Transcript Plant Hormones

Plant Hormones
AP Biology – LAHS
What are Hormones?

Chemical signals
that coordinate the
various parts of an
organism
 Chemicals are made
in one region and
are target for some
other region of the
organism
The Discovery of Plant
Hormones
 Plant
hormones were discovered as
scientists were studying how it is that
plants grow towards light
 Phototropism – growth of a shoot
towards light
Darwin’s Experiments with
Phototropism
Coleoptile – term for the sheath that encloses
a grass seedling
 Studied growth of the coleoptile in different
conditions
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Darkness – grew straight
Illuminated uniformly from all sides – grew straight
Illuminated from one side only – grew towards the
lighted side
Question: What Causes the
Coleoptile to Bend towards Light?
 Hypothesis:
cells on the darker side of
the coleoptile elongate faster than those
on the lighted side. This causes the
coleoptile to bend toward light.
 How does this happen?
Darwin’s Ideas
 The
part of the coleoptile responsible for
sensing light is the TIP.
 Growth response that caused curvature
of the coleoptile was BELOW the tip.
 Hypothesis: some signal was
transmitted downward from the tip
Diagrams of Experiments
Testing Darwin’s Hypothesis

Peter Boysen-Jensen
 Tip was separated from the coleoptile
 Control treatment: A gelatin block separated
the tip form the lower parts of the plant
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
The gelatin block allowed the plant to be cut as it
would be in the experimental treatment, but still
allowed the chemicals from the tip to pass down
Resulted in curvature as normal
Testing Darwin’s Hypothesis
 Experimental
treatment:
 An
impermeable barrier was placed
between the coleoptile tips and the lower
parts of the plants

Prevented the chemicals made at the tip from
moving down the plant
 Result:
Curvature growth did not occur
Diagrams of Experiments
Went’s Experiments

Extracted the chemical messenger from the
coleoptile tip

Removed the tip and allowed it to diffuse onto a
piece of agar

Removed and discarded growing tip from
other coleoptile seedlings
 Placed the agar block evenly centered onto
the “decapitated” seedlings


They grew straight
Placed the agar block Uncentered onto other
decapitated seedlings

Their growth caused them to curve AWAY from
the side with the agar block
Went’s
Experiments
Went’s Conclusions
 The
chemical in the tip stimulated
growth as it passed down the coleoptile
 The coleoptile curved toward light
because of a HIGHER concentration of
the growth-promoting chemical on the
DARKER side of the coleoptile
 Went named the chemical messenger
that he studied AUXIN
Tropisms

Growth responses that result in curvatures of
whole plant organs toward or away from
some stimulus.
 Mechanism


Elongation of cells on the OPPOSITE side of the
organ region that is receiving the stimulus
Stimulii


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Gravity
Light
Touch
How Does Auxin Stimulate
Growth?

Causes cell walls to become “looser” and
more malleable. Then they can be
expanded/elongated
Plant Hormones - Auxin

IAA (indoleacetic acid)
 Found:
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Meristems of apical buds
Major Functions:
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stimulation of stem elongation
Root growth, differentiation,
branching
Apical dominance
 Growth of a stem occurs only
at the tip unless the tip is cut
off
 Absence of auxin from tip will
allow lateral buds to emerge
 This is why we prune
Actively transorted from
cell to cell in a specific
direction
Plant Hormones - Auxin

(IAA) cont.

Found
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Embryos within seeds
Major Functions
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Stimulate growth of
fruit from ovary
Influences responses
to light & gravity
Plant Hormones - Cytokinins
Found
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In actively growing tissues
Produced in roots, transported
elsewhere
Major function:
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Stimulate cytokinesis (cell division)
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Work with auxins to control plant
growth
Plant tissue treated with auxin w/o
cytokiinin – cells will grow but not
divide
Control of apical dominance –
supports lateral buds (weakens apical
dominance)
Anti-aging hormones

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Delays senescence (aging) of leaves
Slow deterioration of leaves – used by
florists
Plant Hormones - Gibberellins

Found in:
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Apical meristems; young
leaves/embryos
Major function:
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Simulates growth in leaf and
stem
Stem bolting – rapid elongation
Fruit growth

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Germination of seeds

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Grapes are sprayed with gib
to cause them to grow larger
and further apart
After water is imbibed,
gibberellins are released in
embryo to break from
dormancy
Inhibition of aging leaves
Plant Hormones – Abscisic Acid

Found in:
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Leaves, stems, roots
Seeds, green fruit
Major function:

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Slow down growth
Dormancy for overwintering
 Suspends primary and secondary growth
 Promotes abscision of leaves (falling off)
In seeds – inhibits growth until ABA can be overcome or diminished by
favorable conditions
 Heavy rain may wash out ABA
 Light may degrade
 Increased gib to ABA ratio may determine germination
 growth
Stress hormone
 When a plant wilts, ABA accumulates causing stomata to close
Plant Hormones - Ethylene

Found in
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Tissues of ripening fruit
Nodes of stems
Ageing leaves and flowers
Major functions

Changes of ovary to become fruit
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Leaf abscission

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Degradation of cell walls;
softening
Dropping from plant
Loss of leaves to prevent water
loss
Tissue at base of petiole dies
Senescence (aging)

Autumn leaves; withering flowers
Tropisms

Growth responses that result in curvatures of
whole plant organs toward or away from
some stimulus.
 Mechanism


Elongation of cells on the OPPOSITE side of the
organ region that is receiving the stimulus
Stimulii



Gravity
Light
Touch
Tropisms - Phototropsm
 Phototropism:
response to light
 Achieved
through
auxin
 When all sides
equally lit, straight
growth
 When stem
unequally lit,
differential growth
Tropisms - Gravitropism

Also geotropism
 Response to gravity
be stems & roots
 Gibberellins & Auxin
involved (relative
concentrations)
Tropisms - Gravitropism
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If stem is horizontal: auxin at apical meristem moves down and
concentrates on lower side – stem bends upwards
If root is horizontal, auxin produced at apical meristem moves up
roots and concentrates on lower side – inhibits growth in roots
Special starch-storing plastics (staloliths) settle at lower ends of
cells to influence auxin movement
Tropisms - Thigmotropism

Response to touch
 Seen in climbing
vines, venus fly trap,
etc.
Photoperiodism

Response of plants
to changes in the
photoperiod (relative
length of day/night)
 A plant maintains
circadian rhythm:
internal clock that
measures length of
day/night
Phytochromes
 Chemicals
that function as
photoreceptors in plants and allow
plants to “measure” photoperiod
Phytochromes

The name given to the photoreceptor that is
responsible for the reversible effects of red
and far-red light is phytochrome
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Phytochrome = a light absorbing protein
2 forms
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Pr = absorbs red light
Pfr = absorbs far red light
The two forms are photoreversible
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When Pr is exposed to red, it becomes Pfr
When Pfr is exposed to far red, it becomes Pr
Phytochrome
Phytochrome
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Pr is form of photochrome synthesized in plant
cells. Pr synthesized in leaves.
Pr and Pfr in equilibrium during daylight. Pr ->
Pfr since red light present in sunlight.
Pr accumulates at night. No sunlight for Pr -> Pfr.
Pfr breaks down faster. Cells continue to make Pr at
night.
Daybreak, light rapidly converts to accumulated
Pr into Pfr. Equilibrium again.
Phytochromes
 The
Pr <-> Pfr
interconversion
acts as a
switching
mechanism that
controls various
events in the life
of a plant.
Phytochromes

Red light - 660nm
 Wavelength of light that is most effective at
interrupting the critical night length of a short
day (long night) plant.
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Exposure at night will cause the plant NOT to
flower
HOWEVER, if this light briefly interrupts the
night of a long day (short night) plant, the
plant will flower

The red flash will shorten the plants perception of
night length
Phytochromes
 The
shortening of night length can be
negated by providing a flash of light at
730nm wavelength.
 This
is called the far-red part of the
spectrum
Phytochrome

Night length is responsible for resetting the
circadian rhythm clock
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If daylight is interrupted with dark there is no effect
If dark is interrupted with flashes of red or far-red
the clock can be affected
Red-light shortens night length
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Because it converts Pr to Pfr – which would not normally
accumulate at night
Far-red light restores – as though night was not
broken

Because far red light flashes convert Pfr back to Pr
Phytochrome
 Plants
synthesize phytochrome as Pr
 If left in the dark, nothing happens to
this pigment
 If the pigment is illuminated with
sunlight, Pr changes to Pfr
 Thus the plant can detect the presence
of sunlight
Phytochrome

Pr = Pfr during daylight
 If shade of larger trees were to block sunlight
from a smaller tree, the radiation most
blocked by canopy is red (not far red)
 Pigments in the plant would be converted to
Pr
 This cue would stimulate the plant to grow
taller.
Phytochrome
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If ample sunlight were
available, the reverse
would happen –
Pfr proportions would
increase and the plant
would “sense” that it was
in sun.
It would be cued to
branch and vertical
growth would be
inhibited
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