Gold nanoparticles synthesized using melatonin suppress cadmium uptake and alleviate its toxicity in rice

Melatonin is a general organizer that promotes plant development and moderates defense responses to abiotic stresses; e.g., heat, cold, and heavy metals. Cadmium (Cd) is a heavy metal that can be uptaken by cereal crops. It causes toxicity in plants and poses serious health risks due to its accumulation in the human body through the food chain. This study demonstrates that melatonin in the form of gold nanoparticles (Mel-AuNPs) can alleviate Cd toxicity to a greater extent in hydroponically grown rice plants compared to melatonin applied alone. When rice was grown in a medium supplemented with 200 μM Mel-AuNPs, the melatonin content significantly increased in the roots (18.0%) and leaves (20.3%), and the Cd level significantly decreased by 33.0% and 46.2%, respectively. Furthermore, the Mel-AuNP supplement restored chlorophyll biosynthesis and mitigated Cd-induced oxidative stresses, while the anti-oxidant enzyme activities were significantly increased. Further study showed that supplying Mel-AuNPs inhibited the Cd-induced gene expression of metal transporter-related genes (OsHMA2, OsHMA3, OsIRT1, OsIRT2, OsNramp1, OsNramp5, and OsLCT1) in rice roots. Overall, this study suggests that Mel-AuNP application could mitigate Cd toxicity in rice seedlings by enhancing melatonin absorption and suppressing Cd uptake.