PHYRE, a Translated uORF, is Required for Phytochrome B Translation and Photomorphogenesis in Arabidopsis

Translational control allows cells to adjust protein output independently of transcription, yet the mRNA-encoded mechanisms that govern this regulatory layer remain poorly understood. Upstream open reading frames (uORFs) are among the most prevalent translational regulatory elements in eukaryotic genomes; however, characterised examples of activating uORFs that promote rather than inhibit downstream translation are exceedingly rare. Here we show that efficient expression of phytochrome B (phyB), the master red-light photoreceptor of Arabidopsis thaliana, requires translation of a short uORF in its own 5′UTR, which we name PHYTOCHROME REGULATOR (PHYRE). Loss of PHYRE function phenocopies phyB null mutants across germination, hypocotyl growth, leaf development, and flowering time. PHYRE activity depends on its encoded peptide sequence, and the spacing between PHYRE and the PHYB start codon is a critical determinant of downstream translation efficiency, consistent with a position-dependent reinitiation mechanism. PHYRE-like elements are conserved across eudicots, establishing uORF-mediated translational gating as a broadly maintained regulatory layer controlling a master photoreceptor.