PURPOSE: A gene(CYB5-1 to CYB5-7) isolated from castor cytochrome B5 is provided to promote hydroxyl fatty acid production in a plant and to develop a transgenic plant which produces hydroxyl fatty acid. CONSTITUTION: A castor cytochrome B5 polypeptide has an amino acid sequence of sequence number 2, 4, 6, 8, 10, 12, or 14. A castor cytochrome B5 gene encoding the polypeptide has a nucleic acid sequence of sequence number 1, 3, 5, 7, 9, 11, or 13. The castor cytochrome B5 gene is derived from castor. A composition for promoting hydroxyl fatty acid production in a plant contains the polypeptide or the gene. A method for promoting hydroxyl fatty acid production in the plant comprises a step of transducing the gene into a plant by transforming the plant with a vector containing the gene.
