Light-dependent Br-org production in terrestrial plants under acetaminophen stress and the bromination mechanisms mediated by photosystem

Abstract

Due to the endocrine toxicity, neurotoxic, and reproductive toxicity to organisms, the sources and risks of brominated organic pollutants have attracted widespread attention. However, knowledge gaps remain in the bromination processes of emerging phenolic pollutants in plants, which may increase the potential health risk associated with food exposure. Our study discovered that light induced generation and accumulation of more toxic brominated organic compounds (Br-org) in lettuce leaves under the stress of acetaminophen (ACE) than that without light, as evidenced by an increase in C-Br bond intensity in FTIR analysis. This result can be explained by the oxidation of bromide ions (Br^-) by reactive species (ROS and ³Chl*) of chloroplast into reactive bromine species (RBS). The main mechanism is that the redox of Br^- reduced the oxidative damage of ACE to the structure and function of chloroplasts, providing good conditions for light energy uptake and utilization and promoting the increase of pigments and active species. Compared with the dark group exposed to 5 mg/L Br^-, the pigment content, H₂O₂ and ¹O₂ level of the light group increased by 56%, 84% and 69%, respectively. On the other hand, RBS attacks certain electrophilic organic compounds in leaves to generate Br-org. Triple excited state of chlorophyll (³Chl*) was the dominant species for the transformation of ACE, while RBS is a key factor in the generation of Br-org in the Br^-/light/chlorophyll system. A total of six transformation products were identified by HPLC-MS/MS, which were involved in three transformation pathways: methylation, hydroxyl oxidation and hydroxylation followed by bromination. This is the first report that Br^- could enter the chloroplast and improved chloroplast structure under ACE stress, and elucidated the bromination mechanism of organics in terrestrial plant involving of biophotochemical bromination in chloroplast besides enzyme-catalyzed bromination. This study is beneficial for risk assessment and prevention of emerging phenolic pollutants.