Inositol polyphosphate sensor that associates with transcription factors to regulate Pi starvation responses (PubMed:27080106). The SPX domain provides a basic binding surface for inositol polyphosphate signaling molecules (PubMed:27080106). Interacts with PHR2 to inhibits its translocation to the nucleus and repress its DNA-binding activity, and then negatively regulate Pi signaling (PubMed:24692424). Homodimer (PubMed:24692424). Interacts (via N-terminus) with PHR2 (via C-terminus) in the presence of inositol polyphosphate (PubMed:24692424, PubMed:27080106). Interacts with BHLH6 (PubMed:33128314). Unlike PubMed:19566645, PubMed:24692424 cannot find a membrane localization. Widely expressed (PubMed:19566645). Detected in root cells, with the exception of epidermis, and in mesophyll and vascular bundles in leaves (PubMed:24692424). Up-regulated during cold stress (PubMed:19508276). Not regulated by Pi-starvation (PubMed:19566645, PubMed:24692424). Degraded under Pi starvation conditions through the ubiquitin/26S proteasome pathway. Growth inihibition, Pi accumulation in shoots and up-regulation of phosphate starvation-induced (PSI) genes downstream of PHR2 under high phosphate conditions. Truncated N-terminus.