Phototropism and the Cholodny-Went Hypothesis

Phototropism is a tropism — a directional growth response to a directional stimulus — in which a plant organ curves relative to the direction of incident light. Growth toward a unidirectional light source is positive phototropism (typical of stems and coleoptiles); growth away from light is negative phototropism (typical of some roots). The response is graded: the degree of curvature depends on light fluence and direction, not an all-or-none switch, and it is restricted to elongating tissue, most classically the grass coleoptile (the protective sheath around the embryonic shoot of grasses such as oat, Avena sativa) and the hypocotyls of dicot seedlings. The mechanistic core is differential growth: cells on the shaded flank of the organ elongate more, and faster, than cells on the illuminated flank, so the organ curves toward the light even though total elongation of the organ as a whole may barely change. Because plant cell elongation is driven by turgor-powered stretching of a wall loosened under acid conditions, anything that changes the rate of wall loosening on one side relative to the other will produce curvature. Auxin (indole-3-acetic acid, IAA) is the molecule that does this: it activates plasma-membrane H⁺-ATPases, acidifying the cell wall (the acid-growth hypothesis), which loosens the wall's load-bearing bonds and permits turgor-driven expansion. The Cholodny-Went hypothesis, formulated independently by Nikolai Cholodny and Frits Went in the late 1920s, ties these pieces together into a single causal chain: a directional stimulus (light or gravity) causes auxin, synthesized in the tip and normally transported straight down the organ's axis, to instead be redistributed laterally — more molecules routed to the shaded or lower side before continuing basipetally. That lateral asymmetry in auxin concentration produces an asymmetry in wall-loosening and elongation rate, and the organ bends. Bending is therefore not a report of "how much light fell on each side" directly, but a downstream consequence of an internal hormone gradient the light gradient creates.

Step 1 — Perception at the tip. Blue light (400–500 nm peak sensitivity, closely matching an action spectrum with a fine-structure of peaks around 450, 425, and 475 nm) is absorbed by phototropins (phot1 and phot2 in Arabidopsis), plasma-membrane-associated Ser/Thr kinases that use a flavin mononucleotide (FMN) chromophore bound in LOV (Light-Oxygen-Voltage) domains. Light absorption triggers a conformational change and autophosphorylation of the kinase domain, initiating a signaling cascade. Phototropins are most concentrated in the extreme apex of the coleoptile or the elongation zone just behind it — precisely the region Darwin identified as the "sensitive" zone.

Step 2 — Establishing a lateral light gradient. Because a coleoptile is a small, roughly cylindrical, slightly light-scattering structure, unilateral illumination creates a real difference in fluence rate between the lit and shaded flanks — the shaded side receives measurably less blue light, partly attenuated and partly refracted through the tissue acting as a weak lens. This physical asymmetry is the only external input the plant needs.

Step 3 — Lateral auxin redistribution. Auxin is synthesized mainly in young leaves and the shoot apex and is moved basipetally (tip-to-base) by polar auxin transport, a cell-to-cell relay driven by asymmetrically localized PIN-FORMED (PIN) efflux carriers on the basal membrane of transporting cells and AUX1/LAX influx carriers, as established in prior work on auxin transport. Phototropin activation at the tip changes the lateral (radial) distribution of these carriers — PIN3 and PIN7 in particular relocalize within the cells of the tip's vascular transition zone so that more auxin is channeled toward the shaded flank before it resumes its normal downward, cell-file-restricted journey. The mechanistic upshot: auxin flux down the shaded side of the organ becomes larger than auxin flux down the lit side, typically in a ratio in the range of roughly 60:40 to 70:30 (shaded:lit) in classic Avena measurements, sufficient to drive substantial curvature without requiring any destruction of auxin.

Step 4 — Differential cell elongation. In the elongation zone below the tip, the extra auxin arriving on the shaded side binds the nuclear receptor TIR1/AFB, promoting degradation of Aux/IAA repressor proteins and de-repressing ARF transcription factors, which upregulate genes including those encoding expansins and driving rapid, non-genomic activation of plasma-membrane H⁺-ATPases. Proton extrusion acidifies the cell wall (down to roughly pH 4.5 from a resting apoplastic pH near 5.5–6), activating expansins that disrupt hydrogen bonding between cellulose microfibrils and cross-linking glucans, loosening the wall. Turgor pressure (typically several bar in these cells) then stretches the loosened wall preferentially on the shaded side.

Step 5 — Macroscopic curvature. Because shaded-side cells elongate faster than lit-side cells over the same time interval, the organ's growing region curves, tip swinging toward the light source. Curvature continues until the light gradient across the tip disappears (i.e., until the organ points directly at the source), at which point auxin distribution symmetrizes and net lateral transport ceases — a simple negative-feedback endpoint, not a "goal" the plant computes.

Darwin and Darwin (1880), The Power of Movement in Plants. Charles Darwin, assisted by his son Francis, worked with canary-grass (Phalaris) and oat (Avena) coleoptiles. They observed that intact seedlings bent toward a lateral light source, and then performed a sequence of decisive manipulations. Covering only the extreme tip with an opaque cap (a small hood of foil or a blackened glass tube) abolished bending entirely, even though the rest of the coleoptile — the part that actually elongates and curves — remained fully illuminated. Covering only the base while leaving the tip exposed still produced normal bending. Removing the tip altogether also abolished the response. From these results Darwin concluded that "when seedlings are freely exposed to a lateral light, some influence is transmitted from the upper to the lower part, causing the latter to bend." This was the first clean separation, in any tropism, of the site of perception from the site of response — and it explicitly proposed a transmissible internal signal decades before that signal was chemically identified.

Boysen-Jensen (1910s–1913). The Danish plant physiologist Peter Boysen-Jensen tested whether Darwin's "influence" was electrical (as some contemporaries proposed) or a diffusible chemical. He decapitated coleoptiles and inserted a thin block of gelatin — permeable to small molecules but not electrically continuous in the same way as living tissue — between the excised tip and the stump, then replaced the tip on top of the gelatin. Unilateral illumination still produced normal curvature: the signal crossed the gelatin gap. He then repeated the experiment inserting an impermeable mica sheet on the shaded side only, between tip and stump; bending was suppressed on that side, while inserting mica on the illuminated side did not block bending. This asymmetric result was crucial: it showed not just that a chemical could cross a permeable barrier, but that the substance responsible for bending specifically traveled down the shaded flank — the earliest direct evidence for lateral, side-specific movement of the growth signal, exactly what the later Cholodny-Went model requires.

Went (1928), the avena curvature test. Frits Went, working in his father F. A. F. C. Went's laboratory in Utrecht, cut the tips off oat coleoptiles and let them stand for a defined interval on small blocks of plain agar, allowing the diffusible growth substance to pass from cut surface into the agar in the dark. He then placed these agar blocks — containing no plant tissue at all — asymmetrically on top of decapitated coleoptile stumps, covering only one side of the cut surface. In total darkness, with no light gradient whatsoever, the stumps bent away from the side bearing the agar block. Placing the block centered on the stump caused straight, symmetric growth; placing it fully off-center produced maximal curvature. Went further showed the amount of curvature was quantitatively related to the number of tips that had diffused into the agar and to the diffusion time, allowing him to construct a bioassay — the Avena curvature test — that became the standard method for quantifying auxin for the next several decades before chemical assays existed. This experiment proved two things simultaneously: that the tip produces a diffusible chemical sufficient by itself to cause curvature (no light or nervous signal required), and that an asymmetric application of that single chemical, with everything else held constant, reproduces phototropic-type bending. Went named the substance auxin (from Greek auxein, "to increase"). Together, these three results build an airtight chain of inference: Darwin shows perception and response are spatially separated and something is transmitted; Boysen-Jensen shows the transmitted thing is a diffusible chemical moving preferentially down the shaded side; Went shows that chemical, applied asymmetrically with no light at all, is sufficient to cause the bending. Cholodny, working independently on gravitropism with root and shoot tips in the same period, integrated these findings with his own auxin-distribution measurements into the general hypothesis that both phototropism and Gravitropism result from lateral auxin redistribution rather than any all-or-none signal. A modern refinement worth flagging honestly: direct radiolabeled-auxin measurements in intact coleoptiles (Briggs and colleagues, 1960s) confirmed a lateral asymmetry of roughly 60:40 favoring the shaded side under moderate unilateral light, enough by the acid-growth relationship to account for observed curvature rates — but the very high asymmetries (approaching total block on the lit side) that some early workers reported are now understood to be partly an artifact of the isolated-tip diffusion assay, and in some systems photodestruction of a small fraction of auxin on the illuminated side may make a modest additional contribution. The core lateral-transport mechanism, however, remains the dominant and best-supported explanation, and its molecular basis — light-triggered relocalization of PIN3/PIN7 auxin efflux carriers in the tip — has since been directly visualized with GFP-tagged PIN proteins in Arabidopsis hypocotyls, closing the loop between Went's 1928 bioassay and a defined molecular mechanism.

Example 1 — Predicting the outcome of a Boysen-Jensen-style barrier experiment. A student decapitates ten oat coleoptiles and re-attaches the tips using four different treatments, then exposes each to unilateral blue light for 90 minutes and measures curvature.

Reasoning through this table is exactly Boysen-Jensen's logic: the only variable that matters is whether the shaded-side channel is physically open. If curvature depended on an electrical signal, an impermeable mica sheet would have blocked it on either side equally, since mica interrupts the living cytoplasmic continuum regardless of which flank it is placed on. The fact that only the shaded-side block matters shows the causal channel is a diffusible substance moving asymmetrically, not a symmetric electrical signal.

Example 2 — Quantitative Avena bioassay calculation. In a Went-style bioassay, an experimenter collects auxin diffusate from a batch of 50 coleoptile tips into a standard 4 mm × 4 mm agar block over 90 minutes in the dark. A calibration curve (built by applying known concentrations of pure IAA to decapitated stumps and measuring resulting curvature after a fixed interval) gives: curvature (degrees) ≈ 8.4 × log₁₀(auxin concentration in ng/block) + 2 The experimenter applies the unknown block off-center to a decapitated, non-illuminated stump and measures 14° of curvature after the standard interval. Solve for the auxin content: 14 = 8.4 × log₁₀(x) + 2 12 = 8.4 × log₁₀(x) log₁₀(x) = 1.4286 x ≈ 10^1.4286 ≈ 26.8 ng of IAA-equivalent per block Dividing by the 50 tips that contributed diffusate, the average tip released roughly 0.54 ng of auxin into the agar over 90 minutes — a number of exactly the same order of magnitude Went reported and later chemical assays confirmed (typical estimates from Avena coleoptile tips run in the range of a few tenths of a nanogram to roughly 1 ng per tip per hour). This worked calculation illustrates why the bioassay was trusted quantitatively for decades: curvature magnitude scaled predictably (approximately log-linearly) with auxin dose, letting Went back-calculate real hormone quantities from a purely visual, angle-measuring assay long before HPLC or mass spectrometry existed.

Nikolai Cholodny (1882–1953) was a Ukrainian-Russian plant physiologist working at the Timiryazev Institute and later in Kyiv, whose careful microsurgical experiments on root and coleoptile tips in the 1920s measured growth-substance distribution directly by grafting tip segments asymmetrically onto decapitated stumps and recording resulting curvature — essentially a gravitropism-focused mirror image of Went's phototropism work. Frits Warmolt Went (1903–1990), Dutch-born and later a long-time professor at Caltech and director of the Missouri Botanical Garden, built his 1927 doctoral thesis in Utrecht around the diffusible-substance bioassay described above; he coined the name "auxin" and, throughout a long career spanning phytotronics and desert ecology, remained the figure most identified with turning Darwin's qualitative "influence" into a quantifiable molecule. The two scientists never collaborated directly, arriving at compatible conclusions from opposite ends of the plant (root versus coleoptile) and opposite continents, which is part of why the hypothesis carries both their names.

Understanding lateral auxin transport underlies practical plant manipulation across agriculture and biotechnology. Synthetic auxin herbicides (2,4-D, dicamba) exploit the same TIR1/AFB-Aux/IAA signaling axis described here, overwhelming normal growth regulation in broadleaf weeds while grasses are comparatively resistant due to differences in auxin metabolism and tissue anatomy. Rooting hormones used in horticultural cuttings (typically indole-3-butyric acid, IBA) work because polar auxin transport concentrates applied hormone at a cut base, promoting adventitious root initiation — a direct commercial application of the polar-transport machinery phototropism research first made legible. In controlled-environment agriculture (vertical farms, greenhouses), understanding phototropin action spectra guides LED spectral design: growers add blue light specifically to control stem architecture, keeping crops compact rather than etiolated and bent, since excess far-red-dominated or blue-deficient light produces weak, overly elongated, poorly self-supporting stems. In basic and translational research, PIN-protein trafficking dynamics discovered through phototropism and gravitropism studies are now central to understanding plant architecture engineering — breeders and CRISPR-based genome editors target PIN and AUX1/LAX gene family members to modify branching angle, root architecture, and canopy shape for higher-density planting and improved light capture, directly extending the Cholodny-Went logic from a coleoptile in a dark room to the design of modern crop ideotypes.

Ask the tutor: How does the mechanism of PIN3/PIN7 relocalization in phototropism compare mechanistically to statolith-driven auxin redistribution in root gravitropism?