Molecular cloning and tissue-specific expression of two genes that encode caffeic acid O-methyltransferases from Populus kitakamiensis

Two genes (homt1 and homt2) for caffeic acid: S-adenosyl-L-methionine O-methyltransferase (OMT) were isolated from Populus kitakamiensis, Nucleotide sequencing analysis revealed that the coding sequence of homt1 exhibited 99% homology to the cDNA for lignin bi-specific OMT from quaking aspen. The nucleotide sequence of the coding region of homt2 was 89% homologous to this cDNA. Northern blot analysis showed that the transcript of homt1 was abundant in stems while the transcript of homt2 accumulated in leaves. Immunohistochemical analysis with OMT-specific antiserum showed that the enzyme was localized in the xylem of stems and leaves but not in other tissues. Since the xylem is known as a lignifying tissue, it seems likely that homt1 and homt2 are involved in the biosynthesis of lignin and that the expression of these genes is subject to tissue-specific control.