Down-regulation of fatty acid desaturase-6 (FAD6) in green alga Chlamydomonas reinhardtii

The proportion of unsaturated fatty acids such as oleic acid (18:1) is an important factor for increasing oxidative stability of algal oil during storage. Oleic acid and palmitoleic acid (16:1) are desaturated to more unsaturated fatty acids in chloroplast by FAD6 (plastid-pathway) and cytosol by FAD2 (ER-pathway) in higher plants. In this study, we isolated FAD6 from Chlamydomonas reinhardtii, named CrFAD6 and generated CrFAD6-downregulated transgenic lines using RNAi to increase accumulation level of oleic acid in C. reinhardtii. In CrFAD6- RNAi lines, oleic acid and palmitoleic acid content were increased ~2.1- and ~5.6-fold, respectively, compared to the wild-type control. Interestingly, both linoleic acid (18:2) and linolenic acid (18:3) contents in CrFAD6-RNAi lines were reduced ~32% in comparison with the control; in fad6 mutant of Arabidopsis, palmitoleic acid, oleic acid and linoleic acid content were induced, whereas linolenic acid content was reduced, indicating that desaturation of oleic acid to linoleic acid is mainly regulated by ER pathway in higher plants. In this study, remarkable phenomenon compared to higher plants is that downregulation of CrFAD6 resulted in the reduction of linoleic acid content (18:2) in C. reinhardtii. These results also suggest that the plastidic pathway may play a major role in desaturation of oleic acid to linoleic acid/linolenic acid in green alga C. reinhardtii.