INOSITOL (1,3,4) TRIPHOSPHATE 5/6 KINASE1-dependent inositol polyphosphates regulate auxin responses in Arabidopsis.

The combinatorial phosphorylation of myo-inositol results in the generation of different inositol phosphates (InsPs), of which phytic acid (InsP6) is the most abundant species in eukaryotes. InsP6 is also an important precursor of the higher phosphorylated inositol pyrophosphates (PP-InsPs), such as InsP7 and InsP8, which are characterized by a diphosphate moiety and are also ubiquitously found in eukaryotic cells. While PP-InsPs regulate various cellular processes in animals and yeast, their biosynthesis and functions in plants has remained largely elusive because plant genomes do not encode canonical InsP6 kinases. Recent work has shown that Arabidopsis (Arabidopsis thaliana) INOSITOL (1,3,4) TRIPHOSPHATE 5/6 KINASE1 (ITPK1) and ITPK2 display in vitro InsP6 kinase activity and that, in planta, ITPK1 stimulates 5-InsP7 and InsP8 synthesis and regulates phosphate starvation responses. Here we report a critical role of ITPK1 in auxin-related processes that is independent of the ITPK1-controlled regulation of phosphate starvation responses. Those processes include primary root elongation, root hair development, leaf venation, thermomorphogenic and gravitropic responses, and sensitivity to exogenously applied auxin. We found that the recombinant auxin receptor complex, consisting of the F-Box protein TRANSPORT INHIBITOR RESPONSE1 (TIR1), ARABIDOPSIS SKP1 HOMOLOG 1 (ASK1), and the transcriptional repressor INDOLE-3-ACETIC ACID INDUCIBLE 7 (IAA7), binds to anionic inositol polyphosphates with high affinity. We further identified a physical interaction between ITPK1 and TIR1, suggesting a localized production of 5-InsP7, or another ITPK1-dependent InsP/PP-InsP isomer, to activate the auxin receptor complex. Finally, we demonstrate that ITPK1 and ITPK2 function redundantly to control auxin responses, as deduced from the auxin-insensitive phenotypes of itpk1 itpk2 double mutant plants. Our findings expand the mechanistic understanding of auxin perception and suggest that distinct inositol polyphosphates generated near auxin receptors help to fine-tune auxin sensitivity in plants.

Arabidopsis PFA-DSP-Type Phosphohydrolases Target Specific Inositol Pyrophosphate Messengers.

Inositol pyrophosphates are signaling molecules containing at least one phosphoanhydride bond that regulate a wide range of cellular processes in eukaryotes. With a cyclic array of phosphate esters and diphosphate groups around myo-inositol, these molecular messengers possess the highest charge density found in nature. Recent work deciphering inositol pyrophosphate biosynthesis in Arabidopsis revealed important functions of these messengers in nutrient sensing, hormone signaling, and plant immunity. However, despite the rapid hydrolysis of these molecules in plant extracts, very little is known about the molecular identity of the phosphohydrolases that convert these messengers back to their inositol polyphosphate precursors. Here, we investigate whether Arabidopsis Plant and Fungi Atypical Dual Specificity Phosphatases (PFA-DSP1-5) catalyze inositol pyrophosphate phosphohydrolase activity. We find that recombinant proteins of all five Arabidopsis PFA-DSP homologues display phosphohydrolase activity with a high specificity for the 5-ß-phosphate of inositol pyrophosphates and only minor activity against the ß-phosphates of 4-InsP7 and 6-InsP7. We further show that heterologous expression of Arabidopsis PFA-DSP1-5 rescues wortmannin sensitivity and deranged inositol pyrophosphate homeostasis caused by the deficiency of the PFA-DSP-type inositol pyrophosphate phosphohydrolase Siw14 in yeast. Heterologous expression in Nicotiana benthamiana leaves provided evidence that Arabidopsis PFA-DSP1 also displays 5-ß-phosphate-specific inositol pyrophosphate phosphohydrolase activity in planta. Our findings lay the biochemical basis and provide the genetic tools to uncover the roles of inositol pyrophosphates in plant physiology and plant development.

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

In plants, phosphate (Pi) 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 Pi and dependent on the inositol phosphate kinase ITPK1. We found that inositol pyrophosphates are more responsive to Pi than lower inositol phosphates, a response conserved across kingdoms. Using the capillary electrophoresis electrospray ionization mass spectrometry (CE-ESI-MS) we could separate different InsP7 isomers in Arabidopsis and rice, and identify 4/6-InsP7 and a PP-InsP4 isomer hitherto not reported in plants. We found that the inositol pyrophosphates 1/3-InsP7, 5-InsP7, and InsP8 increase several fold in shoots after Pi resupply and that tissue-specific accumulation of inositol pyrophosphates relies on ITPK1 activities and MRP5-dependent InsP6 compartmentalization. Notably, ITPK1 is critical for Pi-dependent 5-InsP7 and InsP8 synthesis in planta and its activity regulates Pi starvation responses in a PHR-dependent manner. Furthermore, we demonstrated that ITPK1-mediated conversion of InsP6 to 5-InsP7 requires high ATP concentrations and that Arabidopsis ITPK1 has an ADP phosphotransferase activity to dephosphorylate specifically 5-InsP7 under low ATP. Collectively, our study provides new insights into Pi-dependent changes in nutritional and energetic states with the synthesis of regulatory inositol pyrophosphates.

Extraction and Quantification of Soluble, Radiolabeled Inositol Polyphosphates from Different Plant Species using SAX-HPLC.

The phosphate esters of myo-inositol, also termed inositol phosphates (InsPs), are a class of cellular regulators playing important roles in plant physiology. Due to their negative charge, low abundance and susceptibility to hydrolytic activities, the detection and quantification of these molecules is challenging. This is particularly the case for highly phosphorylated forms containing 'high-energy' diphospho bonds, also termed inositol pyrophosphates (PP-InsPs). Due to its high sensitivity, strong anion exchange high-performance liquid chromatography (SAX-HPLC) of plants labeled with [3H]-myo-inositol is currently the method of choice to analyze these molecules. By using [3H]-myo-inositol to radiolabel plant seedlings, various InsP species including several non-enantiomeric isomers can be detected and discriminated with high sensitivity. Here, the setup of a suitable SAX-HPLC system is described, as well as the complete workflow from plant cultivation, radiolabeling and InsP extraction to the SAX-HPLC run and subsequent data analysis. The protocol presented here allows the discrimination and quantification of various InsP species, including several non-enantiomeric isomers and of the PP-InsPs, InsP7 and InsP8, and can be easily adapted to other plant species. As examples, SAX-HPLC analyses of Arabidopsis thaliana and Lotus japonicus seedlings are performed and complete InsP profiles are presented and discussed. The method described here represents a promising tool to better understand the biological roles of InsPs in plants.