Auxin• History• Biochemistry• Transport of Auxin• Physiological action

of Auxin• Mode of action

History

• 1st hormone discovered

• Darwin's work • Went's work • Phototropism

Paal & Went

Biochemistry

• Derived from tryptophan

• Indole 3-acetic acid • weak acid, pk=5 • conjugated IAA • degradation • produced in

meristems

Conjugation and Degradation

Transport of Auxin

• basipetal transport from meristems

• evidence for basipetaltransport

Polar Transport of Auxin

• chemosmoticdiffusion model

• proton pumps lower cell wall pH

• transport proteins at base of cell

• cell pH=7, so IAA-

PIN1 protein

Physiological action of Auxin

• Growth in stems and coleoptiles

• Inhibits growth in roots

Phototropism in coleoptiles

• lateral redistribution of IAA to shaded side

• evidence for this, agar blocks

• type of photoreceptor • blue light, flavin

molecule

Gravitropism

• statoliths• starch containing

amyloplasts• effect of Ca ions • supra-optimal effect

Distribution of Auxin

Physiological effects

• apical dominance • rooting of cuttings • tissue culture • herbicides- synthetic

auxin, agent orange

Auxin effects

fruit development

•Wounding and xylem regeneration

Mode of Action

• hormone receptor on membrane

• Short term effects • acid growth

hypothesis • wall loosening • proton pumps • Long term effects • protein synthesis

Lag time for auxin induced growth

Auxin induced acidification

• Auxin induces extrusion of H+

• Acidifies the cell wall

Model for Auxin cell elongation