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Responses to Light
Photomorphogenesis refers to nondirectional, light-triggered development
Phototropisms are directional growth
responses to light
Both compensate for plants’ inability to move
The phytochrome molecule exists in two
interconvertible forms:
-Pr is the inactive form
-Absorbs red light at 660 nm
-Pfr is the active form
-Absorbs far-red light at 730 nm
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Responses to Light
1. Seed germination
-Inhibited by far-red light
and stimulated by red light
in many plants
2. Shoot elongation
-Etiolation occurs when shoot internodes
elongate because red light and active Pfr are not
available
3. Detection of plant spacing
-Crowded plants receive far-red light bounced
from neighboring plants
-This increases plant height in competition for
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sunlight
Responses to Light
Phytochromes are involved in many signaling
pathways that lead to gene expression
-Pr is found in the cytoplasm
-When it is converted to Pfr it enters the nucleus
-Pfr binds to transcription factors, leading to
expression of light-regulated genes
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Phototropisms
Phototropic responses including the bending of
growing stems to sources of light with blue
wavelengths (460-nm range)
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Circadian Clocks
Circadian rhythms (“around the day”) are
particularly common among eukaryotes
Have four characteristics:
1. Continue in absence of external inputs
2. Must be about 24 hours in duration
3. Cycle can be reset or entrained
4. Clock can compensate for differences in
temperature
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Responses to Gravity
Gravitropism is the response of a plant to
the gravitational field of the Earth
Four general steps of a gravitropic response:
1. Gravity is perceived by the cell
2. A mechanical signal is transduced into a
gravity-perceiving physiological signal
3. Physiological signal is transduced to
other cells
4. Differential cell elongation occurs in the
“up” and “down” sides of root and shoot
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Responses to Gravity
Shoots exhibit negative
gravitotropism
In shoots, gravity is sensed in
endodermal cells surrounding the
vascular tissue
-Signaling is in the outer
epidermal cells
Roots have a positive gravitropic
response
In roots, the cap is the site of
gravity perception
-Signaling triggers
differential
cell elongation and division in
the elongation zone
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Response to Gravity
Auxin accumulates on
lower side of the stem
-Results in asymmetrical
cell elongation and
curvature of the stem
upward
Lower cells in root cap are
less elongated than
those on upper side
-Upper side cells grow
more rapidly causing the
root to ultimately grow
downward
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Responses to Mechanical Stimuli
Thigmomorphogenesis permanent form
change in response to mechanical stresses
Thigmotropism directional growth of a plant
in response to contact
Thigmonastic responses occur in same
direction independent of the stimulus
Examples of touch responses:
-Snapping of Venus flytrap
leaves curling of tendrils around
objects
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Responses to Mechanical Stimuli
Some touch-induced plant movements involve
reversible changes in turgor pressure
-Touch triggers movement of ions to outer side of
pulvini
-Water follows by osmosis
-Decreased interior turgor
pressure causes the
leaf to fold
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Responses to Mechanical Stimuli
Some turgor movements are triggered by light
-This movement maximizes photosynthesis
Bean leaves are
horizontal during the
day when their pulvini
are rigid
-But become more
or less vertical at
night as the pulvini
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lose turgor
Water and Temperature Responses
Abscission involves changes at the petiole’s base
Protective layer = Consists of several layers of
suberin-impregnated cells
Separation layer =
Consists of 1-2
layers of swollen,
gelatinous cells
ind and rain
separate the leaf
from the stem
-Nutrient sinks are
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discarded, conserving resources
Seed Dormancy
Accumulating food reserves, forming a protective
seed coat and dehydration
Offspring to wait until conditions for germination
are optimal
-Legume seeds last
decades or longer
-Seeds that are
thousands of years
old have been
successfully
germinated
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Auxin
Discovered in 1881 by Charles and Francis
Darwin
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In 1926, Frits Went performed an experiment
Auxin
that explained all of the previous results
-He named the
chemical
messenger
auxin
-Cell elongation causes the plant
to bend towards light
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Auxin
Winslow Briggs later demonstrated
that auxin molecules migrate away from the light into
the shaded portion of the shoot
-Barriers in a shoot tip revealed equal amounts of
auxin in both the light and dark sides
-However, different auxin concentrations
produced different degrees of curvature
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How Auxin Works
Indoleacetic acid (IAA) is the most common natural
auxin
-Probably synthesized from tryptophan
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Synthetic Auxins
Naphthalene acetic acid (NAA) and
indolebutyric acid (IBA) have many uses
in agriculture and horticulture
-Prevent abscission in apples and berries
-Promote flowering & fruiting in pineapples
2,4-dichlorophenoxyacetic acid (2,4-D) is
a herbicide commonly used to kill weeds
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Cytokinins
Are purines that appear to be
derivatives of adenine
Cytokinins are produced in the
root apical meristems and fruits
-Stimulate cell division and
differentiation & promote the
growth of lateral buds into
branches
Synthetic cytokinins
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Gibberellins
Named after the fungus Gibberella fujikuroi which
causes rice plants to grow very tall
Gibberellins belong to a large class of over 100
naturally occurring plant hormones
-All are acidic
-Have important effects on stem elongation
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Brassinosteroids
First discovered in the pollen of Brassica spp.
-Are structurally similar to steroid hormones
-Elongation, cell division, stem bending,
vascular tissue development, delayed
senescence and reproductive development
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Oligosaccharins
Are complex plant cell wall carbohydrates that
have a hormone-like function
-Can be released from the cell wall by
enzymes secreted by pathogens
-Signal the hypersensitive response (HR)
In peas, oligosaccharins inhibit auxinstimulated elongation of stems
-While in regenerated tobacco tissue, they
inhibit roots and stimulate flowers
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A gaseous hydrocarbon (H2C–CH2)
-thus retards their growth in lateral
bud
-suppresses stem and root elongation
Ethylene controls leaf,
flower and fruit
abscission
It hastens fruit ripening
-Indeed, an antisense
copy of the gene has
been used to create
transgenic tomato
-These stay fresh
longer
Ethylene
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Abscisic Acid
Abscisic acid is synthesized mainly in mature
green leaves, fruits and root caps
-induces formation of dormant winter buds
-necessary for dormancy in seeds
-important in the opening and closing of stomata
-Triggers movement of K+ out of guard cells
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