| Extracellular lipid based barriers, such as the cuticle, suberin, and sporopollenin, protect plants from the environment. Despite the importance of these barriers, many aspects of extracellular lipid biosynthesis remain uncharacterized. I have identified a four member family of thioesterases from Arabidopsis thaliana termed MODIFIERS OF EXTRACELLULAR LIPIDS (MEL1-4). The gene expression patterns of three members of this family, MEL1, MEL2, and MEL4, were found to correlate with the deposition of cuticle, suberin, and sporopollenin, respectively. Artificial microRNA silenced and over-expression lines were created to determine if MEL1 influences cuticle composition. The cuticular wax and cutin composition were analyzed, but no effect on the load or composition was apparent. This may be due to gene redundancy or insufficient silencing or over-expression of MEL1. It is also possible that the effects on extracellular compounds are not detectable using common methods. MEL1 was shown in vitro to hydrolyze palmitoyl-CoA (16:0) into a free fatty acid and CoA. Members of the MEL family were also shown in E. coli to have acyl-ACP thioesterase activity toward beta-ketoacyl-ACP and non-oxygenated acyl-ACP substrates. These results confirm that MEL proteins possess fatty acyl thioesterase activity and likely fulfill a role in extracellular lipid biosynthesis. I discuss several potential roles for thioesterases in extracellular lipid biosynthesis, such as in termination of fatty acid synthesis in the plastid, synthesis of beta-ketoacids leading to the production of methylketones and/or alkanes, production of long-chain or very-long-chain free fatty acids found in cutin and cuticular wax, respectively, and in regulation of extracellular lipid metabolism. The MEL enzymes are expected to fulfill one or more of these roles. |
