Selective accumulation of heavy rare earth elements in the hyperaccumulator Phytolacca americana: First evidence from spatially resolved analysis

Phytolacca americana is the only reported rare earth element (REE) hyperaccumulator plant capable of selectively accumulating heavy REEs (HREEs, Gd to Lu plus Y), yet the underlying mechanisms remain unclear. Through in situ spatial localization analysis with laser ablation inductively coupled plasma time-of-flight mass spectrometry and subcellular distribution analysis by chemical extraction, the differential distribution patterns of light rare earth elements (LREEs, La to Eu) and HREEs in P. americana are revealed for the first time. In P. americana , the translocation of REEs from roots to shoots increases with their atomic number, with HREEs being translocated more readily than LREEs. Although all REEs are predominantly distributed in leaf veins and mesophyll tissues, the LREEs are preferentially retained within vein vascular tissues, whereas HREEs are unloaded into mesophyll cells more efficiently. Furthermore, with increasing atomic number along the lanthanide series, the proportion of REEs retained in the cell walls gradually decreases (from 64.5% to 56.2% of the total content), while those compartmentalized into vacuoles progressively increases (from 5.5% to 14.5%). These observations indicate that HREEs are more readily unloaded from vascular bundles to mesophyll tissues and more efficiently translocated across cellular membranes into vacuoles. This study underscores the importance of identifying HREE transporters in P. americana and provides a strong incentive for developing biotechnologies selectively targeting REEs for resource recovery.