Transcription factor involved in phosphate starvation signaling (PubMed:11511543, PubMed:17927693, PubMed:26586833). Binds as a dimer to P1BS, an imperfect palindromic sequence 5'-GNATATNC-3', to promote the expression of inorganic phosphate (Pi) starvation-responsive genes (PubMed:11511543, PubMed:20838596, PubMed:26586833). SPX1 is a competitive inhibitor of this DNA-binding (PubMed:25271326). PHR1 binding to its targets is low Pi-dependent (PubMed:25271326). Regulates the expression of miR399 (PubMed:20838596). Regulates the expression of IPS1 (At3g09922), a non-coding RNA that mimics the target of miR399 to block the cleavage of PHO2 under Pi-deficient conditions (PubMed:17643101). Regulates lipid remodeling and triacylglycerol accumulation during phosphorus starvation (PubMed:25680792). Required for the shoot-specific hypoxic response (PubMed:24753539). Regulates FER1 expression upon phosphate starvation, linking iron and phosphate homeostasis (PubMed:23788639). Contributes to the homeostasis of both sulfate and phosphate in plants under phosphate deficiency (PubMed:21261953). Required for adaptation to high light and retaining functional photosynthesis during phosphate starvation (PubMed:21910737). Involved in the coregulation of Zn and Pi homeostasis (PubMed:24420568). Homodimers and heterodimers (PubMed:20838596, PubMed:11511543). Interacts with SPX1 in a Pi-dependent manner (PubMed:25271326). Does not interact with PHL2 or PHL3 (PubMed:26586833). The localization to the nucleus is independent of the Pi status. Only moderately up-regulated by Pi starvation. Sumoylated by SIZ1. Sumoylation controls phosphate deficiency responses. Strongly reduced shoot growth, and slightly increased root growth. Reduced expression of phosphate starvation-induced (PSI) genes, decreased cellular inorganic phosphate (Pi) content and shoot-to-root ratio, and impaired anthocyanin accumulation (PubMed:17927693). Belongs to the MYB-CC family.