Rapid Accumulation of Phosphatidylinositol 4,5-Bisphosphate and Inositol 1,4,5-Trisphosphate Correlates with Calcium Mobilization in Salt-Stressed Arabidopsis

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Abstract
The phosphoinositide phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] is a key signaling molecule in animal cells. It can be hydrolyzed to release 1,2-diacyglycerol and inositol 1,4,5-trisphosphate (IP3), which in animal cells lead to protein kinase C activation and cellular calcium mobilization, respectively. In addition to its critical roles in constitutive and regulated secretion of proteins, PtdIns(4,5)P2 binds to proteins that modify cytoskeletal architecture and phospholipid constituents. Herein, we report that Arabidopsis plants grown in liquid media rapidly increase PtdIns(4,5)P2 synthesis in response to treatment with sodium chloride, potassium chloride, and sorbitol. These results demonstrate that when challenged with salinity and osmotic stress, terrestrial plants respond differently than algae, yeasts, and animal cells that accumulate different species of phosphoinositides. We also show data demonstrating that whole-plant IP3 levels increase significantly within 1 min of stress initiation, and that IP3 levels continue to increase for more than 30 min during stress application. Furthermore, using the calcium indicators Fura-2 and Fluo-3 we show that root intracellular calcium concentrations increase in response to stress treatments. Taken together, these results suggest that in response to salt and osmotic stress, Arabidopsis uses a signaling pathway in which a small but significant portion of PtdIns(4,5)P2 is hydrolyzed to IP3. The accumulation of IP3 occurs during a time frame similar to that observed for stress-induced calcium mobilization. These data also suggest that the majority of the PtdIns(4,5)P2 synthesized in response to salt and osmotic stress may be utilized for cellular signaling events distinct from the canonical IP3 signaling pathway.