Development of melatonin nano-delivery systems to reduce cadmium accumulation in rice (Oryza sativa L.) seedlings: Insights from photosynthetic efficiency, antioxidative response and gene expression

Cadmium (Cd) contamination in paddy soil has become a worldwide problem for rice production. Foliar application has been reported as a practical approach to reducing Cd accumulation in rice. In the present study, we used mesoporous silica nanoparticles (MSN) and MSN-chitosan (MSN-CS) as carriers to deliver melatonin (MT) into rice plants through foliar spray. Both systems showed the potentials as controlled-release delivery vehicles, with MT@MSN-CS performing better in vitro. The hydroponic trial revealed that foliar spray of MT@MSN-CS could significantly reduce Cd concentration in rice leaves by 43.8%, which was better than those of MT and MT@MSN. The nano-delivery systems improved leaf photosynthesis efficiency, decreased malondialdehyde (MDA) content, enhanced the activity of antioxidative enzymes including superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX), as well as regulated the expression levels of Cd transport genes (OsNramp1, OsNramp5, OsHMA2, OsHMA3, OsLCD and OsPCR1). Our results also suggested that the use of stimuli-responsive carriers, MSN-CS, for example, as MT-delivery systems may be a feasible option for the reduction of heavy metal accumulation in plants and can be potentially implemented in soil remediation.