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

Abstract

In plants, phosphate (P i ) 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 i and dependent on the inositol phosphate kinase ITPK1. We found that inositol pyrophosphates are more responsive to P i 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 7 isomers in Arabidopsis and rice, and identify 4/6-InsP 7 and a PP-InsP 4 isomer hitherto not reported in plants. We found that the inositol pyrophosphates 1/3-InsP 7 , 5-InsP 7 , and InsP 8 increase several fold in shoots after P i resupply and that tissue-specific accumulation of inositol pyrophosphates relies on ITPK1 activities and MRP5-dependent InsP 6 compartmentalization. Notably, ITPK1 is critical for P i -dependent 5-InsP 7 and InsP 8 synthesis in planta and its activity regulates P i starvation responses in a PHRdependent manner. Furthermore, we demonstrated that ITPK1-mediated conversion of InsP 6 to 5-InsP 7 requires high ATP concentrations and that Arabidopsis ITPK1 has an ADP phosphotransferase activity to dephosphorylate specifically 5-InsP 7 under low ATP. Collectively, our study provides new insights into P i -dependent changes in nutritional and energetic states with the synthesis of regulatory inositol pyrophosphates.

Tomato plant responses induced by sparingly available inorganic and organic phosphorus forms are modulated by strigolactones

“…We adopted the 80 μM Pi concentration to mimic the range plants are likely subjected to in the field (Hinsinger 2001). This concentration revealed sufficient for the WT but could uncover a defect in SL-depleted plants, likely due to altered perception and/or production of internal P stocks: it would be worth investigating whether this defect may be directly linked to the production or perception of the signalling molecule InsP8, the inositol pyrophosphate that is directly perceived by the endogenous sensing system, switching the PSR off (Riemer et al 2021). Alternatively, or in parallel, SL-defective plants may be less efficient in P utilization, which may keep feedback repression of PSR responses off; the miR399-PHO module is indeed dysregulated in SL-deficient plants at 80 μM Pi (Santoro et al 2021).…”

Inside Cover: Stable Isotope Phosphate Labelling of Diverse Metabolites is Enabled by a Family of 18O‐Phosphoramidites (Angew. Chem. Int. Ed. 5/2022)

“…In slime mold and plants, an additional isomer could be detected which we have previously tentatively assigned as 4/6-InsP 7 . [53] Once this new isomer will become accessible as its synthetic isotopologue following the procedures described herein, in-depth studies into its dynamic regulation will become possible.…”

Arabidopsis PFA-DSP-type phosphohydrolases target specific inositol pyrophosphate messengers

“…The question of how plants synthesize 5-InsP7 has been partially solved by work on Arabidopsis inositol (1,3,4) triphosphate 5/6 kinases ITPK1 and ITPK2. Notably, ITPK1 and ITPK2 were reported to catalyse the synthesis of 5-InsP7 from InsP6 in vitro [24][25][26] and consequently itpk1 mutant plants display reduced 5-InsP7 levels 11,13 .…”

“…InsP8-bound SPX receptors inactivate the MYB-type transcription factors PHR1 and PHL1, which control the expression of a majority of Pi starvation-induced (PSI) genes to regulate various metabolic and developmental adaptations induced by Pi deficiency 28,[31][32][33] . The tissue levels of various PP-InsPs, including 5-InsP7 and InsP8, respond sensitively to the plant's Pi status 9,11 , suggesting that their synthesis and degradation are tightly regulated. While the steps involved in the synthesis of PP-InsPs in plants are now better understood, still little is known about how these molecules are degraded.…”

“…Although Arabidopsis ITPK1 harbors no phosphatase domain, under conditions of low adenylate charge it can shift its activity in vitro from kinase to ADP phosphotransferase activity using 5-InsP7 but no other InsP7 isomer 11,26 . Apart from relying on the reversible activities of ITPK1 and Vip1/PPIP5Ks, the degradation of PP-InsPs may also be controlled by specialized phosphohydrolases.…”