Transcript video slide - Chicagoland Jewish High School

Plant Responses to
Internal and External
Signals
Overview: Stimuli and a
Stationary Life
 Plants, being rooted to the ground,
must respond to environmental
changes that come their way
 For example, the bending of a
seedling toward light begins with
sensing the direction, quantity,
and color of the light
Signal transduction pathways
link signal reception to
response
 Plants have cellular receptors that
detect changes in their
environment
 For a stimulus to elicit a response,
certain cells must have an
appropriate receptor
 A potato left growing in darkness
produces shoots that look
unhealthy and lacks elongated
roots
 These are morphological
adaptations for growing in
darkness, collectively called
etiolation
LE 39-2
Before exposure to light. A
dark-grown potato has tall,
spindly stems and
nonexpanded leaves—
morphological adaptations
that enable the shoots to
penetrate the soil. The roots
are short, but there is little
need for water absorption
because little water is lost by
the shoots.
After a week’s exposure to
natural daylight. The potato
plant begins to resemble a
typical plant with broad
green leaves, short sturdy
stems, and long roots. This
transformation begins with
the reception of light by a
specific pigment,
phytochrome.
De-Etioloation
(“Greening”) Proteins
 Many enzymes that function in
certain signal responses are
directly involved in photosynthesis
 Other enzymes are involved in
supplying chemical precursors for
chlorophyll production
Plant hormones help
coordinate growth,
development, and responses
to stimuli
 Hormones are chemical signals
that coordinate different parts of
an organism
The Discovery of Plant
Hormones
 Any response resulting in
curvature of organs toward or
away from a stimulus is called a
tropism
 Tropisms are often caused by
hormones
LE 39-5a
Shaded
side of
coleoptile
Control
Light
Illuminated
side of
coleoptile
LE 39-5b
Darwin and Darwin (1880)
Light
Tip
Tip
removed covered
by opaque
cap
Base covered
Tip
covered by opaque
by trans- shield
parent
cap
LE 39-5c
Boysen-Jensen (1913)
Light
Tip separated
Tip
separated by by mica
gelatin block
LE 39-6
Excised tip placed
on agar block
Growth-promoting
chemical diffuses
into agar block
Control
Control
(agar block
lacking
chemical)
has no
effect
Agar block
with chemical
stimulates growth
Offset blocks
cause curvature
A Survey of Plant
Hormones
 In general, hormones control plant growth
and development by affecting the division,
elongation, and differentiation of cells
 Plant hormones are produced in very low
concentration, but a minute amount can
greatly affect growth and development of a
plant organ
Auxin
 The term auxin refers to any
chemical that promotes cell
elongation in target tissues
 Auxin transporters move the
hormone from the basal end of
one cell into the apical end of the
neighboring cell
The Role of Auxin in Cell
Elongation
 According to the acid growth
hypothesis, 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
LE 39-8a
Cross-linking
cell wall
polysaccharides
Cell wall
enzymes
Expansin
CELL WALL
Microfibril
ATP
Plasma membrane
CYTOPLASM
Lateral and Adventitious
Root Formation
 Auxin is involved in root formation
and branching
 An overdose of auxins can kill
dicots
 Auxin affects secondary growth by
inducing cell division in the
vascular cambium and influencing
differentiation of secondary xylem
Cytokinins
 Cytokinins are so named because
they stimulate cytokinesis (cell
division)
 Cytokinins are produced in actively
growing tissues such as roots,
embryos, and fruits
 Cytokinins work together with
auxin
Control of Apical
Dominance
 Cytokinins, auxin, and other
factors 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
LE 39-9
Axillary buds
“Stump” after
removal of
apical bud
Lateral branches
Intact plant
Plant with apical bud removed
Anti-Aging Effects
 Cytokinins retard the aging of
some plant organs by inhibiting
protein breakdown, stimulating
RNA and protein synthesis, and
mobilizing nutrients from
surrounding tissues
Gibberellins
 Gibberellins have a variety of
effects, such as stem elongation,
fruit growth, and seed germination
 Gibberellins stimulate growth of
leaves and stems
 In stems, they stimulate cell
elongation and cell division
Fruit Growth
 In many plants, both auxin and
gibberellins must be present for
fruit to set
 Gibberellins are used in spraying of
Thompson seedless grapes
Germination
 After water is imbibed, release of
gibberellins from the embryo
signals seeds to germinate
LE 39-11
Aleurone
Endosperm
a-amylase
GA
Water
Scutellum
(cotyledon)
GA
Radicle
Sugar
Abscisic Acid
 Two of the many effects of abscisic
acid (ABA):
 Seed dormancy
 Drought tolerance
Seed Dormancy
 Seed dormancy ensures that the
seed will germinate only in optimal
conditions
 Precocious germination is observed
in maize mutants that lack a
transcription factor required for
ABA to induce expression of
certain genes
LE 39-12
Coleoptile
Ethylene
 Plants produce ethylene in
response to stresses such as
drought, flooding, mechanical
pressure, injury, and infection
The Triple 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
LE 39-14
ein mutant
ctr mutant
ein mutant. An ethylene-insensitive
(ein) mutant fails to undergo the triple
response in the presence of ethylene.
ctr mutant. A constitutive triple-response
(ctr) mutant undergoes the triple response
even in the absence of ethylene.
Apoptosis: Programmed Cell
Death
 A burst of ethylene is associated
with apoptosis, the programmed
destruction of cells, organs, or
whole plants
Leaf Abscission
 A change in the balance of auxin
and ethylene controls leaf
abscission, the process that occurs
in autumn when a leaf falls
Fruit Ripening
 A burst of ethylene production in a
fruit triggers the ripening process
Responses to light are critical
for plant success
 Light cues many key events in plant
growth and development
 Effects of light on plant morphology
are called photomorphogenesis
 Plants detect not only presence of light
but also its direction, intensity, and
wavelength (color)
Phototropic effectiveness relative to 436 nm
LE 39-17
1.0
0.8
0.6
0.4
0.2
0
400
450
500
550
600
Wavelength (nm)
Light
Time = 0 min.
Time = 90 min.
650
700
 There are two major classes of
light receptors: blue-light
photoreceptors and phytochromes
Blue-Light Photoreceptors
 Various blue-light photoreceptors
control hypocotyl elongation,
stomatal opening, and
phototropism
Phytochromes as
Photoreceptors
 Phytochromes regulate many of a
plant’s responses to light
throughout its life
 Studies of seed germination led to
the discovery of phytochromes
LE 39-18
Dark (control)
Red
Dark
Red Far-red Red
Red Far-red
Dark
Dark
Red Far-red Red Far-red
 The photoreceptor responsible for
the opposing effects of red and
far-red light is a phytochrome
 Phytochromes exist in two
photoreversible states, with
conversion of Pr to Pfr triggering
many developmental responses
LE 39-20
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 the “shade avoidance” response, the
phytochrome ratio shifts in favor of Pr when
a tree is shaded
Biological Clocks and
Circadian Rhythms
 Many plant processes oscillate
during the day
 Many legumes lower their leaves in
the evening and raise them in the
morning
LE 39-21
Noon
Midnight
 Cyclical responses to
environmental stimuli are called
circadian rhythms and are about
24 hours long
 Circadian rhythms can be
entrained to exactly 24 hours by
the day/night cycle
The Effect of Light on the
Biological Clock
 Phytochrome conversion marks
sunrise and sunset, providing the
biological clock with environmental
cues
Photoperiodism and
Responses to Seasons
 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
Photoperiodism and
Control of Flowering
 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
 In the 1940s, researchers discovered that
flowering and other responses to
photoperiod are actually controlled by night
length, not day length
LE 39-22
Darkness
Flash of
light
Critical
dark
period
Light
“Short-day” plants
“Long-day” plants
LE 39-23
24
20
R
FR
R
16
12
8
4
0
Short-day (long-night) plant
Long-day (short-night) plant
R
FR
R
FR
R
FR
R
A Flowering Hormone?
 The flowering signal, not yet
chemically identified, is called
florigen
 Florigen may be a hormone or a
change in relative concentrations
of multiple hormones
LE 39-24
Graft
Time
(several
weeks)
Gravity
 Response to gravity is known as
gravitropism
 Roots show positive gravitropism
 Stems show negative gravitropism
 Plants may detect gravity by the settling
of statoliths, specialized plastids
containing dense starch grainsVideo:
Gravitropism
Mechanical Stimuli
 The term thigmomorphogenesis refers
to changes in form that result from
mechanical perturbation
 Rubbing stems of young plants a couple
of times daily results in plants that are
shorter than controls
 Thigmotropism is growth in response to
touch
 It occurs in vines and other climbing
plants
 Rapid leaf movements in response to mechanical
stimulation are examples of transmission of
electrical impulses called action potentials
 NASTIC RESPONSE-DUE TO CHANGE IN TUGOR PRESSURE
LE 39-27
Unstimulated
Stimulated
Side of pulvinus with
flaccid cells
Leaflets
after
stimulation
Side of pulvinus with
turgid cells
Vein
Pulvinus
(motor
organ)
Motor organs
0.5 mm
Defenses Against
Pathogens
 A plant’s first line of defense against
infection is its “skin,” the epidermis or
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 inherited ability to recognize certain
pathogens
Gene-for-Gene
Recognition
 A virulent pathogen is one that a plant has
little specific defense against
 An avirulent pathogen is one that may harm
but not kill the host plant
 Gene-for-gene recognition involves
recognition of pathogen-derived molecules
by protein products of specific plant disease
resistance (R) genes
 A pathogen is avirulent if it has a
specific Avr gene corresponding to
an R allele in the host plant
LE 39-30a
Signal molecule (ligand)
from Avr gene product
Receptor coded by R allele
R
Avr allele
Avirulent pathogen
Plant cell is resistant
If an Avr allele in the pathogen corresponds to an R
allele in the host plant, the host plant will have
resistance, making the pathogen avirulent.
 If the plant host lacks the R gene
that counteracts the pathogen’s
Avr gene, then the pathogen can
invade and kill the plant
LE 39-30b
R
No Avr allele;
virulent pathogen
R allele; plant cell
becomes diseased
Avr allele
Avr allele;
virulent pathogen
No R allele; plant cell
becomes diseased
No Avr allele;
virulent pathogen
No R allele; plant cell
becomes diseased
If there is no gene-for-gene recognition because of
one of the above three conditions, the pathogen
will be virulent, causing disease to develop.
Plant Responses to
Pathogen Invasions
 A hypersensitive response against
an avirulent pathogen seals off the
infection and kills both pathogen
and host cells in the region of the
infection
LE 39-31
Signal
Hypersensitive
response
Signal transduction
pathway
Signal
transduction
pathway
Acquired
resistance
Avirulent
pathogen
R-Avr recognition and
hypersensitive response
Systemic acquired
resistance
Systemic Acquired
Resistance
 Systemic acquired resistance
(SAR) is a set of generalized
defense responses in organs
distant from the original site of
infection
 Salicylic acid is a good candidate
for one of the hormones that
activates SAR