Transcript Plant Response to Signals

Plant
Responses to
Internal and
External
Signals
Chapter 31
Following the Big Ideas
 Energy
and Homeostasis- Growth and
timing responses are essential to plant
energy acquisition and survival.
 Information and Signaling- Plant
hormones typically work by affecting
gene expression in some manner.
Plant Response
 Stimuli

& a Stationary Life
animals respond to stimuli by changing
behavior
 move
toward positive stimuli
 move away from negative stimuli

plants respond to stimuli by adjusting
growth & development
Plant hormones
 Chemical
signals that coordinate different
parts of an organism





only tiny amounts are required
produced by 1 part of body
transported to another part
binds to specific receptor
triggers response in target cells & tissues
The Discovery of Plant Hormones
Any response resulting in curvature of organs
toward or away from a stimulus is called a
tropism
 In the late 1800s, Charles Darwin and his son
Francis conducted experiments on
phototropism, a plant’s response to light
 They observed that a grass seedling could bend
toward light only if the tip of the coleoptile was
present
 They postulated that a signal was transmitted
from the tip to the elongating region

Figure 31.2
Results
Shaded
side
Control
Inquiry: What part of a grass
coleoptile senses light, and
how is the signal transmitted?
Light
Boysen-Jensen
Illuminated
side
Light
Darwin and Darwin
Gelatin
(permeable)
Light
Tip
removed
Opaque
cap
Transparent
cap
Opaque
shield
over
curvature
Mica
(impermeable)
In 1913, Peter
Boysen-Jensen
demonstrated
that the signal
was a mobile
chemical
substance
 In 1926, Frits
Went extracted
the chemical
messenger for
phototropism,
auxin, by
modifying
earlier
experiments

2005-2006
Plant hormones
 auxins
 cytokinins
 gibberellins
 brassicosteroids
 abscisic
acid
 ethylene
YouTube: Bozeman Science
Plant Control
Table 31.1
Response to light:
Positive Phototropism
 Physiological
event that involves interactions between
the environment and internal molecular signals
 Growth towards light
 Hormone: Auxin


asymmetrical distribution of auxin
cells on darker side elongate faster
than cells on brighter side
Auxin- in detail!
The term auxin refers to any
chemical that promotes
elongation of coleoptiles
 Indoleacetic acid (IAA) is a
common auxin in plants; in
this lecture the term auxin
refers specifically to IAA
 Auxin is produced in shoot
tips and is transported down
the stem
 Auxin transporter proteins
move the hormone from the
basal end of one cell into
the apical end of the
neighboring cell

Figure 31.4
Results
Cell 1
100 m
Cell 2
Epidermis
Cortex
Phloem
Xylem
Pith
25 m
Basal end
of cell
Response to light:
Phototropism
 Changes
in the
light source
lead to
differential
growth, resulting
in maximum
exposure of
leaves to light
for
photosynthesis.
IAA is a naturally occurring
auxin
 The
role of auxin in cell elongation: Polar
transport of auxin stimulates proton pumps
in the plasma membrane
 According to the acid growth hypothesis,
the proton pumps lower the pH in the cell
wall, activating expansins, enzymes that
loosen the wall’s fabric
 With the cellulose loosened, the cell can
elongate
Figure 39.8
Cross-linking
polysaccharides
Cell wall–loosening
enzymes
Expansin
CELL WALL
Cellulose
microfibril
H2O
H
Plasma
membrane
H
H
H
ATP
H
H
H
Cell wall
H
H
Plasma membrane
CYTOPLASM
Nucleus Cytoplasm
Vacuole
2005-2006
Apical dominance
 Auxin
promotes apical
dominance
 axillary
buds do no grow while
apical bud exerts control
shoot
root
Auxin also alters gene expression and stimulates a
sustained growth response
 Auxin’s role in plant development: Polar transport of
auxin controls the spatial organization of the
developing plant




Reduced auxin flow from the shoot of a branch
stimulates growth in lower branches
Auxin transport plays a role in phyllotaxy, the
arrangement of leaves on the stem
Practical Uses of Auxin


Tomato growers spray their plants with synthetic auxins to
stimulate fruit growth
An overdose of synthetic auxins can kill plants
 For example 2,4-D is used as an herbicide on eudicots
Cytokinins
 Cytokinins
are so named because they
stimulate cytokinesis (cell division)
 Control of cell division and differentiation:
Cytokinins work together with auxin to
control cell division and differentiation
 Cytokinins are produced in actively
growing tissues such as roots, embryos,
and fruits
Gibberellins
 Family


of hormones
over 100 different gibberellins identified
Work in concert with auxins to promote cell growth
 Effects



stem and leaf elongation
fruit growth
seed germination- After water is
imbibed, release of gibberellins
from the embryo signals seeds
to germinate
plump grapes in grocery
stores have been treated
with gibberellin hormones
while on the vine
Brassinosteroids
 Brassinosteroids
are chemically similar to
cholesterol and the sex hormones of animals
 They induce cell elongation and division in
stem segments and seedlings
 They slow leaf abscission (leaf drop) and
promote xylem differentiation
Abscisic acid (ABA)
 Effects



slows growth
counteracts the breaking of dormancy during a
winter thaw
seed dormancy
 high

concentrations of Abscisic acid
germination only after ABA is inactivated down or
leeched out
 survival
value:
seed will germinate only
under optimal conditions


light, temperature, moisture
drought tolerance
 rapid
stomate closing
Ethylene
 Ethylene
is a hormone gas released by plant cells
 Multiple effects

response to mechanical stress
 triple




response
slow stem elongation
thickening of stem
curvature to stem growth
leaf drop (like in Fall)
 Facilitates

apoptosis
Promotes fruit ripening
Apoptosis & Leaf drop
 Ethylene


Senescence: Senescence is the
programmed death of cells or
organs
many events in plants involve
apoptosis (programmed
destruction of cells, organs, or
whole plants)
death of annual plant after flowering
 differentiation of xylem vessels



loss of cytosol
shedding of autumn leaves- Leaf
abscission: A change in the balance
of auxin and ethylene controls leaf
abscission, the process that occurs in
autumn when a leaf falls
What is the
evolutionary
advantage
of loss of
leaves in
autumn?
Fruit ripening
 Adaptation
 hard,
tart fruit protects
developing seed from herbivores
 ripe, sweet, soft fruit attracts
animals to disperse seed
 Ethylene
 triggers
ripening process
 breakdown

softening
 conversion

of cell wall
of starch to sugar
sweetening
 positive
feedback system
 ethylene
triggers ripening
 ripening stimulates more ethylene production
2005-2006
Applications
 Truth

in folk wisdom…..
one bad apple spoils the whole bunch
 ripening
apple releases ethylene to speed ripening of
fruit nearby
 Ripen
green bananas by bagging them with an
apple
 Climate control storage of apples

high CO2 storage = reduces ethylene production
Responses to light and other cues are
critical for plant success

Light triggers many key events in plant growth
and development, collectively known as
photomorphogenesis



A potato left growing in darkness produces shoots
that look unhealthy, and it lacks elongated roots
These are morphological adaptations for growing in
darkness, collectively called etiolation
After exposure to light, a potato undergoes changes
called de-etiolation, in which shoots and roots grow
normally
Figure 31.11
(a) Before exposure to light
(b) After a week’s exposure
to natural daylight
Light-induced de-etiolation (greening) of dark-grown potatoes
Plants detect not only presence of light but also its
direction, intensity, and wavelength (color)
 Different plant responses can be mediated by the
same or different photoreceptors
 There are two major classes of light receptors: bluelight photoreceptors and phytochromes,
photoreceptors that absorb mostly red light
 Various blue-light photoreceptors control
phototropism (movement in response to light),
stomatal opening, and hypocotyl elongation
 Phytochrome Photoreceptors respond to
phytochromes which are pigments that regulate
many of a plants responses to light

 Phytochromes
exist in two photoreversible
states, with conversion of Pr to Pfr triggering
many developmental responses
 Red light at sunrise triggers the conversion
of Pr to Pfr
 Far-red light at sunset triggers the
conversion of Pfr to Pr
 The conversion to Pfr is faster than the
conversion
to Pr
 Sunlight increases the ratio of Pfr to Pr and
triggers germination
Figure 31.14
Red light
Synthesis
Pr
Pfr
Far-red
light
Slow conversion
in darkness
(some species)
Responses to Pfr:
• Seed germination
• Inhibition of vertical
growth and stimulation of branching
• Setting internal clocks
• Control of flowering
Enzymatic
destruction
 Phytochromes
and shade avoidance: The
phytochrome system also provides the plant
with information about the quality of light
 Leaves in the canopy absorb red light
 Shaded plants receive more far-red than red
light
 In the “shade avoidance” response, the
phytochrome ratio shifts in favor of Pr when a
tree is shaded
 This shift induces the vertical growth of the
plant
Circadian rhythms
 Circadian
rhythms are cycles that are about 24
hours long and are governed by an internal
“clock”
 Circadian rhythms can be entrained to exactly
24 hours by the day/night cycle
 The clock may depend on synthesis of a protein
regulated through feedback control
 Phytochrome conversion marks sunrise and
sunset, providing the biological clock with
environmental cues

The conversion from one form to the other allows
the plant to keep track of time!
Circadian rhythms
 Internal
(endogenous) 24-hour cycles
4 O’clock
Morning glory
Noon
Midnight
Photoperiodism



Photoperiod, the relative lengths of night and day,
is the environmental stimulus plants use most often
to detect the time of year
Photoperiodism is a physiological response to
photoperiod.
Critical night length: In the 1940s, researchers
discovered that flowering and other responses to
photoperiod are actually controlled by night
length, not day length



Long day plants- flower only when the light period is
longer than a certain number of hours (short nights)
Short day plants - flower only when the days are
shorter and the nights longer
Day neutral plants- don’t care one way or the other!
Flowering Response- Photoperiodism


Physiological event that involves the interaction between
environmental stimuli and internal molecular signals
Triggered by photoperiod- relative lengths of day & night

night length—“critical period”— is trigger
Plant is
sensitive to
red light
exposure
Florigen is a
hypothetical
hormone
that
promotes
flowering
Helps
plants
prepare
for winter
What is the
evolutionary
advantage of
photoperiodism?
Short-day plants
Long-day plants
Synchronizes
plant responses
to season
2005-2006
Response to gravity

How does a sprouting shoot “know” to grow towards
the surface from underground?

environmental
cues?



roots = positive
gravitropism
shoots = negative
gravitropism
settling of statoliths
(dense starch
grains) may
detect gravity
2005-2006
Response to touchThigmotropism
 Thigmotropism
Mimosa (Sensitive plant)
closes leaves in response to
touch
Caused by changes in
osmotic pressure =
rapid loss of K+ =
rapid loss of H2O =
loss of turgor in cells
Environmental Stresses
Environmental stresses have a potentially adverse
effect on survival, growth, and reproduction
 Stresses can be abiotic (nonliving) or biotic (living)
 Abiotic stresses include drought, flooding, salt stress,
heat stress, and cold stress
 Biotic stresses include herbivores and pathogens

Abiotic Stresses
During drought, plants reduce transpiration by
closing stomata, reducing exposed surface area,
or even shedding their leaves
 During flooding enzymatic destruction of root
cortex cells creates air tubes that help plants
survive oxygen deprivation
 Salt can lower the water potential of the soil
solution and reduce water uptake


Plants respond to salt stress by producing solutes
tolerated at high concentrations

This process keeps the water potential of cells more
negative than that of the soil solution
Abiotic Stresses
 Excessive

heat can denature a plant’s enzymes
Heat-shock proteins, which help protect other
proteins from heat stress, are produced at high
temperatures
 Cold
temperatures decrease membrane
fluidity



Altering lipid composition of membranes is a
response to cold stress
Freezing causes ice to form in a plant’s cell walls
and intercellular spaces
Water leaves the cell in response to freezing,
leading to toxic solute concentrations in the
cytoplasm
Plant DefensesBiotic Stresses
2005-2006
 Defenses
against herbivores include
thorns, hairy leaves, toxins, bad odors
or tastes… others?
When herbivores
eat the leaves of a
plant that has
hairy leaves the
new leaves will
have a greater
density of hairs.
Plant defenses
 Defenses
against herbivores
Parasitoid wasp larvae
emerging from a
caterpillar
coevolution
Defenses Against Pathogens
A plant’s first line of defense against infection is the
barrier presented by the epidermis and periderm
 If a pathogen penetrates the dermal tissue, the
second line of defense is a chemical attack that kills
the pathogen and prevents its spread
 This second defense system is enhanced by the
plant’s ability to recognize certain pathogens
 Host-Pathogen Coevolution



A virulent pathogen is one that a plant has little
specific defense against
An a virulent pathogen is one that may harm but
does not kill the host plant
 Gene-for-gene
recognition involves
recognition of effector molecules by the
protein products of specific plant disease
resistance (R) genes
 An R protein recognizes a corresponding
molecule made by the pathogen’s Avr gene
 R proteins activate plant defenses by
triggering signal transduction pathways
 These defenses include the hypersensitive
response and systemic acquired resistance
The Hypersensitive Response
 The
hypersensitive response
 Causes localized cell and tissue death
near the infection site
 Induces production of phytoalexins and PR
proteins, which attack the specific
pathogen
 Stimulates changes in the cell wall that
confine the pathogen
Systemic Acquired Resistance
 Systemic
acquired resistance
 Causes plant-wide expression of defense
genes
 Protects against a diversity of pathogens
 Provides a long-lasting response
 Methylsalicylic acid travels from an
infection site to remote areas of the plant
where it is converted to salicylic acid,
which initiates pathogen resistance
Figure 31.24
Infected tobacco leaf with lesions
4
2
Signal
5
Hypersensitive
3
response
Signal transduction pathway
6
Signal
transduction
pathway
7
Acquired
resistance
1
R protein
Avirulent
pathogen
Avr effector protein
R-Avr recognition and
hypersensitive response
Systemic acquired
resistance
Connecting the Concepts
With the Big Ideas
 Energy



and Homeostasis-
Phototropism facilitates plant response to
light changes using molecular signals that
maximize photosynthetic surface area.
Photoperiodism involves phytochrome’s
control of flowering and seasonal changes.
Circadian rhythms, including stomata
openings, allow plants to adjust to
environmental conditions.
Connecting the Concepts
With the Big Ideas
 Information



and Signaling-
Cytokines trigger mitosis and cytokinesis by
regulating gene expression.
Increase in ethylene levels induce enzyme
production that promotes fruit ripening.
Gibberellins promote signal transmissions
that affect specific genes, triggering seed
germination.