Transcript Chapter 39 Presentation-Plant Responses to Internal and External

Chapter 39
Plant Responses to Internal and
External Signals
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Response to Stimuli
Plants are sensitive to a wide range of
stimuli.
They elicit a response.
They use a signal transduction pathway.
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Response to Stimuli
 Consider a forgotten
potato in the cupboard.
The eyes of the potato
(axillary buds) sprout
shoots that are suited to
their function. They are
pale and lack broad
green leaves. They lack
elongated roots.
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Etiolation
These adaptations for growing in the
dark are called etiolation.
The “stimulus” for growth is complete
darkness.
The plant uses all energy to elongation
of the stem so the leaves can open after
reaching the surface.
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Etiolation
During etiolation, there is no evaporative
loss of water.
Leaves would be a hindrance to the
shoot passing through the soil.
There’s no need for chlorophyll--there’s
no light.
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De-etiolation
When the shoot hits
the light, de-etiolation
occurs.
Leaves now expand.
Elongation of the
stem slows.
Roots elongate.
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Signal Reception
Signals are detected by receptors.
Proteins change in response to the
stimulus.
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Signal Transduction
 Second messengers are small, internally
produced chemicals.
 They transfer and amplify signals from the
receptor to the other proteins causing a
response.
 One signal receptor protein can give rise to
hundreds of specific enzymes.
 In this way, 2nd messenger signal
transduction leads to rapid amplification of the
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signal
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Response
Signal transduction leads to one or more
cellular pathways being regulated.
Usually, this leads to an increase in the
activity of certain enzymes.
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Response: 2 Main Mechanisms
1. Stimulating transcription of mRNA.
2. Activating existing enzyme
molecules.
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1. Stimulating Transcription of
mRNA
This is called transcriptional regulation.
These transcription factors bind directly
to DNA molecules and control the
transcription of specific genes.
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The De-etiolation Response and
Phytochrome
The receptor involved in de-etiolation is
a phytochrome.
The mutant tomato studied has lower
levels of phytochrome.
They green less when exposed to light
than normal tomatoes.
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The De-etiolation Response and
Phytochrome
When the mutants were injected with
phytochrome from other plants they
exhibited a normal de-etiolation
response when exposed to light.
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The De-etiolation Response and
Phytochrome
Small amounts of light can trigger the
de-etiolation response.
In the phytochrome example, small
amounts of light give rise to activated
phytochrome.
This gives rise to hundreds of second
messenger molecules which leads to
hundreds of activated
enzymes.
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The Change in Phytochrome
 Light causes the conformation of phytochrome to
change.
 This leads to an increase in cGMP (2nd messenger)
and Ca2+ influx.
 cGMP activates protein kinases.
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If we inject the mutant tomatoes with
cGMP, we get a partial de-etiolation
response--even without the addition of
phytochrome.
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Protein kinases are activated by cGMP
and Ca2+, and can act to phosphorylate
and activate other enzymes.
These can be used to stimulate or shut
down transcription.
When transcription is affected, the
enzymes can now synthesize proteins
for chlorophyll production and other detravismulthaupt.com
greening proteins.
The mechanism by which a signal
promotes a new developmental course
depends on the activation of positive or
negative control factors.
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Post-translational modification involves
activating existing enzyme molecules.
This is where existing proteins are
modified--usually via phosphorylation.
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Kinases
Often, kinases become activated by
phosphorylation which activates more
kinases, and so on.
Eventually, the cascades link initial
stimuli to responses at the gene level
where they are expressed.
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Signal Pathways
Signal pathways lead to both a turning
on and off of genes.
For example, putting a potato back into
the cupboard activates many
phosphatases which dephosphorylate
specific proteins and switch off certain
pathways.
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The Idea of Signal Pathways
Classic experiments studying grass
uncovered the notion of chemical
messengers.
The movement of a plant shoot toward
or away from a stimulus is called
tropism.
The hormone pathway.
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Signal Pathways and Grass
Seedlings
Phototropism is the process
that directs plants toward
sunlight for photosynthesis.
Grass shoots kept in the
dark will grow straight up.
So will those illuminated
equally on all sides.
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Signal Pathways and Grass
Seedlings
 If illuminated from only one side, the plant will grow
toward the stimulus.
 This results in differential growth on the opposite side
of the stimulus.
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The Darwins’ Experiments
Observations:
 Plants will only bend toward the light
source if the coleoptile is present--no tip, no
curve.
 Covering the tip with an opaque cap
prevents curving.
 Covering the tip with a transparent cap or
placing a cover below the tip--curving.
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The Darwins’ Experiments
Their conclusions:
The tip of the coleoptile is responsible
for curvature.
Also, the curvature of the plant actually
was the result of differential growth
some distance below the coleoptile.
Some signal must be responsible for
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elongation of the
coleoptile.
Peter Boysen-Jensen
A few decades later:
He separated the tip
of the coleoptile with
a block of gelatin.
The cells still showed
the normal growth
response.
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Peter Boysen-Jensen
Using mica, there
was no response.
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Frits Went
A Dutchman that modified BoysenJensen’s experiment to extract the
chemical messenger.
He removed the coleoptile tip and placed
it on an agar block.
If the messenger could diffuse into the
block, then it could be substituted for the
tip and placed on the decapitated
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coleoptile resulting
in normal growth.
Frits Went
 Results:
 Decapitated
coleoptile--no
growth.
 Decapitated
coleoptile + agar
only--no growth.
 Decapitated
coleoptile + agar with
hormones--growth. travismulthaupt.com
Frits Went
Also, placing the block on one side of
the coleoptile or the other caused
unequal growth on the side containing
the block causing curvature in the
opposite direction.
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Frits Went
Conclusions:
 The plants curved due to the higher
concentration of growth promoting chemical
on the dark side of the plant.
 Went named this hormone auxin.
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Went’s Model
This model doesn’t necessarily occur in
all plants.
There is still an unequal distribution of
auxin in a plant causing curvature.
Some plants show an increase in growth
inhibitors on the light side of the plant.
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Plant Hormones
There are many different classes of plant
hormones.
They all have different effects on plants.
Most are produced in very small
amounts and often have profound
effects on the plant.
The hormones are often amplified and
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acts to alter gene
expression.
3 Common Plant Hormones
Auxin-stimulate growth. Produced in the
embryo, growth tissue, and meristematic
tissue.
Gibberillins--produced in the apical
meristems of buds and roots, young
leaves and embryos.
Ethylene--promotes ripening. Opposes
auxin.
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Light
Light is an important environmental
factor in the growth and development of
plants.
Photomorphogenesis is the effect of light
on plant morphology.
The ability of a plant to perceive light
allows plants to measure the passage of
days and seasons.
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Circadian Rhythms
Common to all eukaryotic life, and is not
governed by an known environmental
factor.
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Photoperiodism
Is the physiological response of plant
due to a change in the lengths of night
and day--a photoperiod.
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Different Types of Plants
There are 3 general varieties of plants
classified according to their light
requirements for flowering:
 1. Short-day plants
 2. Long-day plants
 3. Day-neutral plants.
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Short-Day Plants
Respond the long nights.
A.k.a. long-night plants.
They usually flower in the late summer,
fall, or winter as the light period is
shorter than 14 hours.
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Long-Day Plants
Respond to short nights.
A.k.a. short-night plants.
They flower when the light period is
longer than 14 hours.
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Day-Neutral Plants
These are unaffected by the light period,
and flower when they reach maturity.
Tomatoes, rice, and dandelions.
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Classic Experiments
In the 1940’s scientists began
experimenting with photoperiods.
They looked at the length of the night
and day.
They found that short-day plants flower
when days are 16 hours or shorter
(nights are 8 hours or longer).
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Classic Experiments
 In the short-day plants, they looked at
flowering:
 They found that if the daytime portion of
photoperiod is broken by a brief period of
darkness, there is no effect.
 However, if the nighttime portion of the
photoperiod is interrupted by a short period
of dim light, the plant doesn’t flower.
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Classic Experiments
The opposite is true for long-day plants:
 When long day plants are grown in a
photoperiod of a long night, flower doesn’t
occur.
 However, if the long night portion of the
experiment is interrupted by a brief period
of dim light, flowering will occur.
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From These Experiments
Red light is most effective at interrupting
the nighttime portion of the photoperiod.
Scientists have demonstrated that
phytochrome is the pigment that
measures the photoperiod.
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Extending the Experiments
Scientists at the USDA conducted these
experiments.
Phytochrome was demonstrated to be
the pigment responsible for seed
germination.
From this, they were able to elucidate
the flowering cycle.
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USDA Flowering Experiments
Seeds were subjected to a variety of
monochromatic light.
Red and far-red light opposed each
other in their germinating ability.
One induced germination, the other
inhibited it.
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USDA Flowering Experiments
It was determined that the two different
forms of light switched the phytochrome
back and forth between two isomeric
forms.
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USDA Flowering Experiments
One form caused seed germination, the
other inhibited the germination response.
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USDA Flowering Experiments
The question: How do plants in nature
illicit a response to light and begin
germination?
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USDA Flowering Experiments
If seeds are kept in the dark, they synthesize
Pr.
When seeds are illuminated with sunlight, they
begin to be converted to Pfr.
The appearance of Pfr is one of the ways
plants detect sunlight.
Adequate sunlight converts Pr to Pfr and
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triggers germination.
USDA Flowering Experiments
In the flowering response, scientists
were able to show the effects of the red
and far red light on the flowering ability
in plants.
Again, the 2 forms of light canceled each
other.
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Other Stimuli
There are also a wide variety of stimuli
other than light that effects plant growth.
Gravity, mechanical stimuli, and
environmental stress also play a role in
plant growth and development.
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