Triglycerides As Products of Photosynthesis. Genetic Engineering, Fatty Acid Composition and Structure of Triglycerides

Abstract

In this chapter we will review the contributions of recombinant DNA technology to the study of triglyceride synthesis. The success of this technology will firstly be illustrated by research concerned with fatty acid chain length. Until recently, there was no explanation for the preferential accumulation of fatty acids of a given chain length by certain species. Only through gene cloning and gene transfer technologies was it possible to prove unequivocally that specific thioesterases controlled the elongation of fatty acids. So, the diversity of fatty acids accumulated by different species can now be explained by the diversity of thioesterases active in these species. Research on other enzymes involved in fatty acid synthesis and lipid assemblage is also progressing rapidly, following the path established for the study of thioesterases. In all likelihood, cloning ‘through homology’ will soon deliver the entire ketoacyl-ACP synthases gene family and provide detailed characteristics on the role of each KAS in fatty acid elongation. As for lipid assemblage, recent experiments with a coconut lysophosphatidylacyltransferase expressed in canola seeds support the expectations that future manipulations of specific acyltransferases will enable the production of a variety of new structured triglycerides. With respect to fatty acid desaturation, the genetic manipulation of desaturase genes has already created potentially new oil products. The isolation of membrane-bound desaturase genes, however, remains hindered by their relative lack of homology. Instead of strict homology, the different desaturases share common motifs such as the ‘histidine box’ which was used to identify and isolate a delta-6 desaturase gene from borage. Since the sequences of entire plant genomes are now available, motif recognition could become a general approach to the cloning of new genes.