Rare earth elements,aluminium and silicon distribution in the fern Dicranopteris linearis revealed by μPIXE Maia analysis

BackgroundThe fern Dicranopteris linearis is a hyperaccumulator of rare earth elements (REEs), aluminium (Al) and silicon (Si). However, the physiological mechanisms of tissue-level tolerance of high concentrations of REE and Al, and possible interactions with Si, are currently incompletely known.MethodsA particle-induced X-ray emission (μPIXE) microprobe with the Maia detector, scanning electron microscopy with energy-dispersive spectroscopy and chemical speciation modelling were used to decipher the localization and biochemistry of REEs, Al and Si in D. linearis during uptake, translocation and sequestration processes.ResultsIn the roots >80 % of REEs and Al were in apoplastic fractions, among which the REEs were most significantly co-localized with Si and phosphorus (P) in the epidermis. In the xylem sap, REEs were nearly 100 % present as REEH₃SiO₄²⁺, without significant differences between the REEs, while 24–45 % of Al was present as Al-citrate and only 1.7–16 % Al was present as AlH₃SiO₄²⁺. In the pinnules, REEs were mainly concentrated in necrotic lesions and in the epidermis, and REEs and Al were possibly co-deposited within phytoliths (SiO₂). Different REEs had similar spatial localizations in the epidermis and exodermis of roots, the necrosis, veins and epidermis of pinnae of D. linearis.ConclusionsWe posit that Si plays a critical role in REE and Al tolerance within the root apoplast, transport within the vascular bundle and sequestration within the blade of D. linearis.