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J. Biol. Chem., Vol. 278, Issue 9, 6854-6861, February 28, 2003

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Daphnetin Methylation by a Novel O-Methyltransferase Is Associated with Cold Acclimation and Photosystem II Excitation Pressure in Rye^*

Christian NDong, Dominique Anzellotti^§, Ragai K. Ibrahim^§, Norman P. A. Huner^¶, and Fathey Sarhan

From the  Département des Sciences biologiques, Université du Québec à Montréal, C.P. 8888 Succursale Centre-ville, Montréal, Québec H3C 3P8, Canada, the ^§ Plant Biochemistry Laboratory and Centre for Structural-Functional Genomics, Concordia University, Montréal, Québec H3G 1M8, Canada, and the ^¶ Department of Plant Sciences, University of Western Ontario, London, Ontario N6A 5B7, Canada

In plants, O-methylation of phenolic compounds plays an important role in such processes as lignin synthesis, flower pigmentation, chemical defense, and signaling. However, apart from phenylpropanoids and flavonoids, very few enzymes involved in coumarin biosynthesis have been identified. We report here the molecular and biochemical characterization of a gene encoding a novel O-methyltransferase that catalyzes the methylation of 7,8-dihydroxycoumarin, daphnetin. The recombinant protein displayed an exclusive methylation of position 8 of daphnetin. The identity of the methylated product was unambiguously identified as 7-hydroxy-8-methoxycoumarin by co-chromatography on cellulose TLC and coelution from high performance liquid chromatography, with authentic synthetic samples, as well as by UV, mass spectroscopy, ¹H NMR spectral analysis, and NOE correlation signals of the relevant protons. Northern blot analysis and enzyme activity assays revealed that the transcript and corresponding enzyme activity are up-regulated by both low temperature and photosystem II excitation pressure. Using various phenylpropanoid and flavonoid substrates, we demonstrate that cold acclimation of rye leaves increases O-methyltransferase activity not only for daphnetin but also for the lignin precursors, caffeic acid, and 5-hydroxyferulic acid. The significance of this novel enzyme and daphnetin O-methylation is discussed in relation to its putative role in modulating cold acclimation and photosystem II excitation pressure.

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