A novel type of O-methyltransferase (OMT) cDNA was isolated from maturing seeds of Carthamus tinctorius (safflower). The deduced sequence of the OMT protein showed moderate sequence identity (52%) with C. tinctorius 5-hydroxyconiferaldehyde O-methyltransferase 1 (CAldOMT1). Phylogenetic analysis showed that the novel OMT did not belong to the typical CAldOMT [=caffeic acid OMT (CAOMT)] cluster. The recombinant protein of the OMT catalyzed 3(or 5-) O-methylation of hydroxycinnamaldehydes and hydroxycinnamyl alcohols, while it showed only weak or moderate activity toward hydroxycinnamates and hydroxycinnamoyl coenzyme A esters. There fore, this OMT was designated as C. tinctorius 5-hydroxyconiferaldehyde/5-hydroxyconiferyl alcohol OMT (CtAAOMT). The time profile of CtAAOMT gene expression in C. tinctorius matched the patterns of lignin accumulation. Taken together, our data strongly suggest that along with CtCAldOMT1, CtAAOMT is involved in biosynthesis of syringyl lignin.