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BIO 102 CHAPTER 33
Control Systems in Plants
Learning Objectives
Plant Hormones
33.1 Describe the experiments and conclusions of the phototropism research
performed by the Darwins, Boysen-Jensen, and Went. Explain why auxin does not seem to
play the same role in sunflowers and other eudicots as it does in grasses.
33.2–33.7 Describe the functions of the five major types of plant hormones.
33.8 Describe the uses of plant hormones in modern agriculture and the ethical
issues associated with their use.
Responses to Stimuli
33.9 Define phototropism, gravitropism, and thigmotropism. Explain how these
reactions occur and describe their significance to plants.
33.10 Explain how biological clocks work and how they influence the lives of plants.
33.11 Distinguish between short-day plants and long-day plants. Explain why these terms can be
misleading.
33.12 Describe the roles of phytochromes in plants.
33.13 Explain how plants defend themselves against herbivores. Describe the
systemic acquired resistance defense response in plants.
Key Terms
abscisic acid (ABA)
auxin
biological clock
circadian rhythm
cytokinin
ethylene
gibberellin
gravitropism
herbivore
hormone
long-day plant
photoperiod
phototropism
phytochrome
short-day plant
systemic acquired resistance
thigmotropism
tropism
Word Roots
ab- = off; -scin = to cut (abscisic acid: a plant hormone that inhibits cell division, promotes
dormancy, and interacts with gibberellins in regulating seed germination)
aux- = grow, enlarge (auxin: a plant hormone, such as indoleacetic acid or a related compound, whose chief
effect is to promote seedling elongation)
circ- = a circle (circadian rhythm: in an organism, a biological cycle of about 24 hours that is
controlled by a biological clock, usually under the influence of environmental cues)
cyto- = cell; -kine = moving (cytokinin: one of a family of plant hormones that promotes cell
division, retards aging in flowers and fruits, and may interact antagonistically with auxins in regulating plant
growth and development)
gibb- = humped (gibberellin: one of a family of plant hormones that triggers the germination of seeds and interacts with auxins in regulating growth and fruit development)
gravi- = gravity; -trop = turn, change (gravitropism: a plant’s growth response to gravity)
herbi- = plants; -vora = eat (herbivore: an animal that eats only plants or algae)
photo- = light (phototropism: the growth of a plant shoot toward or away from light)
phyto- = a plant; -chromo = color (phytochrome: a colored protein in plants that contains a special set of atoms
that absorbs light)
thigmo- = a touch (thigmotropism: the growth movement of a plant in response to touch)
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Outline
I. Introduction
A. Soy protein is one of the few plant proteins that provide all of the essential amino acids.
B. Benefits of consuming soy include
1. lowered risk of heart disease,
2. high levels of antioxidants and fiber,
3. low levels of fat, and
4. lowering bad LDL cholesterol while maintaining good HDL levels.
C. Soy contains phytoestrogens, hormones that can reduce the symptoms of menopause in
women and can help
1. reduce the risks of heart disease and
2. sustain bone mass.
D. However, high levels of estrogens appear to increase the risk of
1. breast cancer and
2. ovarian cancer.
II. Plant Hormones
A. 33.1 Experiments on how plants turn toward light led to the discovery of a plant hormone
1. Any growth response that results in plant organs curving toward or away from stimuli is
called a tropism.
2. The growth of a shoot in response to light is called phototropism.
a. Moving toward sunlight helps a growing plant use sunlight to drive
photosynthesis.
b. Phototropism can result when the cells on the dark side of a plant stem elongate faster
than those on the light side.
3. Studies of plant responses to light led to the first evidence of plant hormones, a chemical
signal
a. produced in one part of the body and
b. transported to other parts,
c. where it acts on target cells to change their functioning.
4. Charles Darwin and his son Francis conducted experiments that showed that the shoot
tips of plants controlled their ability to grow toward light.
5. The Darwins’ experiments
a. When plant tips were removed, plants did not grow toward light.
b. When plant tips were covered with an opaque cap, they did not grow toward light.
c. When plant tips were covered with a clear tip, they did grow toward light.
6. Peter Boysen-Jensen later conducted experiments that showed that chemical signals
produced in shoot tips were responsible for phototropism.
7. Jensen’s experiment
a. When a gelatin block that allowed chemical diffusion was placed below the shoot tip,
plants grew toward light.
b. When a mica block that prevented chemical diffusion was placed below the shoot tip,
plants did not grow toward light.
8. A graduate student named Frits Went isolated the chemical hormone responsible for
phototropism.
a. Plant tips were placed on an agar block to allow the chemical signal molecules to
diffuse from the plant tip to the agar.
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b. When agar blocks containing chemical signals were centered on the ends of
“decapitated” plants, they grew straight.
c. When agar blocks were offset to one side of the “decapitated” plants, they bent away
from the side with the agar block.
d. 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.
9. The chemical signal responsible for phototropism is a hormone that Went called auxin.
B. 33.2 Five major types of hormones regulate plant growth and development
1. Plant hormones
a. are produced in very low concentrations but
b. can have a profound effect on growth and development.
2. The binding of hormones to cell surface receptors triggers a signal transduction
pathway that
a. amplifies the hormonal signal and
b. leads to a response or responses within the cell.
3. Plant biologists have identified five major types of plant hormones.
a. Other important hormones exist, but will not be discussed here.
b. Some of the hormones listed in Table 33.2 represent a group of related hormones.
4. As indicated in Table 33.2, each hormone has multiple effects, depending on
a. its site of action,
b. its concentration, and
c. the developmental stage of the plant.
C. 33.3 Auxin stimulates the elongation of cells in young shoots
1. Auxin is used for any chemical substance that promotes seedling elongation.
2. Indoleacetic acid (IAA) is the
a. major natural auxin found in plants and
b. type of auxin referred to in Chapter 32.
3. Auxin is produced in apical meristems at the tips of shoots.
4. At different concentrations, auxin
a. stimulates or inhibits the elongation of shoots and roots,
b. may act by weakening cell walls, allowing them to stretch when cells take up
water,
c. stimulates the development of vascular tissues and cell division in the vascular
cambium, promoting growth in stem diameter, and
d. is produced by developing seeds and promotes the growth of fruit.
D. 33.4 Cytokinins stimulate cell division
1. Cytokinins
a. promote cytokinesis, or cell division,
b. are produced in actively growing organs such as roots, embryos, and fruits, and
c. move upward from roots through a plant,
i. balancing the effects of auxin from apical meristems and
ii. causing lower buds to develop into branches.
E. 33.5 Gibberellins affect stem elongation and have numerous other effects
1. Gibberellins
a. promote cell elongation and cell division in stems and leaves and
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b. 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.
c. There are more than 100 distinct gibberellins produced primarily in roots and young
leaves.
2. Gibberellins also promote
a. fruit development and
b. seed germination.
3. In some plants, gibberellins interact antagonistically with abscisic acid.
F. 33.6 Abscisic acid inhibits many plant processes
1. Abscisic acid (ABA) is a plant hormone that inhibits growth.
2. High concentrations of ABA promote seed dormancy.
a. ABA must be removed for germination to occur.
b. The ratio of ABA to gibberellins controls germination.
3. ABA also acts as a “stress hormone,” causing stomata to close when a plant is
dehydrated.
G. 33.7 Ethylene triggers fruit ripening and other aging processes
1. Ethylene is a
a. gaseous by-product of coal combustion and
b. naturally occurring plant hormone.
2. Plants produce ethylene, which triggers
a. fruit ripening and
b. programmed cell death.
3. Ethylene is also produced in response to stresses such as drought, flooding, injury, or infection.
4. A changing ratio of auxin to ethylene, triggered mainly by shorter days, probably causes
a. autumn color changes and
b. the loss of leaves from deciduous trees.
H. 33.8 CONNECTION: Plant hormones have many agricultural uses
1. Agricultural uses of plant hormones include
a. control of fruit production, ripening, and dropping,
b. production of seedless fruits, and
c. use as weed killers.
2. Agricultural uses of plant hormones
a. help keep food prices down and can benefit the environment in aspects such as soil
erosion, but
b. may have dangerous side effects for humans and the environment.
III. Response to Stimuli
A. 33.9 Tropisms orient plant growth toward or away from environmental stimuli
1. Tropisms are responses that cause plants to grow in response to environmental
stimuli.
a. Positive tropisms cause plants to grow toward a stimulus.
b. Negative tropisms cause plants to grow away from a stimulus.
2. Plants respond to various environmental stimuli.
a. Phototropism is a response to light.
b. Gravitropism is a response to gravity.
c. Thigmotropism is a response to touch.
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B. 33.10 Plants have internal clocks
1. Plants display rhythmic behavior including the
a. opening and closing of stomata and
b. folding and unfolding of leaves and flowers.
2. A circadian rhythm
a. is an innate biological cycle of about 24 hours and
b. may persist even when an organism is sheltered from environmental cues.
3. Research on a variety of organisms indicates that circadian rhythms are controlled by internal timekeepers known as biological clocks.
4. Environmental cues such as light/dark cycles keep biological clocks precisely
synchronized.
5. For most organisms, including plants, we know little about
a. where the clocks are located or
b. what kinds of cells are involved.
C. 33.11 Plants mark the seasons by measuring photoperiod
1. Biological clocks can influence seasonal events including
a. flowering,
b. seed germination, and
c. the onset of dormancy.
2. The environmental stimulus plants most often use to detect the time of year is called photoperiod, the relative lengths of day and night.
3. Plant flowering signals are determined by night length.
4. Short-day plants, such as chrysanthemums and poinsettias
a. generally flower in the late summer, fall, or winter
b. when light periods shorten.
5. Long-day plants, such as spinach, lettuce, and many cereal grains
a. generally flower in late spring or early summer
b. when light periods lengthen.
D. 33.12 Phytochromes are light detectors that may help set the biological clock
1. Phytochromes
a. are proteins with a light-absorbing component and
b. may help plants set their biological clock and monitor photoperiod.
2. Phytochromes detect light in the red and far-red wavelengths.
a. One form of phytochrome absorbs red light (Pr).
b. One form detects far-red light (Pfr).
c. When Pr absorbs light, it is converted into Pfr.
d. When Pfr absorbs light, it is converted into Pr.
e. Pr is naturally produced during dark hours, while Pfr is broken down.
f. The relative amounts of Pr and Pfr present in a plant change as day length changes.
E. 33.13 EVOLUTION CONNECTION: Defenses against herbivores and infectious
microbes have evolved in plants
1. Herbivores are animals that mainly eat plants.
2. Plants use chemicals to defend themselves against herbivores and pathogens.
3. Plants counter herbivores with
a. physical defenses, such as thorns, and
b. chemical defenses, such as distasteful or toxic compounds.
4. Plants defend themselves against pathogens at several levels.
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a. The first line of defense against infection is the physical barrier of the plant’s
epidermis.
b. If that fails, plant cells damaged by infection
i. seal off the infected areas and
ii. release microbe-killing chemicals that signal nearby cells to mount a similar chemical
defense.
iii. In addition, hormones trigger generalized defense responses in other organs in the
process of systemic acquired resistance.
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