ER (microsomal) omega-6 fatty acid desaturase introduces the second double bond in the biosynthesis of 18:3 fatty acids, important constituents of plant membranes (PubMed:14593172, PubMed:7907506). Delta(12)-desaturase with regioselectivity determined by the double bond (delta(9) position) and carboxyl group of the substrate. Can use both 16:1 and 18:1 fatty acids as substrates (PubMed:8685264). It is thought to use cytochrome b5 as an electron donor and to act on fatty acids esterified to phosphatidylcholine (PC) and, possibly, other phospholipids (PubMed:14593172, PubMed:7907506, PubMed:1730697). Very low constitutive hydroxylation activity (PubMed:11864983). Required for desaturation of fatty acids present in extraplastidial membranes, including mitochondria (PubMed:11104757, PubMed:17507388). Required for salt tolerance during seed germination and early seedling growth (PubMed:22279586). (9Z)-octadecenoyl-CoA + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (9Z,12Z)-octadecadienoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O (9Z)-hexadecenoyl-CoA + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (9Z,12Z)-hexadecadienoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O a (9Z)-octadecenoyl-containing glycerolipid + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = a (9Z,12Z)-octadecadienoyl-containing glycerolipid + 2 Fe(III)-[cytochrome b5] + 2 H2O (9Z)-octadecenoyl-CoA + AH2 + O2 = (9Z,12Z)-octadecadienoyl-CoA + A + 2 H2O (9Z)-hexadecenoyl-CoA + AH2 + O2 = (9Z,12Z)-hexadecadienoyl-CoA + A + 2 H2O (9Z)-tetradecenoyl-CoA + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (9Z,12Z)-tetradecadienoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O (9Z)-pentadecenoyl-CoA + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (9Z,12Z)-pentadecadienoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O (9Z)-heptadecenoyl-CoA + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (9Z,12Z)-heptadecadienoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O Lipid metabolism; polyunsaturated fatty acid biosynthesis. Homo- and heterodimer (PubMed:24811169). Interacts with FAD3 but not with FAD6 (PubMed:24811169). FAD2-FAD3 heterodimers can form a metabolic channel in which 18:1-PC is converted to 18:3-PC without releasing a free 18:2-PC intermediate (PubMed:24811169). Expressed in shoots and roots (PubMed:7907506, PubMed:22279586). Expressed in leaves, stems, flowers and siliques (PubMed:22279586). Up-regulated by osmotic stress (PubMed:22279586). Not regulated by cold stress (PubMed:7907506). The histidine box domains may contain the active site and/or be involved in metal ion binding (Probable). The C-terminal sequence (-YNNKL) is necessary and sufficient for maintaining localization in the endoplasmic reticulum (PubMed:14690501). Enhancement of both microviscosity and lipid/protein ratio of mitochondrial membranes, which in turn were responsible for the change in lateral mobility of lipids and for bioenergetic parameter modifications (PubMed:11104757). Increased membrane rigidity and cold sensitivity (PubMed:17507388). Hypersensitivity to salt or osmotic stress (PubMed:22279586). Introduction of Ile at position 146 or 324 has a dominant role in specifying hydroxylation activity. Belongs to the fatty acid desaturase type 1 family.