Surface coating-modulated toxic responses to silver nanoparticles in Wolffia globosa

With the omnipresence of silver nanoparticles (AgNPs) in our daily consumer products, their release has raised serious concerns. However, the biochemical mechanisms by which plants counteract the toxicity of nanoparticles are largely unknown. This study investigated the exposure of aquatic Wolffia globosa to ATP-nAg (AgNPs coated with adenosine triphosphate), cit-nAg (AgNPs coated with citrate), and Ag⁺. Hill reaction activity was basically lost in W. globosa treated with 10mg/L ATP-nAg and Ag⁺, while the activity was still maintained at 38.7%-38.9% of the respective controls at 10mg/L cit-nAg. The reduction of amounts of chlorophyll and soluble protein were shown in response to the Ag stresses. This was accompanied by the accumulation of sugar in W. globosa treated with cit-nAg. By contrast, the depletion of sugar was recorded after 10mg/L ATP-nAg and Ag⁺ treatments. The superoxide dismutase and peroxidase activities were significantly increased after exposure to 10mg/L ATP-nAg and Ag⁺, which did not occurred in W. globosa treated with cit-nAg. The ratio between NADPH/NADP⁺ was higher after cit-nAg and Ag⁺ stresses than the respective controls. The accumulation of Ag was found to increase in a concentration-dependent manner. Ag⁺ and ATP-nAg inhibited the uptake of P and K, and promoted the uptake of Fe and Cu. In contrast, cit-nAg only promoted the uptake of Cu. Our results implied that surface coating induced different physiological responses of W. globosa to AgNPs. Based on above results, we speculated that after exposure to cit-nAg, citrate possibly could serve as the substrate for the tricarboxylic acid cycle and accumulated sugar may promote pentose phosphate pathways. For ATP-nAg treatments, ATP would act as an exogenous energy source of plant metabolisms. Our findings demonstrate that surface coating regulates the physiological responses of plants to AgNPs through distinct mechanisms.