Identification and characterization of a novel plant phospholipase D that requires polyphosphoinositides and submicromolar calcium for activity in Arabidopsis.

Phospholipase D (PLD; EC 3.1.4.4) has been proposed to be involved in a number of cellular processes including transmembrane signaling and membrane deterioration. PLD previously described from various plant sources generally requires millimolar ranges of calcium for optimal activity. In this study, we genetically suppressed the expression of this conventional PLD in Arabidopsis by introducing an antisense PLD cDNA. However, both the antisense transgenic and wild-type plants showed comparable PLD activity in the presence of submicromolar concentrations of calcium and phosphatidylinositol 4, 5-bisphosphate using phosphatidylcholine as a substrate. This new PLD activity was partially stimulated by phosphatidylinositol 4-phosphate, but not by other phospholipids, including phosphatidylinositol, phosphatidylserine, phosphatidylglycerol, phosphatidic acid, or phosphatidylcholine. Its requirement for polyphosphoinositides was further supported by its ability to be inhibited by neomycin. The polyphosphoinositide-dependent PLD requires calcium for activity, but no magnesium. The calcium stimulation was observed in the nanomolar range and reached a plateau at 5 microM calcium. The findings of this study demonstrate the presence of different PLDs that are regulated in a distinct manner in plants. The potential significance of a PLD that is regulated by polyphosphoinositides and physiological concentrations of Ca2+ is discussed.