Characterization of a flavonol 3-O-methyltransferase in the trichomes of the wild tomato species Solanum habrochaites

Abstract

The glandular trichomes of the wild tomato species Solanum habrochaites accumulate the polymethylated flavonol aglycones, 3,7,3'-O-methyl myricetin, 3,7,3',5'-O-methyl myricetin, and 3,7,3',4',5'-O-methyl myricetin. Partially methylated flavonol aglycones and partially methylated flavonol glycones containing a methyl group at the 3 position have been previously reported from a variety of plants. The 3-O-methyltransferase (3-OMT) activity has been previously partially purified from plants, but a gene transcript encoding an enzyme capable of methylating flavonols at the 3 position has not yet been identified, nor have been such proteins purified and characterized. We previously identified two gene transcripts expressed in the glandular trichomes of S. habrochaites and showed that they encode enzymes capable of methylating myricetin at the 3' and 5' and the 7 and 4' positions, respectively. Here we report the identification of gene transcripts expressed in S. lycopersicum (cultivated tomato) and in S. habrochaites glandular trichomes that encode enzymes capable of methylating myricetin, and its partially methylated derivatives exclusively at the 3 position. The S. habrochaites gene transcript is preferentially expressed in the glandular trichomes and it encodes a protein with high similarity to the S. habrochaites, 3'/5' O-methyltransferase which is also present in glandular trichomes. Phylogenic analysis suggests that the 3-OMT activity has probably evolved from an ancestral 3'/5' methyltransferase activity. The discovery and characterization of 3-OMT provides a more complete picture of the series of reactions leading to highly methylated myricetin compounds in S. habrochaites glandular trichomes.