Plant 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 1: 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 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.

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 3: 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 4: 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.

• Question 5: 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 6: 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 7: What new

information does this tell us about why

plants lean toward the light?

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 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?

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

Auxin activityAuxins stimulate genes in cells associated with plant growth.

Auxin roles• Auxins carry out multiple roles

having to do with plant growth including:• Tropisms• Apical dominance• Growth of adventitious roots• Fruit growth

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

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.

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.

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?

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?

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.

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.

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.)

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.

Adventitious roots

• Adventitious roots are those growing out of places where roots don’t normally grow.

• Auxins stimulate root growth on the end of a houseplant cutting..

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?

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• 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?

Cytokinins

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.

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.

Lateral bud growth

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, especially flowers, to age and die (senescence).

• Inhibits stem elongation.

Flower drop• Ethylene is

released after a flower is pollinated.

• The flower senesces, dropping petals and allowing fruit to ripen.

Fruit Ripening• After the

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

Effects on Fruit• Ethylene signals

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

Abscisic Acid

Functions of Abscisic Acid

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

Seed Dormancy• Seeds remain

dormant until germination conditions are ideal.

• Abscisic acid signals continued dormancy, while gibberellins break dormancy.

Promoting senescence• Senescence

and death is a normal part of a annual plant’s life cycle.

• Production of abscisic acids stimulates senescence.

Nastic Movements

Nastic movement

in the sensitive

plant (Mimosa pudica)

Hinge control in Venus Fly Trap - Nastic movement

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.

Movies

• Sensitive Plant: http://www.youtube.com/watch?v=BVU1YuDjwd8

• Venus Fly Trap: http://www.youtube.com/watch?v=ktIGVtKdgwo&feature=related

Other examples

• Sunflowers follow the sun during the day.

• Leaves of many plants turn to follow the sun.

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.

Action of phytochrome on flowering time.

Pfr to Pr switch is how plants “tell time.”

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.