Hyperosmotic stress rapidly generates lyso-phosphatidic acid in Chlamydomonas

Plant cells are continuously exposed to environmental stresses such as hype r-osmolarity, and have to respond in order to survive. When P-32-labelled C hlamydomonas moewusii cells were challenged with NaCl, the formation of a n ew radiolabelled phospholipid was stimulated, which was barely detectable b efore stimulation. The phospholipid was identified as lyso-phosphatidic aci d (LPA), and was the only lyso-phospholipid to be accumulated. The increase in LPA was dose- and time-dependent. When other osmotically active compoun ds were used, the formation of LPA was also induced with similar kinetics, although salts were better inducers than non-salts. At least part of the LP A was generated by phospholipase A(2) (PLA(2)) hydrolysing phosphatidic aci d (PA). This claim is based on PA formation preceding LPA production, and F LAP inhibitors decreasing the accumulation of LPA and promoting the convers ion of PA to diacylglycerol pyrophosphate. The latter is another metabolic derivative of PA that is implicated in cell signalling. The involvement of multiple lipid-signalling pathways in hyperosmotic stress responses is disc ussed.