ITPK1 is an InsP/ADP phosphotransferase that controls phosphate signaling in Arabidopsis.

In plants, phosphate (P) homeostasis is regulated by the interaction of PHR transcription factors with stand-alone SPX proteins, which act as sensors for inositol pyrophosphates. In this study, we combined different methods to obtain a comprehensive picture of how inositol (pyro)phosphate metabolism is regulated by P and dependent on the inositol phosphate kinase ITPK1. We found that inositol pyrophosphates are more responsive to P than lower inositol phosphates, a response conserved across kingdoms. Using the capillary electrophoresis electrospray ionization mass spectrometry (CE-ESI-MS) we could separate different InsP isomers in Arabidopsis and rice, and identify 4/6-InsP and a PP-InsP isomer hitherto not reported in plants. We found that the inositol pyrophosphates 1/3-InsP, 5-InsP, and InsP increase several fold in shoots after P resupply and that tissue-specific accumulation of inositol pyrophosphates relies on ITPK1 activities and MRP5-dependent InsP compartmentalization. Notably, ITPK1 is critical for P-dependent 5-InsP and InsP synthesis in planta and its activity regulates P starvation responses in a PHR-dependent manner. Furthermore, we demonstrated that ITPK1-mediated conversion of InsP to 5-InsP requires high ATP concentrations and that Arabidopsis ITPK1 has an ADP phosphotransferase activity to dephosphorylate specifically 5-InsP under low ATP. Collectively, our study provides new insights into P-dependent changes in nutritional and energetic states with the synthesis of regulatory inositol pyrophosphates.