Involved in delta-9 desaturation of fatty acids (Probable) (PubMed:15240892). Plays a role in the production of very-long-chain monounsaturated fatty acids (VLCMUFAs) in seed lipids and in membrane phospholipids and sphingolipids (PubMed:23175755). Acts as C-16:0 desaturase for monogalactosyl diacylglycerol (MGDG) and phosphatidylglycerol (PG) (PubMed:23585650). Is an essential component for cold adaptation (PubMed:23585650). Is essential to adjust the acyl composition of organelle membrane lipid composition in response to cold stress (PubMed:23585650). a 1-hexacosanoyl-2-acyl-phosphoglycerolipid + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = a 1-[(17Z)-hexacos-17-enoyl]-2-acyl-phosphoglycerolipid + 2 Fe(III)-[cytochrome b5] + 2 H2O a 1-tetracosanoyl-2-acyl-phosphoglycerolipid + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = a 1-[(15Z)-tetracos-15-enoyl]-2-acyl-phosphoglycerolipid + 2 Fe(III)-[cytochrome b5] + 2 H2O Lipid metabolism; polyunsaturated fatty acid biosynthesis. Strongly expressed in flowers, roots, leaves, seedpods, and inflorescence meristems. Up-regulated by cold. The histidine box domains may contain the active site and/or be involved in metal ion binding. No visible phenotype under normal growth conditions, but mutant plant display a dwarf and sterile phenotype when grown at 6 degrees Celsius and also show increased sensitivity to freezing temperature. Substrate specificity shifts from delta-9 to delta-7 desaturation when the protein is retargeted to the chloroplast. Belongs to the fatty acid desaturase type 1 family.