CHAPTER 41
LECTURE
SLIDES
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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
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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
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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
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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 with other proteins that form a
transcription complex, leading to the
expression of light-regulated genes
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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
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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
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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
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Phototropisms
• Directional growth responses
• Connect environmental signal with cellular perception of
the signal, transduction into biochemical pathways, and
ultimately an altered growth response
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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
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Gravitropism
• Response of a plant to the gravitational
field of the Earth
• Shoots exhibit negative gravitropism
• 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
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
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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
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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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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
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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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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
– 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
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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
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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
– 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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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
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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
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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
• 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
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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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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
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Cytokinins
• Plant hormone that, in combination with
auxin, stimulates cell division and
differentiation
Synthetic cytokinins
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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
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• Plant tissue can
form shoots,
roots, or an
undifferentiated
mass
depending on
the relative
amounts of
auxin and
cytokinin
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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
• Enhanced if auxin present
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• 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 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
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Brassinosteroids
• First discovered in the pollen of Brassica spp.
• Are structurally similar to steroid hormones
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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
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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
• 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
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Abscisic Acid
• Necessary for dormancy in seeds
– Prevents precocious germination called
vivipary
• Important in the opening and closing of
stomata
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