Download Plant Response

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 8 Outline (Ch. 39)
I.
Plant Signaling & Hormones
II.
Auxins (and Cytokinins)
A. Responses to Light
B. Responses to Gravity
C. Other responses
III. Ethylene and senescence
IV. Gibberellins and germination
V.
Abscisic acid and dormancy
VI. Responses to Touch
IV. Preparation for next lecture
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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
Signaling
molecule
Relay
molecules
Atomic model of the plant
steroid receptor BRI1
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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
Dormancy, germination,
primary growth,
secondary growth,
production and
maturation of
reproductive structures,
phototropism,
geotropism, leaf
abscission, stomatal
regulation, cell
differentiation, cell death,
branching structure, etc.
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
- 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
– The ability of a terminal bud to suppress development of
axillary buds
• If the terminal bud is removed
– Plants become bushier
“Stump”
after
removal
of
apical
bud
Axillary buds
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Lateral branches
Response to Gravity
• Response of a plant to the gravitational field of the Earth
• Shoots exhibit negative gravitropism
• Roots have a positive gravitropic response
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Gravity Response
How Do Plants Detect Gravity?
•
•
Starch-filled plastids
– In specialized stem
cells and root caps
– Orient within cells
toward gravity
Changing plastid
root
orientation
triggers elongation –
 involves auxin!
cell in
root cap
plastids
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Photoperiodism
Other major class of light receptors:
Phytochromes – red/far-red receptor
• shade avoidance response
• photoperiodism
Photoperiodism:
is the response to time
of year (seasons)
A phytochrome consists of
two identical proteins joined
Photoreceptor
activity.
Enzyme - kinase
activity.
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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
• phytochrome is the pigment that
receives red light, which can interrupt
the nighttime portion of the 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
Circadian Rhythms
• Cyclical responses to stimuli
– about 24 hours long
– entrained to external clues of the day/night cycle
• Phytochrome changes mark sunrise and sunset
– Providing the biological clock with environmental cues
Many legumes
– Lower their
leaves in the
evening and
raise them in
the morning
Noon
Midnight
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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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Thigmotropism
• Thigmotropism is directional growth of a plant or plant
part in response to contact
• Examples of touch responses:
Venus flytrap leaves
Tendrils around objects
Mimosa tree leaves
Often due to differential
elongation or manipulated
water/turgor pressure
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Things To Do After Lecture 8…
Reading and Preparation:
1.
Re-read today’s lecture, highlight all vocabulary you do not
understand, and look up terms.
2.
Ch. 39 Self-Quiz: #1, 2, 6, 7 (correct answers in back of book)
3.
Read chapter 39, focus on material covered in lecture (terms,
concepts, and figures!)
4.
Skim next lecture.
“HOMEWORK” (NOT COLLECTED – but things to think about for studying):
1.
Describe in detail how auxin regulates stem directional growth.
2.
Explain the difference between phototropism and photoperiodism –
what wavelengths (colors) of light control each?
3.
Diagram the direction of growth in relation to positive vs. negative
phototropism, gravitropism, and thigmotropism
4.
List the five main plant hormones and give a once sentence description
of the main function/job of each one.