Nitro-oxidative signalling induced by chemically synthetized zinc oxide nanoparticles (ZnO NPs) in Brassica species

Due to their release into the environment, zinc oxide nanoparticles (ZnO NPs) may come in contact with plants. In elevated concentrations, ZnO NPs induce reactive oxygen species (ROS) production, but the metabolism of reactive nitrogen species (RNS) and the consequent nitro-oxidative signalling has not been examined so far. In this work, Brassica napus and Brassica juncea seedlings were treated with chemically synthetized ZnO NPs (∼8 nm, 0, 25 or 100 mg/L). At low dose (25 mg/L) ZnO NP exerted a positive effect, while at elevated concentration (100 mg/L) it was toxic to both species. Additionally, B. juncea was more tolerant to ZnO NPs than B. napus. The ZnO NPs could enter the root cells due to their small (∼8 nm) size which resulted in the release of Zn2+ and subsequently increased Zn2+ content in the plant organs. ZnO NPs disturbed superoxide radical and hydrogen peroxide homeostasis and modulated ROS metabolic enzymes (NADPH oxidase, superoxide dismutase, ascorbate peroxidase) and non-enzymatic antioxidants (ascorbate and glutathione) inducing similar changes in oxidative signalling in both Brassica species. The homeostasis of RNS (nitric oxide, peroxynitrite and S-nitrosoglutathione) was also altered by ZnO NPs; however, changes in nitrosative signalling proved to be different in the examined species. Moreover, ZnO NPs triggered changes in protein carbonylation and nitration. These results suggest that ZnO NPs induce changes in nitro-oxidative signalling which may contribute to ZnO NP toxicity. Furthermore, difference in ZnO NP tolerance of Brassica species is more likely related to nitrosative than to oxidative signalling.