Hyperosmotic stress induces a rapid and transient increase in inositol 1,4,5-trisphosphate independent of abscisic acid in Arabidopsis cell culture

Phospholipid metabolism is involved in hyperosmotic-stress responses in plants. To investigate the role of phosphoinositide-specific phospholipase C (PI-PLC) - a key enzyme in phosphoinositide turnover-in hyperosmotic-stress signaling, we analyzed changes in inositol 1,4,5-trisphosphate (Ins(1,4,5)P₃) content in response to hyperosmotic shock or salinity in Arabidopsis thaliana T87 cultured cells. Within a few s, a hyperosmotic shock, caused by mannitol, NaCl, or dehydration, induced a rapid and transient increase in Ins(1,4,5)P₃. However, no transient increase was detected in cells treated with ABA. Neomycin and U73122, inhibitors of PI-PLC, inhibited the increase in Ins(1,4,5)P₃ caused by the hyperosmotic shock. A rapid increase in phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P₂) in response to the hyperosmotic shock also occurred, but the rate of increase was much slower than that of Ins(1,4,5)P₃. These findings indicate that the transient Ins(1,4,5)P₃ production was due to the activation of PI-PLC in response to hyperosmotic stress. PI-PLC inhibitors also inhibited hyperosmotic stress-responsive expression of some dehydration-inducible genes, such as rd29A (lti78/cor78) and rd17 (cor47), that are controlled by the DRE/CRT cis-acting element but did not inhibit hyperosmotic stress-responsive expression of ABA-inducible genes, such as rd20. Taken together, these results suggest the involvement of PI-PLC and Ins(1,4,5)P₃ in an ABA-independent hyperosmotic-stress signal transduction pathway in higher plants.