Hyperosmotic stress stimulates phospholipase D activity and elevates the levels of phosphatidic acid and diacylglycerol pyrophosphate

In mammalian cells, phospholipase D (PLD) and its product phosphatidic acid (PA) are involved in a number of signalling cascades, including cell proli feration, membrane trafficking and defence responses. In plant cells a sign alling role for PLD and PA is also emerging. Plants have the extra ability to phosphorylate PA to produce diacylglycerol pyrophosphate (DGPP), a newly discovered phospholipid whose formation attenuates PA levels, but which co uld itself be a second messenger. Here we report that increases in PA and i ts conversion to DGPP are common stress responses to water deficit. Increas es occur within minutes of treatment and are dependent on the level of stre ss. Part of the PA produced is due to PLD activity as measured by the in vi vo transphosphatidylation of 1-butanol, and part is due to diacylglycerol k inase activity as monitored via P-32-PA formation in a differential labelli ng protocol. Increases in PA and DGPP are found not only in the green alga Chlamydomonas moewusii and cell-suspension cultures of tomato and alfalfa w hen subjected to hyperosmotic stress, but also in dehydrated leaves of the resurrection plant Craterostigma plantagineum. These results provide furthe r evidence that PLD and PA play a role in plant signalling, and provide the first demonstration that DGPP is formed during physiological conditions th at evoke PA synthesis.