#44
Chapter
Plant Responses
How plants move and communicate
Early Inquiry
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?
Darwin’s Oats
• The Darwins
studied the leaning
phenomenon in
oats.
• Oat coleoptiles are
highly light
sensitive, and
growth is fairly
rapid.
The Oat Experiments
• In the next several slides, you’ll 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.
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
doesn’t the shoot
with the cap bend
toward the light? List
several possible
reasons that could
be tested with a
scientific study.
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.
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?
Boysen-Jensen
• Several decades later, Peter BoysenJensen 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.
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?
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?
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?
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?
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.
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.
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?
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?
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.
Plant Hormones
Plant Hormones
•
Plant hormones can be divided into
two classes:
• Growth promoters: Auxins,
•
Gibberellins, Cytokinins
• Growth inhibitors: Ethylene gas,
•
Abscisic acid
Growth promoters
• Hormones can promote plant growth
in two ways:
• Stimulating cell division in meristems
to produce new cells.
• Stimulating elongation in cells.
Auxins
Gibberellins
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.
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.
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.
Commercial Uses
• Gibberellins are
sometimes used to
cause fruits to grow
larger or develop
without seeds.
• Thompson seedless
grapes are one
example.
Cytokinins
In search of a growth factor
•
In the early 1950’s, 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.
Isolating the factor
•
Miller ordered a new bottle of herring sperm DNA, but the
new sample didn’t 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 Skoog’s work, similar
molecules have been found, and grouped together under
the name of “cytokinins.”
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.
Ethylene Gas
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 didn’t ripen as fast.
“Illuminating gas”
• In the 1800’s, 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.
Functions of Ethylene
• Released by fruits and causes the fruits
to ripen faster.
• Causes plant parts to age and die
(senescence).
• Inhibits stem elongation.
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.
Abscisic Acid
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.
Functions of Abscisic Acid
• Controls seed and bud dormancy.
• Inhibits gibberellins.
• Promotes senescence in plants.