Transcript Plant Hormones & Plant DefensesV.2

BIOL 100C:
Introductory Biology III
Plant Hormones & Plant Defenses
Dr. P. Narguizian
Fall 2012
Principles of Biology
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
Shaded
side of
shoot
Light
Illuminated
side of
shoot
Experiments on how plants turn toward light led
to the discovery of a plant hormone
 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
Light
Control
1
2
Tip
removed
Tip covered
by opaque
cap
3
4
Tip covered Base
by transcovered
parent cap by opaque
shield
Darwin and Darwin (1880)
5
6
Tip separated by
gelatin
block
Tip separated
by mica
Boysen-Jensen (1913)
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
Five major types of hormones regulate plant
growth and development
 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
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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Auxin stimulates the elongation of cells in young
shoots
 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
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1
Plasma
membrane
Cell
wall
Vacuole
H+
Proton
pump
(protein)
Cell wall
Cellulose
molecule
H+
2
H2O
3
Cell
elongation
Enzyme
Cytoplasm
Cellulose loosens;
cell can elongate Cellulose
molecule
Cross-linking
molecule
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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33.4 Cytokinins stimulate cell division
 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
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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 affect stem elongation and have
numerous other effects
 Gibberellins also promote fruit development and
seed germination
 Gibberellins act antagonistically against another
plant hormone called abscisic acid
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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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Abscisic acid inhibits many plant processes
 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
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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 triggers fruit ripening and other aging
processes
 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
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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 touch
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Gravity
Light
Phototropism
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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EVOLUTION CONNECTION: Defenses against
herbivores and infectious microbes have
evolved in plants
 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
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5
Wasp
lays
eggs
4
Recruitment of wasp
3
1
Damage to plant
and chemical in
caterpillar saliva
Synthesis
and release
of chemical
attractants
Plant cell
2
Signal
transduction
pathway
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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TALKING ABOUT SCIENCE: Plant biochemist
Eloy Rodriguez studies how animals use
defensive chemicals made by plants
 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
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