How we used coleoptiles to discover how auxin drives phototropism
Germinating oat seed
coleoptile: a sheath that protects the new leaves (rolled up inside) until they grow out of the soil.
roots
seed
Coleoptiles grow toward light, and were the experimental victims used for over a century of research on phototropism and on the hormone auxin.
Tip of coleoptile (2-4 mm)
Darwin #1 (1880’s)coleoptile growth in the dark
Conclusion: Coleoptiles do not need light to grow
later
Darwin #2effect of removing the tip
Conclusion: Coleoptile tips provide something that is necessary for the rest of the coleoptile to grow
later
Darwin #3effect of unilateral light
Conclusion #1: Coleoptiles grow toward light
later
Conclusion #2: The bending is below the tip
LIGHTLIGHT
Darwin #4effect of covering the tip
Conclusion: Light on the tip is required for directional growth, but not for uniform lengthwise growth
later
LIGHTLIGHT
LIGHT
Darwin #5effect of a light-proof barrier on the coleoptile except the tip
Conclusion: Light perception is only on the tip, while the response is lower down
later
LIGHT
Boysen-Jensen #1 (1913)effect of mica block of chemicals (not light) on dark side
Conclusion: Something chemical moves down the dark side to promote growth there
later
LIGHT LIGHT
Boysen-Jensen #2effect of mica block of chemicals (not light) on light side
Conclusion: Differential growth does not depend on a chemical moving down the light side to
inhibit growth there
later
LIGHT LIGHT
Boysen-Jensen #3effect of permeable agar on movement in unilateral light
Conclusion: Differential growth depends on a chemical moving from the tip to the rest of the coleoptile
later
LIGHT LIGHTLIGHT
Paal #1 (1919)effect of offset coleoptile tip in dark
Conclusion: Coleoptile tips provide the chemical that causes differential growth of coleoptile sides
Paal named the substance “auxin” (increase).
later
Oat coleoptile bioassay for auxin
Coleoptile tip placed on agar block
Auxin diffuses into block (wait for standard number of hours)
Block placed offset on decapitated coleoptile
Auxin diffuses
from block into coleoptile
(wait)
Measure angle and compare to angles from known concentrations
Does light change amount of auxin?
Diffusion in dark, then bioassay.
LIGHT
Diffusion in unilateral light, then bioassay.
Conclusion: Amount of auxin produced is the same in dark and unilateral light.
Angle = 25.8 degrees Angle = 25.6 degrees
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.
Conclusion: Unilateral light causes auxin to move to dark side. This explains all the observations and experiments.
LIGHT LIGHT
Angles: left = 11.2 degreesright = 11.5 degrees Angles: left = 15.4 degrees
right = 8.1 degrees
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 don’t 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.