Inositol Pyrophosphate InsP8 Acts as an Intracellular Phosphate Signal in Arabidopsis
- The maintenance of cellular phosphate (Pi) homeostasis is of great importance in living organisms. The SPX domain-containing proteins, SPX1, from both Arabidopsis and rice have been proposed to act as sensors of Pi status. The molecule signaling the cellular Pi status to regulate Pi homeostasis in plants, however, remains to be identified, as Pi itself does not bind to the SPX domain. Here, we report the identification of the inositol pyrophosphate InsP8 as a signaling molecule that regulates Pi homeostasis in Arabidopsis. Polyacrylamide gel electrophoresis profiling of InsPs revealed that InsP8 level positively correlates with cellular Pi concentration. We demonstrated that the homologs of diphosphoinositol pentakisphosphate kinase PPIP5K, VIH1 and VIH2, function redundantly to synthesize InsP8, and the vih1 vih2 double mutants over-accumulate Pi. SPX1 directly interacts with PHR1, the central regulator of Pi starvation responses, to inhibit its function under Pi-replete condition. However, this interaction is compromised in vih1 vih2 double mutant, resulting in the constitutive induction of Pi starvation-induced genes, indicating that plant cells cannot sense cellular Pi status without InsP8. Furthermore, InsP8 can directly bind to the SPX domain of SPX1 and is essential for the interaction between SPX1 and PHR1. Our study revealed that InsP8 acts as the ligand that binds to the intracellular Pi sensor SPX1 to control Pi homeostasis in plants.