CoachThe Year 1 volume described what the plant hormones do; here is how they are heard. Several act by regulated destruction. Auxin (indole-3-acetic acid) binds the F-box protein TIR1, gluing it to the Aux/IAA repressors, which are then ubiquitinated and destroyed by the proteasome: the auxin-response genes they were repressing switch on within minutes — the hormone is a molecular glue, and the receptor is part of the degradation machinery. Gibberellin does the same with the DELLA repressors of growth, through its receptor GID1: the dwarf wheats and rices of the Green Revolution carry DELLA proteins that cannot be destroyed, and so stay short however much gibberellin they make. Jasmonate follows the same logic with the JAZ repressors. Abscisic acid (ABA), the hormone of drought and dormancy, binds the PYR/PYL receptors, which then inhibit a phosphatase (PP2C), releasing a kinase (SnRK2) that phosphorylates ion channels and transcription factors — a double negative that makes the response steep. Ethylene, a gas, binds copper-containing receptors in the ER membrane that in its absence actively repress the response; binding switches them off, and the ripening, senescence and stress genes come on. Cytokinins act through histidine-kinase receptors like bacterial two-component systems; brassinosteroids, the plant’s steroids, through a surface receptor kinase, not a nuclear receptor — the only steroid hormones anywhere heard at the cell surface. Plants have no glands: each hormone is made where it is needed, or moved by specific carriers, and its concentration is set locally by synthesis, conjugation and destruction.
Examples
Example 22.5 (Gravitropism and phototropism)
Turn a seedling on its side. In the root cap, dense starch-filled plastids settle onto the new lower side of the columella cells within minutes; the PIN3 carriers of those cells relocate to the lower face; auxin flows preferentially down the lower flank of the root, where, above the optimum for root cells, it inhibits elongation, and the root curves downward. In the shoot the same lateral flow to the lower side promotes elongation (shoot cells’ optimum is higher), and the shoot curves upward. Phototropism: phototropin on the lit side of a shoot alters PIN placement so that auxin accumulates on the shaded side, which grows faster, bending the shoot toward the light. Both movements are slow — hours — because they are growth, not motion, and both are irreversible in the tissue that has grown. Darwin (1880) showed the tip of a grass seedling perceives the light and the region below bends; Went (1928) collected the influence in an agar block placed on a cut tip and showed a block placed asymmetrically on a decapitated shoot made it bend: the influence was a diffusible substance, which was then named auxin.