MUTANTS OF ARABIDOPSIS DEFICIENT IN THE SYNTHESIS OF ALPHA-LINOLENATE- BIOCHEMICAL AND GENETIC-CHARACTERIZATION OF THE ENDOPLASMIC-RETICULUM LINOLEOYL DESATURASE

The overall fatty compositions of leaf and root lipids from a mutant o f Arabidopsis thaliana were characterized by reduced levels of linolen ate (18:3) and correspondingly elevated levels of linoleate (18:2) as a consequence of a single nuclear mutation at the fad3 locus. Comparis on of the fatty acid compositions of individual lipids from wild type and mutant plants showed that chloroplast lipids were largely unaffect ed by the mutation, whereas each of the phospholipids synthesized on t he endoplasmic reticulum in the mutant exhibited a marked reduction in the proportion of 18:3 relative to wild type. These and other results indicate that the fad3 mutants are deficient in the activity of an en doplasmic reticulum 18:2 desaturase. In root tissues, this enzyme appe ars to account for over 80% of the 18:3 present. However, in young lea ves of the mutant, phosphatidylcholine (the major phospholipid) contai ns 16% 18:3 compared with 29% in the wild type, and the proportion of 18:3 in this lipid increases with plant age in both wild type and muta nt plants. These results reflect an exchange of lipid between the chlo roplast and endoplasmic reticulum that allows the chloroplast desatura ses to provide highly unsaturated lipids to the extrachloroplast membr anes of leaf cells.