Detoxification responses of poplar under oxidative stress induced by drought or ozone : focus on the role of glutathione metabolism.
PI : Marie-Noëlle Vaultier (UMR 1137 Ecologie et Ecophysiologie Forestières – EEF)
Co-applicants : Y. Jolivet, D. Le Thiec (UMR 1137-EEF) N. Rouhier (UMR 1136 « Interactions Arbres-Mocroorganismes » IAM)
Collaboration : Pr V.Hurry, Department of Plant Physiology, Umea University, Umea Plant Science Center (UPSC)
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Context —Many forest trees are already affected by the first impacts of progressive climate change which has already caused major loss of forests worldwide. Future issues in forestry will concern the capacity of trees to survive and to acclimate to a changing environment. Given the economic challenges, it is imperative that we better understand how trees respond to environmental variations, notably to drought and increased ozone, two major future threats which generate oxidative stress. More research on the parameters of cell detoxification will help us to better understand the mechanisms involved in the tree detoxification processes.
Objective —The main objective of this project is to study the role of glutathione metabolism, an antioxidant molecule, in the detoxification processes of trees exposed to drought or chronic doses of ozone by using two poplar genotypes differing in their oxidative stress sensitivity. Helping us to understand how poplar respond to abiotic stress will be isolating key elements specific to this detoxification network and seeking common mechanisms to these responses.
Approach —We planned experiments to compare the responses of two genotypes with different susceptibility to oxidative stress, specifically to drought and ozone respectively. Our study focused on both the stomata and the whole leaf with a view to : i) analyzing the specificity of each stress at the cellular level; ii) studying molecular regulation and protein of enzymes involved in glutathione metabolism and iii) identifying new actors involved in glutathione metabolism.
Key results —
- Metabolite pools (17 days). Differences between the two genotypes:
For Robusta we noted a higher level of oxidized ascorbate under O3 and drought stresses. The effect is also significative for the level of oxidized glutathione under drought. For Carpaccio, the glutathione content increased under O3 and the combined stresses (D+O3).
- Gene expression (17 days) for cytosolic GR: For Carpaccio, higher transcript levels under O3 was observed. This stimulation had been noted in Robusta after 2 days but it was not maintained after 17 days.
- Enzymes of the Halliwell-Asada-Foyer cycle: Based on enzyme activities, the two genotypes displayed a quite similar response. O3 tended to stimulate MDHAR, implied in ASA regeneration, while the activity of DHAR was inhibited. Concerning glutathione regeneration, the GR activity was stimulated under O3. Drought alone did not modify the activity of the 3 enzymes and the combined stresses (D+O3) often implied an attenuated effect compared to O3 alone.
Main findings — This work highlights differences in poplar genotypes responses based on detoxification processes. Thus, under O3, the more tolerant genotype (Carpaccio) displayed higher amount of glutathione, a stimulation of GR activity which could be associated to a higher expression of a gene coding for a cytosolic isoform. The response to drought differed from the response to O3 and the combination of the two constraints did not imply a synergistic effect, the effect of O3 being globally attenuated.
Future perspectives — Data acquired on drought and ozone stress separately represent a good basis for understanding responses to a single stress before integrating them in a more complex situation, i.e. the combination of multiple abiotic stresses. These data will help to improve detoxification parameters, and are notably needed to improve the indices for ozone risk assessment on plants currently used at the European level.
