S-Adenosyl-i-Methionine:(lso)Eugenol O-Methyltransferase and Phenylpropanoid Emission

We have previously shown (R.A. Raguso, E. Pichersky [1995] Plant Syst Evol 194: 55-67) that the strong, sweet fragrance of Clarkia breweri (Onagraceae), an annual plant native to California, consists of 8 to 12 volatile compounds, including 4 phenylpropanoids. Although some C. breweri plants emit all 4 phenylpropanoids (eugenol, isoeugenol, methyleugenol, and isomethyleugenol), other C. breweri plants do not emit the latter 2 compounds. Here we report that petal tissue was responsible for the bulk of the phenylpropanoid emission. The activity of S-adenosyl-i-methionine: (iso)eugenol O-methyltransferase (IEMT), a nove1 enzyme that catalyzes the methylation of the para-4’-hydroxyl of both eugenol and (iso)eugenol to methyleugenol and isomethyleugenol, respectively, was also highest i n petal tissue. IEMT activity was absent from floral tissues of plants not emitting (iso)methyleugenol. A C. breweri cDNA clone encoding IEMT was isolated, and i ts sequence was shown to have 70% identity to S-adenosyl-L-methionine:caffeic acid Omethyltransferase. lhe protein encoded by this cDNA can use eugenol and isoeugenol as substrates, but not caffeic acid. Steadystate IEMT mRNA levels were positively correlated with levels of IEMT activity in the tissues, and no IEMT mRNA was observed in flowers that do not emit (iso)methyleugenol. Overall, the data show that the floral emission of (iso)methyleugenol is controlled at the site of emission, that a positive correlation exists between volatile emission and IEMT activity, and that control of the level of IEMT activity i s exerted at a pretranslational step.