Light Rare Earth Elements in Plants: Environmental Sources, Bioavailability, Uptake, Nutrient Interactions, Mechanisms, and Concentration-Dependent Physiological and Signaling Reactions

Rare earth elements (REEs) are traditionally regarded as emerging environmental contaminants. Due to their increasing anthropogenic release and interactions with essential mineral nutrients, they are attracting growing interest for their potential applications in sustainable agriculture. The REEs are a unique group of 17 chemically similar elements, including scandium (Sc), yttrium (Y), and lanthanides. Most existing reviews treat REEs as a single group, with limited emphasis on concentration-dependent uptake mechanisms or species-related responses. However, based on atomic weight, REEs are classified into light and heavy rare earth elements (LREEs/HREEs). LREEs enter the plant system through multiple routes-fertilizer, irrigation, and mining residues; however, their cellular roles and modes of action remain insufficiently understood. Accumulating evidence indicates that LREEs exert concentration-dependent effects on plant growth and metabolism, including decreased growth, oxidative stress, modulation of photosynthesis, chlorophyll and carotenoid accumulation, antioxidant metabolism, secondary metabolite production, and enzyme activities. LREE uptake occurs predominantly via calcium-permeable channels, with limited involvement of transporters, followed by root sequestration and restricted long-distance translocation. At the cellular level, LREEs influence redox balance, membrane stability, chloroplast ultrastructure, and mineral nutrient homeostasis, and interact with multiple signaling pathways, including calcium- and hormone-mediated networks. Most studies on LREEs focused on lanthanum and cerium, attributed to their abundance in soil. Unlike previous reviews addressing REEs broadly, this review specifically focuses on LREE-mediated regulation of plant growth and development, mineral nutrition with emphasis on concentration-dependent responses, uptake and transport, and associated signaling mechanisms and hormonal interactions. Key gaps in current knowledge regarding LREE molecular targets, transport, dose-dependent mechanisms, molecular regulation, and species-specific responses are highlighted to guide future research. By integrating environmental entry routes with cellular and molecular responses, this review provides a mechanistic framework for future studies on LREE plant interactions.