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Ch. 39 Plant Responses to
Internal and External Signals
Objectives:
LO 2.29 The student can create representations and models to describe immune responses.
LO 2.30 The student can create representations or models to describe nonspecific immune
defenses in plants and animals.
LO 2.35 TSIAT design a plan for collecting data to support the scientific claim that the timing
and coordination of physiological events involve regulation.
LO 2.36 TSIAT justify scientific claims with evidence to show how timing and coordination of
physiological events involve regulation.
LO 2.37 TSIAT connect concepts that describe mechanisms that regulate the timing and
coordination of physiological events.
LO 2.38 TSIAT analyze data to support the claim that responses to information and
communication of information affect natural selection.
LO 2.39 TSIAT justify scientific claims, using evidence, to describe how timing and coordination
of behavioral events in organisms are regulated by several mechanisms.
LO 2.40 TSIAT connect concepts in and across domain(s) to predict how environmental
factors affect responses to information and change behavior.
Overview

Since plants cannot move (away from threats
or toward a resource) they respond to cues
by adjusting their individual patterns of
growth and development.
◦ Ex: opening of flowers for pollinator when they
are active.
39.1 Signal Transduction Pathways
Link Signal Reception to Response

Etiolation: morphological adaptations for
growing in the dark.
◦ Ex: a potato puts its resources into producing
stems because it is located under the soil.
 When exposed to light, the stem stops growing and
leaves with chlorophyll are produced (de-eiolation)
(a) Before exposure to light
(b) After a week’s exposure
to natural daylight
Reception

Phytochrome receptors in the cytoplasm of plants.
Transduction

Phytochrome:
◦ opens Ca2+ channels (increases its concentration)
◦ changes shape activating cyclic GMP
(These are second messengers relay and amplify the signal to
response proteins)
Response
Post-translational – activates preexisting enzymes
 Transcriptional – increases mRNA synthesis

Figure 39.4-3
2 Transduction
1 Reception
3 Response
Transcription
factor 1 NUCLEUS
CYTOPLASM
Plasma
membrane
cGMP
Second
messenger
Phytochrome
P
Protein
kinase 1
Transcription
factor 2
P
Cell
wall
Protein
kinase 2
Transcription
Light
Translation
Ca2 channel
Ca2
De-etiolation
(greening)
response proteins
39.2 Plant Hormones Help Coordinate Growth,
Development, and Response to Stimuli
The discovery of plant hormones
 Tropism: growth response toward or away from a
stimulus.
◦ Ex: plants grow toward light (phototropism)
 (Darwins) A hormone is produced in the coleoptile that is transmitted
down the stem to have cells facing the light slow growth and cells not
facing the light grow faster.
RESULTS
Shaded
side
Control
Light
Illuminated
side
Boysen-Jensen
Light
Darwin and Darwin
Light
Gelatin
Mica
(permeable) (impermeable)
Opaque
Tip
removed cap
Transparent
cap
Opaque
shield over
curvature
RESULTS
Excised tip on
agar cube
Growth-promoting
chemical diffuses
into agar cube



Control
(agar cube
lacking
Control chemical)
Offset
cubes

Frits Went placed the
coleoptile tip on agar.
The agar contained a
hormone.
When placed on one side
of the plant, that side
began to grow causing it
to bend.
Auxin (indoleacetic acid) is
the hormone produced
which causes cell
elongation (a plant growth
hormone).
Video: Phototropism
© 2011 Pearson Education, Inc.
Hormone
Where produced/found
Major Functions
Auxin
Shoot apical meristems and
young leaves
Stimulates stem elongation
Cytokinins
Roots
Regulate cell division
Gibberellins
Meristems of buds and roots
Stimulates stem elongation,
reproduction
Brassinosteroids
All tissue
Promote cell expansion
Abscisic acid (ABA)
All tissue
Inhibits growth
Strigolactones
Roots
Promote seed germination
Ethylene
All tissue
Promotes ripening of fruits
39.3 Responses to Light Are Critical
For Plant Success

Blue-Light Photoreceptors
◦ Phototropism
◦ Opening stomata
◦ Slowing hypocotyl growth once seedling
breaks ground.
Light
Time  0 min
Time  90 min
(b) Coleoptiles before and after light exposures

Phytochormes as Photoreceptors
◦ Protein which absorb red light which stimulates
germination.
◦ Discovered by exposing seeds to different
colors of light then observing them.
RESULTS
Red
Dark
Red Far-red
Dark
Dark (control)
Red Far-red Red
Dark
Red Far-red Red Far-red
◦ Phytochromes also serves as a shade avoidance
indicator.
◦ When in shade, far-red light is more abundant
leaving the protein “inactive.”
◦ This tells the plant it is in shade and needs to
grow taller.
Pr
Pfr
Red light
Synthesis
Responses:
seed
germination,
control of
flowering, etc.
Far-red
light
Slow conversion
in darkness
(some plants)
Enzymatic
destruction
Biological Clocks and Circadian
Rhythms

Circadian rhythms are cycles that
occur every 24 hours which are not
directly controlled by any known
environmental variable.
◦ Putting plants in a controlled environment (24
hours of light) only slightly get the rhythm off
course (21-26 hour rhythms).
Noon
Midnight
Photoperiodism

24 hours
A physical response to
the relative lengths of
night and day.
◦ Night time
requirements not to be
broken by light or
flowering won’t occur.
◦ Ex: short day plants
need uninterrupted
long nights to flower
(Long-night plants)
(a) Short day
(long-night) plant
Light
Critical
dark period
Flash Darkness
of
light
(b) Long-day
(short-night) plant
Flash
of light
39.4 Plants Respond to a Wide
Variety of Stimuli Other Than Light

Gravity (gravitropism)
◦ Positive = downward growth of roots
◦ Negative = upward growth of shoots
◦ Detected by statoliths (cytoplasmic components
that settle to the bottom of the cell).
Statoliths
(a) Primary root of maize
bending gravitropically
(LMs)
20 m
(b) Statoliths settling to
the lowest sides of
root cap cells (LMs)
Video: Gravitropism
© 2011 Pearson Education, Inc.

Mechanical Stimuli (being “touched”)
◦ Thigmomorphogensis are changes in the
form of a plant due to mechanical stimuli
(wind, touch, water, etc)
(a) Unstimulated state
(b) Stimulated state
Video: Mimosa Leaf
© 2011 Pearson Education, Inc.
Environmental Stresses (abiotic)

Drought
◦ Plants lose water by transpiration and cannot replenish it.
◦ Responses: shed leaves, curl leaves, close stomata, root growth

Flooding
◦ suffocates roots/no oxygen

Salt
◦ Loss of water in soil
◦ Toxic to plants

Heat
◦ Denatures enzymes

Cold
◦ Loss of fluidity of cell membranes slowing/stopping transport in/out
of cell.
39.5 Plants Respond to Attacks by
Herbivores and Pathogens

Defense against herbivores
◦ Physical defenses
 Thorns
◦ Chemical defenses
 Distasteful or toxic compounds
 Canavanine (replaces arginine when ingested)
 Attraction of parasitoid wasps
4 Recruitment of
parasitoid wasps
that lay their eggs
within caterpillars
1 Wounding
1 Chemical
in saliva
2 Signal transduction
pathway
3 Synthesis
and release
of volatile
attractants

Defense against pathogens
◦ First line of defense is the
epidermis and periderm of
plant body.
◦ Plants can recognize
pathogen-derived molecules
(effectors) causing the plant
to either respond defensively
or be taken over by the
pathogen.
◦ The Hypersensitivity
Response
 cells near infection site
secrete toxins then die to
prevent the spread of
infection.
Infected tobacco leaf with lesions
4
3
Signal
Hypersensitive
response
2 Signal transduction pathway
1
R protein
Avirulent
pathogen
Avr effector protein
R-Avr recognition and
hypersensitive response

Systemic Acquired Resistance
◦ General response putting whole plant “on alert”
◦ Production of salicyclic acid in areas away from infection,
activating a signal transduction pathway to produce
pathogenesis-related (PR) proteins
4
3
Signal
5
Hypersensitive
response
2 Signal transduction pathway
Signal
transduction
pathway
6
7 Acquired
resistance
1
R protein
Avirulent
pathogen
Avr effector protein
R-Avr recognition and
hypersensitive response
Systemic acquired
resistance