Foliar silicon application enhances rare earth element accumulation in the hyperaccumulator Phytolacca americana

Phytolacca americana is a known rare earth element (REE) hyperaccumulator, notable for its high biomass and rapid growth rate. However, its capacity to tolerate and accumulate REEs under high-exposure conditions is relatively limited, which restricts its utilization in agromining on soils heavily contaminated with REEs. Silicon (Si) has been shown to effectively mitigate metal(loid)s stress in various plants, and to promote REE tolerance in the well-known hyperaccumulator fern Dicranopteris linearis. This study aimed to assess the effects of Si on REE agromining using P. americana and to elucidate the underlying mechanisms. The results show that foliar Si application significantly increases the REEs accumulation of P. americana by as much as 73.6 % and 172.4 % in hydroponic and pot experiments respectively. Notably, Si application enhances the retention of REEs within the cell walls, primarily through the formation of silicate particles and / or modifications of the cell walls. Transcriptome analysis indicates that foliar Si application initially mitigates the toxicity of REEs by inducing an upregulation of genes associated with stress response and antioxidant enzymes, which facilitates the rapid elimination of accumulated reactive oxygen species. With increasing Si application, the abnormal overexpression of stress response genes, as well as genes involved in flavonoid biosynthesis and cell division, is alleviated, further implying that the entry of REE into the cytoplasm can be effectively hindered. This study shows that foliar Si application represents a potentially cost-efficiency agronomic practice for improvement of REE agromining using P. americana.