Expression of the Yeast Delta-9 Fatty Acid Desaturase in Nicotiana tabacum

To examine the processes of plant cytoplasmic fatty acid desaturation and glycerolipid biosynthesis, the protein coding sequence of the endoplasmic reticulum cytochrome b₅-dependent, Δ-9 fatty acid desaturase gene from Saccharomyces cerevisiae was introduced into Nicotiana tabacum via Agrobacterium transformation. All transformed plants expressing the yeast gene at the mRNA level exhibited an approximately 10-fold increase in the levels of palmitoleic acid (16:1) in leaf tissue. This fatty acid species is found in very low levels (less than 2%) in wild-type plants. These results indicate that the yeast desaturase can function in plants, presumably by using a leaf microsomal cytochrome b₅-mediated electron transport system. Lipid analysis demonstrated that the overproduced 16:1 is incorporated into most of the major polar lipid classes, including the cytoplasmically produced “eukaryotic” fraction of the chloroplast galactolipids. 16:1 was not found, however, in phosphatidyl glycerol, which is considered to be produced almost exclusively in the chloroplast. Despite these changes in membrane lipid composition, no obvious phenotypic differences were apparent in the transformed plants. Positional analysis shows that the cytoplasmically produced 16:1 is found primarily in the sn-2 position of phosphatidylcholine, phosphatidylethanolamine, monogalactosyldiacylglycerol, and digalactosyldiacylglycerol. The positional data suggest that the sn-2 acyltransferases responsible for the “eukaryotic” arrangement of 16- and 18- carbon fatty acids in glycerolipids are selective for unsaturated fatty acids rather than chain length.