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: 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 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 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 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.
Thinking question • How does pinching back a plant, such as this chrysanthemum, cause it to become more bushy?
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..
Thinking question • When people grow new plants from cuttings, they often dip the end of the cutting in rooting compound to stimulate root growth. • What hormone is in the compound? • How does it work?
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?
Thinking question • You’re at the State Fair, looking at the giant vegetable competition, and you notice a super-tall sunflower entered by your favorite biology professor. You notice that it has the same number of leaves as its competitors, just extra-long internodes. Should you alert the judges?
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.
Thinking question • You’re a plant scientist preparing a plant tissue callus in a petri dish. You want to have lots of rapidly dividing, undifferentiated cells in the dish. What hormone will you use and why?
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.