DISCOVERY OF AN EPIDERMAL STEAROYL-ACYL CARRIER PROTEIN THIOESTERASE - ITS POTENTIAL ROLE IN WAX BIOSYNTHESIS

Plant epicuticular, or surface, waxes are synthesized primarily, if no t exclusively, by epidermal cells. The epicuticular wax constitutes al most 20% of the chloroformextractable lipids in developing leek leaf a nd is derived predominantly from saturated fatty acids. The significan t requirement for saturated fatty acids in epidermal tissues led us to investigate whether or not epidermal extracts have thioesterase activ ities that prefer saturated acyl-acyl carrier protein (ACP) substrates , rather than the 18:1-ACP more commonly hydrolyzed by total leaf extr acts. Epidermal extracts from Brassica, pea, and leek exhibited higher activities toward saturated acyl-ACPs relative to 18:1-ACP when compa red to total leaf or leaf parenchymal extracts. We identified and puri fied a stearoyl-ACP (18:0-ACP)-specific thioesterase from leek epiderm al extracts which could be separated from 18:1-ACP thioesterase using hydroxyapatite chromatography. The stearoyl-ACP thioesterase exhibited a high preference for 18:0-ACP, having less than 10% of the 18:0-ACP hydrolyzing activity when presented with 18:1-ACP, 16:0-ACP, or 18:0-C oA substrates. The stearoyl-ACP thioesterase was predominantly, if not exclusively, expressed in epidermis and may play a role in generating the saturated fatty acid pool required for wax production.