Widespread Presence of SPX and Its Potential Role as a Phosphorus Nutrient Regulator in Dinoflagellates.

SPX domain-containing proteins (SPXc) are crucial for regulating phosphorus (P) homeostasis in plants. Recently, the SPX gene was identified in the diatom model and shown to serve as a negative regulator of P acquisition. Whether occurs in dinoflagellates is unclear. Here, we report the presence and potential functions of genes encoding SPXc in dinoflagellates (). Four classes of SPXc were identified in dinoflagellates, including the three known classes-the stand-alone SPX, SPX-EXS, and SPX-VTC-and SPX-other, with SPX and SPX-EXS being dominant. Using the database, we investigated the taxonomic and geographic distributions of and found variations in expression among size classes of dinoflagellates. The harmful algal bloom-causative species possesses all four classes of SPXc proteins, showing a fluctuating expression pattern under different nutrient conditions and during different phases of the cell cycle and algal bloom. In addition, the genes in Symbiodiniaceae respond not only to P stress but also to thermal variations. These results are consistent with a role of in maintaining P homeostasis in dinoflagellates and suggest the importance of SPX-related genes in enabling dinoflagellates to sustain population growth in nutrient-variable oceans, warranting further research.