Abstract
Inositol pyrophosphates (PP-InsPs) are important signaling molecules that regulate diverse cellular processes in eukaryotes, including energy homeostasis, phosphate (P(i)) signaling, and phytohormone perception. Yet, in plants, the enzymes responsible for their turnover remain largely unknown. Using a non-hydrolysable PP-InsP analog in a pull-down approach, we identified a family of Arabidopsis NUDIX-type hydrolases (NUDTs) that group into two closely related subclades. Through in vitro assays, 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 high-performance liquid chromatography (SAX-HPLC), polyacrylamide gel electrophoresis (PAGE), and capillary electrophoresis electrospray ionization mass spectrometry (CE-ESI-MS), we found that their PP-InsP pyrophosphatase activity is enantiomer selective and Mg(2+) dependent. Specifically, Subclade I NUDTs preferentially hydrolyze 4-InsP(7), while Subclade II NUDTs target 3-InsP(7), with minor activity against other PP-InsPs, including 5-InsP(7). In higher order mutants of Subclade II NUDTs, we observed defects in both P(i) and iron homeostasis, accompanied by increased levels of 1/3-InsP(7) and 5-InsP(7), with a markedly larger increase in 1/3-InsP(7). Ectopic expression of NUDTs from both subclades induced local P(i) starvation responses (PSRs), while RNA-seq analysis comparing wild-type (WT) and Subclade II nudt12/13/16 loss-of-function plants indicates additional PSR-independent roles, potentially involving 1/3-InsP(7) in the regulation of plant defense. Consistently, nudt12/13/16 mutants displayed enhanced resistance to Pseudomonas syringae infection, indicating a role in bacterial pathogen susceptibility. 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 observed that NUDT effectors from pathogenic ascomycete fungi exhibit a substrate specificity similar to Subclade I NUDTs. Collectively, our findings reveal new roles for NUDTs in PP-InsP signaling, plant nutrient and immune responses, and highlight a cross-kingdom conservation of PP-InsP-metabolizing enzymes.