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. It is thought to use cytochrome b5 as an electron donor and to act on fatty acids esterified to phosphatidylcholine and, possibly, other phospholipids. Lipid metabolism; polyunsaturated fatty acid biosynthesis. Strongly expressed in developing seeds. The histidine box domains may contain the active site and/or be involved in metal ion binding. Belongs to the fatty acid desaturase type 1 family.