Characterization of Ca2+-Dependent SOS-like Pathway Components Involved in the Salt Stress Response of an Intertidal Macroalga, Pyropia haitanensis

Intertidal red algae show much stronger tolerance to hypersaline stress than terrestrial plants. However, the characteristics of Ca2+ signaling and the SOS-related ion regulatory mechanisms involved in their hypersaline stress responses remain unclear. In this study, the representative intertidal red alga Pyropia haitanensis was used to characterize Ca2+ signal transduction under hypersaline stress and to identify key components of the SOS pathway. Hypersaline stress rapidly induced a Ca2+ signal mainly driven by extracellular Ca2+ influx, which peaked at 15 min. Inhibition of Ca2+ influx significantly suppressed Na+ efflux, whereas inhibition of SOS1-mediated Na+ efflux enhanced Ca2+ influx. These results suggest that SOS1-associated Na+ efflux is Ca2+-dependent and closely associated with Ca2+ signaling during the hypersaline stress response. Based on the reference genome, 14 SOS-related candidate genes were identified. Most of these genes were significantly upregulated at approximately 10 min after hypersaline treatment, showing a rapid response pattern. Among them, PhSOS1-4 responded significantly at the transcriptional, protein abundance, and phosphorylation levels. PhSOS1-4 interacted with PhSOS3L-5 and PhSOS2L-2, respectively, whereas no direct interaction was detected between PhSOS3L-5 and PhSOS2L-2. These results suggest a possible SOS-like regulatory mode in P. haitanensis that is distinct from the classical SOS pathway in higher plants. In this pathway, PhSOS3L-5 may directly interact with PhSOS1-4 and potentially participate in the rapid regulation of Na+ efflux. Heterologous expression showed that both PhSOS1-4 and PhSOS3L-5 enhanced the hypersaline tolerance of Chlamydomonas reinhardtii. These findings provide new insights into the strong Na+ efflux capacity of intertidal red algae.