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Downregulation of a Pathogen-Responsive Tobacco UDP-Glc:Phenylpropanoid Glucosyltransferase Reduces Scopoletin Glucoside Accumulation, Enhances Oxidative Stress, and Weakens Virus Resistance

^a Institut de Biologie Moléculaire des Plantes, Centre National de la Recherche Scientifique, Université Louis Pasteur, 12 rue du Général Zimmer, 67084 Strasbourg Cedex, France
^b Aventis CropScience, Département des Biotechnologies, 14-20 rue Pierre Baizet, 69009 Lyon, France

³ To whom correspondence should be addressed. E-mail patrick.saindrenan{at}ibmp-ulp.u-strasbg.fr; fax 33-3-88-61-44-42

Plant UDP-Glc:phenylpropanoid glucosyltransferases (UGTs) catalyze the transfer of Glc from UDP-Glc to numerous substrates and regulate the activity of compounds that play important roles in plant defense against pathogens. We previously characterized two tobacco salicylic acid– and pathogen-inducible UGTs (TOGTs) that act very efficiently on the hydroxycoumarin scopoletin and on hydroxycinnamic acids. To identify the physiological roles of these UGTs in plant defense, we generated TOGT-depleted tobacco plants by antisense expression. After inoculation with Tobacco mosaic virus (TMV), TOGT-inhibited plants exhibited a significant decrease in the glucoside form of scopoletin (scopolin) and a decrease in scopoletin UGT activity. Unexpectedly, free scopoletin levels also were reduced in TOGT antisense lines. Scopolin and scopoletin reduction in TOGT-depleted lines resulted in a strong decrease of the blue fluorescence in cells surrounding TMV lesions and was associated with weakened resistance to infection with TMV. Consistent with the proposed role of scopoletin as a reactive oxygen intermediate (ROI) scavenger, TMV also triggered a more sustained ROI accumulation in TOGT-downregulated lines. Our results demonstrate the involvement of TOGT in scopoletin glucosylation in planta and provide evidence of the crucial role of a UGT in plant defense responses. We propose that TOGT-mediated glucosylation is required for scopoletin accumulation in cells surrounding TMV lesions, where this compound could both exert a direct antiviral effect and participate in ROI buffering.

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