Figure 39.0 A grass seedling growing toward a candle’s light
Plant Responses to Internal and External Signals
1
Figure 39.1 Light-induced greening of dark-sprouted potatoes: a dark-grown potato
(left), after a week's exposure to natural sunlight (right)
2
Figure 39.2 Review of a general model for signal-transduction pathways
3
Figure 39.3 An example of signal transduction in plants: the role of phytochrome in
the greening response (Layer 1)
Receptor
During transduction we are
activating second messengers.
In this case it is a G protein
that activates cGMP
One Pathway Activated
4
Figure 39.3 An example of signal transduction in plants: the role of phytochrome in
the greening response (Layer 2)
2nd Pathway Activated
5
Figure 39.3 An example of signal transduction in plants: the role of phytochrome in
the greening response (Layer 3)
6
Figure 39.4 Early experiments of phototropism
7
Figure 39.5 The Went experiments
Obtain chemical
from the tip and
store it in an agar
block
Block, when
placed on one
side, will cause
curvature even
when placed in
the dark.
Block substitutes
for the tip.
8
Table 39.1 An Overview of Plant Hormones
9
Polar Auxin Transport
1. Auxin is normally (-) charged.
2. Picks up a H+ and then becomes neutral and can pass
through the cell membrane.
3. Within the cell the auxin now ionizes to become A-.
4. Auxin can exit the cell at one specific end where there
are carrier proteins
10
Figure 39.6 Polar auxin transport: a chemiosmotic model (Layer 3)
11
Figure 39.7 Cell elongation in response to auxin: the acid growth hypothesis
Auxin can cause H+ to be pumped into the cell wall,
activating expansins, enzymes that break H bonds of
cellulose microfibrils
12
Cytokinins
1. Modified form of adenine (nucleic acid)
2. Plants have cytokinin receptor. One may be at cell
membrane and the other within the cytoplasm
3. Act by opening Ca2+ channels.
13
Effects of Cytokinins
1. Produced in roots and will move up the root in xylem.
2. Acting with auxin they will influence:
a) cell division
b) differentiation
3. Appears that the ratio of auxin / cytokinin is important
in just what the exposed cells will do.
4. Apical Dominance: Auxin travels down and suppresses
lateral bud growth and the shoot lengthens but no
branching. Cytokinins signal the axillary buds to
develop.
14
Figure 39.8 Apical dominance: with apical bud (left), apical bud removed (right)
Axillary buds are inhibited
Cytokinins stimulate axillary bud growth
15
Figure 39.10 Treating pea dwarfism with a growth hormone
Effect of Gibberellins:
increase in stem
elongation in dwarf
plants; little response in
normal plants
Little effect on root
growth
16
Figure 39.11 The effect of gibberellin treatment on seedless grapes
Thompson
seedless
grapes:
makes
grapes
grow
larger.
17
Abscisic Acid
1. Role in seed dormancy
a) High levels of ABA inhibit germination as the seed
matures.
b) High levels also cause production of proteins that help
seed withstand the dehydration conditions of the seed.
c) When ABA levels decrease, germination occurs. Levels
can decrease by rain, light inactivation or cold
inactivation.
2. Drought Stress
a) ABA ensures drought survival
b) ABA will cause stomata to close rapidly as wilting
18
begins. Huge exodus of potassium from the guard cells.
Ethylene Production
1. Plants produce ethylene in response to various stressors:
a) drought
b) flooding
c) mechanical pressure (next slide)
d) injury and infection
2. Ripening of fruit
19
Figure 39.13 Ethylene induces the triple response in pea seedlings
Ethylene exposure will cause stems to
elongate less rapidly, thicken and grow
horizontally and this occurs when a seedling
encounters objects as it tries to germinate
and sprout.
20
Leaf Abscission
1. Leaf loss occurs in the fall to prevent desiccation. The
roots cannot absorb water from frozen ground.
2. Abscission layer: base of petiole
3. Enzymes degrade polysaccharides in cell walls
4. All of this is controlled by auxin and ethylene (not
abscisic acid)
5. Apoptosis: programmed cell death
21
Figure 39.17 Action spectrum for blue-light-stimulated phototropism
Responsible for:
1) phototropisms
2) stomatal opening
3) hypocotyl
After 90 minutes
22
Figure 39.18 Phytochrome regulation of lettuce seed germination
Phytochromes: another photoreceptor
Involved in seed germination
There are two forms: Pr and Pfr
Red light stimulates germination; Far red light inhibits it.
Last flash of light controls the result.
23
Figure 39.19 Structure of a phytochrome
One of two domains of the
protein
Second of two domains of the
protein
This is the linking of light24
to a chemical response.
Figure 39.20 Phytochrome: a molecular switching mechanism
25
Phytochromes and Shade Avoidance
• Phytochromes also provide the plant with info about the “quality”
of the received light. . . That is, the light’s wavelength.
• Eventually the Pr and Pfr reach a dynamic equilibrium.
• For a tree that requires lots of light and it is shaded, its level of Pr
is high because the canopy is absorbing the red wavelengths of
light for PS.
• The ratio of Pr to Pfr changes and this induces the plant to use
more of its energy to grow taller.
• Direct sunlight increases Pfr levels which stimulates branching
while inhibiting vertical growth.
26
Figure 39.21 Sleep movements of a bean plant
27
Figure 39.x1 Biological clocks
28
Figure 39.22 Photoperiodic control of flowering
29
Figure 39.23 Reversible effects of red and far-red light on photoperiodic response
30
Figure 39.24 Experimental evidence for a flowering hormone(s)
31
Phytochrome is a molecular switch
 Switch for: Seed germination, stomatal opening and flowering
 Phytochrome indicates if light is present
 It is synthesized in the Pr form
 And then with light Pr  Pfr and the appearance of Pfr is used to
detect or indicate the presence of light.
 Pfr triggers seed germination by activating the genes for alpha amylase
production to digest the endosperm of seeds.
32
 Overview:
Pr
equilibrium
At night Pfr  Pr so Pr increases in concentration and Pfr is degraded.
In the daytime Pr  Pfr and this marks the end of the dark period.
33
Phytochrome’s Role in Measuring Darkness
 Phytochrome is the pigment thought to measure the length of night.
 We know that red light at a wavelength of 660 nm interrupts
darkness. That is, red light shortens the night.
 Therefore, phytochrome must be sensitive to this wavelength.
 A long night plant fails to flower with exposed to 660 nm (it
“breaks up” the long night)
 A short night plant will flower if a “long night” is interrupted by
660 nm.
34
Phytochrome’s Role in Measuring Darkness
 A flask of 730 nm or far red cancels the effect of 660 nm.
 So we “see” this pigment existing in two forms:
Pr
Pfr
Sensitive
sensitive
to 660 nm
to 730 nm
35
Phytochrome’s Role in Measuring Darkness
Examples
“Mums” or SD/LN Plants:
LN is interrupted by 660 nm  no flowering (Pr  Pfr)
LN gets 660, then 730 nm  flowering because Pfr  Pr
So the plant detected “no dark interruption.”
36
Phytochrome’s Role in Measuring Darkness
SD / LN Plant
 Expose to 730 nm and this causes Pfr  Pr
This maintains a long night situation /environment so flowering
occurs.
 Now expose to 660 nm and Pr  Pfr so this is the same as
“shortening the night” so no flowering will occur.
 So it is the last exposure that controls the plants actions.
37
Figure 39.25 The statolith hypothesis for root gravitropism
Statoliths are plastids
containing starch granules
38
Plant Responses to Environmental Stimuli
1. Responses to Gravity (Gravitropism)
a) Place a seedling on its side and:
(i) shoot grows upward (- gravitropism)
(ii) root grows downward (+ gravitropism)
b) Statoliths
(i) plastids containing starch that settle to lower
portions of cells
(ii) this triggers redistribution of auxin
(iii) auxin accumulates on lower of shoot and stimulate
cell elongation while at the root portion it inhibits
growth and causes the upper portion to grow
39
downward.
Figure 39.26 Altering gene expression by touch in Arabidopsis
Thigmomorphogenesis:
touching of stem in a young
plant will cause the stem to
DECREASE in length.
40
Plant Responses to Envir. Stimuli (cont’d)
2. Thigmomorphogenesis: changes due to touch or pressure
(mechanical stress)
a) Wind sensed by one side of a tree will cause the trunk to
grow thicker
b) Rubbing stems of young plants produces shorter plants
than controls
3. Thigmotropism: directional growth in response to touch
a) vines (ivy) have tendrils that will grow towards
something once they touch it. This produces a coiling
response.
41
Figure 39.27 Rapid turgor movements by the sensitive plant (Mimosa pudica)
42
Plant Responses to Envir. Stimuli (cont’d)
4. Mimosa plant and wind / touch response.
a) collapses and leaflets fold together to prevent water loss,
possibly be less conspicuous to herbivores.
b) due to loss of turgor in specialized “motor” organs at
the joints of the leaf.
(i) cells will lose K+ ions, H2O flows out.
c) an electrical signal called an action potential can be
detected that passes the signal through the leaf.
5. Venus fly-trap and the closing of its leaflets
a) Action potentials are transmitted from sensory hairs in
the trap to closing mechanism.
43
Plant Responses to Envir. Stimuli (cont’d)
6. Drought Stress: initial sense is too much water lost by
transpiration and not enough can be taken up by root
hairs.
a) Guard cells close
b) Increase synthesis of abscisic acid which maintains
closure of stomata
c) Young leaf growth is inhibited (this decreases surface
area for transpiration)
d) Roots will grow deeper rather than stay shallow
44
Plant Responses to Envir. Stimuli (cont’d)
7. Flooding
a) Water Logged soils
(i) lack of oxygen causes ethylene production which
induces apoptosis
b) Adaptations: some plants (mangroves) have aerial
roots that are continuous with submerged roots
Some plants, corn, will develop air tubes.
45
Figure 39.28 A developmental response of corn roots to flooding and oxygen
deprivation
46
Plant Responses to Envir. Stimuli (cont’d)
8. Salt Stress
a) some plants will produce solutes that will counteract the
external water potential decrease
b) this keeps the internal part of the plant more negative
than outside and allows water to continue to flow into the
plant.
c) halophytes have salt secreting glands or pumps
47
Plant Responses to Envir. Stimuli (cont’d)
9. Heat Stress
a) Transpiration and evaporative cooling cannot do it all
b) Heat Shock Proteins
10. Cold Stress
a) Cell membrane fluidity decreases
(i) solute transport through membrane is affected
b) Lipid composition is changed as it gets colder
(i) increase in unsaturated fatty acids which prevents
the packing of fats as temperature drops.
c) Solutes and the lowering of freezing point (fpd)
d) Some plants accumulate specific solutes to prevent
freezing
48
Plant Defense
1. To herbivores:
a) thorns (physical defense)
b) chemicals (canavanine)
(i) Canavanine has a structure similar to the amino acid
arginine and therefore canavanine, produced by the
plant and ingested by an insect, is incorporated into the
insect’s proteins. This alters the shape of the protein.
You’re dead.
c) Recruitment of organisms against herbivores
(i) leaf eaten by certain caterpillars will release volatile
chemicals that attract a specific kind of wasp (parasitoid
wasp)
49
Plant defense cont’d
(ii) Wasps then lay their eggs in the caterpillars
(iii) Eggs hatch within caterpillars and eat their way
out of the caterpillars and the plant benefits.
50
Figure 39.29 A corn leaf recruits a parasitoid wasp as a defensive response to an
herbivore, an army-worm caterpillar
51
Plant defense cont’d
2. To Bacteria
a) Phytoalexins: antimicrobial agents that are released
upon cell wall damage by the pathogen
b) PR (pathogenesis related) proteins:
(i) attack the cell wall in the invading bacteria
(ii) lets neighboring cells of an invading pathogen
and other cells lignify their cell wall to barricade the
pathogen.
c) Salicylic acid: this is a system-wide, nonspecific
activating or warning agent for several days
52
Figure 39.30 Gene-for-gene resistance of plants to pathogens
53
Figure 39.31 Defense responses against an avirulent pathogen
54