Phospholipid signaling responses in salt-stressed rice leaves

Salinity is one of the major environmental factors limiting growth and productivity of rice plants. In this study, the effect of salt stress on phospholipid signaling responses in
rice leaves was investigated. Leaf cuts were radiolabeled with 32 P-orthophosphate and the lipids extracted and analyzed by thin-layer chromatography, autoradiography
and phosphoimaging. Phospholipids were identifi ed by co-migration of known standards. Results showed that 32 P i was rapidly incorporated into the minor lipids, phosphatidylinositol bisphosphate (PIP 2 ) and phosphatidic acid (PA) and, interestingly, also into the structural lipids phosphatidylethanolamine (PE) and phosphatidylglycerol
(PG), which normally label relatively slowly, like phosphatidylcholine (PC) and phosphatidylinositol (PI). Only very small amounts of PIP 2 were found. However, in
response to salt stress (NaCl), PIP 2 levels rapidly (<30 min) increased up to 4-fold, in a time- and dose-dependent manner. PA and its phosphorylated product, diacylglycerolpyrophosphate (DGPP), also increased upon NaCl stress, while cardiolipin (CL) levels decreased. All other phospholipid levels remained unchanged. PA signaling
can be generated via the combined action of phospholipase C (PLC) and diacylglycerol kinase (DGK) or directly via phospholipase D (PLD). The latter can be measured in
vivo, using a transphosphatidylation assay. Interestingly, these measurements revealed that salt stress inhibited PLD activity, indicating that the salt stress-induced PA
response was not due to PLD activity. Comparison of the 32 P-lipid responses in salt-tolerant and salt-sensitive cultivars revealed no signifi cant differences. Together
these results show that salt stress rapidly activates several lipid responses in rice leaves but that these responses do not explain the difference in salt tolerance between sensitive and tolerant cultivars.