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
–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
G.)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
–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
–Control of fruit production, ripening, and dropping
–Production of seedless fruits
–Use as weed killers
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
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