Étude transcriptomique et physiologique des effets de l’arsenic sur plusieurs espèces végétales utilisées en phytoremédiation

The arsenic is a well-described element with high toxicity at low concentration. Its omnipresence around the world is mostly resulting from human activity. When the concentration reaches the toxicity thresholds imposed by the government, it’s imperative to decontaminate the soil before considering using the contaminated area. At this moment, several remediation options can be used. The traditional technics generally involving ground excavation and soil chemical or physical treatment have the advantage of time efficiency at the expense of their cost and their noxious impact on the site ecosystems. A durable, environmentally friendly option is using some plants natural abilities to tolerate and accumulate pollutants from ground. While being a low-cost alternative, this approach requires nonetheless optimization to be able to challenge traditional technics efficiency. The objective of this thesis is to give a better understanding of how some plant species react to arsenic in their growth substrate, at the physiological and molecular levels. In this context, the work done was based on a multi-disciplinary approach merging field observations and controlled analysis in laboratory to generate a list of genes that are differentially expressed in presence of arsenic. In order to compare the physiological responses of several plant candidates (Salix purpurea, Medicago sativa, Festuca arundinacea and Brassica juncea), a study was performed on a former CCA and PCP treated wood storage site. Soil analyses before plants establishment revealed a poly contamination of organic pollutants and metals including arsenic. In parallel, the effect of nitrogen fertilization on plant establishment was monitored. While all the selected species were able to establish on the contaminated site, physiological results suggested a low benefice of nitrogen fertilization. Despite arsenic was not detected in the aerial parts of the plants, its quantitative analysis in ground at the end of the four-year study revealed a decreased concentration. This suggests a potential roots accumulation. An interesting, yet worrying, result is the presence of dioxin and furans in the tested specimens. This suggests that plants grown on dioxin and furans polluted sites could potentially contain those highly toxic pollutants and need to be treated accordingly. Another experience performed in hydroponic and controlled conditions aimed to explore the molecular response from Salix purpurea exposed to different levels of arsenic. Transcriptomes from plants cultivated in a 5mg.kg-1 arsenic substrate were compared to transcriptomes from non-exposed plants in order to get a list of differentially expressed genes that are specifically linked to arsenic response. In addition to give a better understanding of the arsenic path in plants tissues, those results strongly suggest a tannin involvement as an arsenic toxicity management. This PhD thesis has generated innovative tools and data that will be useful to assist plant selection in arsenic contaminated site restoration context.

Keyphrases: Arsenic, Fétuque, Transcriptome, Willow, Tall fescue, Alfalfa, Mustard, Trace elements, Dioxins, Furans