Phospholipase Activities Associated with the Tonoplast from Acer Pseudoplatanus Cells: Identification of a Phospholipase A₁ Activity

Abstract

In higher plants, the lipolytic enzymes and their physiological functions are not well characterized [1]. Most reports demonstrated that phospholipid catabolism in plants is achieved by the concerted actions of membrane-bound enzymes including phospholipase D, phosphatidate phosphatase, lipolytic acyl hydrolases and lipoxygenases [1,2]. With the exception of the phospholipase D, the literature on plant phospholipases is still very limited. We previously reported that tonoplast from Acer pseudoplatanus cells contains small amounts of phosphatidc acid and lysophospholipids, which were produced together with free fatty acids, particularly after addition of Ca²⁺[3]. These data suggested the possible involvement of phospholipase D and phospholipase A in the metabolism of vacuolar membrane lipids. The phospholipase activities were studied by following the hydrolysis of added sn-2-[14C]linoleyl-PC to tonoplast vesicles. Tonoplast was obtained by osmotic lysis of pure preparations of vacuoles isolated from protoplasts derived from Acer pseudoplatanus cells [4], This present work demonstrated clearly the presence of phospholipase D and phospholipase A₁ activities associated with the tonoplast of Acer. The phospholipase A₁ showed an optimal activity at pH about 6–6.5, did not necessarily require divalent cations, but was stimulated by Mg²⁺ and particularly by Ca²⁺. This work presents the first evidence for the presence of phospholipases A₁ in plant cells.