Calcium Signaling via Phospholipase C Is Essential for Proline Accumulation upon Ionic But Not Nonionic Hyperosmotic Stresses in Arabidopsis

Proline (Pro) accumulation occurs in various plant organisms in response to environmental stresses. To identify the signaling components involved in the regulation of Pro metabolism upon water stress in Arabidopsis (Arabidopsis thaliana), a pharmacological approach was developed. The role of phosphoinositide-specific phospholipases C (PLCs) in Pro accumulation was assessed by the use of the aminosteroid U73122, a commonly employed specific inhibitor of receptor-mediated PLCs. We found that U73122 reduced pyrroline-5-carboxylate synthetase transcript and protein as well as Pro levels in salt-treated seedlings. Inhibition of PLC activity by U73122 was quantified by measuring the decrease of inositol 1,4,5-trisphosphate (InsP(3)) levels. Moreover, the utilization of diacylglycerol kinase and InsP(3)-gated calcium release receptor inhibitors suggested that InsP(3) or its derivatives are essential for Pro accumulation upon salt stress, involving calcium as a second messenger in ionic stress signaling. This observation was further supported by a partial restoration of Pro accumulation in salt- and U73122-treated seedlings after addition of extracellular calcium, or when calcium homeostasis was perturbed by cyclopiazonic acid, a blocker of plant type IIA calcium pumps. Taken together, our data indicate that PLC-based signaling is a committed step in Pro biosynthesis upon salinity but not in the case of mannitol stress. Calcium acts as a molecular switch to trigger downstream signaling events. These results also demonstrated the specific involvement of lipid signaling pathway to discriminate between ionic and nonionic stresses.