Phosphorus is a macronutrient taken up by cells as inorganic phosphate (Pi). How cells sense cellular Pilevels is poorly characterized. Here we report that SPX domains, which are found in eukaryotic phosphate transporters, signaling proteins and inorganic polyphosphate polymerases, provide a basic binding surface for inositol polyphosphate signaling molecules (InsPs), whose concentrations change in response to Piavailability. Substitutions of critical binding surface residues impair InsP binding in vitro, inorganic polyphosphate synthesis in yeast and Pitransport inArabidopsis In plants, InsPs trigger the association of SPX proteins with transcription factors to regulate Pistarvation responses. We propose that InsPs communicate cytosolic Pilevels to SPX domains and enable them to interact with a multitude of proteins to regulate Piuptake, transport and storage in fungi, plants and animals.