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

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

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

Darwin #5 effect 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

Boysen-Jensen #2 effect 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

Boysen-Jensen #3 effect 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

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 degreesAngle = 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 Angles: left = 11.2 degrees right = 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.