Rare pictures of Rare-Earth Elements in live hyper-accumulating fern roots and shoots: silicon might play a role in detoxification

There are 14 rare-earth elements (REEs) that are, despite their names, rather common on Earth. For example, cerium is more common than lead. At low concentrations, these elements are important for stress tolerance in plants but only 21 plant species are known to hyper-accumulate REEs in their aboveground parts. 

Drs Wen-Shen Liu from Sun Yat-Sen University and Antony van der Ent from the University of Queensland and colleagues from the University of Melbourne, CSIRO and INRAE, investigated where REEs and light elements (aluminum and silicon) accumulate in a common fern in the (sub)tropics. The researchers demonstrate how particle-induced X-ray emission (μPIXE) analysis can show where different elements are in live plant tissues. 

Based on these images, additional microscopy, and chemical analyses, Liu and colleagues suggest that Si is involved in the REE and Al detoxification process and this fern could help restore mining sites by agromining. Last year, the researchers visualised two REEs and other elements in D. linearis and found that this fern can accumulate more REEs (0.2% in the shoots and 0.5% in the dead fern leaves) than the other 20 known hyper-accumulator plant species. 

Liu and colleagues collected plant samples from D. linearis growing on REE and non-REE mine tailings (leftover material from ore extraction) in China. They also collected soil and xylem sap samples for chemical analyses. The root, stem, and leaf samples were quickly slammed against a solid copper block that has been cooled by liquid nitrogen and was shipped to Australia for μPIXE analysis and Scanning-Electron Microscopy (SEM). 

Liu and colleagues found that the native fern, D. linearis hyper-accumulated REEs, Al, and Si. All elements were highly concentrated in the roots, followed by leaf tissues (pinnae) whilst stem areas (stolon and rachis) had the lowest concentrations as those are involved in nutrient transportation. Silicon was co-localised with REEs in the xylem sap, stolon, rachis, midvein and endodermis but the element visualisation revealed that Si was mainly seen at the leaf margins and blades, but REEs and Mn were found around necrotic lesions and veins. 

“[C]onsidering the co-localisation of REE and Si […], we posit that Si plays a key role in coping with the high concentrations of REEs and Al in the root, in REE transport in the xylem sap, and in REE and Al compartmentalization in the pinnule of D. linearis,” Liu and colleagues wrote. 

In a previous BotanyOne interview, Dr van der Ent highlighted the challenges and opportunities of using micro-X-ray fluorescence (µ-XRF) to visualise different elements in live plant tissues. The current research used the state-of-the-art Maia detector at the University of Melbourne that enables quick, high-resolution elemental image capture (read more in this review). 

The visualisation of REEs in live plant tissues suggests that Si is likely to be involved in REE and Al detoxification in a hyper-accumulating fern. By understanding how this fern can take up and store large amounts of these elements, scientists can explore using this fern for growing these ferns to extract REE from REE contaminated mine soils (agromining).