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Plant Responses to Stimuli
Signal Transduction
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Environmental signals
trigger specific responses in
plants
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Etiolation- adaptations for
darkness
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Potatoes sprouting in a dark
pantry
De-etiolation- “greening”;
occurs when a shoot reaches
sunlight
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Potato leaves expand and
roots elongate when exposed
to sunlight
From Reception to Response
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Reception
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Transduction
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Receptor proteins change
shape in response to stimulus
(hormonal, chemical, or
environmental)
Signal is amplified by second
messengers
Specific protein kinases
activated
Response
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Regulation of cell activity
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Regulation of transcription or
translation
Modification of proteins (posttranslation)
Tropisms
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Tropism- growth
response that orients
plant toward or away
from a stimuli
Growth of a shoot
toward light =
phototrophism
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Coleoptile senses light
and transmits signal
(Darwin and Darwin/
Boyen-Jensen)
Plant Hormones
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Growth caused by the
concentration of
horomones within plant
cells
Phototropism caused by
a higher concentration of
auxin (growth-promoting
horomone) on the dark
side
Fritts Went
Auxin
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Indoleacetic Acid (IAA)
Promotes elongation of
coleoptiles
Synthesized in apical
meristem; moves down the
shoot
Activates expansins (break
down cellulose microfibrils)
Alters gene expression,
causing cells to produce new
proteins
Used in vegetative propigation
of roots and also as a herbicide
Cytokinins
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Promote cytokinesis (cell
division)
Work with auxin to
promote growth (not
effective alone)
Used in the control of
axillary bud growth;
promotes apical dominance
Anti-aging effects
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Inhibit protein breakdown,
stimulate transcription and
protain synthesis, and
mobilize nutrients
Gibberellins
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Stimulate growth of roots
and leaves
Present in seed embryos,
roots, and young leaves
Little effect on roots (can
cause plants to topple if the
concentration is too high)
Stimulates cell-wall
loosening enzymes
Causes grapes to grow
larger because stems are
elongated, giving fruit more
room to grow
Brassinosteroids and Abscisic Acid
Brassinosteroids
 Induce cell elongation and
division in stem segments
 Retard loss of leaves
(abscission)
 Promote xylem differentation
Abscisic Acid (ABA)
 Slows growth
 Keeps seeds dormant/
prevents early sprouting
 Increases drought tolerance
Ethylene
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Response to mechanical stress
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Apoptosis
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Programmed cell death
Shedding leaves/ death of xylem
components, etc.
Leaf abscission
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Occurs if an emerging shoot meets
resistance (rock, etc.)
Triple response- slows elongation,
thickens stem, and causes it to curve
Separates leaf from tree at petiole
Fruit ripening
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Triggers ripening of fruit; causes chain
reaction that ripens all fruit on the
plant
Photomorphogenesis
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Plants can detect the
direction, intensity, and
wavelength of light
Action spectrum- different
wavelengths are more
effective at driving
particular processes
Blue-light photoreceptors
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Phototrophism, opening of
stomata, slowing of
hypocotyl elongation
Phytochromes (red light)
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Seed germination, shade
avoidance in trees
Circadian Rhythms
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Plants “cycle” their processes
and responses daily
Length of day determines
growth of plants
Must have darkness for a
certain amount of time…
critical dark period
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Short-day plants- flower when
darkness is longer than CDP (late
summer/fall/winter)
Long-day plants -flower when
darkness is shorter than CDP (late
spring/summer)
Day-neutral plants- unaffected by
photoperiods
Touchy, touchy…
Gravitropism
 Adjust growth according to
gravity
 Detect gravity by settling of
statoliths (dense starch grains)
 Roots- positive gravitropism
 Shoots- negative gravitropism
Thigmomorphogenesis
 Change in form due to touch
 Inhibition of growth/ change
in leaf conformation
Environmental Stress
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Drought
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Flood
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Can cause desiccation
Heat
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O2 deprivation stimulates
ethylene production
Salt
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Reduce transpiration, inhibit
new growth
Denatures proteins
Cold
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Alters solute transport
Freezing can cause cell death
due to dehydration
Plant Defenses
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Herbivores
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Physical (thorns, etc.)
Chemical (Distasteful compounds,
"recruitment” of predatory animals)
Pathogens
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Virulent (little specific defense) and
avirulent (cause mild harm)
Gene-for-gene recognitionresistance (R) genes
Elicitors (oligosaccharins)
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Stimulate antimicrobial compounds
(phytoalexins) and produce
pathogenesis-related (PR) proteins
Hypersensitive response- seals off
infected region
Systemic Acquired Resistanceprotects entire plant from infection
due to phytoalexin and PR protein
production