Plant Response to Internal and
External Signals
Chapter 39
Figure 39.3
CELL
WALL
1 Reception
Receptor
CYTOPLASM
2 Transduction
3 Response
Relay proteins and
Activation
of cellular
responses
second messengers
Hormone or
environmental
stimulus
Plasma membrane
An Example:
• 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
Reception
• Internal and external signals are detected by
receptors, proteins that change in response to
specific stimuli
• In de-etiolation, the receptor is a
phytochrome capable of detecting light
Figure 39.4-1
1
Reception
CYTOPLASM
Plasma
membrane
Phytochrome
activated
by light
Cell
wall
Light
Transduction
• Second messengers transfer and amplify signals
from receptors to proteins that cause responses
• Two types of second messengers play an
important role in de-etiolation: Ca2+ ions and
cyclic GMP (cGMP)
• The phytochrome receptor responds to light by
– Opening Ca2+ channels, which increases Ca2+ levels in
the cytosol
– Activating an enzyme that produces cGMP
Figure 39.4-2
1
Reception
Transduction
2
CYTOPLASM
Plasma
membrane
cGMP
Protein
kinase 1
Second
messenger
produced
Phytochrome
activated
by light
Cell
wall
Protein
kinase 2
Light
Ca2+ channel
opened
Ca2+
Response
• A signal transduction pathway leads to regulation of one or more
cellular activities
• In most cases, these responses to stimulation involve increased
activity of enzymes
• This can occur by
– 1. transcriptional regulation
• specific transcription factors bind directly to specific regions of DNA and control transcription of
genes
• some transcription factors are activators that increase the transcription of specific genes
• other transcription factors are repressors that decrease the transcription of specific genes
– 2. post-translational modification
•
•
•
•
involves modification of existing proteins in the signal response
modification often involves the phosphorylation of specific amino acids
the second messengers cGMP and Ca2+ activate protein kinases directly
protein phosphatases “switch off” the signal transduction pathways by dephosphorylating
proteins
Figure 39.4-3
1
Reception
3
Transduction
2
Transcription
factor 1
NUCLEUS
CYTOPLASM
Plasma
membrane
cGMP
Protein
kinase 1
Second
messenger
produced
Phytochrome
activated
by light
Response
P
Transcription
factor 2
P
Cell
wall
Protein
kinase 2
Transcription
Light
Translation
Ca2+ channel
opened
Ca2+
De-etiolation
(greening)
response
proteins
De-Etiolation (“Greening”) Proteins
• De-etiolation activates enzymes that
– Function in photosynthesis directly
– Supply the chemical precursors for chlorophyll
production
– Affect the levels of plant hormones that regulate
growth
Hormones help coordinate growth,
development, & responses to stimuli
• Plant hormones are chemical signals that modify
or control one or more specific physiological
processes within a plant
• Produced in very low concentrations, but can
have profound effects on growth and
development
– Each hormone has multiple effects, but multiple
hormones can influence a single process
• Plant responses depend on amount and
concentration of specific hormones and often on
the combination of hormones present
A Survey of Plant Hormones
• The major plant hormones include
– Auxin*
– Cytokinins*
– Gibberellins*
– Abscisic acid*
– Ethylene*
– Brassinosteroids
– Jasmonates
– Strigolactones
Table 39.1
Auxins: any chemical that promotes
elongation of coleoptiles
• Any response resulting in curvature of organs toward
or away from a stimulus is called a tropism
• Charles Darwin and his son Francis conducted
experiments on phototropism, a plant’s response to
light (late 1800s)
– 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
• In 1913, Peter Boysen-Jensen demonstrated that the
signal was a mobile chemical substance
Figure 39.5
Results
Shaded
side
Control
Light
Illuminated
side
Darwin and Darwin
Boysen-Jensen
Light
Light
Tip
removed
Opaque
cap
Transparent
cap
Opaque
shield
over
curvature
Gelatin
(permeable)
Mica
(impermeable)
Auxin II
• 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
• Auxin stimulates proton pumps in the plasma membrane
• 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 39.7
CELL WALL
4 Cell wall-loosening
enzymes cleave
cross-linking
polysaccharides.
3 Low pH
activates
expansins.
H 2O
H+
2
Acidity
increases.
Plasma
membrane
H+
Cell
wall
H+
H+
H+
H+
H+
H+
1 Proton
pump
activity
increases.
Nucleus
ATP
Plasma membrane
H+
CYTOPLASM
Cytoplasm
Vacuole
5
Sliding cellulose
microfibrils allow
cell to elongate.
Cytokinins
• Stimulate cytokinesis
• Cytokinins are produced in actively growing tissues such as
roots, embryos, and fruits
• Cytokinins work together with auxin to control cell division
and differentiation
• Cytokinins, auxin, and strigolactone interact in the control
of apical dominance, a terminal bud’s ability to suppress
development of axillary buds
– If the terminal bud is removed, plants become bushier
• Cytokinins slow the aging of some plant organs by inhibiting
protein breakdown, stimulating RNA and protein synthesis,
and mobilizing nutrients from surrounding tissues
Figure 39.8
Lateral branches
“Stump” after
removal of
apical bud
(b) Apical bud removed
Axillary buds
(a) Apical bud intact (not shown in photo)
(c) Auxin added to decapitated stem
Gibberellins: Stem Elongation
• Gibberellins have a
variety of effects
• Stem elongation
– Produced in young roots
and leaves
– Stimulate growth of
leaves and stems
• In stems, they stimulate
cell elongation and cell
division
Gibberellins: Fruit Growth
• Fruit Growth
– In many plants, both
auxin and gibberellins
must be present for fruit
to develop
– Used in spraying of
Thompson seedless
grapes
Gibberellins: Seed Germination
• Seed Germination
– After water is imbibed, release of gibberellins from the
embryo signals seeds to germinate
Abscisic Acid
• Abscisic acid (ABA) slows growth
• Seed dormancy
– Ensures that the seed will
germinate only in optimal
conditions
– Dormancy is broken when ABA is
removed by heavy rain, light, or
prolonged cold
– Precocious (early) germination can
be caused by inactive or low levels
of ABA
• Drought tolerance
– Primary internal signal that enables
plants to withstand drought
– Accumulation causes stomata to
close rapidly
Ethylene
• Plants produce ethylene in response to
stresses such as drought, flooding, mechanical
pressure, injury, and infection
• The effects of ethylene include response to
mechanical stress, senescence, leaf abscission,
and fruit ripening
Ethylene Response to Mechanical
Stress
• Ethylene induces the
triple response, which
allows a growing shoot
to avoid obstacles
– The triple response
consists of a slowing of
stem elongation, a
thickening of the stem,
and horizontal growth
Ethylene: Senescence & Leaf
Abscission
• Senescence is the
programmed death of cells
or organs
• A burst of ethylene is
associated with apoptosis,
the programmed
destruction of cells, organs,
or whole plants
• A change in the balance of
auxin and ethylene controls
leaf abscission, the process
that occurs in autumn when
a leaf falls
Ethylene: Fruit Ripening
• A burst of ethylene production in a fruit
triggers the ripening process
• Ethylene triggers ripening, and ripening
triggers release of more ethylene
• Fruit producers can control ripening by picking
green fruit and controlling ethylene levels
Response to Light
• Light cues many key events in plant growth and development
• Effects of light on plant morphology are called
photomorphogenesis
• Plants detect the presence of light, its direction, intensity, and
wavelength (color)
– An action spectrum depicts relative response of a process to different
wavelengths
– Action spectra are useful in studying any process that depends on light
• Different plant responses can be mediated by the same or different
photoreceptors
– Blue-light photoreceptors
•
control hypocotyl elongation, stomatal opening, and phototropism
– Phytochromes
• Pigments that regulate many of a plant’s responses to light throughout its life
• These responses include seed germination and shade avoidance
Phytochromes and Seed Germination
• Many seeds remain dormant until light and other
conditions are near optimal
• Red light increased germination, while far-red
light inhibited germination
• The effects of red and far-red light are reversible;
the final light exposure determines the response
• The photoreceptors responsible for the opposing
effects of red and far-red light are phytochromes
Figure 39.16
Results
Red
Dark
Red
Far-red
Dark
Dark (control)
Red
Far-red
Red
Dark
Red
Far-red
Red
Far-red
How Does It Work?
• Phytochromes exist in two photoreversible
states, with conversion of Pr to Pfr triggering
many developmental responses
• Red light triggers the conversion of Pr to Pfr
• Far-red light triggers the conversion of Pfr to Pr
• The conversion of Pr to Pfr is faster than the
reverse process
• Sunlight, containing both red and far-red light,
increases the ratio of Pfr to Pr and triggers
germination
Figure 39.17
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 vertical growth
Biological Clocks and Circadian Rythms
• Many plant processes oscillate during the day in
response to light and temperature changes
• Circadian rhythms are cycles that are about 24 hours
long and are governed by an internal “clock”
– Can be set to exactly 24 hours by the day/night cycle
– May depend on synthesis of a protein regulated through
feedback control and may be common to all eukaryotes
• Phytochrome conversion marks sunrise and sunset,
providing the biological clock with environmental
cues
Photoperiodism and Flowering
• 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
• Some processes, including flowering in many species,
require a certain photoperiod
– Plants that flower when a light period is shorter than a
critical length are called short-day plants
– Plants that flower when a light period is longer than a
certain number of hours are called long-day plants
– Flowering in day-neutral plants is controlled by plant
maturity, not 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
• Short-day plants are
governed by whether the
critical night length sets a
minimum number of hours
of darkness
• Long-day plants are
governed by whether the
critical night length sets a
maximum number of hours
of darkness
Effects of Red Light
• Red light can interrupt the
nighttime portion of the
photoperiod
– A flash of red light followed
by a flash of far-red light
does not disrupt night
length
• Action spectra and
photoreversibility
experiments show that
phytochrome is the pigment
that detects the red light
– Some plants flower after
only a single exposure to
the required photoperiod
– Other plants need several
successive days of the
required photoperiod
Responses to Stimuli Other than Light
• Because of immobility, plants must adjust to a
range of environmental circumstances through
developmental and physiological mechanisms
• Response to gravity is known as gravitropism
– Roots show positive gravitropism; shoots show
negative gravitropism
– Plants may detect gravity by the settling of statoliths,
dense cytoplasmic components
• Thigmotropism is growth in response to touch
– It occurs in vines and other climbing plants
Response to Abiotic Stresses
• During drought, plants reduce transpiration by closing stomata, reducing
exposed surface area, and in some species, shedding leaves
• Enzymatic destruction of root cortex cells creates air tubes that help
plants survive oxygen deprivation during flooding
• Salt can lower the water potential of the soil solution and reduce water
uptake
– Produce solutes tolerated at high concentrations
– Keeps the water potential of cells more negative than that of the soil solution
• 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
• Many plants have antifreeze proteins that prevent ice crystals from growing and
damaging cells
• Excessive heat can denature a plant’s enzymes
– Transpiration helps cool leaves by evaporative cooling
– Heat-shock proteins help protect other proteins from heat stress
Figure 39.UN05
Environmental Stress
Major Response
Drought
ABA production, reducing water loss by
closing stomata
Flooding
Formation of air tubes that help roots survive
oxygen deprivation
Salt
Avoiding osmotic water loss by producing
solutes tolerated at high concentrations
Heat
Synthesis of heat-shock proteins, which reduce
protein denaturation at high temperatures
Cold
Adjusting membrane fluidity; avoiding
osmotic water loss; producing antifreeze
proteins
Figure 39.25
Vascular
cylinder
Air tubes
Epidermis
(a) Control root
(aerated)
100 µm
(b) Experimental root
(nonaerated)
100 µm
Defense Against Pathogens
• A plant’s first line of defense against infection is the barrier presented by
the epidermis and periderm
• Pathogens can enter through wounds or natural openings, such as stomata
• The first line of immune defense depends on the plant’s ability to
recognize pathogen-associated molecular patterns (PAMPs)
– These molecular sequences are specific to certain pathogens
• PAMP recognition starts a chain of signaling events leading to the
production of antimicrobial chemicals and toughening of the cell wall
– Pathogens that have evolved the ability to deliver effectors into plant cells &
can suppress PAMP-triggered plant immunity
• Effectors are pathogen-encoded proteins that cripple the host’s innate immune system
• A second level of plant immune defense evolved in response to these
pathogens
The Hypersensitivity Response
• The hypersensitive response
– Causes cell and tissue death near the infection site
– Induces production of enzymes that attack the
pathogen
– Stimulates changes in the cell wall that confine
the pathogen
Systemic Acquired Resistance
• Systemic acquired resistance causes systemic
expression of defense genes and is a longlasting response
• Methylsalicylic acid is synthesized around the
infection site and carried in the phloem to
other remote sites where it is converted to
salicylic acid
• Salicylic acid triggers the defense system to
respond rapidly to another infection
Figure 39.26a
4
3
Signal
5
Hypersensitive
response
2 Signal transduction pathway
Signal
transduction
pathway
7
Acquired
resistance
1
R protein
Pathogen
Effector protein
Hypersensitive
response
Systemic acquired
resistance
6