Cellulose and lignin biosynthesis is altered by ozone in wood of hybrid poplar (Populus tremula×alba)

Wood formation in trees is a dynamic process that is strongly affected by environmental factors. However, the impact of ozone on wood is poorly documented. The objective of this study was to assess the effects of ozone on wood formation by focusing on the two major wood components, cellulose and lignin, and analysing any anatomical modifications. Young hybrid poplars (Populus tremula×alba) were cultivated under different ozone concentrations (50, 100, 200, and 300 nl l−1). As upright poplars usually develop tension wood in a non-set pattern, the trees were bent in order to induce tension wood formation on the upper side of the stem and normal or opposite wood on the lower side. Biosynthesis of cellulose and lignin (enzymes and RNA levels), together with cambial growth, decreased in response to ozone exposure. The cellulose to lignin ratio was reduced, suggesting that cellulose biosynthesis was more affected than that of lignin. Tension wood was generally more altered than opposite wood, especially at the anatomical level. Tension wood may be more susceptible to reduced carbon allocation to the stems under ozone exposure. These results suggested a coordinated regulation of cellulose and lignin deposition to sustain mechanical strength under ozone. The modifications of the cellulose to lignin ratio and wood anatomy could allow the tree to maintain radial growth while minimizing carbon cost

Cell wall rearrangement in wood of poplars subjected to abiotic stresses

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Condensed Lignins Are Synthesized in Poplar Leaves Exposed to Ozone

Poplar (Populus tremula × 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

Deciphering the ozone-induced changes in cellular processes: a prerequisite for ozone risk assessment at the tree and forest levels

International audienceOzone, one of the major atmospheric pollutants, alters tree growth, mainly by decreasing carbon assimilation and allocation to stems and roots. To date, the mechanisms of O-3 impact at the cellular level have been investigated mainly on young trees grown in controlled or semi-controlled conditions. In the context of climate change, it is necessary to introduce a valuable defence parameter in the models that currently predict O-3 impact on mature trees and the carbon sequestration capacity of forest ecosystems.Air pollution is an important factor that affects negatively forest ecosystems. Among oxidative air pollutants, ozone is considered as the most toxic in terms of impact on vegetation.This paper focuses on the negative impacts of ozone on trees in controlled conditions or in their natural environment. The current knowledge of the responses at cell level is presented and ways to improve their use for ozone risk assessment of forest stands are discussed.Information was collected from original papers or reviews, providing an overview of the research conducted over the last 60 years.The negative effects of ozone on carbon assimilation and tree biomass production were reviewed and discussed, with a focus on effects on cell processes implied in cell defence, including stomatal regulation, detoxification, signalling, and biosynthesis of wood compound.In the context of increasing significance of O-3 flux approach, this review intends to shed light into the black box of defence processes, which are playing a crucial part within the effective O-3 dose modelling. Today, it is recognized that tropospheric ozone inhibits tree growth and its role on the future carbon sink of the forest ecosystem is discussed along with the combination of other environmental factors like elevated temperature, water, and nitrogen supply, likely to be modified in the context of climate change