Plant Hormones:
Control of Growth & Flowering
DP Biology 2009
Plant Hormones
Many aspects of plant function are controlled
and regulated through the action of hormones.
What hormones have we already discussed?
Plant Hormones
Many aspects of plant function are controlled
and regulated through the action of hormones.
 Abscisic acid: closes guard cells
 Gibberellin: stimulates embryo growth and
production of amylase.
Plant Hormones
Many aspects of plant function are controlled
and regulated through the action of hormones.
 Abscisic acid: closes guard cells
 Gibberellin: stimulates embryo growth and
production of amylase.
Just one other one you need to know for IB …
 Auxin: stimulates stem & fruit growth, root
formation, and phototropism.
Other Plant Hormones (FYI)
 Brassinosteroids: growth of stem & pollen
tubule, vascular tissue differentiation
 Cytokinins: work with auxin, promote growth &
inhibit senescence
 Ethylenes: promotes fruit ripening and leaf
abscission
 Jasmonates / Oligosaccharins / Salicylic
Acid : stimulates protection against herbivores
and/or pathogens
Auxin & Plant Growth
 Auxin promotes the elongation of cells.
(Not the formation of new cells … why?)
Auxin & Plant Growth
 Auxin promotes the elongation of cells.
 Plant cell elongation is usually prevented by
…?
Auxin & Plant Growth
 Auxin promotes the elongation of cells.
 Plant cell elongation is usually prevented by
the cell wall.
Auxin & Plant Growth
 Auxin promotes the elongation of cells.
 Plant cell elongation is usually prevented by
the cell wall.
 Auxin loosens the cell wall of plants, allowing
the central vacuole to take up more water and
stretch the cell by tugor pressure.
Auxin & Plant Growth
 Auxin promotes the elongation of cells.
 Plant cell elongation is usually prevented by
the cell wall.
 Auxin loosens the cell wall of plants, allowing
the central vacuole to take up more water and
stretch the cell by tugor pressure.
 The cell wall can be re-strengthened by the
addition of more cellulose fibers.
Auxin & Plant Growth
How does auxin ‘loosen’ cell walls?
 Auxin increases the H+ concentration in cell wall
(with what effect on pH?)
Auxin & Plant Growth
How does auxin ‘loosen’ cell walls?
 Auxin increases the H+ concentration in cell wall
(lowering the pH)
Auxin & Plant Growth
How does auxin ‘loosen’ cell walls?
 Auxin increases the H+ concentration in cell wall
(lowering the pH)
 H+ triggers proteins called expansions to break
up hydrogen bonds between cellulose fibers,
allowing them to slide past each other.
See animation:
http://bcs.whfreeman.com/thelifewire/content/chp38/3802003
.html
Auxin & Phototropism
 Tropisms are directional growth responses.
 Auxin mediates phototropism -- the growth of
the stem towards light.
QuickTime™ and a
decompressor
are needed to see this picture.
QuickTime™ and a
decompressor
are needed to see this picture.
Auxin & Phototropism
 Photoreceptors in shoot tip absorb light.
 Detection of light causes proteins that redistribute
auxin to be produced.
 Auxin is moved from
the tip (where it is
produced) down
through the stem.
photoreceptors
QuickTime™ and a
decompressor
are needed to see this picture.
Auxin & Phototropism
 Auxin is concentrated in the shady side of the stem,
promoting greater elongation there.
 Uneven elongation
promotes curving of
stem towards light.
See animation:
http://trc.ucdavis.edu/
biosci10v/bis10v/media/
ch19/auxin_phototropism.html
photoreceptors
QuickTime™ and a
decompressor
are needed to see this picture.
Other Tropisms (FYI)
 Can you think of any other plant
tropisms?
Other Tropisms (FYI)
 Geotropism: Growth towards or away from
gravity.
 Thigmotropism: Growth towards touch.
Time lapse videos for fun …
http://plantsinmotion.bio.indiana.edu/plantmotion
/movements/tropism/tropisms.html
Control of Flowering
 Flowering in many plants is seasonal, and
closely related to day-length.
 “Long-day” plants flower in the summer,
whereas “Short-day” plants flower in the
winter.
Control of Flowering
 Flowering in many plants is seasonal, and
closely related to day-length.
 “Long-day” plants flower in the summer,
whereas “Short-day” plants flower in the
autumn.
 Experiments have shown that it is not the
length of the day that matter, but rather the
length of the night.
Control of Flowering
Plants have a critical night length that
determines the timing of flowering.
What does this mean?
Control of Flowering
Plants have a critical night length that
determines the timing of flowering.
 Long-day plants flower if the night is shorter
than critical value.
 Short-day plants flower if night is longer than
critical value.
Control of Flowering
How can we tell that it is night length that
matters?
Control of Flowering
Experimenters interrupted dark and light
periods with a flash of light or a brief period of
dark. What do their results suggest?
QuickTime™ and a
decompressor
are needed to see this picture.
D
Control of Flowering
 A flash of light during night affects flowering
 Dark spell during day does not affect flowering.
QuickTime™ and a
decompressor
are needed to see this picture.
D
Measuring “dark”
Pigments in leaves called phytochromes absorb light.
 PR absorbs red light (660 nm) and converts to PFR
 PFR absorbs far-red light (730nm) and converts to PR
 PFR slowly degrades into PR
QuickTime™ and a
decompressor
are needed to see this picture.
Measuring “dark”
Sunlight contains far more red light than far-red light, so
while exposed to sunlight, most of phytochrome is in PFR
state.
At the end of the
night, much of
the PFR has
converted back
to PR.
QuickTime™ and a
decompressor
are needed to see this picture.
Measuring “dark”
The amount of PFR in the leaves at the start of the
day determines whether plants flower.
How does the response differ in short- and longday plants?
 Flowering is inhibited by PFR in short-day plants
 Flowering is promoted by PFR in long-day plants
Measuring “dark”
Experiments
support the
role of PFR in
determining
timing of
flowering.
QuickTime™ and a
decompressor
are needed to see this picture.
‘Florigen’ discussion
 Phytochromes are found in leaves, but their
effects are seen elsewhere in a plant.
 A single leaf is enough to control flowering
in a large plant -- even several plants
grafted together.
‘Florigen’ discussion
 Scientists have long suspected that a
hormone is involved in controlling this
response -- its even been given a name:
florigen.
 Yet, despite decades of intense research,
no one has been able to isolate and identify
this hormone.
‘Florigen’ discussion
 Scientists have long suspected that a
hormone is involved in controlling this
response -- its even been given a name:
florigen.
 Yet, despite decades of intense research,
no one has been able to isolate and identify
this hormone.
‘Florigen’ discussion
 Why would the theory persist, even though
scientists have failed to identify the
hormone?
 Does a failure to identify the hormone
indicate a lack of evidence for its existence?
‘Florigen’ discussion
 Most scientists agree that a theory is not
‘scientific’ unless it is falsifiable. What does
this mean?
 What do you think of the following quote
from the 1970s:
“"Flowering is a religion based on the totally
unfounded dogma of florigen."
‘Florigen’ discussion
 What would a scientist have to do to show
that florigen doesn’t exist?
 Can you think of any other non-falsifiable
‘theories’ in science?
Recent Florigen Developments
Recently a few molecules (proteins & RNAs)
have been discovered that either are the
elusive florigen -- or are otherwise involved in
generating the signal.