buds
• Apical dominance
– Promotes growth of roots and fruit
– Prevents loss of leaves and fruit
– Promotes positive phototropism of stems
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Auxin and Phototropism
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1. Coleoptile tip is
intact.
2. Coleoptile tip is
removed.
3. Tips are placed on
agar, and auxin
diffuses into the agar.
4. Agar block is placed
to one side of the
coleoptile.
5. Curvature occurs
beneath the block.
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Plant Hormones
• How Auxins Cause Stems to Bend
• When a stem is exposed to unidirectional
light, auxin moves to the shady sides
• Auxin binds to plasma membrane receptors;
the complex leads to the activation of a proton
pump
• Activated proton pumps H+ out of cell
– Cell wall loosens
– Turgor pressure increases due to the entry of
water
– Cell enlarges
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Expansion of the Cell Wall
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cellulose fiber
in cell wall
auxin
nucleus
H+
H+
enzyme
(inactive)
H+
H+
chloroplast
1
Cytoplasm
H+
H+
H+
active
enzyme
H2O
H+
2
turgor
turgor
3
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Plant Hormones
• Gibberellins are growth-promoting
hormones
– Gibberellins cause stem elongation
– There are about 70 gibberellins
• Each differ slightly chemically
• The most common is gibberellic acid
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Gibberellins Cause Stem
Elongation
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a.
b.
a: © Robert E. Lyons/Visuals Unlimited; b: © Sylvan Whittwer/Visuals Unlimited
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Plant Hormones
• The cytokinins are a class of hormones that
promote cell division
– found in dividing tissues of roots, in seeds, and in
fruits
– have been used to prolong the life of flower cuttings
as well as vegetables in storage
– Auxin and cytokinins interact
– prevent senescence (aging process)
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Interaction of Hormones
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a.
b.
c.
d.
Courtesy Alan Darvill and Stefan Eberhard, Complex Carbohydrate Research Center, University of Georgia
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Plant Hormones
• Abscisic acid (ABA) is produced by any
“green tissue” (i.e., tissue containing
chloroplasts)
– sometimes called the stress hormone
• initiates and maintains seed and bud dormancy
• brings about the closure of stomata
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Dormancy and Winter Buds
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Dormancy and Germination
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Abscisic Acid Promotes Closure
of Stomata
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inside
outside
H2O
K+
K+
K+
Ca2+
ABA
Open stoma
Guard cell plasma
membrane
Closed stoma
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Plant Hormones
• Ethylene (H2C = CH2) is a gas formed from the
amino acid methionine.
• Effects of ethylene
– Abscission
• Ethylene stimulates certain enzymes, such as cellulase,
which helps cause leaf, fruit, or flower drop
– Ripening of fruits
• Increases the activity of enzymes, such as cellulase, that
soften fruits
• It also promotes the activity of enzymes that produce the
flavor and smell of ripened fruits.
– Axillary bud inhibition
– Suppression of stem and root elongation
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Ethylene and Abscission
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No abscission
© Kingsley Stern
Abscission
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Ethylene and Fruit Ripening
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gene for ethylene
biosynthesis enzyme
ripe tomatoes
harvested
DNA
transcription
mRNA
translation
functional
enzyme for
ethylene
biosynthesis
ethylene synthesis (in plant)
green tomatoes
harvested
no ethylene
synthesis
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Arabidopsis Is a Model
Organism
• Arabidopsis thaliana
– A small flowering plant related to cabbage and
mustard plants
– Has no commercial value
– It has become a model organism for the study of
plant molecular genetics, including signal
transduction.
• It is small, so many hundreds of plants can be
grown in a small amount of space.
• Generation time is short – 5-6 weeks until maturity.
• It normally self-pollinates, but it can easily be crosspollinated.
• The number of base pairs in its DNA is relatively small.
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Overall Appearance of Arabidopsis thaliana
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Arabidopsis thaliana
Courtesy Elliot Meyerowitz/California Institute of Technology
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26.2 Plant Responses
• Tropism
– Plant growth toward or away from a
unidirectional stimulus
• Positive tropism is growth toward the stimulus
• Negative tropism is growth away from the stimulus
– Gravitropism - movement in response to
gravity
– Phototropism - movement in response to
light
– Thigmotropism - movement in response to
touch
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Plant Responses
• Gravitropism
– When a plant is placed on its side, the stem
grows upward, opposite of the pull of gravity
– Stems with root caps grow downward
• Response depends on sensors called statoliths
– Auxin may be responsible for:
• gravitropism of roots and shoots
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Gravitropism
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a.
gravity
b.
c.
25 mm
a: © Kingsley Stern; b: Courtesy Malcolm Wilkins, Glascow University; c: © BioPhot
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Plant Responses
• Phototropism
– Positive phototropism of stems
• Occurs because cells on the shady side of the
stem elongate due to the presence of auxin
• A pigment absorbing blue light initiates
phototropism
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Phototropin
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1
cytoplasm
blue light
2
3
blue light
phot
blue light
phot
ADP
transduction
pathway
phot
P
plasma
membrane
ATP
ATP
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Plant Responses
• Thigmotropism
– Unusual growth due to contact with solid
objects
• Coiling of tendrils
– Thigmomorphogenesis occurs when the
entire plant responds to the presence of
environmental stimuli
• Wind
• Rain
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Coiling Response
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© John D. Cunningham/Visuals Unlimited
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Plant Responses
• Nastic movements:
– Do not involve growth and
– Are not dependent on the stimulus direction
• Turgor movements result from touch, shaking, or
thermal stimulation
– Mimosa pudica
– Venus flytrap
• Sleep movements:
– Occur daily in response to light and dark changes
– Circadian rhythm
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Turgor Movement
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pulvinus
Before
vascular tissue
cell retaining
turgor
cell losing
turgor
After
© John Kaprielian/Photo Researchers, Inc.6
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Plant Responses
• Circadian rhythms:
– Biological rhythms with a 24-hour cycle
– Tend to be persistent
• Rhythm is maintained in the absence of
environmental stimuli
• Caused by a biological clock
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Sleep Movements and Circadian Rhythms
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Prayer plant (morning)
a.
Prayer plant (night)
Morning glory (morning)
b.
Morning glory (night)
Circadian Rhythm
flowers
open
flowers
close
Period
(about 24 hours)
0
12
24
36
48
Time (hours)
c.
(Top, both): © Tom McHugh/Photo Researchers, Inc.(Bottom left): © BIOS A. Thais/Peter Arnold, Inc.; (Bottom right): © BIOS Pierre Huguet/Peter Arnold, Inc.
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Plant Responses
• Photoperiodism:
– Any physiological response prompted by
changes in day or night length
– influences flowering in some plants
– requires participation of a biological clock and
a plant photoreceptor called phytochrome
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Plant Responses
• Phytochrome is a blue-green leaf pigment
that alternately exists in two forms
– Phytochrome red (Pr) is inactive
– Phytochrome far-red (Pfr) is active
• Conversion of forms allows a plant to
detect photoperiod changes
• Also promotes seed germination and
inhibits stem elongation
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Phytochrome Conversion Cycle
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lightsensitive
region
red light
far-red light
kinase
inactive Pr
active Pfr
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Phytochrome Control of Shoot
Elongation
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a. Normal growth
b. Etiolation
(both): © Nigel Cattlin/Visuals Unlimited
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Plant Responses
• Flowering
– Flowering plants can be divided into three
groups based on their flowering status.
• Short-day plants flower when the day length is
shorter than a critical length
• Long-day plants flower when the day length is
longer than a critical length
• Day-neutral plants are not dependent on day
length for flowering
– Some plants may require a specific sequence
of day lengths in order to flower
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Photoperiodism and Flowering
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Cocklebur
Clover
night
flash of light
24
hours
critical
length
day
flower
flower
1
2
a. Short-day (long-night) plant
3
4
flower
5
6
b. Long-day (short-night) plant
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Plant Responses
• Responses to the biotic environment:
• Plants are always under attack by herbivores and
parasites.
• Physical and Chemical Defenses
– Cuticle-covered epidermis and bark
– Secondary metabolites
• Tannins
• Alkaloids
• Cyanogenic glycosides
– Wound responses – proteinase inhibitors and systemin
– Hypersensitive response (HR) – initiates wound response
and seals wounded area
– Indirect defenses – prevent egg laying in insects
– Mutualistic relationships with animals - acacia tree and
acacia ant
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Plant Predators and Parasites
Alfalfa plant bug
Fungus infection
Monarch caterpillar and butterfly
(Top left): Courtesy USDA/Agricultural Research Service, photo by Scott Bauer; (top right): © Kingsley Stern;
(bottom left): © The McGraw Hill Companies, Inc./Ken Cavanagh, photographer; (bottom right): © Dwight Kuhn
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Wound Response in Tomato
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systemin
cytoplasm
lipase
wounded
leaf
systemin
release
membranebound
receptor
proteinase
inhibitors
membrane
lipids
salicylic acid
jasmonic acid
transduction pathway
nucleus
activation of
proteinase
inhibitor genes
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