In plants, myoinositol signaling pathways have been associated with several stress, developmental, and physiological processes, but the regulation of these pathways is largely unknown. In our efforts to better understand myoinositol signaling pathways in plants, we have found that the WD40 repeat region of a myoinositol polyphosphate 5-phosphatase (5PTase13; At1g05630) interacts with the sucrose nonfermenting-1-related kinase (SnRK1.1) in the yeast two-hybrid system and in vitro. Plant SnRK1 proteins (also known as AKIN10/11) have been described as central integrators of sugar, metabolic, stress, and developmental signals. Using mutants defective in 5PTase13, we show that 5PTase13 can act as a regulator of SnRK1 activity and that regulation differs with different nutrient availability. Specifically, we show that under low-nutrient or -sugar conditions, 5PTase13 acts as a positive regulator of SnRK1 activity. In contrast, under severe starvation conditions, 5PTase13 acts as a negative regulator of SnRK1 activity. To delineate the regulatory interaction that occurs between 5PTase13 and SnRK1.1, we used a cell-free degradation assay and found that 5PTase13 is required to reduce the amount of SnRK1.1 targeted for proteasomal destruction under low-nutrient conditions. This regulation most likely involves a 5PTase13-SnRK1.1 interaction within the nucleus, as a 5PTase13:green fluorescent protein was localized to the nucleus. We also show that a loss of function in 5PTase13 leads to nutrient level-dependent reduction of root growth, along with abscisic acid (ABA) and sugar insensitivity. 5ptase13 mutants accumulate less inositol 1,4,5-trisphosphate in response to sugar stress and have alterations in ABA-regulated gene expression, both of which are consistent with the known role of inositol 1,4,5-trisphosphate in ABA-mediated signaling. We propose that by forming a protein complex with SnRK1.1 protein, 5PTase13 plays a regulatory role linking inositol, sugar, and stress signaling.

Inositol signaling pathways are important for many different developmental and physiological processes in eukaryotes. In plants, inositol signaling is used during drought stress, with the drought hormone, ABA, causing an increase in second messenger Ins(1,4,5)P3 levels. In order to better understand inositol signaling pathways in plants, we have investigated Arabidopsis inositol polyphosphate 5‐phosphatases (EC info:ddbj-emblgenbank/3.1.3.56; At5PTase). Like some other plant 5PTase enzymes, At5PTase13 is unique in that it contains WD repeats implicated in protein‐protein interactions. We used the WD region as bait in the yeast two‐hybrid system to identify protein partners of At5PTase13. We identified the Arabidopsis SNF1‐like kinase as an interacting protein and confirmed this interaction in vitro. The plant SNF1‐like kinases have been implicated in stress signaling pathways, so we characterized At5PTase13 loss‐of‐function mutants and their response to stresses. We found that two mutants, At5pt13‐1 and At5pt13‐2 display decreased sensitivity to ABA in a concentration dependent manner. Consistent with the decreased sensitivity to ABA, the endogenous Ins(1,4,5)P3 level is reduced in both mutants upon exposure to 6% glucose which induces ABA synthesis. Together these data suggest that plants use an At5PTase13:SNF1‐like kinase complex as an Ins(1,4,5)P3 sensor to control drought‐sensing signal transduction pathways. USDA NRI award to G.G.