Transcript Sensory Systems in Plants

Chapter 40
Lecture Outline
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Sensory Systems in Plants
Chapter 40
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
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PHYA
mRNA
Phytochrome synthesis
Response
(germination, shoot
elongation, etc.)
Red light (660 nm)
Pr
Pfr
Far-red light
(730 nm)
Ubiquitinbinding site
+
Pfr
ATP
Ubiquitin
Proteasome
+
Degraded Pfr
Ubiquitin
5
Responses to Light
• Phytochrome (P) consists of two parts:
– Chromophore which is light-receptive
– Apoprotein which facilitates expression of lightresponse genes
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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
7
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2. Pfr binds to
Transcription
factors of a
light-regulated
gene.
1.Pfr (but not Pr)
can enter the
nucleus.
Cell wall
Pfr
ProteinBinding
site of Pfr
Red light
Plasma
membrane
Nucleus
Transcription
Pfr
Gene
Pr
Pfr
Transcription factors
Far-red light
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
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Cell wall
Nucleus
Red light
P
Pr
Plasma
membrane
Signal
transduction
Pfr
Pfr
Far-red light
Pfr
Pi
ATP
ADP
Gene
P
Transcription factor
Protein kinase
region of Pfr
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
Etiolation
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Light
Wild type
Dark
1 µm
b.
1 µm
det2
a.
c.
d.
1 µm
© Niko Geldner, UNIL
1 µm
13
Phototropisms
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Light with blue
wavelength
Light without
blue wavelength
Green
Coleoptile bends
toward light with
blue wavelength
Coleoptile does
not bend toward
light without
blue wavelength
• Directional growth responses
• Connect environmental signal with cellular perception of
the signal, transduction into biochemical pathways, and
ultimately an altered growth response
14
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
15
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Cell
membrane
Cytosol
ADP
Blue light
PHOT1
PHOT1
Signal
transduction
PHOT1
P
ATP
ATP
ATP
1. Light with blue wavelengths strikes plant
cell membrane with phototropin 1 (PHOT1).
2. Blue light is absorbed by PHOT1, causing
a change in conformation.
3. This conformational change results in autophosphorylation, triggering a signal transduction.
16
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
17
Gravitropism
• Response of a plant to the gravitational
field of the Earth
• Shoots exhibit negative gravitropism
• Roots have a positive gravitropic response
18
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
19
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
20
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Stem
Vascular tissue
Endodermal cells
Epidermal cells
Signal
Gravitysensing cells
Gravity
response cells
Amyloplasts
a.
Gravity
21
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Root
Gravity
Zone of
elongation
Gravity
response
cells
Signal
Columella
cells with
amyloplasts
Gravitysensing
cells in
root cap
b.
22
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
23
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Mutants
Wild Type
Gravity
Gravity
Root sections
shr
2 mm
Endodermis
Epidermis
Wild-Type scr
a.
scr
Loss of
functional
endodermis
Epidermis
b.
2 mm
2 mm
a: © Jee Jung and Philip Benfey
24
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
25
Thigmomorphogenesis
• Permanent form change in response to
mechanical stimuli
• 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
26
27
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
28
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
29
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Leaflet blade
Pulvinus
Cells gaining turgor
Vascular tissue
Petiole
Cl–
Cells losing turgo
a.
K
H2O
b.
Thigmonastic response
30
Responses to Mechanical Stimuli
• Some turgor movements are triggered by light
• This movement maximizes photosynthesis
31
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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12:00 am
6:00 pm
6:00 am
12:00 pm
32
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
33
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
34
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Axillary bud
Abscission
zone
Petiole
Separation layer
Protective layer
35
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
36
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Increase in embryonic ABN
Dormancy
Increase in maternal ABN
Seed coat hardens
Dehydration
RNA and protein
synthesis
Cessation of RNA and protein synthesis
Increase in lipids and proteins
37
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
38
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
39
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
40
Auxin
• Effects of 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
41
Shoot tips perceive unidirectional light.
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Lightproof
cap
Light
Light
a.
b.
Lightproof
collar
Light
Transparent
cap
Light
c.
d.
42
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
43
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Auxin in tip
of seedling
Auxin
Time
Agar
Light-grown seedling
1. Went removed the tips
of oat seedlings and put
them on agar, an inert,
gelatinous substance.
Auxin diffuses
into agar block
Dark-grown seedlings
2. Blocks of agar were then placed
off-center on the ends of other
oat seedlings from which the
tips had been removed.
3. The seedlings bent
away from the side
on which the agar
block was placed.
Frits Went’s experiment
44
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Shaded side
of seedling
Light
Lighted side
of seedling
• Chemical enhanced rather than retarded cell
elongation
• Frits Went named the substance that he had
discovered auxin
45
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
46
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Unidirectional Light
Barrier Blocks Auxin
Development
Dark Grown Shoot
Auxin in
seedling tip
Light
Auxin Induced
Curvature
24º
Light
The same amount of total auxin is produced by a shoot tip grown with directional light, even when
a barrier divides the shoot tip, and a shoot tip grown in the dark. All three blocks of agar cause the
same amount of curvature in a tipless shoot.
Barrier in Auxin Block
Auxin Concentration Dependant
Curvature
12º
31º
B
A
Light
A
B
Separating the base of the shoot tip and the agar block results in two agar blocks with different
concentrations of auxin that produce different degrees of curvature in tipless shoots.
47
How Auxin Works
• Indoleacetic acid (IAA) is the most common
natural auxin
• Probably synthesized from tryptophan
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IAA (Indoleacetic acid)
Tryptophan
NH2
CH2
COOH
CH2
CH
COOH
a.
N
N
H
H
b.
48
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
49
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1. Auxin
binds TIR1
in the SCF
complex if
Aux/IAA is
present.
Auxin
SCF
TIR1
ARF
Transcription
factor
Aux/IAA
Ubiquitin
2. The SCF
complex tags
Aux/IAA
proteins with
ubiquitin.
3. Aux/IAA proteins
are degraded in
the proteasome.
Proteasome
Degraded
Aux/IAA
proteins
ARF
Transcription
factor
4. Aux/IAA proteins
no longer bind
and repress
transcriptional
activators of an
auxin-induced
gene.
Aux/IAA
Ubiquitin
Auxin-induced gene
expression
50
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
51
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
52
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Cytosol
Cellulose fiber
in cell wall
Auxin
Enzyme
(inactive)
1. Auxin causes
cells to pump
hydrogen ions
into the cell wall.
H+
H+
H+
Cross-bridge
H+
H+
H+
Turgor
Active
enzyme
2. pH in the cell
wall decreases,
activating
enzymes that
break crossbridges between
cellulose fibers
in the cell wall.
3. Cellulose fibers
loosen and
allow the cell to
expand as turgor
pressure inside
the cell pushes
against the
cell wall.
53
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
54
Cytokinins
• Plant hormone that, in combination with
auxin, stimulates cell division and
differentiation
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Kinetin
O
CH2
6-Benzylamino Purine (BAP)
CH2
NH
N
N
Adenine
NH2
NH
N
N
N
N
N
N
N
H
Synthetic cytokinins
N
N
N
H
H
55
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
56
Cytokinins stimulate lateral bud growth
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Auxin
Auxin added
Apical bud
removed
Lateral
(axillary)
buds
Lateral
(axillary)
buds
No auxin
Apical bud
removed
Lateral
branches
a.
b.
c.
© Prof. Malcolm B. Wilkins, Botany Dept, Glasgow University
57
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
58
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• Plant tissue can
form shoots,
roots, or an
undifferentiated
mass
depending on
the relative
amounts of
auxin and
cytokinin
Auxin:
high
Cytokinin:
low
a.
Auxin:
low
Cytokinin:
high
b.
Auxin:
intermediate
Cytokinin:
intermediate
c.
59
Strigolactones
• Inhibit axillary bud growth
• Affect extent of branching
• Derived from caretonoids
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Strigolactone
O
Carotenoid (Lutein)
O
OH
C
A
H
B
O
O
O
D
O
HO
60
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
61
• Adding gibberellins to certain dwarf
mutants restores normal growth and
development
62
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
63
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GA
GA
GID1
GID1
SCF
DELLA
DELLA
SCF
GATRXN
GATRXN
Transcription
No transcription
a.
b.
64
Gibberellins
• Hasten seed germination
• Used commercially to extend internode
length in grapes
– Result is larger grapes
65
Brassinosteroids
• First discovered in the pollen of Brassica spp.
• Are structurally similar to steroid hormones
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Plant
Animal
Brassinolide
Testosterone
Cortisol
OH
OH
OH
HO
H3C OH
O
OH
H3C
HO
O
HO
O
O
O
66
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
67
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
68
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
69
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Wild-type Tomatoes
Gene for ethylene biosynthesis enzyme
DNA
Ripe tomatoes
harvested
Transcription
mRNA
Translation
Functional
enzyme for
ethylene
biosynthesis
Ethylene synthesis (in plant)
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Transgenic Tomatoes
Antisense copy of gene
Green tomatoes
harvested
DNA
Transcription
Sense mRNA
Ethylene applied
Antisense mRNA
Hybridization
No translation and
no ethylene synthesis
70
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
71
Abscisic Acid
• Necessary for dormancy in seeds
– Prevents precocious germination called
vivipary
• Important in the opening and closing of
stomata
72
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Seedling shoot
Dormant bud
b.
a.
a: © John Sohlden/Visuals Unlimited; b: From D. R. McCarty, C. B. Carson, P. S. Stinard, and D. S.
Robertson, “Molecular analysis of viviparous-1: an abscisic acid-insensitive mutant of maize,” The
Plant Cell, 1(5):523-32 © 1989 American Society of Plant Biologists
73
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c.
20 µm
© ISM/Phototake
74