Abstract
Many eukaryotic proteins regulating phosphate (Pi) homeostasis contain SPX domains that are receptors for inositol pyrophosphates (PP-InsP), suggesting that PP-InsPs may regulate Pi homeostasis. Here we report that deletion of two diphosphoinositol pentakisphosphate kinases VIH1/2 impairs plant growth and leads to constitutive Pi starvation responses. Deletion of phosphate starvation response transcription factors partially rescues vih1 vih2 mutant phenotypes, placing diphosphoinositol pentakisphosphate kinases in plant Pi signal transduction cascades. VIH1/2 are bifunctional enzymes able to generate and break-down PP-InsPs. Mutations in the kinase active site lead to increased Pi levels and constitutive Pi starvation responses. ATP levels change significantly in different Pi growth conditions. ATP-Mg2+ concentrations shift the relative kinase and phosphatase activities of diphosphoinositol pentakisphosphate kinases in vitro. Pi inhibits the phosphatase activity of the enzyme. Thus, VIH1 and VIH2 relay changes in cellular ATP and Pi concentrations to changes in PP-InsP levels, allowing plants to maintain sufficient Pi levels.
Highlights
Phosphorus is a growth limiting nutrient for plants, taken up from the soil as inorganic phosphate (H2PO4, Pi)
The recent finding that eukaryotic SPX domains are cellular receptors for PP-InsPs (Wild et al, 2016) and that PP-InsP binding can regulate the activity of PHOSPHATE STARVATION RESPONSE (PHR) transcription factors (Puga et al, 2014; Wang et al, 2014; Wild et al, 2016; Qi et al, 2017) and phosphate transporters (Wild et al, 2016; Potapenko et al, 2018) prompted us to investigate if the Arabidopsis diphosphoinositol pentakisphosphate kinases VIH1 and 2 are components of the plant phosphate starvation response pathway
Our experiments demonstrate that VIH1 and VIH2 redundantly regulate Pi homeostasis, growth and development
Summary
Phosphorus is a growth limiting nutrient for plants, taken up from the soil as inorganic phosphate (H2PO4-, Pi). Plants can take up phosphite (Phi), a reduced form of Pi which cannot be used as a source of phosphorus (Ticconi et al, 2001). Many of the proteins involved in these processes contain small hydrophilic SPX domains (Secco et al, 2012). We have previously shown that these domains act as cellular receptors for inositol pyrophosphate (PP-InsP) signaling molecules (Wild et al, 2016). PP-InsPs are composed of a fully phosphorylated inositol ring containing one or two pyrophosphate moieties (Shears, 2018). PP-InsPs bind to a basic surface cluster highly conserved among SPX domains to regulate divers biochemical and cellular processes in fungi, plants and animals: The N-terminal SPX domains of the yeast VTC
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