Formation of Sandwich Complexes between Lanthanides and Chlorophylls Recovers Photosynthetic Activity and Imparts Crop Resistance to UV Stress via Single-Nanodose Seed Treatment

Abstract

Lanthanides (Lns) have been used as fertilizers in several countries over the past 50 years, yet their interaction with plant metabolism remains largely unknown, with only a few confirmed biological roles, such as cofactors in specific dehydrogenases found in methylotrophic organisms. This study investigates the interplay between Ln ions and chlorophylls (Chls), showing that throughout the Ln series, Ln-Chls replace Mg2+ at the center of the pigment structures through the formation of sandwich-like Ln-porphyrin complexes. When applied as seed treatment to four staple crops (i.e., barley, chickpea, corn, and soybean), Lns enhance seedlings’ growth. Mobility of lanthanum from the rhizosphere to plants is favored when the metal is in neutral or weakly charged forms, as studied by changing the salt used (i.e., LaCl3, La(NO3)3, LaPO4, La2(CO3)3, [LaEDTA(H2O)3]K, or [LaDOTA(H2O)]K). These results highlight the role of the Casparian strip in modulating the apoplastic movement of Lns in the stele and suggest that plants possess an uptake system with binding strengths comparable to or stronger than La-DOTA complexes. Additionally, when complexed with trehalose in seed treatments, Ln3+ protected pigments from UV stress, enhancing the resilience of the four staple crops to abiotic stressors.