Plant hormones and tropisms

Concept

Plant hormones and tropisms

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Statement

Directional growth in response to stimuli.

Why it matters

light-reactions and calvin-cycle establish how a plant captures and fixes energy, while cohesion-tension-transpiration and stomatal-regulation establish how it moves water and gas; this result addresses a different problem entirely — how a rooted, immobile organism nonetheless controls the direction of its own growth in response to its environment, despite being unable to relocate the way an animal can. Hormonal signalling is the plant's substitute for the nervous and muscular systems animals use to move directionally, and auxin is the master coordinating signal that makes it possible.

Understanding tropisms as a hormone-redistribution problem, rather than as some vaguer notion of a plant "sensing and reaching toward" a stimulus, is what allows the same underlying mechanism to explain both phototropism (bending toward light) and gravitropism (roots growing down, shoots growing up) using a single, shared signalling logic.

Hypotheses

Auxin (indole-3-acetic acid, IAA) is transported directionally between cells (polar transport), rather than diffusing passively and uniformly.Directional, cell-to-cell transport, driven by efflux carriers concentrated at one end of each cell, is what allows a lateral concentration gradient to form and persist across an organ in response to an asymmetric stimulus; passive diffusion alone would simply equalise concentration and prevent any such gradient. Over the timescale of an observed tropic response, differential cell elongation, not differential cell division, is the primary mechanism producing curvature.This lets an asymmetric auxin distribution translate directly and rapidly into an asymmetric growth rate on the two sides of an organ, rather than requiring new cells to be produced, which would take considerably longer. Gravitropism specifically relies on sedimentation of dense starch-containing organelles (statoliths) within specialised gravity-sensing cells (statocytes), an assumption particular to that tropism rather than to tropisms in general.

Proof

\text{A unidirectional stimulus (light or gravity) is perceived and produces an asymmetric redistribution of auxin across the responding organ.}

For phototropism, blue-light photoreceptors trigger lateral auxin transport toward the shaded side; for gravitropism, statolith sedimentation in root or shoot statocytes provides the directional cue that redirects auxin efflux carriers. A

\text{Auxin concentration becomes higher on the shaded (or lower) side of the organ than on the illuminated (or upper) side.}

Polar transport (Hypotheses) actively moves auxin laterally in response to the perceived asymmetry, generating and actively maintaining a concentration gradient across the organ's width rather than allowing it to equilibrate — the core claim of the Cholodny-Went hypothesis. A

\text{Elongation rate rises with auxin concentration in shoot tissue but is inhibited by the same concentration in root tissue.}

Shoot and root tissues have oppositely tuned dose-responses to auxin: the same signal (a locally higher auxin concentration) promotes elongation on a shoot's shaded side while repressing it on a root's lower side, which is why shoots bend toward light and away from gravity while roots bend toward gravity, using an identical hormone and gradient direction. B

\text{Differential elongation rate across the two sides of the organ produces curvature toward or away from the stimulus.}

Cells on the faster-elongating side lengthen more than cells on the slower side over the same time interval, physically bending the organ; this follows directly from Step 3's asymmetric growth rates acting on the two flanks of a single continuous structure. A

\text{Abscisic acid (ABA) accumulates under water or osmotic stress and triggers stomatal closure and growth-inhibiting responses, acting antagonistically to auxin and gibberellin.}

ABA is the plant's principal stress hormone, redirecting resources from growth toward conservation when water availability threatens survival; stomatal-regulation's guard-cell closure is a direct downstream ABA effect, operating through a distinct regulatory axis from the auxin-driven tropisms of Steps 1–4. A

Result

\text{lateral stimulus} \to \text{asymmetric auxin} \to \text{opposite shoot/root dose-response} \to \text{differential elongation} \to \text{curvature}

Reading. A single mobile hormone signal, redistributed asymmetrically by a perceived directional stimulus, is read out into opposite growth responses by shoot and root tissue, producing the familiar shoots-toward-light, roots-toward-gravity divergence from one shared underlying mechanism.

Scope. Explains fast, differential-elongation-based curvature in actively growing plant organs; does not by itself account for slower, division-based directional growth, nor for the full network of interacting hormones (gibberellins, cytokinins, ethylene, and ABA) that modulate auxin's effects.

Corollaries & converses

stomatal-regulation's guard-cell response to ABA (Step 5) is best understood as part of the same broader plant-hormone signalling system as tropic auxin action, even though the two operate largely through separate downstream pathways.
Because polar auxin transport is directional at the cellular level (Hypotheses), removing the tip of a coleoptile or shoot — the primary source of auxin — abolishes phototropic bending until auxin is experimentally reapplied, classic evidence for auxin's causal role rather than a mere correlation.
Converse: an organ showing no differential elongation between its two flanks under an asymmetric stimulus, despite an intact auxin transport system, points to a defect in the differential dose-response of Step 3 rather than in stimulus perception or auxin transport itself.

Fails without

Drop polar (directional) auxin transport (Hypotheses): auxin would diffuse and equilibrate roughly evenly across the organ regardless of which side is stimulated, so no lateral concentration gradient — and hence no differential growth or curvature — could be sustained, however strong or asymmetric the original stimulus was.
Drop the opposite shoot-versus-root auxin dose-response (Step 3): without this reversal, roots would bend away from gravity in the same direction shoots bend toward light rather than growing into the soil; the single shared signal (auxin asymmetry) would then produce identical, not opposite, tropic outcomes in the two organ types, which is not what is observed.

Common errors

Assuming auxin simply "causes growth" at a single fixed concentration, rather than recognising that its effect is dose- and tissue-dependent, with the identical concentration promoting elongation in shoots while inhibiting it in roots (Step 3).
Describing phototropism or gravitropism as a plant "actively bending toward" a stimulus, which obscures the actual mechanism: it is asymmetric growth, more elongation on one side than the other, not contraction or movement of already-formed tissue.
Confusing tropisms (directional growth responses to a directional stimulus) with nastic movements (non-directional responses to a non-directional stimulus, such as touch-triggered leaf folding, which relies on rapid turgor change rather than differential growth and occurs regardless of the stimulus's direction).
Treating auxin as the only hormone involved in tropic or stress responses, when gibberellins, cytokinins, ethylene and ABA all interact with and modulate auxin's effects rather than acting in isolation.

Discussion

Charles Darwin and his son Francis Darwin's coleoptile-bending experiments, published in The Power of Movement in Plants (1880), first demonstrated that the tip of a grass coleoptile perceives light and transmits a signal to the elongating region below it, long before auxin itself was chemically identified. Frits Went's experiments in the late 1920s — isolating the growth-promoting substance by diffusion into small agar blocks, then showing that an off-centre agar block alone could induce curvature in complete darkness — provided the direct evidence for a mobile chemical messenger underlying the Darwins' observation, and gave the resulting mechanism its name, the Cholodny-Went hypothesis, after Went and Nikolai Cholodny, who independently proposed the lateral-transport mechanism at around the same time.

Modern molecular work has identified specific auxin efflux carriers (PIN proteins) whose asymmetric, regulated relocalisation within cells provides the actual molecular basis for the polar transport the Hypotheses simply assert; PIN protein relocalisation in response to gravity or light signalling is now understood as the proximate mechanistic step underlying Steps 1–2's "asymmetric auxin distribution."

Common misconception: that gravitropism and phototropism are two entirely separate mechanisms. Per the Result, both are variations on the same core auxin-redistribution logic, differing mainly in how the initial directional stimulus is perceived (statolith sedimentation versus photoreceptor light sensing), not in how that perception is converted into curvature.

Worked examples

\text{Went's coleoptile experiment: tip removed, placed on an agar block, block then placed off-centre on a decapitated stump in darkness}

Because the coleoptile is kept in the dark, light perception (Step 1) cannot be responsible for any observed asymmetry; the auxin concentration difference has been imposed directly and artificially by the off-centre placement of the auxin-containing agar block, and the stump curves away from the side bearing the block, confirming that an asymmetric auxin distribution alone (Step 2) is sufficient to cause curvature (Step 4), independent of any actual directional light stimulus. B

\text{Differential elongation: } L_0=10\,\text{mm},\ r_{\text{lit}}=0.05\,\text{h}^{-1},\ r_{\text{shade}}=0.15\,\text{h}^{-1},\ t=2\,\text{h}

Illuminated-side segment length after \(2\,\text{h}\): \(L_{\text{lit}}=10(1+0.05\times2)=11\,\text{mm}\). Shaded-side segment length: \(L_{\text{shade}}=10(1+0.15\times2)=13\,\text{mm}\). The \(2\,\text{mm}\) length difference across the organ's diameter, accumulated over just two hours, is sufficient to produce clearly visible curvature toward the light source, consistent with observed phototropic bending rates in young seedlings. A

L_{\text{shade}}-L_{\text{lit}} = 13-11 = 2\,\text{mm after }2\,\text{h}\ \Rightarrow\ \text{curvature toward the light}

Reading. A modest, sustained difference in elongation rate between the two sides of an organ is enough to produce measurable curvature within hours, illustrating why phototropic bending is one of the faster observable plant growth responses.

Scope. The identical calculation, with the sign of the differential reversed, applies to gravitropic bending in roots (Problem 1).

Problems

A root is placed horizontally. Predict which side (upper or lower) shows greater cell elongation, and in which direction the root will curve, referencing Step 3.
Solution
Statoliths sediment toward the lower side of the root's statocytes, redirecting auxin to accumulate on the lower side (Steps 1–2, the same directional logic as in shoots). Root tissue's dose-response is inhibitory to elongation at that higher auxin concentration (Step 3), so the lower side elongates less than the upper side, and the root curves downward — into the direction of gravity — as differential elongation continues (Step 4).
Explain, using the Cholodny-Went hypothesis, why removing a shoot's tip abolishes its phototropic response, and why reapplying auxin symmetrically (rather than asymmetrically) to the cut stump fails to restore directional bending.
Solution
Without the tip, there is no auxin source available to redistribute asymmetrically in response to light perception (Step 1), so no lateral gradient (Step 2) forms and no differential elongation (Steps 3–4) occurs regardless of the light stimulus. Reapplying auxin evenly on both sides restores elongation-promoting auxin levels generally, but produces no asymmetry between the two flanks, so growth resumes without directional curvature — confirming that it is specifically the asymmetry, not auxin's mere presence, that causes bending.
A researcher observes that under drought stress, a plant's stomata close and shoot growth slows even in well-lit, otherwise favourable conditions. Which hormone (Step 5) most likely mediates this, and what is the adaptive logic?
Solution
Abscisic acid (ABA), which accumulates under water or osmotic stress, triggers stomatal closure (limiting further water loss via transpiration, stomatal-regulation) and antagonises growth-promoting hormones such as auxin and gibberellin, redirecting the plant's priorities from growth toward water conservation and survival until stress conditions ease.