SPX-related genes regulate phosphorus homeostasis in the marine phytoplankton, Phaeodactylum tricornutum

Phosphorus (P) is an essential nutrient for marine phytoplankton. Maintaining intracellular P homeostasis against environmental P variability is critical for phytoplankton, but how they achieve this is poorly understood. Here we identify a SPX gene and investigate its role in Phaeodactylum tricornutum. SPX knockout led to significant increases in the expression of phosphate transporters, alkaline phosphatases (the P acquisition machinery) and phospholipid hydrolases (a mechanism to reduce P demand). These demonstrate that SPX is a negative regulator of both P uptake and P-stress responses. Furthermore, we show that SPX regulation of P uptake and metabolism involves a phosphate starvation response regulator (PHR) as an intermediate. Additionally, we find the SPX related genes exist and operate across the phytoplankton phylogenetic spectrum and in the global oceans, indicating its universal importance in marine phytoplankton. This study lays a foundation for better understanding phytoplankton adaptation to P variability in the future changing oceans.