SUMMARYInositol pyrophosphates are unique cellular signaling molecules with recently discovered roles in energy sensing and metabolism. Studies in eukaryotes have revealed that these compounds have a rapid turnover, and thus only small amounts accumulate. Inositol pyrophosphates have not been the subject of investigation in plants even though seeds produce large amounts of their precursor, myo-inositol hexakisphosphate (InsP 6 ). Here, we report that Arabidopsis and maize InsP 6 transporter mutants have elevated levels of inositol pyrophosphates in their seed, providing unequivocal identification of their presence in plant tissues. We also show that plant seeds store a little over 1% of their inositol phosphate pool as InsP 7 and InsP 8 . Many tissues, including, seed, seedlings, roots and leaves accumulate InsP 7 and InsP 8 , thus synthesis is not confined to tissues with high InsP 6 . We have identified two highly similar Arabidopsis genes, AtVip1 and AtVip2, which are orthologous to the yeast and mammalian VIP kinases. Both AtVip1 and AtVip2 encode proteins capable of restoring InsP 7 synthesis in yeast mutants, thus AtVip1 and AtVip2 can function as bonafide InsP 6 kinases. AtVip1 and AtVip2 are differentially expressed in plant tissues, suggesting nonredundant or non-overlapping functions in plants. These results contribute to our knowledge of inositol phosphate metabolism and will lay a foundation for understanding the role of InsP 7 and InsP 8 in plants.

The Sucrose non-Fermenting Related Kinase 1 (SnRK1) proteins have been linked to regulation of energy and stress signaling in eukaryotes. In plants, there is a small SnRK1 gene family. While the SnRK1.1 gene has been well studied, the role other SnRK1 isoforms play in energy or stress signaling is less well understood. We used promoter:GUS analysis and found SnRK1.1 is broadly expressed, while SnRK1.2 is spatially restricted. SnRK1.2 is expressed most abundantly in hydathodes, at the base of leaf primordia, and in vascular tissues within both shoots and roots. We examined the impact that sugars have on SnRK1 gene expression and found that trehalose induces SnRK1.2 expression. Given that the SnRK1.1 and SnRK1.2 proteins are very similar at the amino acid level, we sought to address whether SnRK1.2 is capable of re-programming growth and development as has been seen previously with SnRK1.1 overexpression. While gain-of-function transgenic plants overexpressing two different isoforms of SnRK1.1 flower late as seen previously in other SnRK1.1 overexpressors, SnRK1.2 overexpressors flower early. In addition, SnRK1.2 overexpressors have increased leaf size and rosette diameter during early development, which is the opposite of SnRK1.1 overexpressors. We also investigated whether SnRK1.2 was localized to similar subcellular compartments as SnRK1.1, and found that both accumulate in the nucleus and cytoplasm in transient expression assays. In addition, we found SnRK1.1 accumulates in small puncta that appear after a mechanical wounding stress. Together, these data suggest key differences in regulation of the SnRK1.1 and SnRK1.2 genes in plants, and highlights differences overexpression of each gene has on the development of Arabidopsis.

Inositol pyrophosphates (PP-InsPs) are an emerging class of "high-energy" intracellular signaling molecules containing one or two diphosphate groups attached to an inositol ring, with suggested roles in bioenergetic homeostasis and inorganic phosphate (Pi) sensing. Information regarding the biosynthesis of these unique class of signaling molecules in plants is scarce, however the enzymes responsible for their biosynthesis in other eukaryotes have been well described. Here we report the characterization of the two Arabidopsis VIP kinase domains, a newly discovered activity of the Arabidopsis ITPK1 and ITPK2 enzymes, and the subcellular localization of the enzymes involved in the synthesis of InsP6 and PP-InsPs. Our data indicate that AtVIP1-KD and AtVIP2-KD act primarily as 1PP-specific Diphosphoinositol Pentakisphosphate Kinases (PPIP5) Kinases. The AtITPK enzymes, in contrast, can function as InsP6 kinases, and thus are the missing enzyme in the plant PP-InsP synthesis pathway.Together, these enzyme classes can function in plants to produce PP-InsPs, which have been implicated in signal transduction and Pi sensing pathways. We measured a higher InsP7 level (increased InsP7/InsP8 ratio) in vip1/vip2 double loss-of-function mutants, and an accumulation of InsP8 (decreased InsP7/InsP8 ratio) in the 35S:VIP2 overexpression line relative to wild-type plants. We also report that enzymes involved in the synthesis of InsPs and PP-InsPs accumulate within the nucleus and cytoplasm of plant cells. Our work defines a molecular basis for understanding how plants synthesize PP-InsPs which is crucial for determining the roles of these signaling molecules in processes such as Pi sensing. 3 SIGNIFICANCE STATEMENTInositol pyrophosphate signaling molecules are of agronomic importance as they can control complex responses to the limited nutrient, phosphate. This work fills in the missing steps in the inositol pyrophosphate synthesis pathway and points to a role for these molecules in the plant cell nucleus. This is an important advance that can help us design future strategies to increase phosphate efficiency in plants.

Inositol phosphates (InsPs) are intricately tied to lipid signaling, as at least one portion of the inositol phosphate signaling pool is derived from hydrolysis of the lipid precursor, phosphatidyl inositol (4,5) bisphosphate. The focus of this review is 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 (referred to as phytic acid). Even though plants produce huge amounts of precursor InsP6 in seeds, almost no attention has been paid to whether PPx-InsPs exist in plants, and if so, what roles these molecules play. Recent work has delineated that Arabidopsis has two genes capable of PP-InsP5 synthesis, and PPx-InsPs have been detected across the plant kingdom. This review will detail the known roles of PPx-InsPs in yeast and animal systems, and provide a description of recent data on the synthesis and accumulation of these novel molecules in plants, and potential roles in signaling.