Chapter 26: Flowering Plants: Control of Growth Responses

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1 Chapter 26: Flowering Plants: Control of Growth Responses AP Curriculum Alignment Big Idea 2 explains that organisms maintain homeostasis by using positive and negative feedback mechanisms. A plant s response to drought is an example of a negative feedback mechanism and fruit ripening is an example of a positive feedback mechanism. An alteration in a feedback mechanism may result in a plant s inability to respond properly to the environment. Chapter 26 reviews the major feedback mechanisms found in flowering plants. Like animals, plants have evolved a variety of defenses against infections and other disruptions to homeostasis including immune responses. At the ecosystem level, disruptions in dynamic homeostasis impact the balance of the ecosystem and the interactions between specific organisms therein. Chapter 26 reviews several plant defenses that help them maintain homeostasis. Physiological events in organisms can involve interactions between environmental stimuli and internal molecular signals; phototropism and photoperiodism in plants are examples. Photoperiodism in plants due to changes in critical night length is an example of timing and coordination of processes that are regulated by several means. Big Idea 3 contains information about the ubiquitous nature of cellular communication. Multicellular organisms have complex processes that are directed by cell-to-cell communication and environment-to-cell communication. The signal of a chemical signaling pathway can be a molecule or a physical or environmental factor. Understanding chemical pathways enables humans to modify these pathways as seen in our ability to control or modify ripening in fruit and agricultural production (growth hormones). Chapter 26 explains specific communication pathways that plants use for growth and development. ALIGNMENT OF CONTENT TO THE CURRICULUM FRAMEWORK Big Idea 2: Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis. Enduring understanding (EU) 2.C: Organisms use feedback mechanisms to regulate growth and reproduction, and to maintain dynamic homeostasis. Essential knowledge (EK) 2.C.1: Organisms use feedback mechanisms to maintain their internal environments and respond to external environmental changes. a. Negative feedback mechanisms maintain dynamic homeostasis for a particular condition (variable) by regulating physiological processes, returning the changing condition back to its target set point. To foster student understanding of this concept, instructors can choose an illustrative example such as: 376 Mader, Biology, 12 th Edition, Chapter 26

2 Operons in gene regulation Temperature regulation in animals Plant responses to water limitations b. Positive feedback mechanisms amplify responses and processes in biological organisms. The variable initiating the response is moved farther away from the initial set-point. Amplification occurs when the stimulus is further activated which, in turn, initiates an additional response that produces system change. Students should be able to demonstrate understanding of the above concept by using an illustrative example such as: Lactation in mammals Onset of labor in childbirth Ripening of fruit Essential knowledge (EK) 2.C.2: Organisms respond to changes in their external environments. a. Organisms respond to changes in their environment through behavioral and physiological mechanisms. To foster student understanding of this concept, instructors can choose an illustrative example such as: Photoperiodism and phototropism in plants Hibernation and migration in animals Taxis and kinesis in animals Chemotaxis in bacteria, sexual reproduction in fungi Nocturnal and diurnal activity: circadian rhythms Shivering and sweating in humans No specific behavioral or physiological mechanism is required for teaching the above concept. Teachers are free to choose the mechanism that best fosters student understanding. Enduring understanding (EU) 2.D: Growth and dynamic homeostasis of a biological system are influenced by changes in the system s environment. Essential knowledge (EK) 2.D.4: Plants and animals have a variety of chemical defenses against infections that affect dynamic homeostasis. a. Plants, invertebrates and vertebrates have multiple, nonspecific immune responses. Students should be able to demonstrate understanding of the above concept by using an illustrative example such as: Invertebrate immune systems have nonspecific response mechanisms, but they lack pathogen-specific defense responses. Plant defenses against pathogens include molecular recognition systems with systemic responses; infection triggers chemical responses that destroy infected and adjacent cells, thus localizing the effects. Vertebrate immune systems have nonspecific and nonheritable defense mechanisms against pathogens. Mader, Biology, 12 th Edition, Chapter

3 Enduring understanding 2.E: Many biological processes involved in growth, reproduction and dynamic homeostasis include temporal regulation and coordination. Essential knowledge (EK) 2.E.2: Timing and coordination of physiological events are regulated by multiple mechanisms. a. In plants, physiological events involve interactions between environmental stimuli and internal molecular signals. Evidence of student learning is a demonstrated understanding of each of the following: 1. Phototropism, or the response to the presence of light 2. Photoperiodism, or the response to change in length of the night, that results in flowering in long-day and short-day plants Memorization of the names, molecular structures and specific effects of all plant hormones are beyond the scope of the course and the AP Exam. Big Idea 3: Living systems store, retrieve, transmit, and respond to information essential to life processes. Enduring understanding (EU) 3.B: Expression of genetic information involves cellular and molecular mechanisms. Essential knowledge (EK) 3.B.2: A variety of intercellular and intracellular signal transmissions mediate gene expression. a. Signal transmission within and between cells mediates gene expression. To foster student understanding of this concept, instructors can choose an illustrative example such as: Cytokines regulate gene expression to allow for cell replication and division. Mating pheromones in yeast trigger mating gene expression. Levels of camp regulate metabolic gene expression in bacteria. Expression of the SRY gene triggers the male sexual development pathway in animals. Ethylene levels cause changes in the production of different enzymes, allowing fruits to ripen. Seed germination and gibberellin. Enduring understanding (EU) 3.D: Cells communicate by generating, transmitting and receiving chemical signals. Essential knowledge 3.D.2: Cells communicate with each other through direct contact with other cells or from a distance via chemical signaling. a. Cells communicate by cell-to-cell contact. To foster student understanding of this concept, instructors can choose an illustrative example such as: Immune cells interact by cell-cell contact, antigen-presenting cells (APCs), helper T-cells and killer T-cells. Plasmodesmata between plant cells that allow material to be transported from cell to cell. b. Cells communicate over short distances by using local regulators that target cells in the vicinity of the emitting cell. To foster student understanding of this concept, instructors can choose an 378 Mader, Biology, 12 th Edition, Chapter 26

4 illustrative example such as: Neurotransmitters Plant immune response Quorum sensing in bacteria Morphogens in embryonic development Essential knowledge 3.D.3: Signal transduction pathways link signal reception with cellular response. a. Signaling begins with the recognition of a chemical messenger, a ligand, by a receptor protein. Evidence of student learning is a demonstrated understanding of each of the following: 1. Different receptors recognize different chemical messengers, which can be peptides, small chemicals or proteins, in a specific one-to-one relationship. 2. A receptor protein recognizes signal molecules, causing the receptor protein s shape to change, which initiates transduction of the signal. To foster student understanding of this concept, instructors can choose an illustrative example such as: G-protein linked receptors Ligand-gated ion channels Receptor tyrosine kinases No particular system is required for teaching the concepts above. Teachers are free to choose a system that best fosters student understanding. b. Signal transduction is the process by which a signal is converted to a cellular response. Evidence of student learning is a demonstrated understanding of each of the following: 1. Signaling cascades relay signals from receptors to cell targets, often amplifying the incoming signals, with the result of appropriate responses by the cell. 2. Second messengers are often essential to the function of the cascade. To foster student understanding of this concept, instructors can choose an illustrative example such as: Ligand-gated ion channels Second messengers, such as cyclic GMP, cyclic AMP, calcium ions (Ca2+), and inositol triphosphate (IP3) 3. Many signal transduction pathways include: i. Protein modifications (an illustrative example could be how methylation changes the signaling process) ii. Phosphorylation cascades in which a series of protein kinases add a phosphate group to the next protein in the cascade sequence. Mader, Biology, 12 th Edition, Chapter

5 Concepts covered in Chapter 26 also align to the learning objectives that provide a foundation for the course, an inquiry-based laboratory experience, class activities, and AP exam questions. Each learning objective (LO) merges required content with one or more of the seven science practices (SP), and one activity or lab can encompass several learning objectives. The learning objectives and science practices from the Curriculum Framework that pertain to the control of growth in flowering plants are shown in the table below. Note that other learning objectives may apply as well. LO 2.15 The student can justify a claim made about the effect(s) on a biological system at the molecular, physiological or organismal level when given a scenario in which one or more components within a negative regulatory system is altered. LO 2.16 The student is able to connect how organisms use negative feedback to maintain their internal environments. LO 2.17 The student is able to evaluate data that show the effect(s) of changes in concentrations of key molecules on negative feedback mechanisms. LO 2.18 The student can make predictions about how organisms use negative feedback mechanisms to maintain their internal environments. LO 2.19 The student is able to make predictions about how positive feedback mechanisms amplify activities and processes in organisms based on scientific theories and models. LO 2.20 The student is able to justify that positive feedback mechanisms amplify responses in organisms. LO 2.21 The student is able to justify the selection of the kind of data needed to answer scientific questions about the relevant mechanism that organisms use to respond to changes in their external environment. LO 2.35 The student is able to design a plan for collecting data to support the scientific claim that the timing and coordination of physiological events involve regulation. LO 2.36 The student is able to justify scientific claims with evidence to show how timing and coordination of physiological events involve regulation. LO 2.37 The student is able to connect concepts that describe mechanisms that regulate the timing and coordination of physiological events. LO 3.22 The student is able to explain how signal pathways mediate gene expression, including how this process can affect protein production. LO 3.23 The student can use representations to describe mechanisms of the regulation of gene expression. LO 3.36 The student is able to describe a model that expresses the key elements of signal transduction pathways by which a signal is converted to a cellular response. 380 Mader, Biology, 12 th Edition, Chapter 26

6 Key Concepts Summary Plant signaling and response Plant use cell communication to respond to the environment and maintain homeostasis. The process of cellular communication involves signal reception, signal transduction, and cellular response. 1. Signal reception: a specific receptor protein receives a specific signal molecule 2. Signal transduction pathway: receptor protein passes the signal through a cascade or secondary messenger molecules to the part of the cell that can respond. 3. Cellular response: can involve the activation of DNA or a particular metabolic pathway Plant hormones Plant hormones serve as chemical signals that initiate cellular communication and a final cellular response. Auxins are a class of plant hormones that promote growth. o Auxins cause phototropism, which is plant bending toward light to maximize the light that the plant receives. o Auxins are responsible for geotropism by causing roots to grow downward and stem to grow upward. o Auxin produced in the apical meristem of the terminal bud is transported downward, inhibiting the growth of lateral or axillary buds. o Synthetic auxins are used commercially as in spraying tomatoes to cause fruit to ripen without pollination, thus creating seedless tomatoes. Gibberellins induce growth in a variety of crops, such as apples, cherries, and sugarcane, and seedless grapes. o Gibberellins can break the dormancy of buds and seeds by inducing the presence of amylase, and enzyme that breaks down starch and makes glucose available as an energy source. Cytokinins influence plant growth by promoting cell division in all tissues of growing plants. o Plant organ formation is influenced by cytokinins and auxin interactions and these two hormones must exist in a specific ration for proper development. o Cytokinins prevent senescence, which is plant aging. Abscisic acid, the stress hormone, initiates and maintains seed and bud dormancy and brings about the closure of stomata. Ethylene gas is the hormone involved in abscission (dropping leaves and fruit) and the ripening of fruits. o Because it is a gas, ethylene moves freely through a plant and air by diffusion. o Ethylene is used commercially to ripen fruit. Mader, Biology, 12 th Edition, Chapter

7 o Plant, can be genetically modified to not produce ethylene to facilitate shipping of agricultural products. Plant defenses While plants have an epidermis and bark that discourage attackers, plants need a variety of defenses that are not dependent on its outer surface. Plants produce secondary metabolites for defense or survival. o Nicotine was produced by tobacco plants to poison herbivores. o Trichomes of some plants produce a sugary substance irresistible to hungry caterpillars. The sugar contains a volatile substance, which makes the caterpillar smell good to predatory ants, which grab the caterpillars and carry them off to their nests. Tropisms Tropism is a plant growth toward or away from a unidirectional stimulus. Phototropism is growth towards a light source. o Blue light wavelengths are absorbed by the pigment portion of the photoreceptor which changes its shape and initiates the reception of auxin in the nucleus. Thigmotropism is a response to touch from another plant, an animal, rocks, or the wind. o Thigmomorphogenesis is when a plant changes its overall shape due to an environmental touch stimulus, such as a barrier, wind, or rain. Gravitropism is the effect of gravity on plant growth. o It is controlled by auxin distribution in the plant. o The location of statoliths, starch granules, directed the placement of auxin in the plant. Turgor movements, the entering or exiting of water from a plant cell, can result from touch, shaking, or thermal stimulation. Plants can sense time in two different ways: time of day and season of the year and respond accordingly. o Light-sensing pigments phototropin and phytochrome detect blue and red wavelengths to detect changes are time of day and season. o Phytochrome senses day length by detecting wavelengths of light in the range of red ( nm) which is present in the daytime and far-red ( nm) which is present at dawn and dusk. o There is a constant conversion between the two forms of phytochrome, P r and P fr. P r (phytochrome red) absorbs red light and is converted into P fr in the daytime. P fr (phytochrome far-red) absorbs far-red light and is converted into P r in the evening. Night length is responsible for resetting the circadian-rhythm clock o If a plant is exposed to a flash of red light during the night P r P fr and a shorter night period is measured. o If a flash of far red light follows the red flash, P fr P r and they cancel, so 382 Mader, Biology, 12 th Edition, Chapter 26

8 the clock is not reset. The presence of P fr indicates to some seeds that sunlight is present and conditions are favorable for germination. Plants experience circadian rhythms. Circadian rhythms are entrained to a daily cycle through the action of phytochrome and blue-light receptors. The internal mechanism by which a circadian rhythm is maintained in the absence of appropriate environmental stimuli is termed a biological clock. Key Terms biological clock circadian rhythm day-neutral plants etiolated long-day plants photoperiodism phytochrome short-day plants statolithes Teaching Strategies Class time: Three 45-minute class periods Day 1: Lecture on plant signaling and response, and the major classes of plant hormones 25 minutes Activity 1 introduction and set-up 20 minutes Day 2: Activity 1, have students check on and record any observations 10 minutes Activity 2, effects of plant hormones 20 minutes Lecture on plant defenses 15 minutes Day 3: Lecture on tropisms and light-sensing in plants 45 minutes Suggested Approaches It can be hard for students to understand that plants produce and are regulated by hormones. An activity (such as Activity1 below) that allows students the opportunity to experience the effects of plant hormones can enable students to gain an appreciation for plant hormones. Mader, Biology, 12 th Edition, Chapter

9 Student Misconceptions and Pitfalls Students will have a misconception about tropism and think that tropism is a growth response to an environmental factor. Tropism refers to the direction of plant growth, rather than general growth, being determined by the direction of an environmental factor, such as light, gravity, or touch. Students may think that photoperiodism is the effect of day length on flowering. Photoperiodism is much more and also refers to such plant activities as seed germination, tuber and bulb formation, leaf abscission, and dormancy. Suggested Activities Activity 1: The Effect of an Auxin on Plant Growth Teacher instructions: Purchase 3-Indoleacetic acid form a chemical supply house. You may also use root-grow from a retail greenhouse. Purchase small plants or have your students plant seeds and wait for germination and growth. Allow student to plan their own investigation into the effect of 3-Indoleacetic acid on plant growth. If your purchase plants, allow a two week period in order to obtain observable results. If you purchase seeds, add the time for germination and maturation to the activity length. Activity 2: Web Search for the Effects of Plant Hormones Have students search for pictures that show the results of plant hormone usage. They should create a gallery of pictures that indicate these effects. 384 Mader, Biology, 12 th Edition, Chapter 26

10 Student Edition Chapter Review Answers Answers to Assess Questions 1. c; 2. d; 3. b; 4. e; 5. a; 6. c; 7. d; 8. a; 9. b; 10. c; 11. a; 12. d; 13. d; 14. b; 15. b Answers to Applying the Big Ideas Questions 1. A small village grows a plant whose fruits and leaves provide much of their essential nourishment. The village is interested in increasing their crop yields of this plant as their population grows. a) Describe TWO kinds of data that could be collected to provide a direct answer to the question, what type of growth response(s) might be capitalized on by the village farmers to increase fruit and leaf production? b) Explain how the data you suggested in part (a) would provide a direct answer to the question. Essential Knowledge Science Practice Learning Objective 2.C.2: Organisms respond to changes in their external environments. 4.1: The student can justify the selection of the kid of data needed to answer a particular scientific question. 2.21: The student is able to justify the selection of the kind of data needed to answer scientific questions about the relevant mechanism that organisms use to respond to changes in their external environment. 4 points maximum. Description of the appropriate kind of data and the appropriately linked explanation may include: Description of data (1 point each) Determine the ideal spacing for growing the plants, or the ideal increments for shading the leaves of the plants to stimulate growth controlled by phytochrome. Explanation (1 point each) Leaf shading increases the amount of far-red light relative to red light received by a plant. Plants measure the amount of far-red light bounced back to them from neighboring plants. The closer together plants are, the more far-red relative to red light they perceive and the more likely they are to grow taller a strategy for outcompeting others for sunlight. Mader, Biology, 12 th Edition, Chapter

11 Determine the photoperiodism of the plant. Artificial darkness or light exposure for the plant can stimulate flowering (and eventual fruiting) depending on the photoperiod of the plant. Expose unripe fruits to ethylene gas to hasten the ripening process to prepare the fruits for eating. Plants can be genetically modified to produce cytokinins at the onset of aging. Gibberellins can be applied externally to plants to promote growth and decrease the time spent by seeds in dormancy. The phytocrome red (inactive form) phytochrome far-red conversion cycle is known to control various growth functions in plans. P fr (activated by the presence of red light) promotes seed germination, inhibits shoot elongation, promotes flowering, and affects plant spacing and accumulation of chlorophyll. Ethylene ripens fruits by increasing the activity of enzymes that softens fruits. In addition to stimulating the production of cellulose, it promotes the activity of enzymes that produce the flavor and smell of ripened fruits and breaks down chlorophyll, inducing the color changes associated with fruit ripening. Cytokinins, which typically enourage mitosis and cytokinesis, can prevent senescence (the aging of plants). This has been found to keep lettuce heads fresher longer and avoid brown and wilting leave and could possibly work for this plant in a similar fashion. Gibberellins applied externally induce growth in a variety of crops, encouraging stem elongation and increase in fruit size. Gibberellins can also break the dormancy period of buds and seeds, so flower bud development can be hastened. 386 Mader, Biology, 12 th Edition, Chapter 26

12 2. Explain how signal pathways mediate gene expression, using THREE pieces of evidence from studies of plant hormones. Essential Knowledge Science Practice Learning Objective 3.B.2: A variety of intercellular and intracellular signal transmissions mediate gene expression. 6.2: The student can construct explanations of phenomena based on evidence produced through scientific practices. 3.22: The student is able to explain how signal pathways mediate gene expression, including how this process can affect protein production. 3 points maximum. Explanations using evidence from studies of plant hormones may include (1 point each): Plant cells respond to stimuli by utilizing signal transduction, the binding of a molecular signal that initiates and amplifies through a transduction pathway (a series of relay proteins or enzymes that pass a signal until it reaches the machinery of the cell) a cellular response, typically the response is made by the plant s genes. Hormones serve as the chemical signals in plant cells to coordinate the response. Increase in ethylene levels induce enzyme production that promotes fruit ripening and in abscission (dropping leaves). A holly twig with leaves placed in a closed system will not drop leaves for a week. However, if under the same jar an ethylene-producing ripe apple is added, abscission of the holly leaves will occur. Tomatoes can be genetically modified to produce no ethylene and stay green for shipping instead of turning red on the vine if the gene for ethylene synthesis is removed, then the functional enzyme for ethylene biosynthesis will not be produced. Cytokines regulate gene expression to allow for cell replication and division by triggering mitosis and cytokinesis. In plants they also help form organs and are influenced by auxin. Gibberellins promote signal transmissions that affect specific genes, stimulating growth (particularly stem elongation) and triggering seed germination. Auxin attaches to a receptor, causing a chain of events that weakens the cell wall so that it can be stretched and rebuilt even larger, causing the stem to elongate particularly on one side so that it can bend toward the light so that the plant can capture more light energy (phototropism). Auxin functions in a similar fashion to stimulate roots to grow downward (gravitropism) into the soil. Mader, Biology, 12 th Edition, Chapter

13 Answers to Applying the Science Practices Questions Think Critically 1. Answers will vary, but students may hypothesize that there is some chemical that is transported from the other plants to the day-neutral plant. 2. Answers will vary, but students may predict that a chemical (or hormone) might be transported from the plant receiving the correct day length to the other plant, stimulating flowering. 3. Answers will vary but should include exposure to series of day lengths that become gradually longer. 388 Mader, Biology, 12 th Edition, Chapter 26

14 Additional Questions for AP Practice 1. Photoperiodism in plants can determine when a plant actually produces a flower. Plants can be short day plants or long day plants. What concept does Panel 3 in the figure below depict about the Cocklebur? A) This is a long-day plant and does not flower if the night is interrupted. B) This is a short-day plant and does not flower if the night is interrupted. C) This is a long-day plant and only flowers when there is more than 12 hours of daylight. D) This is a short-day plant and only flowers if the night is interrupted. 2. Plants as well as animals undergo genetic mutations. A plant that has suffered a mutation in the gene that codes for its potassium gates would likely A) not be able to produce ripened fruit. B) be unable to exhibit phototropism. C) be unable to close its stoma. D) fail to exhibit gravitropism. 3. Describe one way a plant uses negative feedback mechanism to maintain internal conditions. Mader, Biology, 12 th Edition, Chapter

15 4. The opening and closing of morning glory flowers is represented in the graph below. If these morning glories are placed in the dark, choose the best prediction of the pattern of flowers opening and closing. A) Flowers will continue to open and close every 24 hours. B) Flowers will continue to open and close every 12 hours. C) Flowers will remain closed and will start to decay. D) Flowers will continue to open and close every 26 hours. 390 Mader, Biology, 12 th Edition, Chapter 26

16 Answers to Additional Questions for AP Practice 1. The correct answer is B. 2. The correct answer is C. 3. Lack of sunlight causes positive phototropism of stems occurs because the cells on the shady side of the stem elongate due to the presence of auxin. Even though this is termed positive tropism, it is a negative feedback mechanism. The set point for the amount of light needed to grow was not being maintained, so auxin is released to enable the plant cells to elongate on one side and allow the plant to obtain the necessary light. 4. D If organisms are sheltered from environmental stimuli, their biological clock keeps the circadian rhythms going, but the cycle extends. Mader, Biology, 12 th Edition, Chapter