A NOVEL MULTIFUNCTIONAL O-METHYLTRANSFERASE IMPLICATED IN A DUAL METHYLATION PATHWAY ASSOCIATED WITH LIGNIN BIOSYNTHESIS IN LOBLOLLY-PINE

S-adenosyl-L-methionine (SAM)-dependent O-methyltransferases (OMTs) ca talyze the methylation of hydroxycinnamic acid derivatives for the syn thesis of methylated plant polyphenolics, including lignin. The distin ction in the extent of methylation of lignins in angiosperms and gymno sperms, mediated by substrate-specific OMTs, represents one of the fun damental differences in lignin biosynthesis between these two classes of plants. In angiosperms, two types of structurally and functionally distinct lignin pathway OMTs, caffeic acid 3-O-methyltransferases (CAO MTs) and caffeoyl CoA 3-O-methyltransferases (CCoAOMTs), have been rep orted and extensively studied. However, little is known about lignin p athway OMTs in gymnosperms, We report here the first cloning of a lobl olly pine (Pinus taeda) xylem cDNA encoding a multifunctional enzyme, SAM:hydroxycinnamic Acids/hydroxycinnamoyl CoA Esters OMT (AEOMT). The deduced protein sequence of AEOMT is partially similar to, but clearl y distinguishable from, that of CAOMTs and does not exhibit any signif icant similarity with CCoAOMT protein sequences. However, functionally , yeast-expressed AEOMT enzyme catalyzed the methylation of CAOMT subs trates, caffeic and 5-hydroxyferulic acids, as well as CCoAOMT substra tes, caffeoyl CoA and 5-hydroxyferuloyl CoA esters, with similar speci fic activities and was completely inactive with substrates associated with flavonoid synthesis, The lignin-related substrates were also effi ciently methylated in crude extracts of loblolly pine secondary xylem. Our results support the notion that, in the context of amino acid seq uence and biochemical function, AEOMT represents a novel SAM-dependent OMT, with both CAOMT and CCoAOMT activities and thus the potential to mediate a dual methylation pathway in lignin biosynthesis in loblolly pine xylem.