An RWP-RK factor links salt response and nitrogen uptake during stress–recovery in intertidal macroalgae <i>Pyropia yezoensis</i> (Rhodophyte)

Intertidal macroalgae such as Pyropia yezoensis routinely endure drastic and rapid fluctuations in salinity due to tidal cycles. However, the mechanisms by which these algae coordinate salt stress responses with metabolic adaptation remain unclear. In this study, transcriptomic analysis identified key genes and pathways involved in high salinity response and subsequent recovery in P. yezoensis. Among four RWP-RK transcription factors, gene Py02293 was markedly upregulated under high salinity stress and especially during the recovery phase. Functional validation via transgenic overexpression (OE1) and gene silencing (Si1) demonstrated that overexpression of Py02293 enhanced tolerance to high salt stress and accelerated recovery of photosynthetic efficiency (Fv/Fm), whereas silencing impaired recovery. Further transcriptome and qRT-PCR analyses showed that Py02293 negatively regulates several nitrogen metabolism-related genes, including ammonium and high-affinity nitrate transporters and glutamine synthetases. Notably, while overexpression of Py02293 suppressed inorganic nitrogen uptake under high salinity stress, it significantly promoted nitrogen absorption during the recovery phase, indicating a context-dependent regulatory role. These results suggest that Py02293 mediates the crosstalk between salt stress responses and nitrogen metabolism, enabling a rapid metabolic shift that supports environmental resilience and efficient recovery in the fluctuating intertidal zone. This study reveals the role of RWP-RK protein in marine macroalgae, provides valuable insight into the integration of stress and nutrient signaling pathways, and highlights Py02293 as a potential target for improving stress tolerance in economically important seaweeds.