Plays a positive role in plant adaptation to phosphate starvation (PubMed:18315545). Inhibits PHR1 DNA-binding activity in a Pi-dependent manner (PubMed:25271326). Interacts with PHR1 in a highly Pi-dependent manner (PubMed:25271326). Up-regulated under phosphate starvation. No effect on Pi accumulation, due to the redundancy with SPX2. Spx1 and spx2 double mutants have an increased root-to-shoot growth ratio and a reduced rot hair size when grown in Pi-sufficient conditions.