Involved in sphingolipid trihydroxy long-chain base (4-hydroxysphinganine) biosynthesis. Can use C18- and C20-sphinganine as substrates to produce C18- and C20-phytosphinganines (D-ribo-2-amino-1,3,4-trihydroxyoctadecane and -eicosane). a dihydroceramide + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = a phytoceramide + 2 Fe(III)-[cytochrome b5] + H2O Membrane lipid metabolism; sphingolipid biosynthesis. Ubiquitous, with higher levels in flowers and roots. The histidine box domains may contain the active site and/or be involved in metal ion binding. No visible phenotype; due to the redundancy with SBH2. Sbh1 and sbh2 double mutants are severely dwarfed, do not progress from vegetative to reproductive growth and have enhanced expression of programmed cell death associated-genes. Belongs to the sterol desaturase family. Truncated N-terminus. Truncated N-terminus.