Arabidopsis inositol phosphate kinases IPK1 and ITPK1 constitute a metabolic pathway in maintaining phosphate homeostasis.

Emerging studies have suggested that there is a close link between inositol phosphate (InsP) metabolism and cellular phosphate (Pi ) homeostasis in eukaryotes; however, whether a common InsP species is deployed as an evolutionarily conserved metabolic messenger to mediate Pi signaling remains unknown. Here, using genetics and InsP profiling combined with Pi -starvation response (PSR) analysis in Arabidopsis thaliana, we showed that the kinase activity of inositol pentakisphosphate 2-kinase (IPK1), an enzyme required for phytate (inositol hexakisphosphate; InsP6 ) synthesis, is indispensable for maintaining Pi homeostasis under Pi -replete conditions, and inositol 1,3,4-trisphosphate 5/6-kinase 1 (ITPK1) plays an equivalent role. Although both ipk1-1 and itpk1 mutants exhibited decreased levels of InsP6 and diphosphoinositol pentakisphosphate (PP-InsP5 ; InsP7 ), disruption of another ITPK family enzyme, ITPK4, which correspondingly caused depletion of InsP6 and InsP7 , did not display similar Pi -related phenotypes, which precludes these InsP species from being effectors. Notably, the level of d/l-Ins(3,4,5,6)P4 was concurrently elevated in both ipk1-1 and itpk1 mutants, which showed a specific correlation with the misregulated Pi phenotypes. However, the level of d/l-Ins(3,4,5,6)P4 is not responsive to Pi starvation that instead manifests a shoot-specific increase in the InsP7 level. This study demonstrates a more nuanced picture of the intersection of InsP metabolism and Pi homeostasis and PSRs than has previously been elaborated, and additionally establishes intermediate steps to phytate biosynthesis in plant vegetative tissues.

Urine inositol pentakisphosphate 2-kinase and changes in kidney structure in early diabetic kidney disease in type 1 diabetes.

We examined the association of urine inositol 1,3,4,5,6-pentakisphosphate 2-kinase (IPP2K) with the presence and progression of diabetic kidney disease (DKD) lesions. Urine IPP2K was measured at baseline by quantitative liquid chromatography-mass spectrometry in 215 participants from the Renin-Angiotensin System Study who had type 1 diabetes and were normoalbuminuric and normotensive with normal or increased glomerular filtration rate (GFR). Urine IPP2K was detectable in 166 participants. Participants with IPP2K below the limit of quantification (LOQ) were assigned concentrations of LOQ/√2. All concentrations were then standardized to urine creatinine (Cr) concentration. Kidney morphometric data were available from biopsies at baseline and after 5 yr. Relationships of IPP2K/Cr with morphometric variables were assessed by linear regression after adjustment for age, sex, diabetes duration, hemoglobin A1c, mean arterial pressure, treatment assignment, and, for longitudinal analyses, baseline structure. Baseline mean age was 29.7 yr, mean diabetes duration 11.2 yr, median albumin excretion rate 5.0 µg/min, and mean iohexol GFR 129 ml·min-1·1.73m-2. Higher IPP2K/Cr was associated with higher baseline peripheral glomerular total filtration surface density [Sv(PGBM/glom), tertile 3 vs. tertile 1 ß = 0.527, P = 0.011] and with greater preservation of Sv(PGBM/glom) after 5 yr ( tertile 3 vs. tertile 1 ß = 0.317, P = 0.013). Smaller increases in mesangial fractional volume ( tertile 3 vs. tertile 1 ß = -0.578, P = 0.018) were observed after 5 yr in men with higher urine IPP2K/Cr concentrations. Higher urine IPP2K/Cr is associated with less severe kidney lesions at baseline and with preservation of kidney structure over 5 yr in individuals with type 1 diabetes and no clinical evidence of DKD at baseline.

Arabidopsis inositol pentakisphosphate 2-kinase, AtIPK1, is required for growth and modulates phosphate homeostasis at the transcriptional level.

Inositol hexakisphosphate (IP6 ) provides a phosphorous reservoir in plant seeds; in addition, along with its biosynthesis intermediates and derivatives, IP6 also plays important roles in diverse developmental and physiological processes. Disruption of the Arabidopsis inositol pentakisphosphate 2-kinase coding gene AtIPK1 was previously shown to reduce IP6 content in vegetative tissues and affect phosphate (Pi) sensing. Here we show that AtIPK1 is required for sustaining plant growth, as null mutants are non-viable. An incomplete loss-of-function mutant, atipk1-1, exhibited disturbed Pi homeostasis and overaccumulated Pi as a consequence of increased Pi uptake activity and root-to-shoot Pi translocation. The atipk1-1 mutants also showed a Pi deficiency-like root system architecture with reduced primary root and enhanced lateral root growth. Transcriptome analysis indicated that a subset of Pi starvation-responsive genes was transcriptionally perturbed in the atipk1-1 mutants and the expression of multiple genes involved in Pi uptake, allocation, and remobilization was increased. Genetic and transcriptional analyses suggest that disturbance of Pi homeostasis caused by atipk1 mutation involved components in addition to PHR1(-like) transcription factors. Notably, the transcriptional increase of a number of Pi starvation-responsive genes in the atipk1-1 mutants is correlated with the reduction of histone variant H2A.Z occupation in chromatin. The myo-inositol-1-phosphate synthase mutants, atmips1 and atmips2 with comparable reduction in vegetative IP6 to that in the atipk1-1 mutants did not overaccumulate Pi, suggesting that Pi homeostasis modulated by AtIPK1 is not solely attributable to IP6 level. This study reveals that AtIPK1 has important roles in growth and Pi homeostasis.