How we used coleoptiles to discover how auxin drives phototropism

bergv@uni.edu

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.