ABSTRACT In plants, phosphate (P i ) homeostasis is regulated by the interaction of P i starvation response transcription factors (PHRs) with stand-alone SPX proteins, which act as sensors for inositol pyrophosphates (PP-InsPs). Recently, ITPK1 was shown to generate the PP-InsP InsP 7 from InsP 6 in vitro , but the importance of this activity in P i signaling remained unknown. Here, we show that uncontrolled P i accumulation in ITPK1-deficient plants is accompanied by impaired P i -dependent InsP 7 and InsP 8 synthesis. Reciprocal grafting demonstrates that P i starvation responses are mainly controlled by ITPK1 activity in shoots. Nuclear magnetic resonance assays and PAGE analyses with recombinant protein reveal that besides InsP 6 phosphorylation, ITPK1 is also able to catalyze ATP synthesis using 5-InsP 7 but not any other InsP 7 isomer as a P-donor when ATP is low. Additionally, we show that the dynamic changes in InsP 7 and InsP 8 to cellular P i are conserved from land plant species to human cells, suggesting that P i -dependent PP-InsP synthesis is a common component of P i signaling across kingdoms. Together, our study demonstrates how P i -dependent changes in nutritional and energetic states modulate ITPK1 activities to fine-tune the synthesis of PP-InsPs.

ITPK1 is an InsP6/ADP phosphotransferase that controls systemic phosphate homeostasis in Arabidopsis