Chapter 39
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Examples of some hormones
Phototropism experiments
Auxin’s mode of action
Apical dominance
Other tropisms
Seed dormancy, germination
Fruit ripening
Photoperiodism
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Hormone: a chemical substance
produced in one part of the body and has
a physiological response in another part
of the body at very low concentrations.
coordinates metabolic activities
active in small amounts
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Phototropism plants grow towards light
Gravitropism- roots grow down into soil
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statoliths in root cap
Thigomotropism- tendrils curl around branch,
etc. Responds to touch.
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wind blowing
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Darwin and Son, 1880
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Boysen-Jensen, 1913
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signal moving down from tip
Went, 1926
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light sensitivity in tip.
signal is a chemical substance
Thimman isolated the structure of
Auxin
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Differential cell elongation causes
stem to bend. Cells on shaded side
elongate.
How do cells elongate?
Auxin in higher concentration on
shaded side of stem.
Only cells directly underneath
stimulated cells elongate
Auxin moves basipetally down stem
How?
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Cell wall has constitutive enzymes
that loosen connections between
cellulose when activated.
An increase in cell wall acidity
(lower pH) activates the enzymes.
Auxin in cytoplasm causes cell wall
to acidify.
Water moves in by osmosis, swelling
cell.
Cell membrane lays down additional
cell wall material keeping new size /
shape
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How cellulose microfibrils are laid
down determine the direct of
elongation.
Elongation takes place when a cell
only has primary cell wall.
Once proper size and shape is
attained, secondary cell wall
material is added inside the primary
cell wall.
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Determines the
cell shape
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higher pH in
cytoplasm
activates Auxin
Can only leave
cell via transport
proteins at base
of cell.
Diffuse across
into next cell, in
inactive form.
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Entering next
cell down,
auxin is
activated by
pH, causing
proton pumps
to start
Cell wall
acidifies,
activating
enzymes.
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Auxin moves down
to next cell.
Now more diluted
by process
Signal fades out
farther down
Fig. 39-7
RESULTS
Cell 1
100 µm
Cell 2
Epidermis
Cortex
Phloem
Xylem
Pith
25 µm
Basal end
of cell
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Statoliths
fall to
bottom of
cell at root
cap.
Inhibit
auxin
production
in lower
cells
elongate
bending tip
downward.
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Flowers or leaves bend towards sun’s path in
the sky during the day.
Motor cells at base of flower or leaf uptake K+
and other ions causing them to swell, bending
the stem towards light.
Stem continues to respond to direction of light
during the day, and different cells swell, or
relax changing direction.
Similar process cause an opening / closing
response to flowers (poppies) or Leaves
(prayer plants).
Can be under circadian rhythm.
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Shoot apical meristem produces auxin
which moves down stem and inhibits
auxiliary bud from growing.
Roots produce cytokinins that move up
and stimulate buds to grow.
Pinching back tops makes plants bushier
Pruning sends a surge of cytokinins up to
remaining buds- fast growth in spring.
Limiting root growth can stunt plants.
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Many annual’s have abscisic acid (ABA) in
seed coat.
ABA keeps embryo, seed dormant
Rains wash out ABA
Embryo swells produces gibberillins
which cause seed to germinate
Ensures germination after soil is wet
enough.
Other seeds respond to cold, light etc.
Fig. 39-12
Early germination
in red mangrove
Coleoptile
Early germination
in maize mutant
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Ethylene is the only gaseous hormone.
May spread to other plants
Causes fruit to ripen
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“one bad apple…”
Positive feedback loop
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Apples, bananas
not strawberries, mangoes
Organic acids convert to sugars, pectin
in middle lamella breaks down
Ethylene sensitive fruit can be stored
green under carbon dioxide for months
Gassed before sending to market
Potential area for biotechnology
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Growing tips
meets on object
Secrete
ethylene
Causes stem to
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1) slow
elongation
2) thicken
3) grow
sideways
Until around
object and
resumes upward
growth.
Fig. 39-3
CELL
WALL
1 Reception
CYTOPLASM
2 Transduction
Relay proteins and
second messengers
3 Response
Activation
of cellular
responses
Receptor
Hormone or
environmental
stimulus
Plasma membrane
Ethylene
mutants
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Testing in lab
dwarfism in
many plants
Bolting- &
flowering
Fruit set
Stimulate cell
division &
elongation
Promotes seed
germination
Fig. 39-10
(b) Gibberellin-induced
fruit growth
(a)Gibberellin-induced stem
growth
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Leaf abscission
cause by balance
of ethylene and
auxin
Apoptosis cell
death- recycles
many essential
nutrients to plant,
stimulated by burst
of ethylene
How do plants
detect when this
should happen?
Not covered
Table 39-1
Not covered
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Auxins: growth, cell elongation in stem
root, Apical Dominance, seedless fruit
Cytokinins: (roots) root growth,
stimulates cell differentiation & growth
retards senescence (fruit, flower life),
stimulates germination
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Gibberellins; stimulate cell division &
elongation, fruit set, bolting, promotes
seed germination
Ethylene: fruit ripening, opposes some
auxin affects
Absicisc acid; inhibits growth, closes
stomata, dormancy in seeds
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Oligosaccharins- Trigger defense mechanisms
 short sugar chains released from cell wall by
enzymatic breakdown of cellulose and pectin.
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Brassinosteroids- steroids required for normal
growth and development.
 Studied mostly by mutations lacking these
compounds.
Fig. 39-1
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Phototropism
responds to blue
light levels
Many responses
to light detected
by phytochrome
sensitive to red
light.
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Seed germination
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Shade avoidance
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Need light to germinate
Higher PR ratio in shade
Plants grows taller to reach brighter light
Flower response – Florigen
Other photoreceptors sense blue
light: phototropism
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In light Pr converts rapidly to Pfr
In dark, Pfr slowly reverts to Pr
Used to time amount of darkness, or dawn
Resets internal biological clock
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Actually refer to length of darkness
Many plants are day neutral
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Far red
light
counteracts
red light,
erasing
“day”
signal
Fig. 39-23
24 hours
24 hours
Long-day plant
grafted to
short-day plant
Long-day
plant
24 hours
Graft
Short-day
plant
Induced by
photoperiod
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Flowering hormone ?
Structure still not
discovered
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may be a macro
molecule - CONSTANS
protein
Can be induced in one
plant and move to
another
Moves cell to cell,
slower than phloem
Economic
significance?
Not induced
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When underground (in darkness)
young stems etiolate
grow long internodes
 no leaves produced
 yellow, no chlorophyll expressed
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Light detected by phytochrome
reverses etiolation,
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plant sprouts leaves
Fig. 39-3
CELL
WALL
1 Reception
CYTOPLASM
2 Transduction
Relay proteins and
second messengers
3 Response
Activation
of cellular
responses
Receptor
Hormone or
environmental
stimulus
Plasma membrane
Fig. 39-4-3
1
Reception
2
Transduction
3
CYTOPLASM
Plasma
membrane
cGMP
Second messenger
produced
Specific
protein
kinase 1
activated
Response
Transcription
factor 1
NUCLEUS
P
Transcription
factor 2
Phytochrome
activated
by light
P
Cell
wall
Specific
protein
kinase 2
activated
Transcription
Light
Translation
Ca2+ channel
opened
Ca2+
De-etiolation
(greening)
response
proteins
Fig. 39-26
(a) Unstimulated
state
(b) Stimulated
state
Side of pulvinus
With flaccid cells
Leaflets
after
stimulation
Pulvinus
(motor
organ)
Side of pulvinus
With turgid cells
Vein
(c) Cross section of a leaflet pair in the stimulated state (LM)