How we used coleoptiles to discover how auxin

How we used coleoptiles to discover how auxin drives phototropism bergv@uni. edu

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

Darwin #1 (1880’s) coleoptile growth in the dark later Conclusion: Coleoptiles do not need light to grow

Darwin #2 effect of removing the tip later Conclusion: Coleoptile tips provide something that is necessary for the rest of the coleoptile to grow

Darwin #3 effect of unilateral light LIGHT later Conclusion #1: Coleoptiles grow toward light Conclusion #2: The bending is below the tip LIGHT

Darwin #4 effect of covering the tip LIGHT later Conclusion: Light on the tip is required for directional growth, but not for uniform lengthwise growth LIGHT

Darwin #5 effect of a light-proof barrier on the coleoptile except the tip LIGHT later Conclusion: Light perception is only on the tip, while the response is lower down LIGHT

Boysen-Jensen #1 (1913) effect of mica block of chemicals (not light) on dark side LIGHT later Conclusion: Something chemical moves down the dark side to promote growth there LIGHT

Boysen-Jensen #2 effect of mica block of chemicals (not light) on light side LIGHT later Conclusion: Differential growth does not depend on a chemical moving down the light side to inhibit growth there LIGHT

Boysen-Jensen #3 effect of permeable agar on movement in unilateral light LIGHT later Conclusion: Differential growth depends on a chemical moving from the tip to the rest of the coleoptile LIGHT

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).

Oat coleoptile bioassay for auxin 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) Measure angle and compare to angles from known concentrations

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.

Does light change amount of auxin? LIGHT Tip and block divided by mica sheet, blocks assayed separately. Angles: left = 11. 2 degrees right = 11. 5 degrees LIGHT Tip intact while rest and block are divided by mica sheet, then blocks assayed separately. 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.

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.