FLORAL SCENT PRODUCTION IN CLARKIA-BREWERI (ONAGRACEAE) .2. LOCALIZATION AND DEVELOPMENTAL MODULATION OF THE ENZYME S-ADENOSYL-L-METHIONINE-(ISO)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. br eweri plants emit all 4 phenylpropanoids (eugenol, isoeugenol, methyle ugenol, 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-adenos yl-L-methionine: (iso)eugenol O-methyltransferase (IEMT), a novel enzy me that catalyzes the methylation of the para-4'-hydroxyl of both euge nol and (iso)eugenol to methyleugenol and isomethyleugenol, respective ly, was also highest in petal tissue. IEMT activity was absent from fl oral tissues of plants not emitting (iso)methyleugenol. A C. breweri c DNA clone encoding IEMT was isolated, and its sequence was shown to ha ve 70% identity to S-adenosyl-L-methionine:caffeic acid O-methyltransf erase. The protein encoded by this cDNA can use eugenol and isoeugenol as substrates, but not caffeic acid. Steady-state IEMT mRNA levels we re positively correlated with levels of IEMT activity in the tissues, and no IEMT mRNA was observed in flowers that do not emit (iso)methyle ugenol. Overall, the data show that the floral emission of (iso)methyl eugenol is controlled at the site of emission, that a positive correla tion exists between volatile emission and IEMT activity, and that cont rol of the level of IEMT activity is exerted at a pretranslational ste p.