The production of VLCPUFAs in plants

Abstract

Very long-chain polyunsaturated fatty acids (VLCPUFAs) like eicosapentaenoic acid (EPA) or docosahexaenoic acid (DHA) are health-beneficial components in the human diet. However, due to overfishing and pollution of the sea, oily sea fish as the main dietary source for these important fatty acids is decreasingly available. A promising solution to this shortfall is the production of VLCPUFAs in transgenic oilseed crops. Although this goal has already been reached to some extent, there are still challenges which need to be met. These are the specific accumulation of VLCPUFAs in the neutral lipid fraction of seeds as well as absolute product yields, especially in case of DHA. Those aspects of heterologous VLCPUFA production might be improved by transfer of specific acyltransferases and superior desaturases from suitable VLCPUFA-producing organisms into seed lipid metabolism. Therefore, nucleotide sequences from two Ostreococcus species potentially encoding acyltransferases or a Δ4-desaturase, respectively, were characterized by expression in yeast. In these studies, none of the analyzed putative acyltransferases revealed specificity for VLCPUFAs. In contrast, the front-end desaturase Old4p from Ostreococcus lucimarinus was found to prefer VLCPUFAs to shorter-chain fatty acids and to desaturate both (n-3)- and (n-6)-substrates bound to lipids. In addition to the yeast experiments, acyltransferases were also introduced individually and in combination with VLCPUFA-producing enzymes into Arabidopsis thaliana. Upon separate expression, a putative acyl-coenzyme A:lyso-phosphatidylcholine acyltransferase (LPCAT) sequence from O. lucimarinus produced the strongest effects by enhancing levels of polyunsaturated fatty acids at the expense of monounsaturated fatty acids in total seed lipids. However, the combination of acyltransferases with VLCPUFA-producing enzymes did not lead to higher yields in transgenic seeds. Besides the expression studies performed with the microalgal acyltransferase sequences, also endogenous acyltransferase activities of the two different host plants A. thaliana and Camelina sativa were investigated by using both species for VLCPUFA-production and comparing product yields.