Plays a major role as delta(8)-fatty-acid desaturase which introduces a double bond at the 8-position in the long-chain base (LCB) of ceramides with or without a hydroxy group at the 4-position. The enzyme produces both the 8E and 8Z isomers (in a 4:1 ratio). This structural modification contributes to the quantitative partitioning of ceramides between the two major sphingolipid classes, glucosylceramides and glycosylinositolphosphoryl ceramides. Sphingolipids are important membrane components involved in environmental stress responses, such as resistance to chilling, and act as cell signaling molecules. an N-acyl-(4R)-4-hydroxysphinganine + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = a (4R,8E)-4-hydroxysphingenine ceramide + 2 Fe(III)-[cytochrome b5] + 2 H2O an N-acyl-(4R)-4-hydroxysphinganine + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (4R,8Z)-4-hydroxysphing-8-enine ceramide + 2 Fe(III)-[cytochrome b5] + 2 H2O Highly expressed in flowers. Expressed in roots, leaves, stems and siliques. 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 plants have strongly reduced levels of unsaturated LCB sphingolipids in all organs and show enhanced sensitivity to prolonged low-temperature exposure. Belongs to the fatty acid desaturase type 1 family.