Functional repressor of PHR2 (PubMed:24368504). Involved in maintaining cellular Pi homeostasis when plants are exposed to an external change in Pi (PubMed:24368504). Negatively regulates root-to-shoot Pi translocation redundantly with SPX3 (PubMed:24368504). Homo- and heterodimers with SPX3. Predominantly expressed in roots (PubMed:19566645). Expressed in root epidermis, exodermis and the sclerenchymal layer (PubMed:24368504). Under Pi starvation, also detected in mesophyll and phloem in the vascular bundles (PubMed:24368504). Up-regulated under phosphate starvation (PubMed:19566645, PubMed:24368504). Down-regulated by SPX1 and SPX3 (PubMed:19566645). Up-regulated during cold stress (PubMed:19508276).