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). Down-regulates the non-coding RNA IPS1 (At3g09922) and suppresses the responses of miR399 and PHO2 to Pi-starvation (PubMed:19566645). Homo- and heterodimers with SPX5. Predominantly expressed in roots, leaves and seeds (PubMed:19566645). Expressed in root epidermis, exodermis, sclerenchymal layer, cortex and endosperm (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). Up-regulated during cold stress (PubMed:19508276). No visible phenotype, due to the redundancy with SPX5.