Plant Responses

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Plant Responses

• How plants move and communicate

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Early Inquiry
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The houseplant observation

• For years, people noticed that houseplants tended
  to lean toward a source of light.
• Charles Darwin and his son Francis, wondered why.
  How does a plant know where to lean?

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Darwins Oats

• The Darwins studied the leaning phenomenon in
  oats.
• Oat coleoptiles are highly light sensitive, and
  growth is fairly rapid.

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The Oat Experiments

• In the next several slides, youll see
  representations of experiments done by the
  Darwins and other scientists.
• On your own paper, answer the questions on each
  of the slides. After writing your answers,
  discuss them with a neighbor or in a small group.
  You will hand these in at the end of class.

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Darwin Experiment 1
Oat shoots tend to bend toward the light. When
the tip of the shoot is covered with a small cap,
the shoot does not bend.
Question 1 Why doesnt the shoot with the cap
bend toward the light? List several possible
reasons that could be tested with a scientific
study.
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One hypothesis...

• The Darwins speculated that somehow the tip of
  the plant perceives the light and communicates
  chemically with the part of the shoot that bends.
• Question 2 How could they test these two
  alternative explanations?
• The cap itself prevents bending.
• Light further down the shoot, rather than on the
  tip, causes bending.

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Darwin Experiment 2
Some shoots were covered with small caps of
glass. Others were covered with a sleeve that
left the tip exposed but covered the lower shoot.
Questions 3 What new information does this
experiment give us about the cause of shoot
bending? What new questions does it raise?
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Boysen-Jensen

• Several decades later, Peter Boysen-Jensen read
  of the Darwins experiments, and had further
  questions. He designed a set of experiments to
  try to further explain why plants bend toward the
  light.

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Boysen-Jensen 1

• Boysen-Jensen cut the tips off of oat coleoptiles
  and found that they did not bend toward the
  light.
• Question 4 What further information does this
  tell us about the role of the tip in this
  phenomenon? What questions does it raise?

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Boysen-Jensen 2

• Boysen-Jensen then cut the tips off of several
  oat coleoptiles and put the tips back on. These
  coleoptiles bent toward the light.
• Question 5 Why did Boysen-Jensen do this? What
  further information does this experiment give us?

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Boysen-Jensen 3
Boysen-Jensen then tried putting a porous barrier
(agar gel) and an impenetrable barrier (a flake
of mica) between the shoot tip and the rest of
the shoot. The shoot with an agar barrier bent
toward the light. The shoot with the mica
barrier did not.
Question 6 Does this experiment give us new
information or only confirm the results of other
experiments?
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Boysen-Jensen 4
In another experiment, Boysen-Jensen took a tiny,
sharp sliver of mica and pushed it into the
coleoptile so that it cut off communication
between the tip and the rest of the plant on one
side only. If the sliver was on the side that was
lit, it still leaned that toward the light, but
if it was on the opposite side, the plant did not
lean toward the light.
Questions 7 What new information does this tell
us about why plants lean toward the light?
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F.W. Went

• In the early 20th century, F.W. Went worked on
  identifying the factor that was causing plants to
  bend toward the light.
• By building on the work of the Darwins and
  Boysen-Jensen, Went was able to isolate the
  factor and show how it worked.

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F.W. Went 1
Went first cut the tips off of oat coleoptiles
and placed them on a block of agar and allowed
juices from the tip to diffuse into the agar.
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F.W. Went 2
Went then cut blocks from the agar. If he cut a
tip from an oat coleoptile and placed an agar
block on top, then put the coleoptile in the
dark, it grew just as it would if the tip were
intact.
Questions Why use the agar block infused with
plant juice instead of just cutting and replacing
the tip? Why place the plants in the dark instead
of shining light on one side as in the other
experiments?
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F.W. Went 3
Went also compared what happened when he placed
an agar block squarely on top of a clipped
coleoptile versus what happened when he set the
block on one side of the cut tip. In the first
case, the coleoptile grew straight up. In the
second, it bent.
Questions 8 What does this tell us about the
role of juice from the coleoptile tip in plant
growth? What effect do you think the juice is
having at the cellular level?
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The Mystery Factor

• Eventually, F.W. Went was able to isolate a
  chemical from coleoptile juice Indole acetic
  acid (IAA), one chemical in a class of plant
  hormones called auxins.

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Plant Hormones
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Plant Hormones

• Plant hormones can be divided into two classes
• Growth promoters Auxins, Gibberellins,
  Cytokinins
• Growth inhibitors Ethylene gas, Abscisic acid

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Growth promoters

• Hormones can promote plant growth in two ways
• Stimulating cell division in meristems to produce
  new cells.
• Stimulating elongation in cells.

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Auxins
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Auxin activity
Auxins stimulate genes in cells associated with
plant growth.
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Auxin roles

• Auxins carry out multiple roles having to do with
  plant growth including
• Tropisms
• Apical dominance
• Growth of adventitious roots
• Fruit growth

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Tropisms

• Tropisms are the growth of a plant toward or away
  from a stimulus, including
• Phototropism in response to light
• Gravitropism in response to gravity
• Thigmotropism in response to touch

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Tropisms cell elongation

• In general, tropisms involve cell elongation or
  suppression of cell elongation on one side of a
  plant, causing the plant to grow in a particular
  direction.

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Phototropism

• Look at the sprouts in the bottom picture and the
  explanatory diagram at the top. Explain why the
  sprouts are all leaning in the same direction.

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Gravitropism

• In this Impatiens plant, shoots grow upwards and
  roots grow downwards in response to gravity.
• On which side of the shoot and root do you think
  auxins are more concentrated?

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Gravitropism in shoots

• In shoots, auxins are more concentrated on the
  lower side of the stem, causing the cells there
  to elongate.
• Why is this gravitropism and not phototropism?

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Gravitropism in roots

• In roots, however, auxin concentration on the
  lower side of the root suppresses cell
  elongation.
• The upper side of the root continues to grow,
  causing the roots to bend downward.

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Plastids and Gravitropism
How does a root know which way is
down? Plastids, particularly leucoplasts, in the
root cap cell tend to settle on the bottom side
of the cell. This stimulates the release of
auxins.
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Thigmotropism

• In some plants, vining stems or tendrils will
  grow in response to touch.
• Which side of the tendril is elongating? Where
  might the auxin be? (Remember, this is the shoot
  system.)

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Apical dominance

• Auxins are released from the shoot tip. These
  stimulate cell elongation in the stem, but
  suppress the lateral buds.
• Cytokinins, produced in the roots, can stimulate
  lateral buds if the shoot tip is removed.

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Adventitious roots

• Adventitious roots are those growing out of
  places where roots dont normally grow.
• Auxins stimulate root growth on the end of a
  houseplant cutting..

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Fruit growth

• Developing seeds produce auxins that stimulate
  growth of the plant ovary into a fruit.
• Removal of seeds from a strawberry prevents the
  fruit from growing, but add auxin and will grow.
• How could this be used in commercial agriculture?

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Gibberellins
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Foolish rice seedlings

• Gibberellins were discovered when Japanese
  scientists were investigating bakanae, or
  foolish rice seedling disease, that caused
  seedlings to grow excessively tall, then fall
  over.

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Discovery of Gibberellins

• In 1898, Shotaro Hori suggested that the disease
  was caused by a fungus that infected the rice.
• Eiichi Kurosawa in 1926 was able isolate
  secretions from the fungus. The secretions caused
  the same symptoms when applied to other rice
  plants.
• In 1934, Teijiro Yabuta isolated the active
  substance and named it gibberellin.

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Functions of Gibberellins

• Promotes cell elongation in the internodes of
  plants.
• Stimulates growth of the ovary wall into a fruit.
• Stimulates seed germination and release of food
  reserves in seeds.

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Commercial Uses

• On the left are ordinary green grapes with seeds.
  On the right is a cluster of Thompson seedless
  grapes. These both came from the same variety of
  grapevine. How can this be?

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Cytokinins
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Functions of Cytokinins

• Promote cell division in meristems.
• Promote growth of lateral buds when auxin
  concentrations are low.
• Stimulate fruit and seed development.
• Delays senescence of plant parts.

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Delay of Senescence

• The plant on the left has blossomed and is now
  senescing.
• The plant on the right is the same age, but was
  treated with cytokinins.

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Lateral bud growth
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Ethylene Gas
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Gaseous discoveries

• In ancient China, people placed pears or oranges
  in rooms with burning incense to make them ripen
  faster.
• For centuries, people assumed heat or light was
  responsible for fruit ripening. In the 19th
  century, fruit ripening sheds were built using
  gas or kerosene heaters. When these were replaced
  with electric heaters, fruit didnt ripen as fast.

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Illuminating gas

• In the 1800s, gas lighting was first installed
  in cities. People noticed that houseplants
  growing near gas light fixtures grew abnormally.
  Cut flowers aged and wilted quickly.
• Physiologist Dimitry Neljubow analyzed natural
  gas and found that one component, ethylene gas,
  was responsible for the effects.

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Functions of Ethylene

• Released by fruits and causes the fruits to ripen
  faster.
• Causes plant parts, especially flowers, to age
  and die (senescence).
• Inhibits stem elongation.

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Flower drop

• Ethylene is released after a flower is
  pollinated.
• The flower senesces, dropping petals and allowing
  fruit to ripen.

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Fruit Ripening

• After the flower senesces, the plant again
  produces ethylene gas to stimulate fruit ripening.

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Effects on Fruit

• Ethylene signals the release of several enzymes.
  These enzymes break starch into sugars, soften
  pectin, reduce chlorophyll and create other
  pigments.

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Abscisic Acid
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Functions of Abscisic Acid

• Controls seed and bud dormancy.
• Inhibits gibberellins.
• Promotes senescence in plants.

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Seed Dormancy

• Seeds remain dormant until germination conditions
  are ideal.
• Abscisic acid signals continued dormancy, while
  gibberellins break dormancy.

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Promoting senescence

• Senescence and death is a normal part of a annual
  plants life cycle.
• Production of abscisic acids stimulates
  senescence.

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Nastic Movements
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Nastic movement in the sensitive plant (Mimosa
pudica)
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Hinge control in Venus Fly Trap - Nastic movement
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How it works

• Nastic movements are rapid, reversible movements
  in a plant.
• Electrical potentials across cell membranes,
  similar to those in our nerve cells, signal plant
  cells at the base of the Mimosa leaf to rapidly
  lose water. This causes the leaf to droop.

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Movies

• Sensitive Plant http//www.youtube.com/watch?vBV
  U1YuDjwd8
• Venus Fly Trap http//www.youtube.com/watch?vktI
  GVtKdgwofeaturerelated

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Other examples

• Sunflowers follow the sun during the day.
• Leaves of many plants turn to follow the sun.

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Day/Night length

• Some plants flower in response to the length of
  periods of darkness.
• Spring-blooming flowers are long night (short
  day) plants, while summer-blooming flowers are
  short night (long day) plants.
• Some plants are day-neutral.

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Action of phytochrome on flowering time.
Pfr to Pr switch is how plants tell time.
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Plant Communication

• Plants communicate chemically.
• Injured plants send out chemical signals that may
• signal other plants to prepare for an attack.
• attract other insects that eat the insects that
  are attacking the plant.

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