Inositol pyrophosphate InsP8 acts as intracellular phosphate signal in Arabidopsis

The maintenance of cellular phosphate (Pi) homeostasis is of paramount importance in living organisms. Little is known about how cells sense intracellular Pi status in multi-cellular eukaryotes. Here, we report the identification of inositol pyrophosphate InsP8, a ligand that binds to the Pi sensor protein SPX1 to inhibit PHR1, the central regulator of Pi starvation responses in Arabidopsis. The concentration of InsP8, synthesized by diphosphoinositol pentakisphosphate kinases VIH1 and VIH2, depends on cellular Pi level and the double mutants accumulated higher amounts of Pi. The paradox of Pi toxicity and induction of Pi starvation inducible genes indicated that the cells could not sense cellular Pi without InsP8. Our study reveals that InsP8 acts as a Pi signaling molecule and binds to the receptor SPX1 to regulate Pi homeostasis. mRNA profiles of 12-day-old wild-type (Col-0) and double mutant (vih1vih2) Arabidopsis seedlings were generated by deep sequencing, in triplicate, using Illumina HiSeq 2500.