Chapter 39
Plant Responses to Internal and
External Signals
Shawn Koshy
Peter Jandovitz
Jason Lee
Cody Pickel
Edwin Mathieu
Concept 39.1: Signal transduction
pathways link signal reception to
response
• All organisms receive specific environmental signals
and respond to them in ways tat enhance survival
and reproductive success.
• Responses caused by stimuli can nly happen through
certain receptors on cells.
• Etiolation: morphological plant adapttions for growig
in the darkness.
• De-etiolation: the changes a plant shoot undergoes in
response to sunlight; also known informally as
greening.
Reception
• Receptors, or proteins that undergo conformational
changes in response to a specific stimulus, are the
first to detect signals.
• Phytochrome: the receptor involved in de-etiolation
(photoreceptor).
Transduction
• Second messengers: small internally produced chemicals that
transfer and amplify the signal from the receptor to other
proteins that cause the response.
• For example, signal transduction in plants begin by the detection
of the light signal by the phytochrome receptor, which activates
at least 2 signal transduction pathways.
• One pathway uses cGMP as a second messenger that activates
a specific protein kinase.
• The next pathway causes an increase in cytoplasmic Ca2+
levels, ultimately activating another protein kinase.
• Lastly, both pathways will result in an expression of genes for
proteins that function in the de-etiolation response (greening).
Response
• A signal transduction pathway leads to regulation of 1 or more
cellular activities.
• There are 2 main mechanisms by which an enzyme can be
activated by a signal pathway:
– Transcription Regulation: stimulates transcription of mRNA for the
enzyme.
– Post-Translational Modification: activates existing enzyme
molecules.
Transcription Regulation
• Transcription factors bind directly to particular regions of DNA
and control the transcription of certain genes.
• The activation of positive or negative or both types of
transcription factors has an affect on the mechanism by which a
signal promotes a new developmental course.
Post-Translational Modification of
Proteins
• The post-translational modification of existing proteins is as
important as the syntheses of new proteins by transduction and
translation.
• Chains of phosphorylated protein kinases can result in signal
pathways ultimately regulating the synthesis of new proteins
(turning genes on and off).
• Protein phosphatases: enzymes that dephosphorylate specific
proteins (“switch-off” processes).
De-Etiolation (“Greening”) Proteins
• Enzymes that are involved in photosynthesis directly, and
enzymes involved in supplying the chemical precursors for
chlorophyll production, and many more also effect hormones
that regulate plant growth.
• These are either newly transcribed or activated by
phosphorylation during the de-etiolation process.
39.2: Plant hormones help coordinate growth,
development, and responses to stimuli
• Tropism- response that causes changes
in growth away from or toward stimuli
ex) phototropism- growth towards light
• Tropisms are caused by hormoneschemical signals in an organisms
Discovery of Plant Hormones
• In the late 19th century, Charles and Francis
Darwin discovered that a phototropic response
could only be triggered when light could reach
the tip of the coleoptile.
• Boysen-Jensen observed that a phototrophic
response was triggered by a light- activated
mobile chemical
• Later modified experiments by Frits Went led to
the discovery of auxin.
Survey of Plant Hormones
Hormone
Major Functions
Auxins
At lower concentrations, stimulates cell elongation by
increasing the activity of proton pumps; at higher
concentrations, it inhibits cell elongation. Lateral and
adventitious root formation and branching. Induces xylem
differentiation in developing plants. Promotes the growth of
fruits. Can be used as herbicides for broadleaf plants.
Cytokinins Produced in the roots and fruits, and spread throughout the
plant, working with auxin to stimulate cell division and
differentiation. Works against auxin in controlling apical
dominance.
Giberellin
s
Signals a young embryo to break dormancy and begin
germination. Stimulate growth of both leaves and stems,
especially in bolting, rapid growth of the floral stalk.
Stimulates cell elongation by inducing enzymes that
facilitate the expansins that loosen cell walls. Used
commercially to enhance development and growth of fruits.
Plant Hormones (cont’d)
Hormone
Major Functions
Brassinosteroi Induce cell elongation and division in stem segments and
d
seedlings at low concentrations. Retard leaf abscission
and promote xylem differentiation.
Abscisic Acid
Increases in levels to promote seed dormancy . Internal
signal that enables plants to withstand drought. Under
excessive drought, causes stomata to close rapidly,
reducing transpiration.
Ethylene
Produced in response to mechanical stresses such as
drought, flooding, mechanical pressure, and infection.
Instigates the triple response when seed growth reaches
an obstacle. Increased levels associated with apoptosis,
the programmed destruction of organs or tissues in the
plant. Leaf abscission controlled by a balance of auxin and
ethylene (higher ethylene levels promote leaf abscission).
Ethylene triggers fruit ripening, which in turn triggers more
ethylene through positive feedback.
Figure 1: Triple
Response Caused
by Ethylene
The growing shoot on
the left undergoes the
triple response, resulting
in a slowing of stem
elongation, thickening of
the stem, and a
curvature of the stem
that causes it to grow
horizontally. The growing
shoot on the right is
under control conditions,
and continues to grow
vertically.
Figure 2: Commercial
Use of Gibberellins for
Fruit Production
The picture on the right
shows how gibberellins
enhance the
development and growth
of fruits. The grapes on
the right were grown with
daily spraying of
gibberellins. Thompson
grapes are an example
of hoe gibberellins are
used in industry to
increase the size, taste
and overall worth of
fruits to the consumer.
System Biology and Hormone
Interactions
• Interactions between hormones and their
signal transduction pathways makes it difficult
to predict the effect of genetic engineering on
a plant.
• Systems biology strives for better, in-depth
knowledge of plants that will grant better view
of these interactions, making genetic
engineering more effective.
Concept 39.3: Responses to light are
critical for plant success
• Effects of light on plant morphology are what
plant biologists call photomorphogenesis
• Light causes many key events in plant growth
and development
• There are two major classes of light
receptors: blue-light photoreceptors and
phytochromes, which absorb mostly red light
EXPERIMENT Researchers exposed maize (Zea mays) coleoptiles to violet,
blue, green, yellow, orange, and red light to test which wavelengths
stimulate the phototropic bending toward light.
The graph below shows phototropic effectiveness (curvature
per photon) relative to
1.0
effectiveness of light with
a wavelength of 436 nm.
0.8
The photo collages show
coleoptiles before and after
0.6
90-minute exposure to side
lighting of the indicated colors.
0.4
Pronounced curvature occurred
0.2
only with wavelengths below
500 nm and was greatest with
0
blue light.
450 500
550
600 650
700
400
Phototropic effectiveness relative to 436 nm
RESULTS
Wavelength (nm)
Light
Time = 0 min.
Time = 90 min.
CONCLUSION The phototropic bending toward light is caused by a
photoreceptor that is sensitive to blue and violet light, particularly blue
light.
•
An action spectrum depicts the relative effectiveness of different wavelengths of radiation in
driving a particular process
Blue-Light Photoreceptors
• Blue-light receptors initiate diverse
responses in plants including:
• The light induced opening of stomata
• The light-induced slowing of hypocotyl
elongation that occurs when a seedling
breaks ground
• Phototropism
Phytochromes as Photoreceptors
• Phytochromes are responsible for many
of a plant’s responses to light
throughout its lifetime
• De-etiolation is regulated by
phytochromes
Phytochromes and Seed
Germination
• Phytochromes were discovered during
studies of seed germination
• In the 1930s, scientists at the U.S.
Department of Agriculture determined
the action spectrum for light-induced
germination of lettuce seeds
EXPERIMENT During the 1930s, USDA scientists briefly exposed batches of
lettuce seeds to red light or far-red light to test the effects on germination. After
the light exposure, the seeds were placed in the dark, and the results were
compared with control seeds that were not exposed to light.
The bar below each photo indicates the sequence of red-light
exposure, far-red light exposure, and darkness. The germination rate increased
greatly in groups of seeds that were last exposed
to red light (left). Germination was inhibited in groups of seeds that were last
exposed to far-red light (right).
RESULTS
Dark (control)
Red
Dark
Red Far-red Red
CONCLUSION
Red Far-red
Dark
Dark
Red Far-red Red Far-red
Red light stimulated germination, and far-red light inhibited
germination.
The final exposure was the determining factor. The effects of red and far-red
light were reversible.
• Figure 3: Phytochromes exist in two
photoreversible states - Pr and Pfr
Pr
Pfr
Red light
Responses:
seed germination,
control of
flowering, etc.
Synthesis
Far-red
light
Slow conversion
in darkness
(some plants)
Enzymatic
destruction
Phytochromes and Shade
Avoidance
• The phytochrome system also provides
the plant with information about the
quality of light
• In a shade avoidanc response of a tree,
the phytochrome ratio shifts in favor of
Pr
Biological Clocks and Circadian
Rhythms
• Many plant processes oscillate during
the day
• Ex: Transpiration
• Synthesis of certain enzymes
• Figure 4: Some plants lower leaves in the
evening and raise them in the morning
Noon
Midnight
• Cyclical responses with a frequency of
about 24 hours and not directly paced
by environmental variables are called
circadian rhythms
• approximately 24 hours long
• can be made to be exactly 24 hours by
the day/night cycle
The Effect of Light on the
Biological Clock
• Phytochrome conversion marks sunrise
and sunset
• This provides the biological clock with
environmental cues
Photoperiodism and Responses
to Seasons
• Photoperiod - the relative lengths of
night and day
• Many plants use the photoperiod to
detect the time of year
• Photoperiodism - a physiological
response to photoperiod
Photoperiodism and Control of
Flowering
• Some developmental processes require a
certain photoperiod
• Short-day plants flower in fall or winter due to
the shorter day lengths and longer nights
• Long-day plants flower in late spring or early
summer do to the long hours of daylight.
• Day-neutral plants are unaffected by
photoperiod and flower regardless of
daylength
Critical Night Length
•
In the 1940s, researchers discovered that responses to photoperiod are controlled by night
length, not day length
During the 1940s, researchers conducted experiments in which
periods of darkness were interrupted with brief exposure to light to test how the
light and dark portions of a photoperiod affected flowering in “short-day” and
“long-day” plants.
EXPERIMENT
RESULTS
Darkness
Flash of
light
Critical
dark
period
Light
(a) “Short-day” plants
flowered only if a period of
continuous darkness was
longer than a critical dark
period for that particular
species (13 hours in this
example). A period of
darkness can be ended by a
brief exposure to light.
(b) “Long-day” plants
flowered only if a
period of continuous
darkness was shorter
than a critical dark
period for that
particular species (13
hours in this example).
The experiments indicated that flowering of each species was determined by
a critical period of darkness (“critical night length”) for that species, not by a specific period of
light. Therefore, “short-day” plants are more properly called “long-night” plants, and “longday” plants are really “short-night” plants.
CONCLUSION
A Flowering Hormone?
• The flowering signal, not yet chemically
identified is called florigen
• It may be a ormone or change in
relative concentrations of multiple
hormones
Meristem Transition and
Flowering
• The outcome of the combination of
environmental cues and internal signals
is the transition of a bud’s meristem
from a vegetative to a flowering state
Section 39. 4: Plants
respond to a wide variety of
stimuli other than light
Gravity
•
•
•
•
•
Gravitropism is a response to gravity.
Gravitropism functions as soon as the seed germinates ensuring that
the root grows into the soil and the shoot reaches sunlight regardless of
how the seed happens to be oriented in the soil
Gravitropism may be either positive (toward) or negative (away from).
– In their responses to gravity, roots display positive gravitropism and
shoots exhibit negative gravitropism
The curvature that occurs in reaction to gravity is due to differences in
cell elongation on the opposite sides of a root or shoot.
The molecule called auxin promotes cell elongation in shoot and
inhibits it in roots.
Gravity (cont.)
•
•
•
•
Plants may detect gravity by the settling of statoliths, specialized
plastids containing dense starch grains, to the lower portions of cells
According to one hypothesis, the settling of statoliths in cells of the root
cap triggers movement of calcium, which causes the lateral transport of
auxin.
The calcium and auxin accumulate on the lower side of the growing
root, where the high concentration of auxin inhibits cell elongation,
causing the root to curve downward.
The settling of the protoplast and large organelles may distort the
cytoskeleton and also signal gravitation direction.
Figure #5: Positive gravitropism in roots: the statolith hypothesis
Mechanical Stimuli
•
Thigmomorphogenesis refers to the morphological changes in its
form that result from mechanical stress
– Plants are very sensitive to mechanical stress
•
Mechanical stimulation activates a signal transduction pathway that
increases the cytosolic Ca2+ , which in turn mediates the activation of
specific genes, some of which encode for proteins that affect cell wall
properties.
•
Rubbing the stems of a young plant a couple of times daily results in
plants that are shorter than controls (see Figure #2)
Figure #6:
Thigmomorphogenesis
Mechanical Stimuli (Cont.)
•
•
Thigmotropism is the directional growth as a response to contact with
a solid object
For example, when the compound leaf of the sensitive plant Mimosa
pudica is touched, it collapses and its leaflets fold together (see Figure
#3)
– This response is due to the rapid loss of turgor by cells in
specialized motor organs called pulvini, located at the joints
of the leaf
– These cells lose potassium when stimulated, resulting in
osmatic water loss.
•
The message travels through the plant from the point of stimulation,
perhaps as the result of electrical impulses, called Action potentials
Figure #7:
• Thigmotropism
Environmental Stresses
• Drought
• Water deficit in a leaf causes guard cells to lose turgor, a
simple control mechanism that slows transpiration by closing
stomata
• stimulates increased synthesis and release of abscisic acid in
the leaf, and this hormone helps keep stomata closed by acting
on guard cell membranes.
• inhibits the growth of young leaves, minimizing the
transpirational loss of water by slowing the increase in leaf
surface
• Inhibits the growth of shallow roots while deeper roots in moist
soil continue to grow
Environmental Stresses
(Cont.)
• Flood
• The air spaces of flooded soil lack the oxygen needed for the
cellular respiration of the roots
• Oxygen deprivation stimulates the production of the hormone
ethylene, which causes some of the cells in the root cortex to
undergo apoptosis (programmed cell death).
• Enzymatic destruction of cells creates air tubes that function as
“snorkels,” providing oxygen to the submerged roots
Environmental Stresses
(Cont.)
• Salt Stress
• Lowers the water potential of the soil solution below that of
roots, causing the roots to lose water
• sodium and certain other ions are toxic to plants when their
concentrations are relatively high
– The selectively permeable membranes of root cells prevent the
uptake of most harmful ions, but this only aggravates the problem
of acquiring water from hypertonic soil.
• Plants may respond to moderate soil salinity by producing
compatible solutes that lower the water potential of root cells.
• Halophytes- salt tolerant plants that have salt glands that
pump salts out across the leaf epidermis
Environmental Stresses (Cont.)
• Heat Stress
• Excessive heat can harm and eventually kill a plant by
denaturing its enzymes and damaging its metabolism in other
ways
• Transpiration creates evaporative cooling for a plant, but this
effect may be lost on hot, dry days when stomata close to
reduce water loss
• In high temperatures, plant cells produce heat-shock proteins
that may provide temporary support to reduce protein
denaturation.
Environmental Stresses
(Cont.)
• Cold Stress
• Plants respond to cold stress by increasing the proportion of
unsaturated fatty acids in membrane lipids in order to maintain
the fluidity of cell membranes.
• At subfreezing temperatures, ice forms in the cell walls and
intercellular spaces of most plants, lowering the extracellular
water potential and causing cells to dehydrate
• Plants adapted to cold winters have special adaptations that
enable them to cope with freezing stress, such as changing the
solute composition of the cytosol
39.5: Plants defend themselves against
herbivores and pathogens
• Plants do not exist in isolation but interact
with many species.
• While some of these interactions can be
beneficial, most are harmful and dangerous
to the plant.
• As a producer plants are the base of most
food webs and subject to attack by a wide
range of animals, as well as infection by
pathogenic viruses, bacteria, etc.
Defenses Against Herbivores
• Many plants have physical defenses, such as
thorns, and chemical defenses, such as toxic
compounds
• Some plants recruit predatory animals that
prey on specific herbivore by releasing
volatile chemicals which attract the predator.
• Volatile chemicals also serve as an alert for
nearby plants, which allow them to activate
genes for plant defense
ex) jasmonic acid
Defenses Against Pathogens
• Virulent – host plant has little defense against
pathogen.
• Avirulent- pathogen able to harm, but not kill,
host plant.
1) Gene-for-gene recognition- recognition of
pathogen derived molecules by the protein
products of specific disease resistant (R)
genes.
– R proteins recognize pathogen molecules encoded
from avirulence (Avr) genes, which play a role in the
infection of pathogen.
Defenses (cont’d)
2) Plant Responses
- Elicitors- induce broader type of host
defense; stimulate phytoalexins, antimicrobial
compounds
- PR proteins- spread signals to nearby cells,
as well as aid in attacking pathogens
- Cross linking of cell walls and release of
lignin, which produces a barricade to prevent
further infection
Hypersensitive Response (HR)- enhance
production of elicitors and PR proteins
Systemic Acquired Resistance
(SAR)
• Chemical signals sent throughout whole
plant, stimulating production of phytoalexins
and PR proteins; coupled with HR.
• Salicylic acid- main hormone attributed to
SAR