Lanthanum(III)-amino acid chelate mitigates copper(II) stress in rice (Oryza sativa)

Lanthanum (La(III)) is recognized for its ability to mitigate heavy metal stress in plants. However, the inorganic La(III) salts and lanthanum oxide nanoparticles (La₂O₃ NPs) extensively used in agriculture are prone to soil immobilization, thereby compromising their bioavailability and posing environmental risks. This study synthesized and characterized the lanthanum(III)-amino acid chelate (La(III)-AA) from soybean protein isolate (SPI) hydrolysates. Maximum chelating rate (94.95%) was achieved under the conditions of mole ratio 1:1.5, pH 8.0, 50 ℃ and 5 h. Glu, Asp and Pro represent the primary La(III)-binding ligands. UV-vis and FTIR demonstrated that amino nitrogen and carboxyl oxygen participate in metal-ligand recognition. Scanning and Transmission electron microscopy showed that La(III) chelates with amino acids in a core-shell structure of uniform size. Consequently, a proposed chemical structure for the La(III)-AA complex was presented. A concentration of 20 mg/L La(III)-AA outperforms inorganic La salts in growth promotion and Cu detoxification. La(III)-AA significantly reduces the content of Cu (II) in rice tissues and enhances seedling tolerance to Cu (II) stress. This study provides a novel La(III)-based candidate for crop protection and furthers our understanding of rare earth element-induced mitigation of heavy metal stress.