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Plant Hormones: Fun Facts and Applications Target audience: High School Biology students Time allotted for this activity: Two 45-minute class periods (or one class period and finish for homework); or one 90-minute block. Materials needed: Computer, handouts (optional) National Science Standards: 12CLS1.4 Cell functions are regulated. Regulation occurs both through changes in the activity of the functions performed by proteins and through the selective expression of individual genes. This regulation allows cells to respond to their environment and to control and coordinate cell growth and division. 12CLS6.2 Organisms have behavioral responses to internal changes and to external stimuli. Responses to external stimuli can result from interactions with the organism’s own species and others, as well as environmental changes; these responses either can be innate or learned. The broad patterns of behavior exhibited by animals have evolved to ensure reproductive success. Animals often live in unpredictable environments, and so their behavior must be flexible enough to deal with uncertainty and change. Plants also respond to stimuli. Objectives for this activity: 1. The student will be able to identify five major plant hormones. 2. The student will be able to list at least three effects that each hormone has on plant growth and development. 3. The student will be able to list a commercial application for each plant hormone. To begin the trackstar, go to: http://trackstar.4teachers.org/trackstar/ts/viewTrack.do?number=405358 Click on “View in Frames” at the bottom of the page. Teachers can use the guided questions that correlate with each website in the following ways: 1. Have students copy the questions from the top of each new screen and write out their answers. 2. Teachers can give students a handout with the questions and directions (see student copy). 2 3. Students can copy and paste the questions into a Word document and type their responses. This is helpful for electronic submission of the assignment. Abscisic Acid - http://www.plant-hormones.info/abscisicacid.htm 1. Discuss how abscisic acid (ABA) got its name. Scientists were studying compounds that were responsible for abscission of fruits. The compounds that were isolated were named abscisin I and abscisin II. 2. Where is ABA produced in plants? List both the plant organ and organelles. Construct a flow chart showing the synthesis of this compound from the source to the activated compound.. ABA is produced in chloroplasts and other plastids which are primarily in leaves. The pathway of synthesis is: carotenoids (violaxanthin) →isomerase reaction → oxidation reaction → xanthonin →ABA aldehyde →oxidation →ABA →glucose or oxidation produces the active form. 3. Read the functions of ABA. Based on the original notion that this substance is involved in abscission, why do the listed functions make sense? All of the functions involve inhibition or slowing down the growth processes of plants. Some of the functions also involve dormancy or preparing for dormancy. Auxins- http://plantphys.info/plant_physiology/auxin.shtml 1. Look at the diagrams showing the historical experiments that enabled scientists to learn about the function of auxin (IAA). Briefly describe what each scientist did. Charles Darwin cut the tops off of coleoptiles to see the response of the plant to light. He determined that the tip of the plant responded to light. The BoysenJensen experiments cut the top of the coleoptile off but they inserted a piece of mica between the cut tip and the tip itself. They believed there was a chemical response to light was occurring further down the tip. The Went experiments used agar blocks between the coleoptile tips and the rest of the coleoptile. The blocks were placed off-center and the coleoptile growth responded accordingly. This proved that a chemical was involved the growth pattern of the coleoptiles. 2. What is the optimal concentration for IAA for elongation? 10-5M 3. When a plant produces auxin in excess of the optimal concentration, how does the plant compensate? Look at the diagram showing the sources and sinks of auxin. The auxin is compartmentalized, transported somewhere else in the plant, degraded, or conjugated and then possibly degraded. 3 4. Discuss how auxin moves through a plant. Look at the diagrams showing basipetal movement. What does this word mean? Google this term to find a definition. The auxin always moves from the section of the stem that is towards the top of the plant to the bottom of the plant. The diagrams show that no matter which way the stem sections were turned, the auxin would move from the area closest to the top of the stem towards the bottom part of the stem. Basipetal means to proceed from the apex to the base. Auxins continued - http://www.plant-hormones.info/auxins.htm 5. Scroll down to the “Functions of auxin.” Contrast the main function of this hormone with that of ABA. Auxin promotes growth of cells and cell division. It also promotes growth of flower parts and delays cell death or dormancy of the plant. It also suppresses lateral buds so apical growth is encouraged. ABA promotes cell death or dormancy. In a sense, it has the opposite effects on the plant from auxin. Cytokinins - http://www.plant-hormones.info/cytokinins.htm 1. Before reading the information about cytokinin, speculate what the job of this hormone is based on its name. Students may have learned the term cytokinesis and associate this word with cell division. The role of cytokinins is promotion of cell division. 2. Summarize the history of how cytokinin was discovered. Scientists discovered that substances which promoted cell division were in phloem and in endosperm. The first kinetin was isolated from herring sperm in 1955. The first cytokinin isolated from a plant was zeatin – named for the plant it was taken from (Zea mays - corn). 3. Where is cytokinin concentrated in a plant? The meristematic tissue. 4. Read the functions of this hormone. Contrast one function of cytokinin with that of auxin. Auxin does not promote lateral buds but cytokinin does stimulate lateral growth. 4 Cytokinin continued - http://www.ars.usda.gov/is/pr/2010/100310.htm Read the article on how cytokinin is used for growing cotton. Discuss why the application of this hormone is beneficial to cotton farmers. Farmers that grow cotton in dry areas have found that cytokinin “tricks” the young plants into growing a deeper root system and also the plant produces a waxy coating that prevents water loss. The cytokinin is applied at a low concentration and did not have any deleterious effect on plants grown in wet conditions. The hormone can be applied at the same time that weed killers are applied. Gibberelins - http://4e.plantphys.net/article.php?ch=5&id=372 1. Read the paragraph on commercial uses for gibberellins. What are the various uses for this hormone in the grape, cherry, apple, and citrus crop industries? This hormone is used to promote growth of fruit crops and increase stem length. In the grape industry, the gibberellins substitute for seeds and fruits will form without the seed. It also elongates the rachis of the grape clusters which allows for larger fruit and less fungal growth since the fruits are farther apart and air flow is better. Cherry production is enhanced because the hormone produces bigger fruit. In apples the hormone is used to promote fruiting in the “off years” since apples tend to be biennial producers of fruit. One type of gibberellin is used for prevention of russetting on Golden Delicious apples. For citrus crops, the gibberellic acid can prevent rind-aging in oranges and tangerines and in lemons and limes it helps ripening occur when the fruit is at an optimal size. Ethylene – http://plantphys.info/plant_physiology/ethylene.shtml 1. Scroll half-way down the web page until you reach the section on Ethylene and Fruit Ripening. What process accelerates after an increase in ethylene release? What does the graph show that verifies this conclusion? What is the term that describes this effect? Respiration increases after ethylene is released in plants. The graph shows that carbon dioxide release increases right after ethylene release increases. The term for an increase in respiration shortly after ethylene release is climacteric. 2. Ethylene accelerates cell death and ripening of fruits. Read and summarize how the shelf-life of cut flowers can be extended if ethylene is blocked. Ethylene requires a copper cofactor in the cell-membrane receptor protein that can be eliminated if silver is present instead. By adding silver thiosulfate, the ethylene does not bind to the cells and flowers do not die as quickly. 5 Extra credit – http://www.plant-hormones.info/index.htm Go to the website and click on the links for each plant hormone from this activity. List the compound that is the hormone is synthesized from in the plant. For example, ABA →violaxanthin (a carotenoid). ABA →violaxanthin (a carotenoid) Auxin→tryptophan Cytokinin→adenine Ethylene→methionine Gibberellin→acetyl Co-A