Light signal transduction in plants: insights from phytochrome nuclear translocation and photobody formation

Phytochromes (phys) are essential photoreceptors in plants that regulate growth and development through nuclear translocation upon light activation. PhyA and phyB, representing distinct evolutionary lineages, utilize different nuclear import mechanisms. PhyA, specialized for far-red light sensing, relies on FAR-RED ELONGATED HYPOCOTYL1 (FHY1) and FHY1-LIKE for nuclear entry, whereas phyB employs light-induced unmasking of its nuclear localization signals. Calcium signaling plays a critical role in phyB phototransduction, with calcium-dependent protein kinases CPK6/12 phosphorylating phyB at Ser80/Ser106 to initiate its nuclear import. Once in the nucleus, phytochromes localize to photobodies - membraneless condensates formed through liquid-liquid phase separation - which serve as hubs integrating light and temperature signals to regulate transcription and protein turnover. Evolutionary studies indicate that FHY1-mediated import mechanisms predate the divergence of land plants, while the phyB-like import mechanisms developed later in seed plants. Early diverged land plants, such as liverworts and mosses, retain ancestral phytochromes with dual red/far-red light responsiveness, likely employing hybrid nuclear import strategies. Gene duplication has driven the functional diversification of phytochromes, enabling adaptive specialization to complex light environments. This review synthesizes molecular and evolutionary perspectives on phytochrome nuclear import, highlighting calcium-dependent phosphorylation as a key regulatory mechanism and emphasizing the conserved yet adaptable nature of light signaling.