Involved in plant adaptation to phosphate starvation (PubMed:19000161). Inhibits PHR2 DNA-binding activity via a Pi-dependent protein interaction (PubMed:25271318). Suppresses the regulation on expression of PT2 by PHR2 and accumulation of shoot Pi (PubMed:20149131). Optimizes growth under phosphate-limited conditions through a negative feedback loop of the PSI (phosphate starvation-induced) signaling pathway (PubMed:19000161, PubMed:20149131). Regulates the expression of SPX2, SPX3 and SPX5 (PubMed:19566645). May be an important link between signal transduction pathways related to phosphate starvation and cold stress (PubMed:19000161). Interacts (via SPX domain) with PHR2 (via C-terminus) (PubMed:25271318). Predominantly expressed in roots and leaves. Up-regulated under phosphate starvation (PubMed:19566645, PubMed:19000161). Up-regulated during cold stress (PubMed:19508276). Under negative feedback regulation by PHO2 (PubMed:20149131). 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.