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CHAPTER 41
LECTURE
SLIDES
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Sensory Systems in Plants
Chapter 41
Responses to Light
• Pigments not used in photosynthesis
• Detect light and mediate the plant’s
response to it
• Photomorphogenesis
– Nondirectional, light-triggered development
• Phototropisms
– Directional growth responses to light
• Both compensate for inability to move
3
Responses to Light
• Phytochrome molecule exists in 2 forms
– Pr – absorbs red light at 660 nm
• Biologically inactive
• Converted to Pfr when red photons available
– Pfr – absorbs far-red light at 730 nm
• Active form
• Converted back to Pr when far-red photons
available
4
5
Responses to Light
• Phytochrome (P) consists of two parts:
– Chromophore which is light-receptive
– Apoprotein which facilitates expression of lightresponse genes
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Chromophore
Apoprotein
H2N
COOH
Serine
Protein-binding site
Protein kinase
Ubiquitin-binding site
6
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 with other proteins that form a
transcription complex, leading to the
expression of light-regulated genes
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8
Responses to Light
• Phytochrome also works through protein
kinase-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
9
10
Responses to Light
• Amount of Pfr is also regulated by
degradation
• Ubiquitin tags Pfr for transport to the
proteasome
• Process of tagging and recycling Pfr is
precisely regulated to maintain needed
amounts of phytochrome in the cell
11
Responses to Light
• Phytochrome is involved in
– Seed germination
• Inhibited by far-red light and stimulated by red light
in many plants
• Only germinate when exposed to direct sunlight
– Shoot elongation
• Etiolation is caused by a lack of red light
– Detection of plant spacing
• Plants measure the amount of far-red light
bounced back from neighboring plants
12
Phototropisms
• Directional growth responses
• Connect environmental signal with cellular perception of
the signal, transduction into biochemical pathways, and
ultimately an altered growth response
13
Phototropisms
• Blue-light receptor phototropin 1 (PHOT1)
– 2 light-sensing regions – change
conformation in response to blue light
– Stimulates the kinase region of PHOT1 to
autophosphorylate
– Triggers signal transduction
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Circadian Clocks
• ~ 24-hour rhythms 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
•
Phytochrome action
4. Clock can compensate for differences in
temperature
16
Gravitropism
• Response of a plant to the gravitational
field of the Earth
• Shoots exhibit negative gravitropism
• Roots have a positive gravitropic response
17
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
physiological signal
3. Physiological signal is transduced inside the
cell and to other cells
4. Differential cell elongation occurs in the “up”
and “down” sides of root and shoot
18
Responses to Gravity
• In shoots, gravity is sensed along the
length of the stem in endodermal cells
surrounding the vascular tissue
– Signaling toward 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
• Gravity-sensing cells are located in the
root cap
• Cells that actually undergo asymmetrical
growth are in the distal elongation zone
(closest to root cap)
• Auxin may be involved
– Still occurs if auxin transport is suppressed
24
Thigmomorphogenesis
• 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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26
Responses to Mechanical Stimuli
• Some touch-induced plant movements
involve reversible changes in turgor
pressure
• If water leaves turgid cells, they may
collapse, causing plant movement
• If water enters a limp cell, it becomes
turgid and may also move
27
Responses to Mechanical Stimuli
• Mimosa pudica leaves have swollen
structures called pulvini at the base of their
leaflets
– When leaves are stimulated, an electrical
signal is generated
– 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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29
Responses to Mechanical Stimuli
• Some turgor movements are triggered by light
• This movement maximizes photosynthesis
30
Responses to Mechanical Stimuli
• Bean leaves
– Pulvini are rigid during
the day
– But lose turgor at night
– Reduce water loss from
transpiration during the
night
– Maximize
photosynthetic surface
area during the day
31
Water and Temperature Responses
• Responses can be short-term or long-term
• Dormancy results in the cessation of
growth during unfavorable conditions
– Often begins with abscission – dropping of
leaves
– Advantage is that nutrient sinks can be
discarded, conserving resources
32
Water and Temperature Responses
• Abscission involves changes that occur in
an abscission zone at the petiole’s base
• Hormonal changes lead to differentiation
– Protective layer – consists of several
layers of suberin-impregnated cells
– Separation layer – consists of 1–2 layers of
swollen, gelatinous cells
• Pectins will break down middle lamellae of these
cells
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34
Seed Dormancy
• Seeds allow plant offspring to wait until
conditions for germination are optimal
• Triggers to break seed dormancy
– Water leaching away inhibitor; cracking seed
coat osmotically
• Favorable temperatures, day length, and
amounts of water can release buds,
underground stems and roots, and seeds
from a dormant state
35
36
Responses to Chilling
• Lipid composition of a plant’s membranes
can help predict whether the plant will be
sensitive or resistant to chilling
– The more unsaturated the membrane lipids
are, the more resistant the plant is to chilling
• Supercooling – survive as low as –40oC
– Limits ice crystal formation to extracellular
spaces
• Antifreeze proteins
37
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
38
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
39
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
• 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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• Chemical enhanced rather than retarded cell
elongation
• Frits Went named the substance that he had
discovered auxin
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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
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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
• Two families of proteins mediate auxininduced changes in gene expression
– Auxin response factors (ARFs)
• Can enhance or suppress transcription
– Aux/IAA proteins
• Bind and repress proteins that activate the
expression of ARF genes
• TIR1 is the auxin receptor
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How Auxin Works
• Unlike with animal hormones, a specific
signal is not sent to specific cells, eliciting
a predictable response
• Most likely, multiple auxin perception sites
are present
• Auxin is also unique among the plant
hormones in that it is transported toward
the base of the plant
50
How Auxin Works
• One of the direct effects of auxin is an
increase in the plasticity of the plant cell
wall
– Works only on young cell walls lacking
extensive secondary cell wall formation
• Acid growth hypothesis
– Cells actively transport hydrogen ions from
the cytoplasm into the cell wall space
– Drop in pH activates enzymes that can break
the bonds between cell wall fibers
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52
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 and fruiting in
pineapples
• 2,4-dichlorophenoxyacetic acid (2,4-D) is a
herbicide commonly used to kill weeds
53
Cytokinins
• Plant hormone that, in combination with
auxin, stimulates cell division and
differentiation
Synthetic cytokinins
54
Cytokinins
• Produced in the root apical meristems and
developing fruits
• In all plants, cytokinins, working with other
hormones, seem to regulate growth
patterns
• Promote the growth of lateral buds into
branches
• Inhibit the formation of lateral roots
– Auxin promotes their formation
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Cytokinins
• Promote the synthesis or
activation of cytokinesis
proteins
• Also function as antiaging
hormones
• Agrobacterium inserts genes
that increase rate of cytokinin
and auxin production
– Causes massive cell division
– Formation of crown gall tumor
57
• Plant tissue can
form shoots,
roots, or an
undifferentiated
mass
depending on
the relative
amounts of
auxin and
cytokinin
58
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
• Enhanced if auxin present
59
• Adding gibberellins to certain dwarf
mutants restores normal growth and
development
60
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 GAdependent transcription factors from a
repressor
• These transcription factors can now
directly affect gene expression
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Gibberellins
• Hasten seed germination
• Used commercially to extend internode
length in grapes
– Result is larger grapes
63
Brassinosteroids
• First discovered in the pollen of Brassica spp.
• Are structurally similar to steroid hormones
64
Brassinosteroids
• Functional overlap with other plant
hormones, especially auxins and
gibberellins
• Broad spectrum of physiological effects
– Elongation, cell division, stem bending,
vascular tissue development, delayed
senescence, membrane polarization and
reproductive development
65
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
66
Ethylene
• 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
• Major role in fruit development – hastens
ripening
– Transgenic tomato plant can’t make ethylene
– Shipped without ripening and rotting
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Abscisic Acid
• Synthesized mainly in mature green
leaves, fruits, and root caps
• Little evidence that this hormone plays a
role in abscission
• Induces formation of dormant winter buds
• Counteracts gibberellins by suppressing
bud growth and elongation
• Counteracts auxin by promoting
senescence
69
Abscisic Acid
• Necessary for dormancy in seeds
– Prevents precocious germination called
vivipary
• Important in the opening and closing of
stomata
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