With the increasing applications of metal-based nanoparticles in various commercial products, it is necessary to address their environmental fate and potential toxicity. In this work, we assessed the phytotoxicity of lanthanum oxide (La₂O₃) NPs to cucumber plants and determined its distribution and biotransformation in roots by TEM and EDS, as well as STXM and NEXAFS. LaCl₃ was also studied as a reference toxicant. La₂O₃ NPs and LaCl₃ were both transformed to needle-like LaPO₄ nanoclusters in the intercellular regions of the cucumber roots. In vitro experiments demonstrated that the dissolution of La₂O₃ NPs was significantly enhanced by acetic acid. Accordingly, we proposed that the dissolution of NPs at the root surface induced by the organic acids extruded from root cells played an important role in the phytotoxicity of La₂O₃ NPs. The reactions of active NPs at the nano-bio interface should be taken into account when studying the toxicity of dissolvable metal-based nanoparticles.