Auxins and Gibberellins

Auxin's discovery began with a simple observation Charles Darwin made together with his son Francis: the coleoptile of a canary-grass seedling, when lit from one side only, bends toward the light, but if the very tip of the coleoptile is covered or removed, this bending no longer occurs -- implying that the tip itself perceives the light and sends some influence down to the elongating region below it. Decades later, Boysen-Jensen showed that this influence could pass through a gelatin block inserted between the tip and the lower coleoptile but was blocked by an impermeable barrier such as mica, establishing that the signal travelling down from the tip was a diffusible chemical substance rather than, say, an electrical or purely mechanical one. It was F. W. Went, working with oat (Avena) coleoptiles, who finally isolated this substance: he placed excised coleoptile tips asymmetrically on small agar blocks, allowed the presumed chemical to diffuse into the agar, discarded the tips, and then placed the agar blocks themselves off-centre atop decapitated coleoptiles kept in the dark -- and observed that these coleoptiles bent away from the side bearing the agar block even without any light at all, proving that a genuine chemical, and not light itself, was directly responsible for the differential elongation. Went named this substance auxin; the principal naturally occurring auxin in plants is indole-3-acetic acid (IAA), though several synthetic auxins such as naphthalene acetic acid (NAA) and 2,4-dichlorophenoxyacetic acid (2,4-D) are also widely used. Auxin is synthesised chiefly in the shoot apex and young, actively growing leaves, from where it is transported down toward the elongating and root regions.

Auxin's physiological effects are correspondingly wide-ranging: it promotes cell elongation, particularly on the shaded side of a coleoptile or stem exposed to unilateral light, producing the phototropic bending with which its discovery story began; it maintains apical dominance, the suppression of lateral (axillary) bud growth for as long as the shoot's main apical bud remains active and continues producing auxin, so that removing the apical bud (as gardeners do when they 'pinch out' a shoot tip) releases the lateral buds to grow; it promotes the initiation of adventitious roots, which is why rooting powders used on stem cuttings are typically auxin-based; at low concentration it prevents the premature abscission of young leaves and fruits, while at high concentration it can instead promote abscission; it can induce parthenocarpic (seedless) fruit development, as is exploited commercially in some tomato varieties; and, in the synthetic form 2,4-D, it is widely used as a selective herbicide, because it disrupts the growth of dicot weeds at concentrations that leave monocot cereal crops largely unaffected. …