Transcript Document

Sensory Systems in Plants
Chapter 41
Responses to Light
Pigments other than those used in
photosynthesis can detect light and
mediate the plant’s response to it
Photoperiodism response to changes in the
length of day and night, it is nondirectional
Phototropisms are directional growth
responses to light
Both compensate for plants’ inability to move
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Responses to Light
Phytochrome (P) consists of two parts:
-Chromophore which is light-receptive
-Apoprotein which initiates a signaltransduction pathway
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Responses to Light
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
-Tagged by ubiquitin for degradation in
the proteasome
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Responses to Light
In Arabidopsis, five forms of phytochromes
have been characterized: PHYA to PHYE
-Involved in several plant growth responses
1. Seed germination
-Inhibited by far-red light and stimulated
by red light in many plants
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Responses to Light
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 sunlight
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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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Responses to Light
Phytochrome also works through proteinkinase signaling pathways
-When Pr is converted to Pfr, its protein
kinase domain causes autophosphorylation
or phosphorylation of another protein
-This initiates a signaling cascade that
activates 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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Phototropisms
A blue-light receptor phototropin 1 (PHOT1)
has been characterized
-Has two regions
-Blue-light activates the light-sensing
region of PHOT1
-Stimulates the kinase region of
PHOT1 to autophosphorylate
-Triggers a signal transduction
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Phototropisms
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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
-Shoots exhibit negative gravitotropism;
roots have a positive gravitropic response
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Responses to Gravity
Four general steps lead to 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 17
Responses to Gravity
In shoots, gravity is sensed along the length of
the stem in endodermal cells surrounding
the vascular tissue
-Signaling is in the outer epidermal cells
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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Stem Response to Gravity
Auxin accumulates on lower side of the stem
-Results in asymmetrical cell elongation
and curvature of the stem upward
Two Arabidopsis mutants, scarecrow (scr)
and short root (shr) do not show a normal
gravitropic response
-Due to lack of a functional endodermis
and its gravity-sensing amyloplasts
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Root Response to Gravity
Lower cells in horizontally oriented root cap
are less elongated than those on upper side
-Upper side cells grow more rapidly causing
the root to ultimately grow downward
Auxin may not be the long-distance signal
between the root cap and elongation zone
-However, it has an essential role in root
gravitotropism
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Responses to Mechanical Stimuli
Thigmomorphogenesis is a permanent form
change in response to mechanical stresses
Thigmotropism is directional growth of a
plant or plant part 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
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Responses to Mechanical Stimuli
Some turgor movements are triggered by light
-This movement maximizes photosynthesis
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Responses to Mechanical Stimuli
Bean leaves are horizontal during the day
when their pulvini are rigid
-But become more
or less vertical at
night as the pulvini
lose turgor
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Water and Temperature Responses
When water and temperature affect plants,
responses can be short-term or long-term
Dormancy results in the cessation of growth
during unfavorable conditions
-Often begins with dropping of leaves
Abscission is the process by which leaves
or petals are shed
-One advantage is that nutrient sinks can
be discarded, conserving resources
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Water and Temperature Responses
Abscission involves changes that occur in an
abscission zone at the petiole’s base
-Hormonal changes lead to differentiation of:
-Protective layer = Consists of several
layers of suberin-impregnated cells
-Separation layer = Consists of 1-2
layers of swollen, gelatinous cells
-As pectins break down, wind and rain
separate the leaf from the stem
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Seed Dormancy
Seeds allow plant offspring to wait until
conditions for germination are optimal
-Legume seeds often last decades and
even longer without special care
-Seeds that are thousands of years old
have been successfully germinated
Essential steps leading to dormancy include:
-Accumulating food reserves, forming a
protective seed coat and dehydration
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Responses to Chilling
Plants respond to cold temperatures by:
1. Increasing number of unsaturated lipids in
their plasma membranes
2. Limiting ice crystal formation to
extracellular spaces
3. Producing antifreeze proteins
Some plants can undergo deep supercooling
-Survive temperatures as low as –40OC
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Responses to High Temperatures
Plants produce heat shock proteins (HSPs)
if exposed to rapid temperature increases
-HSPs stabilize other proteins
Plants can survive otherwise lethal
temperatures if they are gradually exposed
to increasing temperature
-Acquired thermotolerance
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Hormones and Sensory Systems
Hormones are chemicals produced in one
part of an organism and transported to
another part where they exert a response
In plants, hormones are not produced by
specialized tissues
-Seven major kinds of plant hormones
-Auxin, cytokinins, gibberellins,
brassinosteroids, oligosaccharins,
ethylene, and abscisic acid
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Auxin
Discovered in 1881 by Charles and Francis
Darwin
-They reported experiments on the
response of growing plants to light
-Grass seedlings do not bend if the tip
is covered with a lightproof cap
-They do bend when a collar is placed
below the tip
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Auxin
The Darwins hypothesized that shoots bend
towards light in response to an “influence”
transmitted downward from the tip
Thirty years later, Peter Boysen-Jensen and
Arpad Paal demonstrated that the
“influence” was actually a chemical
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Auxin
In 1926, Frits Went performed an experiment
that explained all of the previous results
-He named the chemical messenger auxin
-It accumulated on the side of an oat
seedling away from light
-Promoted these cells to grow faster
than those on the lighted side
-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 inserted in a shoot tip revealed
equal amounts of auxin in both the light and
dark sides of the barrier
-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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How Auxin Works
The auxin receptor is the transport inhibitor
response protein 1 (TIR1)
Two families of proteins mediate auxininduced changes in gene expression
-Auxin responses factors (ARFs)
-Aux/IAA proteins
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How Auxin Works
1. Auxin binds TIR1 in the SCF complex if
Aux/IAA is present
2. SCF complex tags Aux/IAA proteins with
ubiquitin
3. These are degraded in the proteasome
4. Transcriptional activators of ARF genes
are released from repression by Aux/IAA
5. Auxin-induced gene expression
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How Auxin Works
One of the downstream effects of auxin is an
increase in plasticity of the plant cell wall
-The acid growth hypothesis provides a
model linking auxin to cell wall expansion
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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
Synthetic cytokinins
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Cytokinins
Cytokinins are produced in the root apical
meristems and developing fruits
-Stimulate cell division and differentiation,
in combination with auxin
Cytokinins promote the growth of lateral buds
into branches
-They inhibit the formation of lateral roots,
while auxin promotes their formation
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Cytokinins
Cytokinins promote the synthesis or
activation of cytokinesis proteins
-They also function as anti-aging hormones
Plant tissue can form shoots, roots, or an
undifferentiated mass depending on the
relative amounts of auxin and cytokinin
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Cytokinins
The plant pathogen Agrobacterium introduces
genes into the plant genome that increase
the production of cytokinin and auxin
-Cause massive cell
division and formation
of a crown gall tumor
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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 and abbreviated GA
-Have important effects on stem elongation
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Gibberellins
Adding gibberellins to certain dwarf mutants
restores normal growth and development
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Gibberellins
GA is used as a signal from the embryo that
turns on transcription of genes encoding
hydrolytic enzymes in the aleurone layer
-When GA binds to its receptor, it frees
GA-dependent transcription factors from a
repressor
-These transcription factors can now
directly affect gene expression
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Gibberellins
GAs hasten seed germination
-They also function as pheromones in ferns
GAs are used
commercially to
extend internode
length in grapes
-The result is
larger grapes
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Brassinosteroids
First discovered in the pollen of Brassica spp.
-Are structurally similar to steroid hormones
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Brassinosteroids
Have a broad spectrum of physiological
effects
-Elongation, cell division, stem bending,
vascular tissue development, delayed
senescence and reproductive development
Additive effects with auxins and gibberellins
have been reported
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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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Ethylene
A gaseous hydrocarbon (H2C–CH2)
Auxin stimulates ethylene production in the
tissues around the lateral bud and thus
retards their growth
Ethylene also suppresses stem and root
elongation
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Ethylene
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
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Abscisic Acid
Abscisic acid is synthesized mainly in mature
green leaves, fruits and root caps
There is little evidence that this hormone plays
a role in abscission
Abscisic acid induces formation of dormant
winter buds
It counteracts gibberellins, by suppressing bud
growth and elongation, and auxin, by
promoting senescence
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Abscisic Acid
Abscisic acid is also necessary for dormancy
in seeds
-Prevents precocious germination called
vivipary
Abscisic acid is important in the opening and
closing of stomata
-Triggers movement of K+ out of guard cells
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