Chapter #44Plant ResponsesHow 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 1Oat 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 2Some 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 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.

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 3Boysen-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 4In 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 1Went 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 2Went 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 3Went 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.