Control Systems in Plants

Control Systems in Plants
Chapter 33 Control Systems in Plants

Introduction Soy protein is one of the few plant proteins that provide all of the essential amino acids. Benefits of consuming soy include lowered risk of heart disease, high levels of antioxidants and fiber, low levels of fat, and lowering LDL (“bad cholesterol”) levels while maintaining HDL levels. © 2012 Pearson Education, Inc. 2

Introduction Soy contains phytoestrogens, hormones that can reduce the symptoms of menopause in women and can help reduce the risks of heart disease and sustain bone mass. However, high levels of estrogens appear to increase the risk of breast cancer and ovarian cancer. © 2012 Pearson Education, Inc. 3

Chapter 33: Big Ideas Plant Hormones Responses to Stimuli
Figure 33.0_1 Chapter 33: Big Ideas Figure 33.0_1 Chapter 33: Big Ideas Plant Hormones Responses to Stimuli 4

Figure 33.0_2 Figure 33.0_2 Several products made from soybeans 5

33.1 Experiments on how plants turn toward light led to the discovery of a plant hormone
Any growth response that results in plant organs curving toward or away from stimuli is called a tropism. The growth of a shoot in response to light is called phototropism. Moving toward sunlight helps a growing plant use sunlight to drive photosynthesis. Phototropism can result when the cells on the dark side of a plant stem elongate faster than those on the light side. Student Misconceptions and Concerns 1. Students are likely to think of plants as static, inert objects that interact passively with their environments. Without careful study and consideration, it can be challenging for students to explain how plants interact with their surroundings. This section on plant hormones reveals some of the mechanisms used by plants to respond to changing environmental conditions. 2. Students are unlikely to know about the many manipulations of plants performed in modern agriculture. Learning about them will familiarize students with some of the more practical (and profitable) applications of scientific knowledge and perhaps reveal new career options. Teaching Tips 1. Consider bringing in a plant that shows a distinct orientation in one direction. Before discussing phototropism, ask students to explain why the plant may have this orientation. You may also want to challenge them to predict how the plant will respond after several days in the classroom. 2. Students might not realize that angled growth can result from accelerated and/or retarded growth on one side. In Module 33.1, the authors note that accelerated growth appears to occur in grass shoots, but retarded growth occurs in sunflowers and other eudicots. Video: Phototropism © 2012 Pearson Education, Inc. 7

Figure 33.1A Figure 33.1A A houseplant growing toward light 8

Illuminated Shaded side of shoot side of shoot Light Figure 33.1B
Figure 33.1B Phototropism in a grass seedling 9

Studies of plant responses to light led to the first evidence of plant hormones, a chemical signal produced in one part of the body and transported to other parts, where it acts on target cells to change their functioning. Charles Darwin and his son Francis conducted experiments that showed that the shoot tips of plants controlled their ability to grow toward light. Student Misconceptions and Concerns 1. Students are likely to think of plants as static, inert objects that interact passively with their environments. Without careful study and consideration, it can be challenging for students to explain how plants interact with their surroundings. This section on plant hormones reveals some of the mechanisms used by plants to respond to changing environmental conditions. 2. Students are unlikely to know about the many manipulations of plants performed in modern agriculture. Learning about them will familiarize students with some of the more practical (and profitable) applications of scientific knowledge and perhaps reveal new career options. Teaching Tips 1. Consider bringing in a plant that shows a distinct orientation in one direction. Before discussing phototropism, ask students to explain why the plant may have this orientation. You may also want to challenge them to predict how the plant will respond after several days in the classroom. 2. Students might not realize that angled growth can result from accelerated and/or retarded growth on one side. In Module 33.1, the authors note that accelerated growth appears to occur in grass shoots, but retarded growth occurs in sunflowers and other eudicots. © 2012 Pearson Education, Inc. 10

The Darwins’ experiments When plant tips were removed, plants did not grow toward light. When plant tips were covered with an opaque cap, they did not grow toward light. When plant tips were covered with a clear tip, they did grow toward light. Student Misconceptions and Concerns 1. Students are likely to think of plants as static, inert objects that interact passively with their environments. Without careful study and consideration, it can be challenging for students to explain how plants interact with their surroundings. This section on plant hormones reveals some of the mechanisms used by plants to respond to changing environmental conditions. 2. Students are unlikely to know about the many manipulations of plants performed in modern agriculture. Learning about them will familiarize students with some of the more practical (and profitable) applications of scientific knowledge and perhaps reveal new career options. Teaching Tips 1. Consider bringing in a plant that shows a distinct orientation in one direction. Before discussing phototropism, ask students to explain why the plant may have this orientation. You may also want to challenge them to predict how the plant will respond after several days in the classroom. 2. Students might not realize that angled growth can result from accelerated and/or retarded growth on one side. In Module 33.1, the authors note that accelerated growth appears to occur in grass shoots, but retarded growth occurs in sunflowers and other eudicots. © 2012 Pearson Education, Inc. 11

Figure 33.1C Light 1 2 3 4 5 6 Control Tip removed Tip covered by opaque cap Tip covered by trans- parent cap Base covered by opaque shield Tip separated by gelatin block Tip separated by mica Figure 33.1C Early experiments on phototropism: detection of light by shoot tips and evidence for a chemical signal Darwin and Darwin (1880) Boysen-Jensen (1913) 12

Light Control Tip removed Tip covered by opaque cap Tip covered
Figure 33.1C_1 Light 1 2 3 4 Control Tip removed Tip covered by opaque cap Tip covered by trans- parent cap Base covered by opaque shield Figure 33.1C_1 Early experiments on phototropism: detection of light by shoot tips and evidence for a chemical signal (part 1) Darwin and Darwin (1880) 13

Peter Boysen-Jensen later conducted experiments that showed that chemical signals produced in shoot tips were responsible for phototropism. Jensen’s experiment When a gelatin block that allowed chemical diffusion was placed below the shoot tip, plants grew toward light. When a mica block that prevented chemical diffusion was placed below the shoot tip, plants did not grow toward light. Student Misconceptions and Concerns 1. Students are likely to think of plants as static, inert objects that interact passively with their environments. Without careful study and consideration, it can be challenging for students to explain how plants interact with their surroundings. This section on plant hormones reveals some of the mechanisms used by plants to respond to changing environmental conditions. 2. Students are unlikely to know about the many manipulations of plants performed in modern agriculture. Learning about them will familiarize students with some of the more practical (and profitable) applications of scientific knowledge and perhaps reveal new career options. Teaching Tips 1. Consider bringing in a plant that shows a distinct orientation in one direction. Before discussing phototropism, ask students to explain why the plant may have this orientation. You may also want to challenge them to predict how the plant will respond after several days in the classroom. 2. Students might not realize that angled growth can result from accelerated and/or retarded growth on one side. In Module 33.1, the authors note that accelerated growth appears to occur in grass shoots, but retarded growth occurs in sunflowers and other eudicots. © 2012 Pearson Education, Inc. 14

Light Tip Tip separated separated by mica by gelatin block
Figure 33.1C_2 Light 5 6 Figure 33.1C_2 Early experiments on phototropism: detection of light by shoot tips and evidence for a chemical signal (part 2) Tip separated by gelatin block Tip separated by mica Boysen-Jensen (1913) 15

A graduate student named Frits Went isolated the chemical hormone responsible for phototropism. Plant tips were placed on an agar block to allow the chemical signal molecules to diffuse from the plant tip to the agar. When agar blocks containing chemical signals were centered on the ends of “decapitated” plants, they grew straight. When agar blocks were offset to one side of the “decapitated” plants, they bent away from the side with the agar block. Student Misconceptions and Concerns 1. Students are likely to think of plants as static, inert objects that interact passively with their environments. Without careful study and consideration, it can be challenging for students to explain how plants interact with their surroundings. This section on plant hormones reveals some of the mechanisms used by plants to respond to changing environmental conditions. 2. Students are unlikely to know about the many manipulations of plants performed in modern agriculture. Learning about them will familiarize students with some of the more practical (and profitable) applications of scientific knowledge and perhaps reveal new career options. Teaching Tips 1. Consider bringing in a plant that shows a distinct orientation in one direction. Before discussing phototropism, ask students to explain why the plant may have this orientation. You may also want to challenge them to predict how the plant will respond after several days in the classroom. 2. Students might not realize that angled growth can result from accelerated and/or retarded growth on one side. In Module 33.1, the authors note that accelerated growth appears to occur in grass shoots, but retarded growth occurs in sunflowers and other eudicots. © 2012 Pearson Education, Inc. 16

Went concluded that a chemical produced in the shoot tip was transferred down through the plant, and high concentration of that chemical increased cell elongation on the dark side of the plant. The chemical signal responsible for phototropism is a hormone that Went called auxin. Student Misconceptions and Concerns 1. Students are likely to think of plants as static, inert objects that interact passively with their environments. Without careful study and consideration, it can be challenging for students to explain how plants interact with their surroundings. This section on plant hormones reveals some of the mechanisms used by plants to respond to changing environmental conditions. 2. Students are unlikely to know about the many manipulations of plants performed in modern agriculture. Learning about them will familiarize students with some of the more practical (and profitable) applications of scientific knowledge and perhaps reveal new career options. Teaching Tips 1. Consider bringing in a plant that shows a distinct orientation in one direction. Before discussing phototropism, ask students to explain why the plant may have this orientation. You may also want to challenge them to predict how the plant will respond after several days in the classroom. 2. Students might not realize that angled growth can result from accelerated and/or retarded growth on one side. In Module 33.1, the authors note that accelerated growth appears to occur in grass shoots, but retarded growth occurs in sunflowers and other eudicots. © 2012 Pearson Education, Inc. 17

A chemical diffuses from the shoot tip into the agar block.
Figure 33.1D_s1 A chemical diffuses from the shoot tip into the agar block. Agar 1 The block stimulates growth. Control No light Figure 33.1D_s1 Went’s experiments: isolation of the chemical signal (step 1) 18

Figure 33.1D_s2 A chemical diffuses from the shoot tip into the agar block. Agar 1 The block stimulates growth. 2 Offset blocks stimulate curved growth. Control No light Figure 33.1D_s2 Went’s experiments: isolation of the chemical signal (step 2) 19

Figure 33.1D_s3 A chemical diffuses from the shoot tip into the agar block. Agar 1 The block stimulates growth. 3 Blocks with no chemical have no effect. 2 Offset blocks stimulate curved growth. Control No light Figure 33.1D_s3 Went’s experiments: isolation of the chemical signal (step 3) 20

33.2 Five major types of hormones regulate plant growth and development
Plant hormones are produced in very low concentrations but can have a profound effect on growth and development. The binding of hormones to cell-surface receptors triggers a signal transduction pathway that amplifies the hormonal signal and leads to a response or responses within the cell. Student Misconceptions and Concerns 1. Students are likely to think of plants as static, inert objects that interact passively with their environments. Without careful study and consideration, it can be challenging for students to explain how plants interact with their surroundings. This section on plant hormones reveals some of the mechanisms used by plants to respond to changing environmental conditions. 2. Students are unlikely to know about the many manipulations of plants performed in modern agriculture. Learning about them will familiarize students with some of the more practical (and profitable) applications of scientific knowledge and perhaps reveal new career options. Teaching Tips In Modules 33.2 and 33.3, the authors note that the effects of a particular hormone are determined by many factors, including its concentration, the developmental stage of the plant, and the presence and concentration of other hormones. Biology is highly complex (although some students might wish otherwise!). As you relate the diversity and complexity of life throughout your course, hormones and their interactions in plants and animals are wonderful examples. © 2012 Pearson Education, Inc. 21

Plant biologists have identified five major types of plant hormones. Other important hormones exist, but will not be discussed here. Some of the hormones listed in Table 33.2 represent a group of related hormones. Student Misconceptions and Concerns 1. Students are likely to think of plants as static, inert objects that interact passively with their environments. Without careful study and consideration, it can be challenging for students to explain how plants interact with their surroundings. This section on plant hormones reveals some of the mechanisms used by plants to respond to changing environmental conditions. 2. Students are unlikely to know about the many manipulations of plants performed in modern agriculture. Learning about them will familiarize students with some of the more practical (and profitable) applications of scientific knowledge and perhaps reveal new career options. Teaching Tips In Modules 33.2 and 33.3, the authors note that the effects of a particular hormone are determined by many factors, including its concentration, the developmental stage of the plant, and the presence and concentration of other hormones. Biology is highly complex (although some students might wish otherwise!). As you relate the diversity and complexity of life throughout your course, hormones and their interactions in plants and animals are wonderful examples. © 2012 Pearson Education, Inc. 22

As indicated in Table 33.2, each hormone has multiple effects, depending on its site of action, its concentration, and the developmental stage of the plant. Student Misconceptions and Concerns 1. Students are likely to think of plants as static, inert objects that interact passively with their environments. Without careful study and consideration, it can be challenging for students to explain how plants interact with their surroundings. This section on plant hormones reveals some of the mechanisms used by plants to respond to changing environmental conditions. 2. Students are unlikely to know about the many manipulations of plants performed in modern agriculture. Learning about them will familiarize students with some of the more practical (and profitable) applications of scientific knowledge and perhaps reveal new career options. Teaching Tips In Modules 33.2 and 33.3, the authors note that the effects of a particular hormone are determined by many factors, including its concentration, the developmental stage of the plant, and the presence and concentration of other hormones. Biology is highly complex (although some students might wish otherwise!). As you relate the diversity and complexity of life throughout your course, hormones and their interactions in plants and animals are wonderful examples. © 2012 Pearson Education, Inc. 23

Table 33.2 Table 33.2 Major Types of Plant Hormones 24

33.3 Auxin stimulates the elongation of cells in young shoots
Auxin is the term for any chemical substance that promotes seedling elongation. Indoleacetic acid (IAA) is the major natural auxin found in plants and type of auxin referred to in this chapter. Auxin is produced in apical meristems at the tips of shoots. Student Misconceptions and Concerns 1. Students are likely to think of plants as static, inert objects that interact passively with their environments. Without careful study and consideration, it can be challenging for students to explain how plants interact with their surroundings. This section on plant hormones reveals some of the mechanisms used by plants to respond to changing environmental conditions. 2. Students are unlikely to know about the many manipulations of plants performed in modern agriculture. Learning about them will familiarize students with some of the more practical (and profitable) applications of scientific knowledge and perhaps reveal new career options. Teaching Tips 1. Ask your students to explain how some seedless fruits are produced. It may be amusing for them to consider this question when they do not know how it is done! 2. In Modules 33.2 and 33.3, the authors note that the effects of a particular hormone are determined by many factors, including its concentration, the developmental stage of the plant, and the presence and concentration of other hormones. Biology is highly complex (although some students might wish otherwise!). As you relate the diversity and complexity of life throughout your course, hormones and their interactions in plants and animals are wonderful examples. © 2012 Pearson Education, Inc. 25

Figure 33.3A Figure 33.3A The effect of auxin (IAA): comparing a wild-type Arabidopsis plant (right) with one that underproduces auxin (left) 26

33.3 Auxin stimulates the elongation of cells in young shoots
At different concentrations, auxin stimulates or inhibits the elongation of shoots and roots, may act by weakening cell walls, allowing them to stretch when cells take up water, stimulates the development of vascular tissues and cell division in the vascular cambium, promoting growth in stem diameter, and is produced by developing seeds and promotes the growth of fruit. Student Misconceptions and Concerns 1. Students are likely to think of plants as static, inert objects that interact passively with their environments. Without careful study and consideration, it can be challenging for students to explain how plants interact with their surroundings. This section on plant hormones reveals some of the mechanisms used by plants to respond to changing environmental conditions. 2. Students are unlikely to know about the many manipulations of plants performed in modern agriculture. Learning about them will familiarize students with some of the more practical (and profitable) applications of scientific knowledge and perhaps reveal new career options. Teaching Tips 1. Ask your students to explain how some seedless fruits are produced. It may be amusing for them to consider this question when they do not know how it is done! 2. In Modules 33.2 and 33.3, the authors note that the effects of a particular hormone are determined by many factors, including its concentration, the developmental stage of the plant, and the presence and concentration of other hormones. Biology is highly complex (although some students might wish otherwise!). As you relate the diversity and complexity of life throughout your course, hormones and their interactions in plants and animals are wonderful examples. © 2012 Pearson Education, Inc. 27

  Stems Promotion Elongation Roots Inhibition 0.9 g/L 108 106 104
Figure 33.3B  Stems Promotion Elongation Roots Inhibition 0.9 g/L  Figure 33.3B The effect of auxin concentration on cell elongation 108 106 104 102 1 102 Increasing auxin concentration (g/L) 28

CELL WALL CYTOPLASM Figure 33.3C
Enzyme that separates cross-linking molecules CELL WALL Cross-linking molecule 2 4 Cellulose microfibril H2O Plasma membrane Cell wall H H H 3 H H H H Enzyme that loosens cell wall H Vacuole 1 Figure 33.3C A hypothesis to explain how auxin stimulates cell elongation Proton pump (protein) Plasma membrane H CYTOPLASM 29

CELL WALL CYTOPLASM Enzyme that separates cross-linking molecules
Figure 33.3C_1 Enzyme that separates cross-linking molecules CELL WALL Cross-linking molecule 2 Cellulose microfibril H H H 3 H H H H Enzyme that loosens cell wall H Figure 33.3C_1 A hypothesis to explain how auxin stimulates cell elongation (detail) 1 Proton pump (protein) Plasma membrane H CYTOPLASM 30

33.4 Cytokinins stimulate cell division
Cytokinins promote cytokinesis, or cell division, are produced in actively growing organs such as roots, embryos, and fruits, and move upward from roots through a plant, balancing the effects of auxin from apical meristems and causing lower buds to develop into branches. Student Misconceptions and Concerns 1. Students are likely to think of plants as static, inert objects that interact passively with their environments. Without careful study and consideration, it can be challenging for students to explain how plants interact with their surroundings. This section on plant hormones reveals some of the mechanisms used by plants to respond to changing environmental conditions. 2. Students are unlikely to know about the many manipulations of plants performed in modern agriculture. Learning about them will familiarize students with some of the more practical (and profitable) applications of scientific knowledge and perhaps reveal new career options. Teaching Tips In Module 33.4, the authors compare a basil plant with and without a terminal bud. In addition to details on the control of axillary bud growth, the experiment demonstrates an important principle of biology: Organisms must compromise. With limited resources, the basil plant’s growth reflects a compromise between growth in height and growth of branches. © 2012 Pearson Education, Inc. 31

Terminal bud No terminal bud Figure 33.4
Figure 33.4 Apical dominance in a basil plant resulting from the action of auxin and cytokinins 32

33.5 Gibberellins affect stem elongation and have numerous other effects
Gibberellins promote cell elongation and cell division in stems and leaves and were named for a genus of fungi that produce the same chemical and cause “foolish seedling” disease, in which rice seedlings grew so tall and spindly that they toppled over before producing grain. There are more than 100 distinct gibberellins produced primarily in roots and young leaves. Student Misconceptions and Concerns 1. Students are likely to think of plants as static, inert objects that interact passively with their environments. Without careful study and consideration, it can be challenging for students to explain how plants interact with their surroundings. This section on plant hormones reveals some of the mechanisms used by plants to respond to changing environmental conditions. 2. Students are unlikely to know about the many manipulations of plants performed in modern agriculture. Learning about them will familiarize students with some of the more practical (and profitable) applications of scientific knowledge and perhaps reveal new career options. Teaching Tips 1. Although the similarities are limited, you may want to challenge students to identify human hormones that have effects like the plant hormones discussed in this chapter. For example, gibberellins and human growth hormone both promote growth. 2. Ask your students to explain how some seedless fruits are produced. It may be amusing for them to consider this question when they do not know how it is done! © 2012 Pearson Education, Inc. 33

33.5 Gibberellins affect stem elongation and have numerous other effects
Gibberellins also promote fruit development and seed germination. In some plants, gibberellins interact antagonistically with abscisic acid. Student Misconceptions and Concerns 1. Students are likely to think of plants as static, inert objects that interact passively with their environments. Without careful study and consideration, it can be challenging for students to explain how plants interact with their surroundings. This section on plant hormones reveals some of the mechanisms used by plants to respond to changing environmental conditions. 2. Students are unlikely to know about the many manipulations of plants performed in modern agriculture. Learning about them will familiarize students with some of the more practical (and profitable) applications of scientific knowledge and perhaps reveal new career options. Teaching Tips 1. Although the similarities are limited, you may want to challenge students to identify human hormones that have effects like the plant hormones discussed in this chapter. For example, gibberellins and human growth hormone both promote growth. 2. Ask your students to explain how some seedless fruits are produced. It may be amusing for them to consider this question when they do not know how it is done! © 2012 Pearson Education, Inc. 34

Dwarf plant (untreated) Dwarf plant treated with gibberellins
Figure 33.5A Figure 33.5A Reversing dwarfism in pea plants with gibberellins Dwarf plant (untreated) Dwarf plant treated with gibberellins 35

Figure 33.5B Figure 33.5B A parsley plant bolting, a result of too much gibberellin 36

Figure 33.5C Figure 33.5C Gibberellin-treated grapes (left) and untreated grapes (right) 37

33.6 Abscisic acid inhibits many plant processes
Abscisic acid (ABA) is a plant hormone that inhibits growth. High concentrations of ABA promote seed dormancy. ABA must be removed for germination to occur. The ratio of ABA to gibberellins controls germination. ABA also acts as a “stress hormone,” causing stomata to close when a plant is dehydrated. Student Misconceptions and Concerns 1. Students are likely to think of plants as static, inert objects that interact passively with their environments. Without careful study and consideration, it can be challenging for students to explain how plants interact with their surroundings. This section on plant hormones reveals some of the mechanisms used by plants to respond to changing environmental conditions. 2. Students are unlikely to know about the many manipulations of plants performed in modern agriculture. Learning about them will familiarize students with some of the more practical (and profitable) applications of scientific knowledge and perhaps reveal new career options. Teaching Tips Much of the content of biology courses may answer questions students have never thought to ask. Module 33.6 raises a few of these questions. What keeps seeds dispersed in autumn from sprouting in the winter? What keeps the seeds in fruit from germinating immediately? As we explore biology, it is useful to encourage curiosity first, to make students receptive to the explanation. © 2012 Pearson Education, Inc. 38

Figure 33.6 Figure 33.6 The Mojave Desert in California blooming after a rain 39

33.7 Ethylene triggers fruit ripening and other aging processes
Ethylene is a gaseous by-product of coal combustion and naturally occurring plant hormone. Plants produce ethylene, which triggers fruit ripening and programmed cell death. Ethylene is also produced in response to stresses such as drought, flooding, injury, or infection. Student Misconceptions and Concerns 1. Students are likely to think of plants as static, inert objects that interact passively with their environments. Without careful study and consideration, it can be challenging for students to explain how plants interact with their surroundings. This section on plant hormones reveals some of the mechanisms used by plants to respond to changing environmental conditions. 2. Students are unlikely to know about the many manipulations of plants performed in modern agriculture. Learning about them will familiarize students with some of the more practical (and profitable) applications of scientific knowledge and perhaps reveal new career options. Teaching Tips Special bags made of a material that absorbs ethylene gas are now commercially available under brand names such as Green Bags. Their manufacturers claim that they extend the life of fruits and vegetables stored inside them. © 2012 Pearson Education, Inc. 40

33.7 Ethylene triggers fruit ripening and other aging processes
A changing ratio of auxin to ethylene, triggered mainly by shorter days, probably causes autumn color changes and the loss of leaves from deciduous trees. When an autumn leaf falls, the base of the leaf separates from the stem. The separation region is called the abscission layer, and the leaf drops off when its weight splits the abscission layer apart. Student Misconceptions and Concerns 1. Students are likely to think of plants as static, inert objects that interact passively with their environments. Without careful study and consideration, it can be challenging for students to explain how plants interact with their surroundings. This section on plant hormones reveals some of the mechanisms used by plants to respond to changing environmental conditions. 2. Students are unlikely to know about the many manipulations of plants performed in modern agriculture. Learning about them will familiarize students with some of the more practical (and profitable) applications of scientific knowledge and perhaps reveal new career options. Teaching Tips Special bags made of a material that absorbs ethylene gas are now commercially available under brand names such as Green Bags. Their manufacturers claim that they extend the life of fruits and vegetables stored inside them. © 2012 Pearson Education, Inc. 41

Figure 33.7A Figure 33.7A The effect of ethylene on the ripening of bananas 42

Leaf stalk (petiole) Stem Protective layer Abscission layer Stem
Figure 33.7B Leaf stalk (petiole) Stem Figure 33.7B The abscission layer at the base of a leaf Protective layer Abscission layer Stem Leaf stalk 43

Leaf stalk (petiole) Stem Figure 33.7B_1
Figure 33.7B_1 The abscission layer at the base of a leaf (part 1) 44

Protective layer Abscission layer Stem Leaf stalk Figure 33.7B_2
Figure 33.7B_2 The abscission layer at the base of a leaf (part 2) Protective layer Abscission layer Stem Leaf stalk 45

33.8 CONNECTION: Plant hormones have many agricultural uses
Agricultural uses of plant hormones include control of fruit production, ripening, and dropping, production of seedless fruits, and use as weed killers. Agricultural uses of plant hormones help keep food prices down and can benefit the environment in aspects such as soil erosion, but may have dangerous side effects for humans and the environment. Student Misconceptions and Concerns 1. Students are likely to think of plants as static, inert objects that interact passively with their environments. Without careful study and consideration, it can be challenging for students to explain how plants interact with their surroundings. This section on plant hormones reveals some of the mechanisms used by plants to respond to changing environmental conditions. 2. Students are unlikely to know about the many manipulations of plants performed in modern agriculture. Learning about them will familiarize students with some of the more practical (and profitable) applications of scientific knowledge and perhaps reveal new career options. Teaching Tips 1. As noted above, the realities of biology mean that modern agriculture requires compromise. Human societies cannot last without a steady food supply, but they must also factor in the cost, quality, and environmental risks associated with its production. Make sure your students appreciate the key role of science and education in informing the important decisions we make as a society about our food and the environment. 2. Ask your students to explain how some seedless fruits are produced. It may be amusing for them to consider this question when they do not know how it is done! © 2012 Pearson Education, Inc. 46

Figure 33.8 Figure 33.8 Using auxins to prevent early fruit drop 47

Figure 33.8_UN Figure 33.8_UN Ripening peaches 48

33.9 Tropisms orient plant growth toward or away from environmental stimuli
Tropisms are responses that cause plants to grow in response to environmental stimuli. Positive tropisms cause plants to grow toward a stimulus. Negative tropisms cause plants to grow away from a stimulus. Plants respond to various environmental stimuli. Phototropism is a response to light. Gravitropism is a response to gravity. Thigmotropism is a response to touch. Student Misconceptions and Concerns Students may struggle with the concept that plants respond actively to their environments. In addition to the internal changes in physiology produced by hormones, plants can also move in response to environmental stimuli. Teaching Tips 1. Photographs or living examples of a germinating seed and a climbing vine are very useful teaching aids for demonstrating gravitropism and thigmotropism, respectively. 2. Short video clips on plant tropisms can be found by searching for “tropisms video” in Google. Such short clips can quickly illustrate these activities. Video: Gravitropism Video: Mimosa Leaf © 2012 Pearson Education, Inc. 50

Figure 33.9A Figure 33.9A Gravitropism in a corn seedling 51

Figure 33.9B Figure 33.9B The “sensitive plant” Mimosa pudica 52

33.10 Plants have internal clocks
Plants display rhythmic behavior including the opening and closing of stomata and folding and unfolding of leaves and flowers. A circadian rhythm is an innate biological cycle of about 24 hours and may persist even when an organism is sheltered from environmental cues. Student Misconceptions and Concerns Students may struggle with the concept that plants respond actively to their environments. In addition to the internal changes in physiology produced by hormones, plants can also move in response to environmental stimuli. Teaching Tips Encourage your students to focus on circadian rhythms by challenging them to identify those in their own lives. Sleep, physical activity, hunger, eating, drinking, and urination are not spread evenly throughout a 24-hour period. Recognizing these cycles in themselves helps students relate to such cycles in plants. © 2012 Pearson Education, Inc. 53

33.10 Plants have internal clocks
Research on a variety of organisms indicates that circadian rhythms are controlled by internal timekeepers known as biological clocks. Environmental cues such as light/dark cycles keep biological clocks precisely synchronized. For most organisms, including plants, we know little about where the clocks are located or what kinds of cells are involved. Student Misconceptions and Concerns Students may struggle with the concept that plants respond actively to their environments. In addition to the internal changes in physiology produced by hormones, plants can also move in response to environmental stimuli. Teaching Tips Encourage your students to focus on circadian rhythms by challenging them to identify those in their own lives. Sleep, physical activity, hunger, eating, drinking, and urination are not spread evenly throughout a 24-hour period. Recognizing these cycles in themselves helps students relate to such cycles in plants. © 2012 Pearson Education, Inc. 54

Figure 33.10 Figure Sleep movements of a bean plant Noon Midnight 55

33.11 Plants mark the seasons by measuring photoperiod
Biological clocks can influence seasonal events including flowering, seed germination, and the onset of dormancy. The environmental stimulus plants most often use to detect the time of year is called photoperiod, the relative lengths of day and night. Student Misconceptions and Concerns Students may struggle with the concept that plants respond actively to their environments. In addition to the internal changes in physiology produced by hormones, plants can also move in response to environmental stimuli. Teaching Tips After addressing circadian rhythms, which occur within a 24-hour day, Module addresses seasonal biological cycles. Challenge students to identify seasonal biological cycles in animals in their region. If winters are cold in your area, annual changes may include the growth of thicker fur in mammals, hibernation, or migration to distant locations. © 2012 Pearson Education, Inc. 56

33.11 Plants mark the seasons by measuring photoperiod
Plant flowering signals are determined by night length. Short-day plants, such as chrysanthemums and poinsettias generally flower in the late summer, fall, or winter when light periods shorten. Long-day plants, such as spinach, lettuce, and many cereal grains generally flower in late spring or early summer when light periods lengthen. Student Misconceptions and Concerns Students may struggle with the concept that plants respond actively to their environments. In addition to the internal changes in physiology produced by hormones, plants can also move in response to environmental stimuli. Teaching Tips After addressing circadian rhythms, which occur within a 24-hour day, Module addresses seasonal biological cycles. Challenge students to identify seasonal biological cycles in animals in their region. If winters are cold in your area, annual changes may include the growth of thicker fur in mammals, hibernation, or migration to distant locations. © 2012 Pearson Education, Inc. 57

Flash of light Critical dark period
Figure 33.11 Time (hrs) 24 Plants bloom only with a longer dark period. 1 Short- day (long- night) plants 2 Flash of light prevents flowering. 3 Light Darkness Flash of light Plants bloom only with a shorter dark period. 4 Figure Photoperiodic control of flowering Long- day (short- night) plants 5 Flash of light induces flowering. 6 Critical dark period 58

Time (hrs) 24 Plants bloom only with a longer dark period. Short- day
Figure 33.11_1 Time (hrs) 24 Plants bloom only with a longer dark period. 1 Short- day (long- night) plants 2 Flash of light prevents flowering. 3 Figure 33.11_1 Photoperiodic control of flowering (part 1) Light Darkness Flash of light 59

Time (hrs) 24 Plants bloom only with a shorter dark period. Long- day
Figure 33.11_2 Time (hrs) 24 Plants bloom only with a shorter dark period. 4 Long- day (short- night) plants 5 Flash of light induces flowering. 6 Figure 33.11_2 Photoperiodic control of flowering (part 2) Critical dark period 60

33.12 Phytochromes are light detectors that may help set the biological clock
Phytochromes are proteins with a light-absorbing component and may help plants set their biological clock and monitor photoperiod. Phytochromes detect light in the red and far-red wavelengths. One form of phytochrome absorbs red light (Pr). One form detects far-red light (Pfr). When Pr absorbs light, it is converted into Pfr. When Pfr absorbs light, it is converted into Pr. Student Misconceptions and Concerns Students may struggle with the concept that plants respond actively to their environments. In addition to the internal changes in physiology produced by hormones, plants can also move in response to environmental stimuli. Teaching Tips Module describes a mechanism used by plants to measure the length of the night and respond to morning light. Figure 33.12A is especially helpful in explaining this reaction. Challenge your students to explain why the many types of receptors and signaling pathways that respond to light have been particularly advantageous to plants. Although light is important to animal activities, plants must be even more sensitive to it because of the important role played by light in photosynthesis. © 2012 Pearson Education, Inc. 61

R a p i d c o n v e r s y l g h t Red light Pr Pfr S l o w c n v e r s
Figure 33.12A R a p i d c o n v e r s y l g h t Red light Pr Pfr Figure 33.12A Interconversion of the two forms of phytochrome S l o w c n v e r s i k a d Far-red light 62

33.12 Phytochromes are light detectors that may help set the biological clock
Pr is naturally produced during dark hours, while Pfr is broken down. The relative amounts of Pr and Pfr present in a plant change as day length changes. Student Misconceptions and Concerns Students may struggle with the concept that plants respond actively to their environments. In addition to the internal changes in physiology produced by hormones, plants can also move in response to environmental stimuli. Teaching Tips Module describes a mechanism used by plants to measure the length of the night and respond to morning light. Figure 33.12A is especially helpful in explaining this reaction. Challenge your students to explain why the many types of receptors and signaling pathways that respond to light have been particularly advantageous to plants. Although light is important to animal activities, plants must be even more sensitive to it because of the important role played by light in photosynthesis. © 2012 Pearson Education, Inc. 63

Time (hrs) 24 R R FR R FR R R FR R FR Short-day (long-night) plant
Figure 33.12B Time (hrs) 24 1 R 2 R FR 3 R FR R Figure 33.12B The reversible effects of red and far-red light 4 R FR R FR Short-day (long-night) plant Long-day (short-night) plant Critical dark period 64

33.13 EVOLUTION CONNECTION: Defenses against herbivores and infectious microbes have evolved in plants Herbivores are animals that mainly eat plants. Plants use chemicals to defend themselves against herbivores and pathogens. Plants counter herbivores with physical defenses, such as thorns, and chemical defenses, such as distasteful or toxic compounds. Student Misconceptions and Concerns 1. Students may struggle with the concept that plants respond actively to their environments. In addition to the internal changes in physiology produced by hormones, plants can also move in response to environmental stimuli. 2. Students may not understand why defenses against herbivores are not found in all plants. The production of distasteful or toxic compounds reveals an important evolutionary compromise. Is it adaptive to use more energy to produce chemical defenses, or better to divert the energy to regrowth? Plants, animals, and all life exist in a world of limited resources. As circumstances and organisms vary, the particular strategy that is most adaptive varies too. Specialization comes at a cost, and all adaptations represent compromise. Teaching Tips Examples of natural selection often include simple predator-prey relationships. The complex relationships depicted in Figure 33.13A provide some additional examples of the evolution of complex systems. Challenge students to explain how the recruitment of parasitic wasps might have evolved in this system. Perhaps some plants under attack by caterpillars emitted variants of a chemical from their leaves that at that time was involved in some other unrelated reaction. However, the few plants that emitted a variant of this chemical attracted parasitic wasps. Such plants would be more likely to survive and reproduce. With each generation, the proportion of plants with this new ability to attract parasitic wasps would increase, and the trait would become common in subsequent generations. © 2012 Pearson Education, Inc. 65

Plant cell Damage to plant and chemical in caterpillar saliva 1
Figure 33.13A_s1 Plant cell 1 Figure 33.13A_s1 Recruitment of a wasp in response to an herbivore (step 1) Damage to plant and chemical in caterpillar saliva 66

Plant cell Damage to plant and chemical in caterpillar saliva Signal
Figure 33.13A_s2 Plant cell 1 Figure 33.13A_s2 Recruitment of a wasp in response to an herbivore (step 2) Damage to plant and chemical in caterpillar saliva 2 Signal transduction pathway 67

Synthesis and release of chemical attractants Plant cell
Figure 33.13A_s3 3 Synthesis and release of chemical attractants Plant cell 1 Figure 33.13A_s3 Recruitment of a wasp in response to an herbivore (step 3) Damage to plant and chemical in caterpillar saliva 2 2 Signal transduction pathway 68

Wasp is attracted Synthesis and release of chemical attractants
Figure 33.13A_s4 4 Wasp is attracted 3 Synthesis and release of chemical attractants Plant cell 1 Figure 33.13A_s4 Recruitment of a wasp in response to an herbivore (step 4) Damage to plant and chemical in caterpillar saliva 2 Signal transduction pathway 69

Wasp is attracted The wasp lays eggs Synthesis and release of chemical
Figure 33.13A_s5 4 5 The wasp lays eggs Wasp is attracted 3 Synthesis and release of chemical attractants Plant cell 1 Figure 33.13A_s5 Recruitment of a wasp in response to an herbivore (step 5) Damage to plant and chemical in caterpillar saliva 2 Signal transduction pathway 70

Figure 33.13A_2 Figure 33.13A_2 Recruitment of a wasp in response to an herbivore (photo) 71

33.13 EVOLUTION CONNECTION: Defenses against herbivores and infectious microbes have evolved in plants Plants defend themselves against pathogens at several levels. The first line of defense against infection is the physical barrier of the plant’s epidermis. If that fails, plant cells damaged by infection seal off the infected areas and release microbe-killing chemicals that signal nearby cells to mount a similar chemical defense. In addition, hormones trigger generalized defense responses in other organs in the process of systemic acquired resistance. Student Misconceptions and Concerns 1. Students may struggle with the concept that plants respond actively to their environments. In addition to the internal changes in physiology produced by hormones, plants can also move in response to environmental stimuli. 2. Students may not understand why defenses against herbivores are not found in all plants. The production of distasteful or toxic compounds reveals an important evolutionary compromise. Is it adaptive to use more energy to produce chemical defenses, or better to divert the energy to regrowth? Plants, animals, and all life exist in a world of limited resources. As circumstances and organisms vary, the particular strategy that is most adaptive varies too. Specialization comes at a cost, and all adaptations represent compromise. Teaching Tips Examples of natural selection often include simple predator-prey relationships. The complex relationships depicted in Figure 33.13A provide some additional examples of the evolution of complex systems. Challenge students to explain how the recruitment of parasitic wasps might have evolved in this system. Perhaps some plants under attack by caterpillars emitted variants of a chemical from their leaves that at that time was involved in some other unrelated reaction. However, the few plants that emitted a variant of this chemical attracted parasitic wasps. Such plants would be more likely to survive and reproduce. With each generation, the proportion of plants with this new ability to attract parasitic wasps would increase, and the trait would become common in subsequent generations. © 2012 Pearson Education, Inc. 72

Binding of the pathogen’s Avr protein to the plant’s R protein
Figure 33.13B_s1 1 Binding of the pathogen’s Avr protein to the plant’s R protein R protein Avirulent pathogen Figure 33.13B_s1 Defense responses against an avirulent pathogen (step 1) Avr protein 73

Binding of the pathogen’s Avr protein to the plant’s R protein
Figure 33.13B_s2 1 Binding of the pathogen’s Avr protein to the plant’s R protein R protein Avirulent pathogen 2 Figure 33.13B_s2 Defense responses against an avirulent pathogen (step 2) Signal transduction pathway Avr protein 74

Recognition between R and Avr proteins,
Figure 33.13B_s3 3 Enhanced local response 1 Binding of the pathogen’s Avr protein to the plant’s R protein R protein Avirulent pathogen 2 Figure 33.13B_s3 Defense responses against an avirulent pathogen (step 3) Signal transduction pathway Avr protein Recognition between R and Avr proteins, leading to a strong local response 75

Figure 33.13B_s4 3 Enhanced local response 1 Binding of the pathogen’s Avr protein to the plant’s R protein R protein 4 Avirulent pathogen 2 Hormones Figure 33.13B_s4 Defense responses against an avirulent pathogen (step 4) Signal transduction pathway Avr protein Recognition between R and Avr proteins, leading to a strong local response 76

Figure 33.13B_s5 3 5 Signal transduction pathway Enhanced local response 1 Binding of the pathogen’s Avr protein to the plant’s R protein R protein 4 Avirulent pathogen 2 Hormones Figure 33.13B_s5 Defense responses against an avirulent pathogen (step 5) Signal transduction pathway Avr protein Recognition between R and Avr proteins, leading to a strong local response 77

Figure 33.13B_s6 3 5 Signal transduction pathway Enhanced local response 1 Binding of the pathogen’s Avr protein to the plant’s R protein R protein 6 Additional defensive chemicals 4 Avirulent pathogen 2 Hormones Figure 33.13B_s6 Defense responses against an avirulent pathogen (step 6) Signal transduction pathway Avr protein Recognition between R and Avr proteins, leading to a strong local response Systemic acquired resistance 78

You should now be able to
Describe the experiments and conclusions of the phototropism research performed by the Darwins, Boysen-Jensen, and Went. Describe the functions of the five major types of plant hormones. Describe the uses of plant hormones in modern agriculture and the ethical issues associated with their use. Define phototropism, gravitropism, and thigmotropism. © 2012 Pearson Education, Inc. 79

You should now be able to
Explain how biological clocks work and how they influence the lives of plants. Distinguish between short-day plants and long-day plants. Describe the roles of phytochromes in plants. Explain how plants defend themselves against herbivores. © 2012 Pearson Education, Inc. 80

Light Gravity Phototropism Gravitropism Thigmotropism Figure 33.UN01
Figure 33.UN01 Reviewing the Concepts, 33.9 Phototropism Gravitropism Thigmotropism 81

Critical dark period Critical dark period Short-day
Figure 33.UN02 Critical dark period Critical dark period Short-day (long-night) plants Long-day (short-night) plants Figure 33.UN02 Reviewing the Concepts, 33.11 82

Cell elongation Axillary bud growth Leaf abscission Seed dormancy
Figure 33.UN03 (a) (c) stimulates inhibits Cell elongation Axillary bud growth enhances opposes stimulates stimulates (b) (d) (e) (g) inhibits inhibits Figure 33.UN03 Connecting the Concepts, question 1 Leaf abscission Seed dormancy opposes opposes stimulates stimulates (f) (h) 83

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