Photomorphogenesis and Phytochrome
Bury a bean seed in the dark and it does something strange. It grows fast, stays pale, and never opens its leaves. Bring that same seedling into light and within hours it transforms. The stem stops elongating, chlorophyll floods in, and the first leaves unfurl. This switch from a blind search for light to a body plan built around it is called photomorphogenesis. It is one of the clearest cases in biology where a single environmental variable rewires development almost instantly. Plants cannot walk toward light. So they must build the right body depending on where light is and how much of it there is. A seed germinating two centimeters underground needs a completely different growth strategy than one whose cotyledons have just broken the soil surface. Getting this decision wrong is lethal. A seedling that "thinks" it is still buried while sitting in full sun wastes energy racing upward instead of building leaves. Meanwhile one that greens up prematurely underground burns its limited reserves making useless chlorophyll. Light is therefore not merely an energy source for plants. It is an information source, arguably as important developmentally as any hormone. The molecule that lets plants read this information is phytochrome. It is a blue-green pigment-protein that acts as a molecular switch between two conformations. One conformation is favored by red light, the other by far-red light. Red and far-red wavelengths tell a plant very different things about its surroundings. They distinguish bare soil versus shade, and open sky versus a leaf canopy overhead. Phytochrome effectively lets the plant ask "where am I, and what is above me?" every few minutes of its life. This article works through the physiological phenomenon of de-etiolation. It covers the biochemistry of the phytochrome switch, and the classic action-spectrum experiments that discovered it. It also presents two worked scenarios — a seedling emergence problem and a canopy-shading problem. These scenarios show phytochrome logic in action. It sets up two ideas explored elsewhere: the reversible red/far-red control system in detail, and how phytochrome timing feeds into Photoperiodism and Flowering and Circadian Rhythms in Plants.
- Photomorphogenesis is light-directed development; its opposite, skotomorphogenesis (etiolation), occurs in darkness.
- Dark-grown seedlings show the etiolated phenotype: long hypocotyl, closed apical hook, unopened cotyledons, no chlorophyll.
- Phytochrome exists in two interconvertible forms — inactive Pr (absorbs red, 660 nm) and active Pfr (absorbs far-red, 730 nm).
- Red light converts Pr → Pfr; far-red light converts Pfr → Pr; this reversibility is the defining proof of phytochrome action.
- Pfr moves into the nucleus, binds PIF transcription factors, and reprograms gene expression to trigger de-etiolation.
- The classic Borthwick–Hendricks lettuce-seed experiments (1952) established red/far-red photoreversibility as a real physiological switch, decades before the phytochrome protein was isolated (Butler, 1959).
Phytochrome is a reversible molecular switch — red light flips it "on" (Pfr), far-red light flips it "off" (Pr) — that lets a plant use light itself as a developmental instruction rather than just an energy source.
The core idea
Photomorphogenesis is the process by which light directs plant growth, form, and cellular differentiation, as distinct from skotomorphogenesis (etiolation), the developmental program that runs in darkness. Both programs use the same genome; light acts as a switch between two alternative developmental outputs, not as a trigger for growth itself. A seed does not "need" light to grow — dark-grown seedlings grow rapidly — it needs light to grow in the correct shape. The dark-grown, or etiolated, phenotype is dramatic and highly conserved across dicots: an elongated, thin hypocotyl; a tightly closed apical hook that shields the delicate shoot apical meristem and folded cotyledons as they are forced up through soil; unexpanded, yellow (chlorophyll-free) cotyledons; and an internal pigment precursor, protochlorophyllide, waiting to be converted into chlorophyll the instant light arrives. This is an energy-conserving, force-maximizing strategy: all resources go into upward extension growth, none into a photosynthetic apparatus that would be useless underground and metabolically wasteful to build in the dark. The moment light reaches the seedling, a suite of changes called de-etiolation begins: hypocotyl elongation slows sharply, the apical hook opens, cotyledons expand and turn green, chloroplasts differentiate, and hundreds to thousands of genes change expression — largely through a family of red/far-red photoreceptors called phytochromes, working alongside blue-light receptors (cryptochromes, phototropins) and UV-B receptors (UVR8) that are covered elsewhere. Phytochrome is the dominant receptor for detecting red and far-red light and for reading out canopy shade, and it is the photoreceptor whose discovery essentially founded the field of plant photobiology.
Mechanism
1. Phytochrome structure and the two-state switch. Phytochrome is a soluble chromoprotein consisting of a protein (apoprotein) covalently linked to a light-absorbing tetrapyrrole chromophore called phytochromobilin. The protein exists as a dimer, and each monomer independently binds one chromophore. Critically, phytochrome is photoreversible between two stable conformers:
| Form | Absorbs maximally | Biological activity |
|---|---|---|
| Pr (red-absorbing) | ~660 nm (red) | Inactive; cytoplasmic |
| Pfr (far-red-absorbing) | ~730 nm (far-red) | Active; translocates to nucleus |
Absorption of a red photon isomerizes the chromophore (a Z→E isomerization at the C15=C16 double bond of the open-chain tetrapyrrole), converting Pr into Pfr. Absorption of a far-red photon reverses this, converting Pfr back to Pr. Pfr also reverts to Pr slowly and spontaneously in the dark in many species (dark reversion), and Pfr is selectively degraded by the ubiquitin-proteasome system, so the Pfr pool naturally decays at night. The ratio of Pfr to total phytochrome (Pfr/Ptotal), often called the phytochrome photoequilibrium, is what the plant effectively "reads" — not simply whether light is present, but what kind of light, because sunlight, shade light, and twilight each set a different equilibrium ratio.
2. Why red versus far-red carries meaning. Chlorophyll absorbs red light strongly for photosynthesis but transmits and reflects far-red light. Light that has passed through or reflected off green leaves is therefore relatively enriched in far-red and depleted in red — it has a low red:far-red ratio (R:FR). Full, unfiltered sunlight has an R:FR ratio of roughly 1.0–1.2; light under a leaf canopy or beside a neighboring plant can drop to 0.1–0.2. Because Pfr is preferentially made by red light and destroyed by far-red light, canopy shade or a nearby competitor pushes the Pfr/Ptotal ratio down, which the plant reads as "I am shaded or competing for light" and responds to with the shade-avoidance syndrome (stem elongation, reduced branching, earlier flowering) — the physiological topic developed in The Red/Far-Red Response.
3. Signal transduction: from chromophore to nucleus. Photoactivation triggers a conformational change in the phytochrome protein that exposes nuclear localization signals. Pfr translocates from the cytoplasm into the nucleus, where it accumulates in discrete photobodies (nuclear speckles) whose size and number correlate with light intensity and duration. Inside the nucleus, Pfr directly binds and inactivates a family of bHLH transcription factors called PIFs (Phytochrome-Interacting Factors). In darkness, PIFs are stable, accumulate, bind promoters of light-responsive genes, and actively promote hypocotyl elongation and hook maintenance — PIFs are, in effect, the molecular drivers of etiolation. When Pfr binds PIFs, it triggers their phosphorylation and subsequent ubiquitination/degradation via the 26S proteasome, removing the etiolation program. Simultaneously, light-activated phytochrome promotes the degradation of COP1 (Constitutive Photomorphogenic 1), an E3 ubiquitin ligase that in darkness marks positive photomorphogenic transcription factors — chiefly HY5 (Elongated Hypocotyl 5) — for destruction. In the dark, COP1 (working from the nucleus) keeps HY5 protein levels low, permitting etiolation. In the light, COP1 is excluded from the nucleus and HY5 accumulates, directly binding thousands of promoters to switch on genes for chlorophyll synthesis, chloroplast development, anthocyanin production, and cell-wall changes that halt elongation. The net effect of light, then, is a double action: destroy the etiolation-promoting PIFs, and stabilize the photomorphogenesis-promoting HY5. These two arms converge on largely overlapping sets of target genes with opposite effects, giving light a switch-like, rather than gradual, control over the transcriptome.
4. Downstream cellular outcomes. With PIFs degraded and HY5 active, cells reduce auxin-driven cell elongation in the hypocotyl, the apical hook straightens (via altered auxin gradients across the hook), proplastids differentiate into chloroplasts, protochlorophyllide is converted to chlorophyllide by the light-dependent enzyme protochlorophyllide oxidoreductase (POR) and then to chlorophyll a, and cotyledon cells expand and begin photosynthesizing. Multiple phytochromes exist in flowering plants (phyA–phyE in Arabidopsis), with phyA mediating responses to very-low-fluence and far-red-rich light (important for detecting light under soil or deep shade) and phyB mediating the classical, more stable red/far-red reversible responses under typical daylight — a division of labor that fine-tunes the system across light environments.
5. Reading the light environment quantitatively. Because the Pr↔Pfr equilibrium depends on the ratio of red to far-red photons actually absorbed, plants effectively perform a real-time photometric measurement. Direct sunlight, shade light, and light filtered through soil each set a different steady-state Pfr fraction, and each fraction corresponds to a different developmental program — from full de-etiolation in open sun to shade-avoidance elongation under a canopy to full etiolation growth underground.
The evidence
The discovery of phytochrome is a textbook example of physiology preceding biochemistry: the switch was proven functionally, with light of two colors, more than a decade before the actual pigment protein was isolated.
Flint and McAlister (1935–1937), Lettuce seed germination. Lewis Flint and E. Darwin McAlister, at the USDA, found that Grand Rapids lettuce seeds — which require light to germinate — germinated best under red light and were inhibited under far-red or blue light. This was the first hint that specific wavelengths, not merely light intensity, controlled a developmental decision, but the mechanism was unclear.
Borthwick, Hendricks, and colleagues (1952), the photoreversibility experiment. The decisive experiment was performed by a USDA team led by Harry Borthwick and Sterling Hendricks. Using a custom spectrograph, they exposed batches of dormant, light-requiring lettuce seeds to narrow wavelength bands and then, crucially, exposed the same seeds to a second pulse of a different wavelength. Their key finding:
- Seeds given red light (~660 nm) alone: germination rate approached 100%.
- Seeds given red light, then immediately far-red light (~730 nm): germination dropped back to baseline (~10-20%, comparable to dark controls).
- Seeds given red, then far-red, then red again: germination was restored to the high rate.
- This could be repeated for many cycles, and the outcome was always determined by whichever wavelength was given last.
This perfect, repeatable reversibility was the proof: it demonstrated that a single photoreversible pigment, switching between two states, controlled the response — not two separate independent pigments, and not a simple additive light-dose effect. Borthwick and Hendricks named the hypothetical pigment "phytochrome" (from Greek phyton, plant, and chroma, color) and correctly predicted its two absorption forms years before anyone had purified the molecule.
Butler, Norris, Siegelman, and Hendricks (1959), physical isolation. Using a sensitive dual-wavelength spectrophotometer that could detect small absorbance changes in intact tissue and crude extracts, Warren Butler and colleagues directly measured light-induced, reversible shifts in absorbance at 660 nm and 730 nm in etiolated seedling tissue — the first physical, spectroscopic demonstration that phytochrome was a real molecule with the predicted two interconvertible chromophore states. They subsequently purified the pigment-protein complex from etiolated maize and oat seedlings (which are especially rich in phytochrome because they accumulate large amounts in darkness, a key practical reason etiolated monocot tissue became the standard source material).
Action spectra confirm red/far-red specificity across many responses. Beyond seed germination, the same red/far-red reversibility was subsequently demonstrated for a wide range of morphogenic responses: de-etiolation of hypocotyls, unfolding of the plumular hook, leaf expansion, anthocyanin synthesis, and control of flowering time (photoperiodism) — all showing the diagnostic signature of induction by red light, reversal by far-red, and reversal-of-reversal by a second red pulse. This generality across taxonomically distant responses was strong evidence that a single, conserved photoreceptor system, rather than a coincidence in one species or tissue, was responsible.
Molecular genetics (1980s–1990s) confirmed the model at the gene level. Cloning of PHY genes showed multiple phytochrome genes in genomes (phyA–phyE in Arabidopsis), and mutants lacking specific phytochromes (e.g., phyB mutants) showed constitutively elongated, partially de-etiolated-in-the-dark or shade-avoidance-like phenotypes even in red light, directly linking the photoreceptor gene to the whole-organism phenotype predicted by the 1952 physiology. Later work identified PIF and HY5/COP1 as the transcriptional machinery downstream, closing the gap between photon absorption and gene expression.
Worked examples
Example 1 — Predicting a seedling's fate from a light regime. A researcher germinates Arabidopsis seeds in complete darkness for 4 days, producing etiolated seedlings (long hypocotyl, closed hook, no chlorophyll). She then applies one of the following light pulses and returns the seedlings to darkness, measuring hook angle 24 hours later. Treatment sequence → predicted hook opening:
| Pulse sequence | Final Pfr status | Predicted hook response |
|---|---|---|
| Red only | High Pfr | Hook opens (de-etiolation triggered) |
| Red → Far-red | Pfr converted back to Pr | Hook stays closed (etiolated) |
| Red → Far-red → Red | High Pfr again | Hook opens |
| Far-red only | Pfr never accumulates (already mostly Pr) | Hook stays closed |
Reasoning: the physiological outcome depends only on the last pulse received, because each pulse resets the Pr/Pfr ratio and the plant's transcriptional machinery (PIF degradation, HY5 stabilization) responds to whichever conformer is present when the light signal is transduced. This is a direct, miniature replication of the Borthwick–Hendricks logic, and it predicts that the sequence "Red → Far-red → Red" behaves identically to "Red only," which is exactly what is observed experimentally — confirming there is a single, reversible photoreceptor rather than two separate, additive pigments.
Example 2 — Interpreting a canopy-shading scenario (shade avoidance). Two seedlings of the same species, Impatiens capensis (jewelweed), germinate 30 cm apart. Seedling A is in an open gap with full sunlight (R:FR ≈ 1.15). Seedling B is directly beneath a dense grass tussock whose leaves have already absorbed most red light, transmitting mostly far-red (R:FR ≈ 0.15). Step 1: Estimate the phytochrome photoequilibrium. Because Pfr is generated by red-photon absorption and lost (via reversion, degradation, and far-red-driven reversal) faster under far-red-enriched light, seedling B will have a substantially lower Pfr/Ptotal fraction than seedling A — perhaps 20-30% of total phytochrome, versus 55-60% for seedling A. Step 2: Translate the Pfr level into gene expression. Lower Pfr means less Pfr available to bind and degrade PIF transcription factors; PIFs accumulate in seedling B's nucleus and remain transcriptionally active there, continuing to drive auxin-responsive elongation genes. Step 3: Predict phenotype. Seedling B is expected to show classic shade avoidance: increased stem/internode elongation, reduced leaf area, upward leaf angle (hyponasty), reduced branching, and earlier reproductive transition, compared with seedling A, which instead invests in leaf expansion and lateral branching. Step 4: Adaptive interpretation (with the necessary caution): this response is not goal-directed foresight — it is the automatic output of a signal transduction cascade shaped by selection over evolutionary time, because genotypes whose phytochrome-PIF circuitry happened to elongate under low R:FR historically out-competed neighbors for light and left more offspring. The elongation is caused mechanistically by reduced Pfr-driven PIF degradation, not by the plant "sensing danger" and "deciding" to grow tall.
Common misconceptions & traps
| Misconception | Correction |
|---|---|
| "Phytochrome only matters for germination." | Phytochrome controls hook opening, cotyledon expansion, chlorophyll synthesis, shade avoidance, and photoperiodic flowering — germination is just the response first discovered. |
| "Far-red light does nothing to plants." | Far-red actively reverses phytochrome to the inactive Pr form; it is a real, opposite-acting signal, not simply "no light." |
| "Etiolation means the plant is dying or diseased." | Etiolation is a normal, adaptive developmental program for growth in darkness (e.g., underground), not a pathology. |
| "Light is only an energy source in plants." | Light is also information; phytochrome responses occur at very low fluence rates, far too dim to matter for photosynthesis. |
| "One phytochrome molecule does everything." | Flowering plants have multiple phytochromes (phyA–phyE in Arabidopsis) with distinct, partly overlapping roles across light intensities. |
| "Phytochrome directly turns genes on/off by binding DNA." | Phytochrome does not bind DNA itself; it regulates transcription indirectly by controlling the stability/activity of PIF and HY5/COP1. |
The scientist behind it
Sterling Hendricks and Harry Borthwick worked together for decades at the USDA's Plant Industry Station in Beltsville, Maryland, an unusually productive partnership between a physical chemist (Hendricks) and a plant physiologist (Borthwick). Hendricks brought rigorous physical-chemistry instrumentation — including a custom-built large spectrograph capable of isolating narrow light wavebands, an unusual investment for an agricultural research station in the 1940s — while Borthwick supplied the botanical expertise in seed physiology and photoperiodism that had originally been the group's practical mandate (they were investigating why some crop varieties failed to flower properly under different day lengths). Their willingness to treat a practical agronomy problem as a fundamental physics-and-chemistry question is a large part of why they succeeded where purely descriptive botany had stalled for two decades after Flint and McAlister's initial red-light germination results.
Origins · 1 minSterling Brown Hendricks and how this was discovered
Hendricks trained as a physical chemist and applied X-ray crystallography and spectroscopy to biological problems at a time when few plant physiologists used such tools. His insistence on quantitative, reversible-switch logic — rather than vague "light quality" descriptions — is what let the phytochrome hypothesis be stated as a testable, falsifiable physical model years before the pigment was isolated.
Where this matters today
Horticulture and greenhouse lighting. LED "grow lights" are engineered around phytochrome logic: growers manipulate red:far-red ratios to control stem length, branching, and flowering time in ornamentals and vegetable transplants, avoiding the leggy, weak-stemmed growth caused by low R:FR light in crowded plantings.
Crop breeding for shade tolerance and yield. Because shade-avoidance elongation diverts resources from grain/fruit production into stem growth, breeders have selected crop varieties (e.g., certain maize and soybean lines) with altered phytochrome or PIF signaling that suppress excessive shade-avoidance elongation, allowing denser planting without yield loss from lodging or wasted biomass — a direct agricultural payoff of understanding the Pfr/PIF/HY5 pathway.
Forestry and weed management. Far-red-rich light reflected from weed canopies triggers shade-avoidance responses (early stem elongation, reduced tillering) in crop seedlings even before physical shading occurs, informing weed-control timing and row-spacing strategies designed to keep R:FR ratios high around young crop plants.
Post-harvest and food science. Manipulating red/far-red exposure during storage and transport is used to control chlorophyll retention, anthocyanin accumulation, and softening in some fruits and vegetables, since these processes are partly under phytochrome-HY5 transcriptional control.
Foundational tool for photobiology and synthetic biology. Phytochrome's clean, reversible, two-state switching behavior has made it a model system for optogenetics: engineered phytochrome-based switches are used in synthetic biology to control gene expression or protein localization in cells using pulses of red and far-red light, borrowing the exact reversible logic first demonstrated in 1952 lettuce seeds.
Ask the tutor: How does the phytochrome Pfr/Pr ratio at dusk actually get used by the plant to measure night length for flowering?
Is phytochrome the only photoreceptor involved in photomorphogenesis?
No. Phytochrome (phyA–phyE in Arabidopsis) handles red and far-red light, but plants also use cryptochromes and phototropins to detect blue/UV-A light, and UVR8 to detect UV-B. These systems overlap and interact — for example, cryptochromes also help stabilize HY5 and destabilize COP1, converging on much of the same transcriptional machinery phytochrome uses. Photomorphogenesis is best understood as an integrated output of several photoreceptor systems working together, with phytochrome being the dominant and best-characterized one for red/far-red responses like de-etiolation and shade avoidance.
Why do etiolated seedlings have an apical hook?
The hook is a mechanical protection structure: it curls the delicate shoot apical meristem and folded cotyledons downward and inward as the seedling pushes upward through soil, preventing abrasion damage to the meristem. The hook is maintained by an auxin gradient sustained by PIF activity in darkness. When light activates phytochrome and degrades PIFs, that auxin gradient collapses, and the hook straightens — one of the fastest, most visually obvious markers of de-etiolation, often complete within hours of light exposure.
Can far-red light alone trigger germination or de-etiolation?
Generally no — far-red light alone keeps phytochrome mostly in the inactive Pr state and suppresses the classic red-light-induced responses, which is exactly why it is used as the "reversing" treatment in Borthwick–Hendricks-style experiments. However, phyA, one of the phytochrome family members, is specifically sensitive to continuous far-red-rich light at very low fluence (relevant to seeds buried in soil sensing faint far-red light filtering down), so under specific conditions far-red signaling through phyA can drive some responses — a nuance beyond the classical phyB-dominated red/far-red story.
How fast is de-etiolation after a plant first sees light?
Some transcriptional changes (PIF degradation, initial gene expression shifts) occur within minutes of a red-light pulse. Visible phenotypic changes — hook opening, slowing of hypocotyl elongation — typically become apparent within a few hours, and full greening (chlorophyll accumulation, chloroplast maturation) takes roughly one to several days depending on species and light conditions. The speed of the transcriptional switch (minutes) versus the visible morphological output (hours to days) reflects the time needed to translate an altered transcriptome into new cell wall properties, pigment synthesis, and organelle differentiation.
Does phytochrome only act in seedlings, or throughout the plant's life?
Throughout life. Beyond seedling de-etiolation, phytochrome continues to monitor the light environment in mature plants, controlling shade-avoidance responses in adult stems and leaves, leaf senescence timing, and — critically — measurement of night length for photoperiodic control of flowering, covered in Photoperiodism and Flowering. Phytochrome's dark-reversion kinetics also feed into the plant's Circadian Rhythms in Plants, since the Pfr level at dusk helps set the phase of the internal clock.
Why did researchers use etiolated maize and oat seedlings to first isolate phytochrome?
Etiolated (dark-grown) monocot seedlings accumulate unusually large amounts of phytochrome protein in their tissue, because in the dark PIF-driven growth continues without the light-triggered degradation of Pfr that would otherwise deplete the pool. This made etiolated maize and oat tissue a far richer, more practical source for spectroscopic detection and biochemical purification in the 1950s-1960s than green, light-grown tissue, where phytochrome levels are much lower due to continuous light-driven turnover.
How does this connect to plant hormones I already know about?
Phytochrome does not act instead of hormones — it acts through them. Pfr's control of PIF stability directly regulates auxin biosynthesis and signaling genes in the hypocotyl and apical hook, and light signaling also cross-talks with gibberellin (GA) levels, since GA degradation of another repressor family (DELLA proteins) is itself light-regulated. In this sense phytochrome sits upstream of the hormonal machinery described in Plant Hormones: An Overview, reprogramming hormone synthesis and sensitivity rather than replacing hormonal control.