Lipid deacylating enzymes in plants: old activities, new genes.

Because lipids are major components of cellular membranes, their degradation under stress conditions compromises compartmentalization. However, in addition to having structural roles, membrane lipids are also implicated in signalling processes involving the activity of lipolytic enzymes. Phospholipases D and C, acting on the polar heads of phospholipids, have been relatively well characterized in plants. In contrast, knowledge of lipid deacylating enzymes remains limited. Lipid acyl hydrolases (LAH) are able to hydrolyse both fatty acid moieties of polar lipids. They differ from phospholipases A(1) or A(2) (PLA) acting on sn-1 or sn-2 positions of phospholipids, respectively, as well as from lipases which de-esterify triacylglycerols. The free polyunsaturated fatty acids generated by deacylating enzymes can be used in the biosynthesis of oxylipins and the lysophospholipids, provided by PLAs, are also bioactive molecules. In the four decades that have passed since the first description of LAH activities in plants some enzymes have been purified. In recent years, the widespread use of molecular approaches together with the attention paid to lipid signalling has contributed to a renewed interest in LAH and has led to the identification of different gene families and the characterization of new enzymes. Additionally, several proteins with putative lipase/esterase signatures have been identified. In the present paper we review currently available data on LAHs, PLAs, triacylglycerol lipases and other putative deacylating enzymes. The roles of lipid deacylating enzymes in plant growth, development and stress responses are discussed in the context of their involvement in membrane deterioration, lipid turnover and cellular signalling.