Phosphatidylinositol 4-phosphate (PI4P) is a minor phospholipid signal molecule involved in diverse biological processes. It is also the main precursor of phosphatidylinositol 4,5-bisphosphate [PI(4,5)P(2)], which is well described as the substrate of the phospholipase C (PLC)-diacylglycerol kinase (DGK) signalling pathway. Recently, we have demonstrated that PI4P plays a role as an extracellular signalling molecule involved in the activation of xylanase-induced defence responses in tomato cell suspensions. Here we make an approach to the possible mode of action of extracellular PI4P by analysing the involvement of PLC/DGK signalling pathway. We show that extracellular PI4P can be incorporated into tomato cells and further metabolized to PI, although its conversion to PI(4,5)P(2) could not be detected. In addition, treatment of tomato cells with the PLC inhibitor U73122 inhibited the oxidative burst triggered by PI4P treatment, suggesting the involvement of PLC in the induction of this response. Nevertheless, we demonstrate that PI4P treatment does not induce DGK activity. These results indicate that PLC but not DKG activation would be triggered by extracellular PI4P. In this sense, a possible mode of action of extracellular PI4P as a direct substrate of PLC to induce this signalling pathway is discussed.
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