The present invention relates to novel, polynucleotides isolated from dwarf plants. The dwf4 polynucleotides that encode all, or a portion of, a DWF4 polypeptide, a cytochrome P450 enzyme that mediates multiple steps in synthesis of brassinosteroids. The present invention also relates to isolated polynucleotides that encode regulatory regions of dwf4. Uses of the dwf4 polypeptides and polynucleotides are also disclosed.