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Hormones and Plant
responses to stimuli
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HOW PLANTS RESPOND TO
ENVIRONMENTAL STIMULI
Tropisms - plant growth toward or awayfrom a stimulus such as light or gravity.
Nastic Movements - response toenvironmental stimuli that are
independent of the direction of thestimulus. Pre-determined response.
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Phototropism ± ability to bend
towards light Auxins - responsible
for plants bending
towards light.
Auxins - move down
shaded side of the
stem and cause cellsto elongate
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Gravitropism (geotropism) ± plant
response to gravity Auxins ± responsible
for plant response to
gravity
Auxins ± move to
lowest side and cause
stem tissue toelongate ± stem
curves upwards
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PhototropismPhototropism
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Phototropisms
Phototropic responses involve bending of
growing stems toward light sources.
± Individual leaves may also displayphototrophic responses.
auxin most likely involved
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Growth movementGrowth movement
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How Phytochrome Works
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Plants Respond to Gravity
Gravitropism is the response of a
plant to the earth¶s gravitational field.
± present at germination
auxins play primary role
± Four steps
gravity perceived by cell
signal formed that perceives gravity
signal transduced intra- and intercellularly
differential cell elongation
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GravitropismGravitropism
Increased auxin concentration on the lower side in stemscauses those cells to grow more than cells on the upper side.
± stem bends up against the force of gravity
negative gravitropism
Upper side of roots oriented horizontally grow more rapidly
than the lower side
± roots ultimately grow downward
positive gravitropism
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Taiz and Zeiger. 2002. Plant Physiology, 3rd Ed.
Auxin and GeotropismAuxin and Geotropism
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Increased auxin concentration on the lower
side in stems causes those cells to grow
more than cells on the upper side
± Stem bends up against the force of gravity
Negative gravitropism
Upper side of roots orientated horizontally
grow more rapidly than the lower side
± Roots ultimately grow downward
Positive gravitropism
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Gravitropism = GeotropismGravitropism = Geotropism
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StatolithsStatoliths
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Plants Respond to Touch
Thigmotropism is directional growth
response to contact with an object.
± tendrils
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ThigmotropismThigmotropism
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Thigmomorphogenesis
± Changes in form of plant as a result of mechanical stress
± Mechanical stimulation activates signaltransduction, increases calcium, activatesgenes, which affects the cell wall
properties
± Strong wind = stockier trunk
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Pulvinus of Pulvinus of Mimosa pudicaMimosa pudica
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NYCTINASTY
sleep movements
prayer plant - lower
leaves during the dayand raises leaves at
night
shamrock (Oxalis)
legumesCredit:(http://employees.csbsju.edu/ssa
upe/biol327/Lab/movie/movies.htm)
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Circadian Clocks
Circadian clocks are endogenoustimekeepers that keep plant responsessynchronized with the environment.
± circadian rhythm characteristics must continue to run in absence of external inputs
must be about 24 hours in duration
can be reset or entrained (to determine or modify the phase or
period of <circadian rhythms entr ai ned by a light cycle>) can compensate for temperature differences
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Other Growth Responses
Thigmotropism - Other Objects Chemotropism - Chemicals
Traumatropism - Wounding
Thermotropism - Temperature Aerotropism - Oxygen
Skototropism - Dark
Geomagnetotropism - Magnetic Fields
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Hormones and
Signal Transduction
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Cells must communicate to coordinate their activities.
Cells may receive a variety of signals,
chemical signals, electromagnetic signals,and mechanical signals.
How plants respond to stimuli
The process by which a signal on a cell¶s
surface is converted into a specific cellular response is a signal-transduction
pathway.
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Signal Transduction Signals from outside a cell can be perceived,
sometimes by receptors that span the plasmamembrane.
After stimulation of such receptors, informationcan be relayed by a series of small molecules or proteins to the cell nucleus, where activation of specific transcription factors can stimulate new
gene expression programs.
Gene expression results in the production of new proteins that can function in the final
biological responses to the signal.
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Signal Transduction
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SignalSignal--transduction pathwaystransduction pathways
in plantsin plants
Auxin interacts with calcium ions which in turn calmodulin, a protein, which
regulates many processes in plants, animals, and microbes.
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Loosening of cell wallLoosening of cell wall
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Signal Transduction Genes
The activated transcription factors
enter the nucleus and promote gene
expression.
Two classes of auxin-induced genes:
±Primary response, or EARLY
genes. ±Secondary response, or LATE
genes.
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Signal Transduction Genes
Primary (EARLY) Response Genes.
± These genes are present in cell at time of
hormone exposure for fast response.
± Usually, they cannot be blocked by
inhibitors.
± Response time: a few minutes to several
hours. ± They regulate transcription of 2r genes.
± Involvement in cell-to-cell signaling.
± Involvement in stress adaptation.
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