NUDIX Hydrolases Target Specific Inositol Pyrophosphates and Regulate Phosphate and Iron Homeostasis, and the Expression of Defense Genes in Arabidopsis

Abstract

Inositol pyrophosphates (PP-InsPs) are important signaling molecules that regulate diverse cellular processes in eukaryotes, including energy homeostasis and phosphate (Pi) signaling. However, the enzymes responsible for their turnover in plants remain largely unknown. Using a non-hydrolysable PP-InsP analog in a pull-down approach, we identified a family of Arabidopsis Nudix hydrolases (NUDTs) that group into two closely related subclades. Throughin vitroassays, heterologous expression systems, and higher-order gene-edited mutants, we explored the substrate specificities and physiological roles of these hydrolases. Using a combination of strong anion exchange (SAX)-HPLC, PAGE, and capillary electrophoresis electrospray ionization mass spectrometry (CE-ESI-MS), we found that their PP-InsP pyrophosphatase activity is enantiomer-selective and Mg2+-dependent. Specifically, subclade I NUDTs preferentially hydrolyze 4-InsP7, while subclade II NUDTs target 3-InsP7, with minor activity against other PP-InsPs, including 5-InsP7. In higher-order mutants of subclade II NUDTs, we observed defects in both Piand iron homeostasis, accompanied by increased levels of 1/3-InsP7and 5-InsP7, with a markedly larger increase in 1/3-InsP7. Furthermore, ectopic expression of NUDTs from both subclades induced local Pistarvation responses (PSRs). However, RNA-seq analysis of WT andnudt12/13/16plants suggests that these NUDTs have PSR-independent roles, potentially involving 1/3-InsP7in the regulation of plant defense. Expanding beyond subclade II NUDTs, we demonstrated susceptibility of the 3PP-position of PP-InsPs to enzymatic activities unrelated to NUDTs, and found that such activities are conserved across plants and humans. Additionally, we found that NUDT effectors from pathogenic ascomycete fungi exhibit a substrate specificity similar to subclade I NUDTs. Collectively, our study provides new insights into the roles of NUDTs in regulating PP-InsP signaling pathways in plants and highlights cross-kingdom conservation of enzymes involved in PP-InsP metabolism.