Rapid accumulation of phosphatidylinositol 4,5-bisphosphate and inositol 1,4,5-trisphosphate correlates with calcium mobilization in salt-stressed Arabidopsis

The phosphoinositide phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P-2] is a key signaling molecule in animal cells. It can be hydrolyzed to relea se 1,2-diacyglycerol and inositol 1,4,5-trisphosphate (IP,), which in anima l. cells lead to protein kinase C activation and cellular calcium mobilizat ion, respectively, in addition to its critical roles in constitutive and re gulated secretion of proteins, PtdIns(4,5)P-2 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) P-2 synthesis in response to treatment with sodium chloride, potassium chlo ride, and sorbitol. These results demonstrate that when challenged with sal inity and osmotic stress, terrestrial plants respond differently than algae , yeasts, and animal cells that accumulate different species of phosphoinos itides. We also show data demonstrating that whole-plant IP3 levels increas e significantly within 1 min of stress initiation, and that IP3 levels cont inue to increase for more than 30 min during stress application. Furthermor e, using the calcium indicators Fura-2 and Flue-3 we show that root intrace llular calcium concentrations increase in response to stress treatments. Ta ken together, these results suggest that in response to salt and osmotic st ress, Arabidopsis uses a signaling pathway in which a small but significant portion of PtdIns(4,5)P-2 is hydrolyzed to IP3. The accumulation of IP3 oc curs during a time frame similar to that observed for stress induced calciu m mobilization These data also suggest that the majority of the PtdIns(4,5) P-2 synthesized in response to salt and osmotic stress may be utilized for cellular signaling events distinct from the canonical IP3 signaling pathway .