principles of biology biol 100c: introductory biology iii plant hormones & plant defenses dr. p....
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Principles of Biology
BIOL 100C:BIOL 100C:Introductory Biology IIIIntroductory Biology IIIPlant Hormones & Plant DefensesPlant Hormones & Plant Defenses
Dr. P. NarguizianFall 2012
Phototropism
Phototropism is a phenomenon by which plants grow toward a light source
Phototropism occurs when the cells on the dark side of a plant stem elongate faster than those on the light side
Charles Darwin and his son Francis conducted experiments that showed that the shoot tips of plants controlled their ability to grow toward light
Peter Boysen-Jensen later conducted experiments that showed that chemical signals produced in shoot tips were responsible for phototropism
Light
Shadedside of shoot
Illuminatedside ofshoot
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
Experiments on how plants turn toward light led to the discovery of a plant hormone
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
Experiments on how plants turn toward light led to the discovery of a plant hormone
Light
Control
1 2 3 4 5 6
Tipremoved
Tip coveredby opaquecap
Tip coveredby trans-parent cap
Basecovered by opaqueshield
Tip sepa-rated bygelatinblock
Tip separatedby mica
Boysen-Jensen (1913)Darwin and Darwin (1880)
Five major types of hormones regulate plant growth and development
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
Binding of hormones to specific cellular receptors triggers a signal transduction pathway
Tiny amounts of hormone can have a big effect
All aspects of plant growth and development are affected by hormones
There are five classes of plant hormones and each class can have multiple effects on plant growth and development
Five major types of hormones regulate plant growth and development
Auxin stimulates the elongation of cells in young shoots
Indoleacetic acid (IAA) is a naturally occurring auxin that promotes seedling elongation
Auxin is produced in shoot apical meristems and transported downward through a plant
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Auxin stimulates the elongation of cells in young shoots
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
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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
Auxin stimulates the elongation of cells in young shoots
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Plasmamembrane
Cellwall
Vacuole
Cytoplasm
Proton pump(protein)
H+
1
H+
2
Cell wall
Cellulosemolecule
3
Cellelongation
H2O
Enzyme
Cellulose loosens;cell can elongate Cellulose
moleculeCross-linkingmolecule
Cytokinins stimulate cell division
Cytokinins promote cytokinesis, or cell division
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
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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
33.4 Cytokinins stimulate cell division
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Terminal bud
No terminal bud
Gibberellins affect stem elongation and have numerous other effects
Gibberellins are plant hormones that promote stem elongation by increasing cell division and elongation
Gibberellins were named for a genus of fungi that produce the same chemical and cause “foolish seedling” disease
There are more than 100 distinct gibberellins produced primarily in roots and young leaves
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Gibberellins also promote fruit development and seed germination
Gibberellins act antagonistically against another plant hormone called abscisic acid
Gibberellins affect stem elongation and have numerous other effects
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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
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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
Abscisic acid inhibits many plant processes
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Ethylene triggers fruit ripening and other aging processes
Ethylene is a gaseous by-product of natural gas combustion and a naturally occurring plant hormone
Plants produce ethylene in response to stresses such as mechanical pressure, injury, infection, and drought or flood
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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
Ethylene promotes leaf abscission in fall by breaking down cells at the base of the petiole
Ethylene triggers fruit ripening and other aging processes
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1
3
2
GROWTH RESPONSES AND BIOLOGICAL RHYTHMS IN
PLANTS
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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—response to light
– Gravitropism—response to gravity
– Thigmotropism—response to touchCopyright © 2009 Pearson Education, Inc.
Light
Phototropism
Gra
vity
Gravitropism Thigmotropism
PLANT DEFENSES
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EVOLUTION CONNECTION: Defenses against herbivores and infectious microbes have evolved in plants
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
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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
EVOLUTION CONNECTION: Defenses against herbivores and infectious microbes have evolved in plants
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5 4
3
21
Wasplayseggs
Damage to plantand chemical incaterpillar saliva
Synthesisand releaseof chemicalattractants
Signaltransductionpathway
Plant cell
Recruitment of wasp
TALKING ABOUT SCIENCE: Plant biochemist Eloy Rodriguez studies how animals use defensive chemicals made by plants
Animals may “medicate” themselves with chemicals produced by plants
Scientists observe which plants animals eat for “medicinal” purposes and how much of each plant they eat
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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
TALKING ABOUT SCIENCE: Plant biochemist Eloy Rodriguez studies how animals use defensive chemicals made by plants
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