PlantMetals

This study sheds light on the accumulation sites in a model of accumulating ferns for light and heavy rare earth elements, emerging metal contaminants in the environment.

Rare earth elements (REEs) are strategic metals increasingly used in many different fields, such as high technologies and low-carbon energy conversion. They are split into light REEs (LREEs, from lanthanum to europium, plus scandium) and heavy REEs (HREEs, from gadolinium to lutetium, plus yttrium). Their intense mining, wide use and low recyclability contribute to their spreading in soils and waters. Therefore, nature-based solutions such as phytoremediation of REEs need to be investigated, to provide remediation tools and to better understand REEs transfer to the biota. Among the few REE-accumulating plant species, tropical ferns studied through invasive techniques, showed REEs distribution in chloroplasts or cell walls. However, the experimental biases inherent to the methods chosen (loss of tissue integrity, indirect detection) urged us to use a more accurate, direct and nondestructive approach by synchrotron µXRF. Our study focused on the promising fern Dryopteris erythrosora given its hardiness and REE accumulation behavior. The complementarity of the two beamlines at DESY (P06) and SOLEIL (Lucia) synchrotrons respectively allowed us to resolve, first, the REEs distribution at the organ level in the pinnae (Figure A, B), with LREEs and HREEs collocated with the veins, and second, to refine this distribution to the conductive vessels instead of the surrounding cells, thanks to thin cryosections of the pinnae (Figure C, D) and the rachis (Figure A, E, F). In both cases, the REEs had a peculiar distribution compared to that of nutrients such as K, Ca or Mn, which were not only retrieved in the veins but also outside the vascular bundles, in the whole organs (Figure C, D, E, F). The thin root cryosections showed no storage of REEs in the root vessels. Conversely, in the gametophyte stage, REEs were only detected in the rhizoids. In conclusion, this study revealed a root-to-shoot transfer of the REEs ending in their storage in the veins of the aerial parts, where they stay trapped. The lack of cell-to-cell mobility of the REEs in the aboveground parts could be due to their immobilization by ligands or the lack of transporters allowing for REEs unloading from the vessels. Comparing the storage sites of REEs in different plants and identifying their ligands in the sap will be the next steps towards understanding REEs transport and accumulation in plants. 

Read more: https://doi.org/10.1021/acs.est.2c06985

Contact: Marie Le Jean (Marie.lejean@univ-lorraine.fr)