Connections Between Inositol Phosphate Signaling and Energy Responses in Plants

Abstract

The ability for an organism to sense and respond appropriately to its environment is often critical for survival. One mechanism for this is the inositol phosphate (InsP) signaling pathway. This work focuses on the role of InsP signaling in maintaining energy homeostasis in the plant. InsP signaling is connected to energy sensing in plants via a protein complex containing both the inositol polyphosphate 5-phosphatases (5PTase13) and the Sucrose non-Fermenting Related Kinase 1 (SnRK1). SnRK1 is considered a fuel gauge for the plant cell that senses energy status and reprograms growth appropriately. While the SnRK1.1 gene has been well studied, the role other SnRK1 isoforms play in energy or stress signaling is less well understood. This work examined the role of 3 SnRK1 isoforms in energy signaling, finding that SnRK1.1 and SnRK1.2 are regulated and function differently in Arabidopsis. The second part of this work focuses on the inositol pyrophosphates, which are a novel group of InsP signaling molecules containing diphosphate or triphosphate chains (i.e. PPx) attached to the inositol ring. These PPx-InsPs are emerging as critical players in the integration of cellular metabolism and stress signaling in non-plant eukaryotes. Most eukaryotes synthesize the precursor molecule, myo-inositol (1,2,3,4,5,6)-hexakisphosphate (InsP6), which can serve as a signaling molecule or as storage compound of inositol, phosphorus, and minerals. Even though plants produce huge amounts of InsP6 in seeds, almost no attention has been paid to whether PPx-InsPs exist in plants, and if so, what roles these molecules play. This work details the presence of PPx-InsPs in plants and delineates two Arabidopsis gene products (AtVip1 and AtVip2) capable of PP-InsP5 synthesis. We further examined the subcellular location of enzymes connected to PPx-InsP synthesis as well as the developmental and tissue specific patterns of expression of the genes that encode these enzymes. We localized the enzymes involved in InsP6 and PPx-InsP production to the nucleus and endoplasmic reticulum (ER). The subcellular compartmentalization of PPx-InsP signaling may be unique to plants. An increased understanding in the pathways involved in energy sensing and metabolic response may reveal novel strategies to improve crops for yield and viability in the future.