Twenty-Five Years of Photobodies: Formation, Composition, and the Two-Compartment Logic of Phytochrome B Signaling

How nuclear condensates encode cell-signaling dynamics remains unclear. Photobodies (PBs) in Arabidopsis offer a genetically tractable paradigm. PBs are light- and temperature-sensing nuclear condensates organized by the thermosensitive photoreceptor phytochrome B (phyB). Recent advances have clarified PB composition and illuminated PB formation and function. PhyB is the dominant component and provides scaffold-like determinants that recruit selective signaling partners as primary clients (direct phyB binders) and secondary clients (recruited via primaries), spanning transcription/splicing regulators, E3-ligase components, kinases/phosphatases, and chaperones. These clients connect phyB condensates to diverse environmental and hormonal pathways, positioning PBs as a central hub for signaling integration. PB assembly is driven by condensation encoded in phyB's output module and modulated by its photosensory module, coupling assembly/dissolution to photostate and temperature. PBs nucleate nonrandomly at preferred seeding sites, producing spatially distinct classes with different occurrence frequencies and thermosensitivities. PB formation partitions signaling between PBs and the surrounding nucleoplasm, establishing a two-compartment photosensory system. Within this architecture, dynamic sequestration in PBs tunes nucleoplasmic transcription-factor stability and activity to expand signaling dynamic range and extends phyB control into the night by stabilizing active phyB. We propose that PBs function as an autoregulatory rheostat, dialing nucleoplasmic light sensitivity in proportion to incident irradiance and thereby enabling continuous discrimination of light-intensity changes across multiple orders of magnitude. We suggest that this two-compartment logic illustrates a general role of membraneless organelles in signaling: using dense-phase dynamics to adjust pathway sensitivity and output in the surrounding dilute phase.