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ENVIRONMENTAL STRESS AND ADAPTATION

Condensed Lignins Are Synthesized in Poplar Leaves Exposed to Ozone

Unité Mixte de Recherche 1137, Institut National de la Recherche Agronomique (INRA)-Université Henri Poincaré "Ecologie et Ecophysiologie Forestière," Université Henri Poincaré-Nancy I, Boite Postale 239, F–54506 Vanduvre-les-Nancy cedex, France (M.C., J.-C.P., E.L., E.W., P.D.); and Laboratoire de Chimie Biologique, Unité Mixte de Recherche 206, INRA-Institut National Agronomique Paris-Grignon Institut National Agronomique, F–78850 Thiverval-Grignon, France (B.P., C.L.)

Poplar (Populus tremula x alba) trees (clone INRA 717-1-B4) were cultivated for 1 month in phytotronic chambers with two different levels of ozone (60 and 120 nL L^–1). Foliar activities of shikimate dehydrogenase (EC 1.1.1.25), phenylalanine ammonia lyase (EC 4.3.1.5), and cinnamyl alcohol dehydrogenase (CAD, EC 1.1.1.195) were compared with control levels. In addition, we examined lignin content and structure in control and ozone-fumigated leaves. Under ozone exposure, CAD activity and CAD RNA levels were found to be rapidly and strongly increased whatever the foliar developmental stage. In contrast, shikimate dehydrogenase and phenylalanine ammonia lyase activities were increased in old and midaged leaves but not in the youngest ones. The increased activities of these enzymes involved in the late or early steps of the metabolic pathway leading to lignins were associated with a higher Klason lignin content in extract-free leaves. In addition, stress lignins synthesized in response to ozone displayed a distinct structure, relative to constitutive lignins. They were found substantially enriched in carbon-carbon interunit bonds and in p-hydroxyphenylpropane units, which is reminiscent of lignins formed at early developmental stages, in compression wood, or in response to fungal elicitor. The highest changes in lignification and in enzyme activities were obtained with the highest ozone dose (120 nL L^–1). These results suggest that ozone-induced lignins might contribute to the poplar tolerance to ozone because of their barrier or antioxidant effect toward reactive oxygen species.

^* Corresponding author; e-mail cabane{at}scbiol.uhp-nancy.fr; fax 33–3–83–68–42–40.

Received August 13, 2003; returned for revision September 8, 2003; accepted November 3, 2003.

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