Inositol Phosphates and Energy Signaling in Plants
Authors: Adepoju Olusegun Adeboye , Williams Sarah Phoebe , Gillaspy Glenda 
Published: November 2023

Inositol phosphates (InsPs) are water soluble signaling molecules in plants whose phosphate profile (number of phosphate and position of attachment) specifies a unique type of cellular language that plants use in intracellular communication and in responding to changes in their environment. These signaling molecules have been implicated in diverse cellular processes including phosphate homeostasis, hormone response, abiotic stress response, pathogen response, DNA repair, membrane trafficking, mRNA export etc. Inositol phosphates are synthesized via two pathways in plants; the lipid dependent and lipid independent pathways that converge at Inositol‐1,3,4,5,6‐pentakisphosphate (InsP5). InsP5 is the precursor for synthesizing InsP6 in plants, with Inositol‐Pentakisphosphate 2‐Kinase (IPK1) enzyme catalyzing the reaction. InsP6 has roles both as a signaling molecule and as a storage molecule. It accumulates to high amounts in plant seeds and is a phosphorus and mineral store used during germination. Our recent work suggests a role for InsP6 in low energy signaling. The InsP profile of Arabidopsis seedling can be determined by growing them in semi‐solid media, 3H‐myo‐inositol is added and after 4 days, InsPs are extracted and resolved with ion‐exchange HPLC. The energy condition of these seedling was controlled using limited light, and either no sucrose or 3% sucrose for low and optimal energy respectively. Our labeling studies show an elevation in InsP6 levels in seedlings grown under low energy conditions relative to optimal energy. Additionally, when seedlings are transferred from optimal to low energy conditions, InsP6 levels rapidly increase. These data suggests InsP6 is generated or accumulated under low energy conditions. Next, we examined Arabidopsis mutants with altered InsP profiles. IPK1 loss‐of‐function mutants (Δipk1) have reduced InsP6 levels and these plants are deficient in responding to low energy conditions. Δipk1 seedling roots are longer under low energy and shorter under high energy conditions. This parallels the phenotype of Arabidopsis plants over‐expressing SnRK1 (Sucrose Non‐fermenting Related Kinase 1). SnRK is the plant homologue of the eukaryotic master metabolic regulator AMP‐dependent protein kinase, which acts as a fuel guage in animals regulating low energy responses. Metabolic profiling of Δipk1 plants indicated changes consistent with carbon starvation, namely reduction in nitrogen and lipid biosynthetic pathways. Conclusively, these data suggest that InsP6 is a signaling molecule for low energy conditions in plants. Additionally, our lab is interested in the potential role of inositol pyrophosphates in energy signaling. Inositol pyrophosphates (PPx‐Ins) contain a diphosphate and triphosphate chains attached to the inositol ring generated from InsP6 by VIP kinases. In yeast, KCS1 is responsible for making InsP7 and Δkcs1 shows metabolic defects similar to yeast cells grown under low energy conditions. We are interested in understanding the effect of inositol phosphates in plant energy sensing as this could help influence development of strategies for improving and optimizing plant growth and development (crop yield) under less than ideal energy conditions.