Final Activity Report Summary - METALS IN SUILLUS (Subcellular localisation of metals in the ectomycorrhizal fungus Suillus bovinus and its host plant Pinus sylvestris: a comparative study ...) Isolates of Suillus sp. thriving on metal-contaminated sites show increased levels of tolerance to the metals that are enriched in the soil. The mechanisms by which these ectomycorrhizal (ECM) fungi provide protection against metal toxicity, to themselves and host plants are still unresolved. Understanding heavy-metal tolerance very often first requires knowledge of the spatial distribution of Zn within the fungus and plant and its chemical forms.To characterise the species and spatial distribution of Zn within the fungus and plant, synchrotron based techniques were used; including micro X-ray fluorescence (miuXRF) and Zn K-edge extended X-ray absorption fine structure spectroscopy, performed in bulk and to micrometer-scale resolution (EXAFS and miuEXAFS).In the fungi three Zn pools were identified including Zn bound to organic acids in solution, Zn bound to polysaccharides of the cell wall and Zn-organic acid complexes in solid state. The presence of organic acids was confirmed by capillary zone electrophoresis. We only recorded slight differences of Zn speciation between Zn tolerant and Zn sensitive isolates. Imaging of Zn localisation using fluorescent probes showed no differential compartmentation between vacuole and cell wall between both isolates. So Zn tolerance seems to rely mostly on a limited Zn uptake or enhanced Zn efflux.In the plant-fungal symbiosis Zn was located predominantly into the fungal mantle around the root and in the external mycelium. On the contrary, in non mycorrhizal roots Zn was located mainly into the vascular cylinder.This study characterised for the first time the different compartments of Zn accumulation within the plant-fungal associations and the speciation of Zn in each compartment. It provided new insights on the mechanisms of Zn tolerance and Zn partitioning between fungal and root cells. Such type of information is important for the design of phytoremediation techniques, in this case for the revegetation of metal-contaminated areas. Our study also shows the great interest of synchrotron tools in environmental biology, and might attract new users to such facilities.
