Plant Responses

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Chapter 44Plant 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 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 How could they test these 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 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 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.
• Questions 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 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 What new information does this tell
us about why plants lean toward the light? What
new hypotheses could be formed?
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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 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

• As early experiments with oat coleoptiles showed,
  Auxin causes elongation of stems on the side of
  the plant where there is light.

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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 just as they do in 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.
• Auxin stimulates cell elongation in the side away
  from the object that is touching the stem.

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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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Thinking question

• How does pinching back a plant, such as this
  chrysanthemum, cause it to become more bushy?

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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. Commercial rooting compounds
  contain auxins to make this happen faster.

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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.

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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

• Gibberellins are sometimes used to cause fruits
  to grow larger or develop without seeds.
• Thompson seedless grapes are one example.

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Cytokinins
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In search of a growth factor

• In the early 1950s, Dr. Folke Skoog and Dr.
  Carlos Miller were in search of a better medium
  in which to grow plant tissues and to manipulate
  cells to grow roots and shoots.
• After experimenting with coconut milk and yeast
  extract, they found evidence that a derivative of
  a nucleotide (DNA component) might be the factor
  in these substances that stimulated cell growth.
• Miller, looking for a source of nucleotides,
  found an old bottle of herring sperm DNA in the
  storeroom. When he used it on plant tissue, he
  found terrific growth.

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Isolating the factor

• Miller ordered a new bottle of herring sperm DNA,
  but the new sample didnt cause cell division as
  the old one had.
• After much work, Skoog and Miller isolated the
  one factor that coconut milk, yeast extract, and
  old DNA had in common, and that stimulated cell
  division. The substance, which they named
  kinetin, was structurally similar to the DNA
  base, adenine and appeared to be a chemical
  derivative of adenine.
• Since the time of Miller and Skoogs work,
  similar molecules have been found, and grouped
  together under the name of cytokinins.

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Functions of Cytokinins

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

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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 to age and die (senescence).
• Inhibits stem elongation.

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Ethylene and fruit

• Many people line green tomatoes up on a
  windowsill to make them ripen.
• However, putting tomatoes in a paper bag to
  concentrate the ethylene gas makes ripening
  happen much faster.

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Abscisic Acid
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In search of an inhibitor

• In separate studies in 1963, F.T. Addicott found
  a substance that stimulated abscission of fruits
  in cotton, and named it abscisin. P.F. Wareing
  found a substance that promoted dormancy in
  sycamore tree leaves and called it dormin.
• By 1967, both teams realized they were studying
  the same substance. At a conference they decided
  to call the substance abscisic acid.

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Functions of Abscisic Acid

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