Thought Question
Plants can’t fight or
hide or run away, so
how do they adapt to a
changing environment?
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Lecture 11 Outline (Ch. 39)
I.
Plant Signal Transduction
II.
Auxins (and Cytokinins)
III. Phytochromes!
IV. Ethylene and senescence
V.
Gibberellins & germination
VI. Abscisic acid & dormancy
VII. Summary
Plant Response - Overview
• Signal transduction pathways link signal reception to response
– Plants have cellular receptors
- detect important changes in environment
• For stimulus to elicit response - cells need specific receptor
Reception
Transduction
Response
Receptor
cellular
response
Relay molecules
Signaling
molecule
Signals of focus in this lecture: light and hormones!
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Plant Hormones
(Plant) Hormone: Chemicals made in one location and
transported to other locations for action
Usually produced in small amounts
Growth
Reproduction
Movement
Water balance
Life cycles
Plant
responses
to
hormones
Example hormone,
there are others
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Plant Orientation
Phototropism – growth in response to directional light
due to: cell ELONGATOIN / DIVISION on the SHADDED versus
SUNNY side of a plant stem
The growth of a
plant part toward
or away from light
(from Greek tropos,
meaning “turn”)
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Phototropism – Light Detection
• Plants not only
detect the presence
of light,
- also direction,
intensity, and
wavelength (color)
• Blue light receptor: Directional growth responses
• Connect environmental signal with cellular perception of
the signal, transduction into biochemical pathways, and
ultimately an altered growth response
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Phototropism – Light Detection
Two major classes of light receptors (we’ll discuss the other
later in this lecture): Blue-light photoreceptors
• stomatal movements
• phototropism
• Blue light receptor: Embedded in cell membrane
• When blue light detected, changes conformation,
signal transduction  differential elongation
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Plant Responses to Light
First hormone
discovered in
phototropism:
Auxin (IAA)
- hormone that
promotes cell
elongation
- Auxin exits basal end of one cell and
enters apical end of adjacent cell
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Cell elongation in response to auxin
Expansins (active at low pH) cleave cellulose
microfibrils from polysaccharides. Exposed
polysaccharides now accessible to enzymes.
Cross-linking
Cell wall
cell wall
enzymes
polysaccharides
Expansin
CELL
WALL
Microfibril
Cell wall
becomes
acidic.
Auxin
increases
activity
of proton
pumps.
H+
H+
H+
H+
H+
H+
H+
H+
ATP
H+
Enzymatic
cleavage of
polysaccharides
allows microfibrils
to slide.Cell wall
can extend. Turgor
causes the cell to
expand.
Plasma membrane
Cytoplasm
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Cell elongation in response to auxin
Plasma
membrane
Cell
wall
H2O
Cytoplasm
Nucleus
Vacuole
With the cellulose loosened, the cell can elongate.
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Other responses to auxin
Other Auxin Stimulated Responses:
• Lateral / branching root formation
• Promote fruit growth (tomato sprays)
• As herbicide, overdose kills dicots
Auxin is produced:
•
At the shoot apex,
seeds, other actively
growing tissues.
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Control of Apical Dominance
• Cytokinins (another plant hormone) and auxins interact in
the control of apical dominance
Apical dominance:
The ability of a
terminal bud to
suppress
development of
axillary buds
Axillary buds
If the terminal bud
is removed plants
become bushier
“Stump”
after
removal
of
apical
bud
Lateral branches
Auxin in the apical bud inhibits the axillary buds.
Cytokinins increase growth of the axillary buds.
On the left is a potato left in the dark;
on the right, a potato left in the light.
Etiolation =
morphological
changes for
growing in dark
De-etiolation
(“greening”)
De-etiolation response in potato
Phytochromes – red light/far-red light receptors
Lettuce seeds
germinate if
exposed to red
light. But if this is
followed by farred light, they will
not germinate.
When light pulses were alternated
between red and far-red, the
results are shown at left.
Phytochromes – red light/far-red light receptors
Phytochromes are receptors for red/far-red light.
The light wavelengths induce conformational changes.
Pfr is the active form for generating cell responses.
The Pr/Pfr ratio is crucial!
Photoperiodism
• Photoperiod - relative lengths of night and day
• Triggers many developmental processes
– Bud break
– Flowering
– Leaf drop in deciduous trees
• Are actually controlled by
night length, not day length
A phytochrome
• Phytochromes also control detection of
night length = photoperiod
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Photoperiodism
• Short-day (long night) plants: flower when nights longer than
critical period
• Long-day (short night) plants: flower when nights shorter than
critical period.
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Photoperiodism
Really it is red light detection
that interrupts the dark period.
Again, depending
on the order of
red/far-red
detection,
responses are
altered!
If you were working your way through college
for a large florist, and in December….
You realize that you left your study materials in the
greenhouse so you turn on the lights in the middle
of the night.
The greenhouse is growing several hundred
poinsettias which are short day plants. Will your
unexpected midnight visit cause a problem, why or
why not?
Senescence
Ethylene – only hormone
that is a gas!
Rapid increase in
ethylene triggers:
apoptosis
fruit ripening
leaf abscission
Abscission
Ethylene stimulates
production of
enzyme that digests
cell walls at base of
petiole
Leaf falls when cells
are sufficiently
weakened
bud
leaf
petiole
abscission layer
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Self-Check
Why will these ripe
bananas help the green
avocados ripen faster?
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Gibberellins stimulate germination
• After water is imbibed, the release of gibberellins from the
embryo signals the seeds to break dormancy and germinate
Responds by synthesizing and
secreting digestive enzymes that
hydrolyze stored nutrients in
the endosperm.
embryo releases
gibberellin as a
signal
Aleurone
Nutrients absorbed from the
endosperm by the cotyledon
are consumed during growth
of the embryo into a seedling.
Endosperm
Embryo
GA
amylase
Sugar
GA
Water
cotyledon
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Abscisic Acid and plant stress
Abscisic Acid:
•
•
Initiates closing stomata in water-stressed plants
Induces and maintains dormancy in buds and seeds
– (inhibits gibberellins)
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Abscisic Acid
Two of the many effects of abscisic acid (ABA) are
• Seed dormancy
– Ensures seeds germinate only when conditions are optimal
• Drought tolerance
– Closes stomata, decreases shoot growth
Coleoptile
K+
K+
K+
Why is that one kernel
(seed) germinating
prematurely?
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Self-Check
Hormone Name
Functions
Auxin
Gibberellin
Cytokinin
Ethylene
Abscisic Acid
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