Light is a dynamic environmental factor that influences nearly every aspect of plant growth. Because plants are sessile, their entire developmental program must be tuned to the light around them. In darkness, seedlings undergo skotomorphogenesis, producing elongated hypocotyls and small, closed cotyledons. Upon illumination they switch to photomorphogenesis, unfolding short hypocotyls and expanded green cotyledons. The phytochromes, a family of photoreceptors that monitor red light at roughly 660 nanometers and far-red light at roughly 730 nanometers, are central to this switch. Arabidopsis possesses five phytochromes, phyA through phyE, and while phyA and phyB dominate most light responses, the roles of the other family members have remained murky.

Phytochromes are synthesized in an inactive Pr form and convert to the biologically active Pfr state upon absorbing red photons; far-red light quickly reverses the process. The Pfr form is also thermodynamically unstable and can revert to Pr even without light, a process called thermal reversion that is accelerated at high temperatures. This property allows phytochromes to act as thermosensors, coupling light perception to ambient temperature. Light-activated phytochromes migrate into the nucleus and assemble into protein complexes called photobodies, which target specific genomic regions to regulate transcription, linking light detection directly to gene expression.

To dissect phyD function, the research team overexpressed a yellow fluorescent protein-tagged phyD, both alone and together with a cyan fluorescent protein-tagged phyB, in genetic backgrounds lacking other functional phytochromes. These lines included a quintuple mutant missing all five endogenous phytochromes and a mutant retaining only endogenous phyB. Seedlings were grown in darkness or under constant red light at 17, 22 or 27 degrees Celsius, and the team measured hypocotyl elongation, a standard readout of photomorphogenic strength, alongside protein abundance and genome-wide transcriptional responses.

The results were striking. phyD-YFP alone could not inhibit hypocotyl elongation at any temperature, confirming its weak standalone activity. phyB-CFP mediated photomorphogenesis at both temperature extremes, though less efficiently than endogenous phyB. But when both photoreceptors were co-expressed, hypocotyl inhibition at 17 degrees Celsius matched the performance of endogenous phyB and nearly matched that of wild-type seedlings. At 27 degrees Celsius, the synergy vanished entirely. The same temperature-dependent enhancement appeared when the transgenic phyD was combined with endogenous phyB, showing that the cooperation is not an artifact of the fluorescent tags.

Immunoblot analyses pointed to a mechanism. Under prolonged red light, phyB-CFP levels dropped dramatically, consistent with the known light-dependent degradation of phyB, whereas phyD-YFP remained stable. In double transgenic seedlings, the presence of phyD-YFP raised phyB-CFP protein levels specifically in red light, not in darkness, and also stabilized endogenous phyB. Notably, phyB did not boost phyD accumulation, indicating that the stabilizing effect runs in one direction. This contrasts with phyD’s previously described partnership with phyC, in which phyC elevates phyD levels rather than the reverse.

RNA sequencing added a transcriptional dimension to the picture. After 24 hours of red light at 17 degrees Celsius, the double transgenic plants showed far more differentially expressed genes than plants expressing phyB alone, with 2807 versus 1529 significantly altered genes, and their expression profiles resembled those of plants with endogenous phyB more closely than those of phyB-CFP plants. At 27 degrees Celsius, however, the transcriptome changes of the double transgenic plants were more similar to those of the phyB-CFP plants than to the endogenous-phyB plants. Gene ontology analysis confirmed that the extra low-temperature genes belonged to light-response and photosynthesis categories, explaining why robust photomorphogenesis emerged only in the cold.

Under pulsed red light, a regime where signaling depends heavily on how long the active Pfr conformer survives between flashes, the partnership proved equally effective. One-minute red light pulses every three hours at 17 degrees Celsius left phyD-YFP plants unresponsive, but co-expression of phyD-YFP with either transgenic or endogenous phyB enhanced hypocotyl inhibition, restoring responses to wild-type levels in the latter case. Protein measurements under pulsed light mirrored the continuous-light results: phyD-YFP prevented the dramatic phytochrome loss that otherwise occurred, expanding the pool of photoreceptor available for signaling.

The authors propose two linked explanations. First, phyD and phyB, which are known to form heterodimers, may assemble light-stable heterodimeric receptors that protect phyB from degradation and support signaling at low temperatures; co-immunoprecipitation confirmed that the proteins form complexes across a broad temperature range. Second, the temperature specificity may arise because these heterodimers either misfold at high temperatures or undergo much faster thermal reversion than phyB homodimers, so even elevated phyB protein levels fail to translate into stronger signaling at 27 degrees Celsius. The study’s authors note that low signal-to-noise ratios prevented direct measurement of thermal reversion rates in these lines, leaving that aspect unresolved.

Together, the findings recast phyD from a dispensable relic of gene duplication into an active modulator of environmental responsiveness. Because phyD cooperates not only with phyB but also with phyC at low temperatures, the work illustrates how apparently redundant photoreceptors can divide labor according to thermal conditions, giving plants developmental flexibility as light and temperature fluctuate together in the natural world.

Subject of Research: Temperature-dependent cooperation between the phytochrome D and phytochrome B photoreceptors in Arabidopsis light signaling

Article Title: Transgenic phytochrome D cooperates with phytochrome B to promote photomorphogenesis at low temperatures

Article References: Sípos, L., Grézal, G., Péter, C., Nagy, F., Silhavy, D., & Viczián, A. (2026). Transgenic phytochrome D cooperates with phytochrome B to promote photomorphogenesis at low temperatures. Plant Cell Reports, 45(11), Article 340. https://doi.org/10.1007/s00299-026-04025-y

Image Credits: AI Generated

DOI: 10.1007/s00299-026-04025-y

Keywords: phytochrome B, phytochrome D, photomorphogenesis, red light signaling, temperature, Arabidopsis thaliana, photoreceptors, hypocotyl elongation, transcriptome, protein stabilization, heterodimers, Transgenic