The Red/Far-Red Response

Phytochrome is a soluble chromoprotein consisting of a protein moiety (apoprotein) covalently attached to a linear tetrapyrrole chromophore called phytochromobilin. It is synthesized in a red-light-absorbing conformation, designated Pr, which absorbs maximally around 660 nm and appears blue-green to our eyes. Absorption of red light causes a photoisomerization of the chromophore (a cis-trans switch at the C15=C16 double bond of the D-ring), which drives a conformational change in the protein and converts it to Pfr, the far-red-absorbing form, with an absorption maximum near 730 nm. Critically, Pfr is not a dead end: it can absorb far-red light and revert back to Pr. Because the absorption spectra of Pr and Pfr overlap substantially, monochromatic red or far-red light never converts the pigment pool completely — each wavelength drives the system toward a photoequilibrium, a steady-state mixture of Pr and Pfr described by the ratio Pfr/Ptotal (often written Pfr/(Pr+Pfr), sometimes abbreviated φ). The essential conceptual move is this: Pfr is the physiologically active signaling form. Pr is essentially inert. This single fact explains why red light promotes so many "light-requiring" responses (germination of light-sensitive seeds, de-etiolation, chloroplast development, inhibition of stem elongation) while far-red light antagonizes them. It also explains photoreversibility: whichever light pulse falls last dictates the Pfr/Ptotal ratio at the moment the cell "reads" the signal, and hence dictates the physiological outcome, regardless of how many alternating flashes preceded it. Phytochrome comes in a small gene family (in Arabidopsis, PHYA through PHYE), which allows plants to fine-tune this system: some phytochromes (like phyA) are labile in light and specialize in detecting very low fluence or continuous far-red-rich light, while others (like phyB) are stable and dominate the classical red/far-red reversible responses and the shade-avoidance syndrome discussed below. This diversity is itself an experimental clue — it was the discovery of several distinct action spectra and mutant phenotypes that revealed multiple phytochromes rather than one.

Step 1 — Synthesis and the ground state. Phytochrome apoprotein is synthesized in the cytosol and auto-catalytically attaches its chromophore, phytochromobilin, made via the heme biosynthetic pathway (heme → biliverdin → phytochromobilin). The resulting holoprotein is synthesized as Pr. Pr is thermodynamically stable in darkness — it does not spontaneously convert to Pfr, and dark-grown ("etiolated") tissue accumulates large pools of Pr-form phytochrome.

Step 2 — Photoconversion. When a Pr molecule absorbs a red photon (peak absorbance ~660 nm), the chromophore's D-ring undergoes a Z→E isomerization at the 15,16 double bond. This chromophore twist forces the surrounding protein through a series of conformational intermediates (this happens on a nanosecond-to-millisecond timescale) that ultimately produce the stable Pfr conformation, which absorbs maximally at ~730 nm. The reverse reaction — a far-red photon driving Pfr back to Pr — is the mirror image, an E→Z isomerization restoring the Pr conformation. Because the Pr and Pfr absorption spectra overlap (Pr still absorbs some far-red; Pfr still absorbs some red), continuous irradiation with either wavelength drives the pigment population to a photoequilibrium rather than to 100% of either form. Under natural sunlight, this equilibrium sits at roughly Pfr/Ptotal ≈ 0.5–0.6; under a leaf canopy, enriched in far-red, it falls to roughly 0.05–0.2.

Step 3 — Dark reversion and destruction. In darkness, Pfr slowly reverts non-photochemically back to Pr in many species (thermal or "dark reversion"), and Pfr is also selectively degraded by the ubiquitin-proteasome system faster than Pr. Both processes erode the Pfr pool over hours, which is part of how plants measure the length of a dark period — a mechanism foreshadowing Photoperiodism and Flowering.

Step 4 — Nuclear translocation. Formation of Pfr exposes a nuclear localization signal and drives translocation of phytochrome from the cytosol into the nucleus, where it accumulates in discrete subnuclear foci called photobodies. This step is essential: phytochrome must physically move to the nucleus to alter transcription, and mutants that block this translocation are photomorphogenically blind even though the chromophore still isomerizes normally.

Step 5 — Direct binding to transcriptional regulators. In the nucleus, Pfr binds directly to a family of basic helix-loop-helix transcription factors called phytochrome-interacting factors (PIFs). In darkness, PIFs accumulate and actively promote the etiolated ("skotomorphogenic") developmental program: elongated hypocotyls, closed cotyledons, undeveloped chloroplasts (etioplasts), and an apical hook. When Pfr binds PIFs, it triggers their phosphorylation and subsequent degradation via the 26S proteasome, and Pfr also physically interferes with PIF binding to DNA. The result is de-repression of the photomorphogenic program: hypocotyl elongation stops, cotyledons expand and green, and thousands of genes controlling chlorophyll synthesis, cell wall properties, and hormone metabolism are switched on or off.

Step 6 — Downstream hormonal integration. Phytochrome signaling reprograms hormone pathways — reducing auxin and gibberellin signaling that drives cell elongation in the dark, and altering ethylene sensitivity — which is how a purely photochemical switch cascades into altered stem length, leaf expansion, and ultimately flowering time and germination.

The discovery of phytochrome is one of the cleanest examples in biology of an action spectrum leading directly to the identification of a molecule. The story begins not with phytochrome itself but with a practical agricultural puzzle: certain lettuce varieties (notably 'Grand Rapids') would germinate well in light but poorly in darkness, and germination was oddly sensitive to the exact wavelength of light used.

The Borthwick–Hendricks lettuce seed experiments (Beltsville, USDA, early 1950s). Harry Borthwick and Sterling Hendricks, working with colleagues including Marion Parker and Elda Toole, built a large spectrograph — essentially a prism-based instrument that spread sunlight or an intense lamp source into its component wavelengths — and exposed batches of imbibed (water-soaked) lettuce seeds to narrow bands of light across the visible and near-infrared spectrum. They then scored germination percentage after a standard dark incubation period. The key findings: 1. An action spectrum for promotion of germination showed a sharp peak of effectiveness at 660 nm (red light). A brief pulse of red light was sufficient to trigger high germination percentages, even though seeds were then returned to darkness. 2. An action spectrum for inhibition of germination showed a peak at 730 nm (far-red light). Far-red-treated seeds germinated at rates no better than seeds kept in complete darkness. 3. Photoreversibility. This was the decisive experiment. Seeds were given alternating pulses of red (R) and far-red (FR) light in various sequences, then returned to darkness and scored for germination:

The rule that emerged was absolute: whichever wavelength came last determined the outcome, regardless of how many reversals preceded it. Borthwick and Hendricks published this result in 1952 (Proceedings of the National Academy of Sciences) and expanded it in 1954, establishing photoreversibility as a defining, reproducible property of a single pigment system rather than two independent pigments.

Why this was proof, not just correlation. Photoreversibility is a very strong test because it rules out the alternative hypothesis that red and far-red light simply act through separate, independent pathways (one promoting, one inhibiting, additively). If that were true, repeated alternation would produce intermediate or cumulative effects. Instead, the all-or-none reversal, with the last pulse always winning, is the behavior expected only of a single interconvertible pigment switching between two states — precisely the behavior later confirmed spectrophotometrically.

Direct spectrophotometric confirmation. In 1959, Warren Butler, Karl Norris, Henry Siegelman, and Sterling Hendricks used a specially built dual-wavelength spectrophotometer to measure light absorbance changes directly in living, dark-grown (etiolated) plant tissue — since etiolated tissue is largely free of interfering chlorophyll. They detected a pigment whose absorbance at 660 nm decreased and whose absorbance at 730 nm increased upon red irradiation, and vice versa upon far-red irradiation — matching, wavelength for wavelength, the action spectra Borthwick and Hendricks had derived from whole-seed physiology. This was the first direct physical detection of phytochrome as a real molecule, confirming that the germination action spectra reflected the photochemistry of an actual pigment rather than a coincidence of two separate sensory systems. The pigment was subsequently purified (from etiolated oat and other tissue, where it is abundant) and named "phytochrome" by Hendricks and Butler.

Molecular confirmation of PIF-mediated signaling. Much later (1990s–2000s), genetic and biochemical work — using Arabidopsis mutants defective in phytochrome B (phyB) and in PIF genes, along with yeast two-hybrid and co-immunoprecipitation assays — showed that Pfr, but not Pr, physically binds PIF proteins, that this binding requires Pfr's conformation specifically, and that PIF phosphorylation and degradation depend on this interaction. This closed the causal chain from photon absorption to transcriptional change, work associated with laboratories including those of Peter Quail and colleagues.

Example 1 — Predicting a lettuce seed's fate from a light protocol. A batch of dormant, imbibed 'Grand Rapids' lettuce seed is given the following treatment, then placed in the dark for 48 hours: Red (1 min) → dark (2 h) → Far-red (1 min) → dark (2 h) → Red (30 s) → dark (2 h) → Far-red (1 min) → 48 h dark. Reasoning: Apply the photoreversibility rule — only the identity of the final light pulse matters, not the number of prior reversals, and not the exact duration of each pulse (above a low threshold fluence). The sequence of pulses is R, FR, R, FR. The last pulse is far-red. Prediction: Germination will be suppressed, at a rate statistically indistinguishable from seeds kept continuously in the dark (typically <10% germination in this classic system, versus >90% for a red-terminated sequence). This is exactly the pattern Borthwick and Hendricks reported, and it is a direct test of whether a student has internalized "last light wins" rather than "more red pulses win" or "cumulative exposure matters" — both of which are wrong.

Example 2 — Quantifying the Pfr/Ptotal ratio under a canopy and predicting shade avoidance. Direct sunlight has a red:far-red (R:FR) ratio of roughly 1.2 (that is, roughly equal red and far-red photon fluence, red slightly favored). Under a dense leaf canopy, chlorophyll absorbs strongly in the red (~660 nm) for photosynthesis but is nearly transparent to far-red (~730 nm), so the light transmitted and reflected by leaves is far-red-enriched. Typical canopy shade has an R:FR ratio as low as 0.05–0.20. Suppose:

• In full sun, photoequilibrium gives Pfr/Ptotal ≈ 0.55 for phytochrome B.
• Under canopy shade with R:FR ≈ 0.10, Pfr/Ptotal falls to roughly ≈ 0.10.

Reasoning: Lower Pfr/Ptotal means less active Pfr available to bind and destabilize PIF transcription factors. PIF proteins (particularly PIF4, PIF5, and PIF7 in the shade-avoidance context) accumulate and remain active, driving auxin biosynthesis genes and cell-wall-loosening genes. Predicted phenotype (shade-avoidance syndrome):

Interpretation: A seedling under a canopy is, in effect, using the R:FR ratio as an early-warning signal of competition for light — well before it is actually shaded from below-canopy darkness — and it responds by elongating stems and internodes to try to outgrow the neighbors, at the metabolic cost of reduced branching and leaf thickening. This anticipatory reallocation of resources is the shade-avoidance syndrome, and it is a direct downstream consequence of the same photoreversible Pr/Pfr switch documented in lettuce seed.

Sterling Brown Hendricks was a physical chemist by training who spent most of his career at the USDA's laboratories in Beltsville, Maryland, applying rigorous spectroscopic and biophysical methods to agricultural problems. Harry Alfred Borthwick was a plant physiologist whose earlier work on photoperiodism in plants (with Marion Parker) had already established that day-length effects on flowering involved a light-sensing pigment, laying essential groundwork before the lettuce seed studies began. Their partnership combined Hendricks's command of spectroscopy and action-spectrum methodology with Borthwick's deep knowledge of whole-plant physiology — a genuinely interdisciplinary collaboration that let them move from a practical seed-germination puzzle to the identification of a universal plant photoreceptor. Their meticulous, quantitative approach — building a custom large-scale spectrograph to resolve narrow wavelength bands, and insisting on reversibility as the decisive test — is a model of how careful physical measurement can crack open a biological mechanism.

Agriculture and horticulture. Growers exploit R:FR manipulation directly. Greenhouse and vertical-farming LED systems are tuned to specific red:far-red ratios to control seedling compactness — suppressing unwanted stem elongation (etiolation) in crowded transplant flats, or conversely promoting rapid stem extension in ornamental cut flowers. Far-red-supplemented "end-of-day" lighting treatments are used commercially to manipulate flowering time and internode length in poinsettias, chrysanthemums, and other photoperiod-sensitive crops.

Weed and crop seed management. Because many weed seeds (like cultivated lettuce) require a red-light cue to break dormancy, tillage practices that bury seed in darkness, or that briefly expose seed to filtered, far-red-shifted light beneath a crop canopy, can suppress weed germination — a principle used in reduced-tillage and shading-based weed-control strategies.

Plant breeding for high-density agriculture. Since shade-avoidance elongation (triggered by low R:FR reflected between neighboring plants) reduces yield in densely planted crops such as maize and soybean by diverting resources into stem growth rather than seed/grain production, breeders have selected and engineered varieties with reduced phytochrome-B-mediated shade-avoidance sensitivity, allowing denser planting without excessive lodging-prone stem elongation.

Forestry and ecology. Understory seedlings in forests use R:FR sensing to detect canopy gaps; conservation and restoration ecologists use this principle when modeling regeneration dynamics and designing selective-logging or gap-creation strategies to promote seedling establishment of light-demanding species.

Biotechnology and optogenetics. The phytochrome photoswitch has been repurposed as a synthetic biology tool: engineered phytochrome-based optogenetic systems allow researchers to switch gene expression or protein interactions on and off with red versus far-red light pulses in cultured cells, borrowing directly from the photoreversible logic first demonstrated in lettuce seed.

Human and public health connections. Because phytochrome and cryptochrome/phototropin systems collectively govern crop nutrient content, growth rate, and stress resilience, red/far-red lighting protocols are increasingly used in controlled-environment agriculture to boost yield and nutritional quality (e.g., anthocyanin or vitamin content) of leafy greens grown in urban and space-agriculture settings, connecting a century-old seed experiment to cutting-edge food security research.

Ask the tutor: How does the slow dark-reversion decay of Pfr let a plant distinguish a short summer night from a long winter one?