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.

Phosphate is a major plant macronutrient and low phosphate availability severely limits global crop productivity. In Arabidopsis, a key regulator of the transcriptional response to low phosphate, phosphate starvation response 1 (PHR1), is modulated by a class of signaling molecules called inositol pyrophosphates (PP-InsPs). Two closely related diphosphoinositol pentakisphosphate enzymes (AtVIP1 and AtVIP2) are responsible for the synthesis and turnover of InsP8, the most implicated molecule. This study is focused on characterizing Arabidopsis vip1/vip2 double mutants and their response to low phosphate. We present evidence that both local and systemic responses to phosphate limitation are dampened in the vip1/vip2 mutants as compared to wild-type plants. Specifically, we demonstrate that under Pi-limiting conditions, the vip1/vip2 mutants have shorter root hairs and lateral roots, less accumulation of anthocyanin and less accumulation of sulfolipids and galactolipids. However, phosphate starvation response (PSR) gene expression is unaffected. Interestingly, many of these phenotypes are opposite to those exhibited by other mutants with defects in the PP-InsP synthesis pathway. Our results provide insight on the nexus between inositol phosphates and pyrophosphates involved in complex regulatory mechanisms underpinning phosphate homeostasis in plants.

The ability of an organism to maintain homeostasis in changing conditions is crucial for growth and survival. Eukaryotes have developed complex signaling pathways to adapt to a readily changing environment, including the inositol phosphate (InsP) signaling pathway. In plants and humans the pyrophosphorylated inositol molecules, inositol pyrophosphates (PP-InsPs), have been implicated in phosphate and energy sensing. PP-InsPs are synthesized from the phosphorylation of InsP6, the most abundant InsP. The plant PP-InsP synthesis pathway is similar but distinct from that of the human, which may reflect differences in how molecules such as Ins(1,4,5)P3 and InsP6 function in plants vs. animals. In addition, PP-InsPs can potentially interact with several major signaling proteins in plants, suggesting PP-InsPs play unique signaling roles via binding to protein partners. In this review, we will compare the biosynthesis and role of PP-InsPs in animals and plants, focusing on three central themes: InsP6 synthesis pathways, synthesis and regulation of the PP-InsPs, and function of a specific protein domain called the Syg1, Pho1, Xpr1 (SPX ) domain in binding PP-InsPs and regulating inorganic phosphate (Pi) sensing. This review will provide novel insights into the biosynthetic pathway and bioactivity of these key signaling molecules in plant and human systems.

Like many institutions around the world, the COVID-19 pandemic prompted us to shift our summer 2020 in-person undergraduate experiential learning program to a remote, virtual format. Here, we present our observations, summarized in 10 best practices, for moving a STEM-focused research experience for undergraduates, experiential learning program or research-based course online. We will also discuss how our program was originally designed and implemented, and how we adapted our activities to deliver an at-home research experience that maintained student engagement, mentorship, and a shared sense of community.

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Phosphate (Pi) is an essential nutrient for plants, required for plant growth and seed viability. Under Pi stress, plants undergo dynamic changes to leverage available Pi. One class of signaling molecules implicated in the Pi sensing pathway is the inositol pyrophosphates (PPx‐InsPs). PPx‐InsPs have high energy bonds, and have been linked to maintaining Pi and energy homeostasis in plants, yeast, and humans. In plants, PPx‐InsPs are thought to function in sensing Pi changes by binding to proteins containing an SPX domain. One particular SPX protein binds to PPx‐InsPs which prevents a transcription factor from activating transcription of the so‐called phosphate starvation response (PSR) genes. Plants are known to accumulate two common PPx‐InsPs, PP‐InsP5 (called InsP7) and PP2‐InsP4 (called InsP8), but the synthesis pathway has not yet been elucidated. We have identified and characterized the plant VIP kinases, enzymes that likely function as PP‐InsP5 kinases. The identity of a plant inositol kinase that can phosphorylate InsP6, resulting in PP‐InsP5, however, has remained elusive. We will describe our recent results in characterizing this pathway, and genetic mutants key for understanding inositol kinase gene function in plants. Our results support a model in which InsP7 and InsP8 function to turn off the PSR in plants.Support or Funding InformationThis work was supported by an award from the NSF to GG and IP:MCB 1615953This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Under changing environmental conditions, plants are able to modulate their lipids to respond to varying nutrient availability. Phosphate (Pi) is an essential nutrient for plants, required for plant growth and seed viability. Under Pi stress, plants undergo dynamic morphological and metabolism changes to leverage available Pi, including the modulation of lipids. Plants have been shown to “remodel” their lipid membrane profiles under phosphate starvation, degrading phospholipids in the cell membranes and utilizing the generated phosphorus for essential biological processes. By concomitantly inducing a phospholipid hydrolysis pathway and galactolipid biosynthetic pathway, membrane phospholipids are replaced by non‐phosphorus containing galactolipids and sulfolipids. The inositol phosphate (InsP) signaling pathway is a crucial element of the plant's ability to respond to changing energy conditions. Inositol hexakisphosphate (InsP6) is the most abundant InsP signaling molecule and can be phosphorylated further by VIP kinases, resulting in inositol pyrophosphates (PP‐InsPs). PP‐InsPs have high energy bonds and have been linked to maintaining Pi and energy homeostasis in yeast and plants. Using liquid chromatography‐mass spectrometry and tandem mass spectrometry, we have examined the lipid profiles of three Arabidopsis PP‐InsP mutants, in response to Pi depletion, to address the role of PP‐InsPs in Pi sensing. Our results suggest that PP‐InsPs play a crucial role in Pi sensing and are involved in the regulation of lipid biosynthesis. Furthermore, the changes in the abundance of lipids suggest a possible direction for future seed oil engineering strategies.

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