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Chapter 33
Control Systems in Plants
Introduction: What Are the Health Benefits
of Soy?
A.)
Soy protein is one of the few plant proteins that provide all
of the essential amino acids
B.)
Benefits of soy include
– It reduces the risk of heart disease
– It is rich in antioxidants and fiber
– It is low in fat and helps increase “good” cholesterol (HDL) while
reducing “bad” cholesterol (LDL)
– Soy contains phytoestrogens, hormones that can reduce the symptoms of
menopause in women
– More research into the use of phytohormones to treat the symptoms of
menopause is needed
PLANT HORMONES
33.1 Experiments on how plants turn toward light led to the
discovery of a plant hormone
A.)
Phototropism is a phenomenon by which plants grow
toward a light source
B.)
Phototropism occurs when the cells on the dark side of a
plant stem elongate faster than those on the light side
C.)
Charles Darwin and his son Francis conducted experiments
that showed that the shoot tips of plants controlled their
ability to grow toward light
D.)
Peter Boysen-Jensen later conducted experiments that
showed that chemical signals produced in shoot tips were
responsible for phototropism
E.)
The Darwins’ experiment
– 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
F.)
Jensen’s experiment
G.)
A graduate student named Frits Went isolated the chemical
hormone responsible for phototropism
– 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
– 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
– 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
H.)
The chemical signal responsible for phototropism is a
hormone that Went called auxin
33.2 Five major types of hormones regulate plant growth and
development
A.)
A hormone is a chemical signal that is produced in one part
of the body and transported to another, where it triggers
responses in target cells
B.)
Binding of hormones to specific cellular receptors triggers a
signal transduction pathway
C.)
Tiny amounts of hormone can have a big effect
D.)
All aspects of plant growth and development are affected by
hormones
E.)
There are five classes of plant hormones and each class can
have multiple effects on plant growth and development
33.3 Auxin stimulates the elongation of cells in young shoots
A.) Indoleacetic acid (IAA) is a naturally occurring auxin that
promotes seedling elongation
B.) Auxin is produced in shoot apical meristems and transported
downward through a plant
C.)
Concentration of auxin and site of activity are important to
auxin’s effects
– In moderate concentrations, auxin promotes cell elongation in stems
– In high concentrations, auxin reduces cell elongation in stems
– Auxins affects cell elongation in roots at lower concentrations
D.)
A hypothesis for the action of auxin
– Auxins stimulate plant cells to take up H+ ions, lowering pH
– Acidity causes separation of cross linkages in cellulose
– As the cell takes up water, the cell elongates because of weakening of
the cellulose cell wall
– Auxins stimulate the plant to produce additional cell wall material
– As pH decreases, the larger cell wall restabilizes
33.4 Cytokinins stimulate cell division
A.) Cytokinins promote cytokinesis, or cell division
B.)
Cytokinins
– Are produced in actively growing organs such as roots, embryos, and
fruits
– Produced in roots move upward through the plant
– Retard aging in leaves and flowers
C.)
Cytokinins and auxins interact to control apical dominance
– Auxins inhibit axillary bud growth, reducing lateral branching
– Cytokinins counter the action of auxin by promoting axillary bud growth
– The ratio of auxins to cytokinins controls axillary bud growth
33.5 Gibberellins affect stem elongation and have numerous
other effects
A.)
Gibberellins are plant hormones that promote stem
elongation by increasing cell division and elongation
B.)
Gibberellins were named for a genus of fungi that produce
the same chemical and cause “foolish seedling” disease
C.)
There are more than 100 distinct gibberellins produced
primarily in roots and young leaves
D.)
Gibberellins also promote fruit development and seed
germination
E.)
Gibberellins act antagonistically against another plant
hormone called abscisic acid
33.6 Abscisic acid inhibits many plant processes
A.) Abscisic acid (ABA) is a plant hormone that inhibits growth
B.)
High concentrations of ABA promote seed dormancy
– ABA must be removed for germination to occur
– The ratio of ABA to gibberellins controls germination
C.)
ABA also influences plant water relations
– Accumulation of ABA in wilted leaves promotes stomatal closure
– ABA produced in roots can signal low soil moisture conditions and
triggers plants to conserve water by closing stomata
33.7 Ethylene triggers fruit ripening and other aging processes
A.) Ethylene is a gaseous by-product of natural gas
combustion and a naturally occurring plant hormone
B.)
Plants produce ethylene in response to stresses such as
mechanical pressure, injury, infection, and drought or flood
C.)
Ethylene promotes aging processes such as fruit ripening
and natural cell death
– It is used commercially to ripen fruits
– Growers inhibit ethylene production using CO2 to inhibit ripening in stored
fruit
D.)
Ethylene promotes leaf abscission in fall by breaking down
cells at the base of the petiole
33.8 CONNECTION: Plant hormones have many agricultural
uses
A.)
Agricultural uses of plant hormones include
B.)
Agricultural uses of plant hormones help keep food prices
down and benefit the environment
C.)
Some consumers are concerned that synthetic plant
hormones may have dangerous side effects for humans
– Control of fruit production, ripening, and dropping
– Production of seedless fruits
– Use as weed killers
GROWTH RESPONSES AND
BIOLOGICAL
RHYTHMS IN PLANTS
33.9 Tropisms orient plant growth toward or away from
environmental stimuli
A.)
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
B.)
Plants respond to various environmental stimuli
– Phototropism—response to light
– Gravitropism—response to gravity
– Thigmotropism—response to touch
33.10 Plants have internal clocks
A.) Circadian rhythms are innate biological cycles of
approximately 24 hours
B.)
Both plants and animals have circadian rhythms
C.)
Circadian rhythms are influenced by environmental cues
such as light, but they are controlled by biological clocks
D.)
The biological clocks of plants are likely the result of
rhythmic production of proteins that influence gene
expression
33.11 Plants mark the seasons by measuring photoperiod
A.) Flowering, seed germination, and dormancy are all seasonal
phenomena in plants
B.)
Plants detect season by measuring photoperiod, the
relative lengths of day and night
C.)
Plant flowering signals are determined by night length
– Short-day plants flower when the dark period is greater than some
critical length
– Long-day plants flower when the dark period is shorter than some
critical length
– Experiments that altered light and dark periods were used to determine
that it is night length and not day length that cues plants to flower
33.12 Phytochrome is a light detector that may help set the
biological clock
A.)
Phytochromes are proteins with a light-absorbing
component
B.)
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
– 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
33.13 TALKING ABOUT SCIENCE: Joanne Chory studies
the effects of light and hormones in the model plant
Arabidopsis
A.)
Scientists often use small and easily manipulated species as models to
learn about biological processes
B.) Arabidopsis is a plant in the mustard family that has been used
extensively to study plant genetics and physiology
C.) Dr. Joanne Chory has used Arabidopsis to study genes that control
hormones and signal transduction pathways; her work has many
applications in science and agriculture
PLANT DEFENSES
33.14 EVOLUTION CONNECTION: Defenses against
herbivores and infectious microbes have evolved in
plants
A.)
Herbivores are organisms that feed on plants; many plant
adaptations have evolved to defend against herbivores
– Production of distasteful or poisonous compounds
– Symbioses with organisms that defend plants
B.)
Plants have also evolved defenses against pathogens
– The epidermis is the first line of defense against infection
– Chemical defenses offer a way to fight pathogens that enter the plant
33.15 TALKING ABOUT SCIENCE: Plant biochemist Eloy
Rodriguez studies how animals use defensive chemicals
made by plants
A.)
Animals may “medicate” themselves with chemicals
produced by plants
B.)
Scientists observe which plants animals eat for “medicinal”
purposes and how much of each plant they eat
C.)
Observation of such animal behavior has led scientists to
study how such chemicals might benefit humans
– Plant chemicals can kill animal parasites
– Some may be useful for treatment of tumors
§ Explain what hormones are and how they work
§ Describe the experiments that led to the discovery of auxins
§ Name the five general classes of plant hormones and describe the
actions of each class
§ Explain what tropisms are and give examples of different kinds of
plant tropisms
§ Describe circadian rhythms and biological clocks; recognize the
innate basis of such rhythms and how they are affected by
environmental cues
§ Explain the difference between short-day and long-day plants
§ Describe the experiments that led to the discovery of the effects
of night length on flowering
§ Explain how plants detect seasons using proteins
§
§
Give examples of plant defenses that have evolved to protect
plants against herbivores and pathogens
Explain how scientists can help treat human diseases by studying
the things that other animals eat