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.
Keywords: Arabidopsis thaliana; d/l-inositol 3,4,5,6-tetrakisphosphate; diphosphoinositol pentakisphosphate; inositol 1,3,4-trisphosphate 5/6-kinase 1 (ITPK1); inositol pentakisphosphate 2-kinase (IPK1); inositol phosphate; phosphate homeostasis; phosphate starvation response; phytate biosynthesis.
© 2018 The Authors The Plant Journal © 2018 John Wiley & Sons Ltd.