Download
1 / 66
660 likes | 915 Views
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.
E N D
Plant Responses • How plants move and communicate
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 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 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?
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 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?
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?
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 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 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.
Auxin activity Auxins 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?
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?
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.
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.
