How we used coleoptiles to discover how auxin drives phototropism

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How we used coleoptiles to discover how auxin
drives phototropism

• bergv_at_uni.edu

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Germinating oat seed
Tip of coleoptile (2-4 mm)
coleoptile a sheath that protects the new leaves
(rolled up inside) until they grow out of the
soil.
Coleoptiles grow toward light, and were the
experimental victims used for over a century of
research on phototropism and on the hormone auxin.
seed
roots
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Darwin 1 (1880s)coleoptile growth in the dark


later
Conclusion Coleoptiles do not need light to grow
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Darwin 2effect of removing the tip


later
Conclusion Coleoptile tips provide something
that is necessary for the rest of the coleoptile
to grow
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Darwin 3effect of unilateral light


LIGHT
LIGHT
later
Conclusion 1 Coleoptiles grow toward light
Conclusion 2 The bending is below the tip
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Darwin 4effect of covering the tip


LIGHT
LIGHT
later
Conclusion Light on the tip is required for
directional growth, but not for uniform
lengthwise growth
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Darwin 5effect of a light-proof barrier on the
coleoptile except the tip


LIGHT
LIGHT
later
Conclusion Light perception is only on the tip,
while the response is lower down
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Boysen-Jensen 1 (1913)effect of mica block of
chemicals (not light) on dark side


LIGHT
LIGHT
later
Conclusion Something chemical moves down the
dark side to promote growth there
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Boysen-Jensen 2effect of mica block of
chemicals (not light) on light side


LIGHT
LIGHT
later
Conclusion Differential growth does not depend
on a chemical moving down the light side to
inhibit growth there
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Boysen-Jensen 3effect of permeable agar on
movement in unilateral light



LIGHT
LIGHT
LIGHT
later
Conclusion Differential growth depends on a
chemical moving from the tip to the rest of the
coleoptile
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Paal 1 (1919)effect of offset coleoptile tip in
dark



later
Conclusion Coleoptile tips provide the chemical
that causes differential growth of coleoptile
sides Paal named the substance auxin
(increase).
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Oat coleoptile bioassay for auxin
Measure angle and compare to angles from known
concentrations
Coleoptile tip placed on agar block
Block placed offset on decapitated coleoptile
Auxin diffuses from block into coleoptile (wait)
Auxin diffuses into block (wait for standard
number of hours)
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Does light change amount of auxin?


LIGHT
Diffusion in dark, then bioassay.
Diffusion in unilateral light, then bioassay.
Angle 25.8 degrees
Angle 25.6 degrees
Conclusion Amount of auxin produced is the same
in dark and unilateral light.
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Does light change amount of auxin?
Tip and block divided by mica sheet, blocks
assayed separately.
Tip intact while rest and block are divided by
mica sheet, then blocks assayed separately.
Angles left 11.2 degrees right 11.5 degrees
Angles left 15.4 degrees right 8.1 degrees
Conclusion Unilateral light causes auxin to move
to dark side. This explains all the observations
and experiments.
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• Generations of plant physiology students all over
  the world spent countless hours in the laboratory
  cutting the tips off oat coleoptiles and placing
  them (or the agar blocks they diffused their
  auxin into) back onto the decapitated
  coleoptiles. It was tedious, fussy work. This is
  one of those things that most people are happy we
  dont have to do any more.
• But our understanding of these matters rests on
  the shoulders of giants of science in the past,
  whose careful (and carefully thought out)
  experiments led us to where we are today.