Ionophorous Functions of Phosphatidic Acid in the Plant Cell

Effects of phosphatidic acid (PA), a product of phospholipase D activity, on Ca 2+ and H + transport were investigated in membrane vesicles obtained from roots and coleoptiles of maize ( Zea mays L.). Calcium flows were measured with fluorescent probes indo-1 and chlorotetracycline loaded into the vesicles and added to the incubation medium, respectively. Phosphatidic acid (50-500 µ M) was found to induce downhill flow of Ca 2+ along the concentration gradient into the plasma membrane vesicles and endomembrane vesicles (tono- plast and endoplasmic reticulum). Protonophorous functions of PA were probed with acridine orange. First, the ionic H + gradient was created on the tonoplast vesicles by means of H + -ATPase activation with Mg-ATP addi- tion. Then, the vesicles were treated with 25-100 µ M PA, which induced the release of protons from tonoplast vesicles and dissipation of the proton gradient. Thus, PA could function as an ionophore and was able to transfer Ca 2+ and H + across plant cell membranes along concentration gradients of these ions. The role of PA in mech- anisms of intracellular signaling in plants is discussed.