The role of inositol pyrophosphates in the regulation of plant stress responses

Abstract

Inositol polyphosphates (InsPs) have been described as important messengers in eukaryotic cells. InsPs with one or more diphospho groups are also termed inositol pyrophosphates (PP-InsPs), which gained special attention as potent cellular regulators. In plants, PP-InsPs were shown to be involved in abiotic and biotic stress responses, such as phosphate (Pi) homeostasis and hormone-dependent pathogen defenses. Phosphorus is an essential element and key nutrient for growth and development in all organisms. In plants, phosphorus is taken up in form of Pi, whose availability is largely restricted. To adjust to changing Pi availabilities, plants have evolved the so-called phosphate starvation response, which is regulated by the interaction of PHOSPHATE STARVATION RESPONSE REGULATOR (PHR) transcription factors with SYG1/Pho81/XPR1-domain containing (SPX) proteins that function as sensors for PP-InsPs. The research herein presented focuses on the Pi-dependent regulation of PP-InsP metabolism, especially in view of INOSITOL 1,3,4-TRISPHOSPHATE 5/6 KINASE 1 (ITPK1). A novel tool for PP-InsP analyses, the capillary electrophoresis electrospray ionization mass spectrometry (CE-ESI-MS), was successfully employed to separate and identify InsP7 isomers in Arabidopsis and rice upon Pi-limited and Pi-resupply conditions. It was shown that 1/3-InsP7, 5-InsP7 and InsP8 accumulate in shoots when Pi-starved plants regained Pi, suggesting a tight link to cellular Pi levels. Notably, ITPK1 plays a crucial role in Pi-dependent synthesis of 5-InsP7, the precursor of InsP8, whose activity is critical for undisturbed Pi signaling. In addition, this work revealed that the generation of 5-InsP7 by ITPK1 requires high ATP concentrations and that under conditions of low adenylate charge, ITPK1 catalyzes the transfer of the β-phosphate of 5-InsP7 to ADP to locally generate ATP. Using CE-ESI-MS, plant 4/6-InsP7 was identified, an isomer that to date was only described in social amoeba Dictyostelium discoideum and is unlikely to play any role in Pi signaling in plants. Further, an unknown PP-InsP species in Pi-starved Arabidopsis and rice roots was observed that most likely represents an unknown PP-InsP4 isomer that was absent in itpk1 roots, suggesting that ITPK1 is involved in the biosynthesis of this unknown PP-InsP. <br /> This thesis provides an insight into the involvement of PP-InsPs in Pi homeostasis and presents the current knowledge about the functions of PP-InsPs in abiotic and biotic stress responses in plants.