Inhibits PHR2 DNA-binding activity via a Pi-dependent protein interaction (PubMed:25271318). Interacts (via SPX domain) with PHR2 (via C-terminus) (PubMed:25271318). Predominantly expressed in roots, leaves and seeds. Up-regulated under phosphate starvation (PubMed:19566645). Up-regulated during cold stress (PubMed:19508276). Up-regulated by the transcription factor PHR2 (PubMed:25271318). The SPX domain is sufficient for inhibition of PHR2 binding to DNA. SPX1 and SPX2 have redundant functions in repressing the activity of PHR2.