Single cell mass-spectrometry: A novel approach to evaluating xenobiotic uptake and fate in plants

Reclaimed wastewater is increasingly used for irrigation of arable crops to alleviate agricultural pressures associated with water scarcity in arid and semi-arid regions across the globe. Treated and untreated wastewater is a known reservoir for pharmaceutical contaminants, which are subsequently introduced into the soil-plant system, where exposure to terrestrial organisms has been shown to have toxicological effects and where plant uptake poses a risk for human exposure. Studies on the fate and uptake of pharmaceuticals are relatively sparse, with investigations only considering 3% of currently prescribed pharmaceuticals. The presented study addresses two significant research gaps. First, a prioritisation approach was established to select priority study compounds derived from the last 15 years of annual prescription data in England. This approach was applied to the antiepileptic class of pharmaceuticals and revealed that the most studied compounds, carbamazepine and lamotrigine, do not represent the most significant chemical exposures. Second, a high resolution assessment of plant uptake and distribution of priority compounds was accomplished through the novel application of Single Cell Mass-Spectrometry (SCMS). For the first time, SCMS was used to analyse the contents of single plant cells exposed to exogenously applied, high-priority pharmaceutical compounds. Using hydroponic exposures, the accumulation and transformation of 1 antiepileptic compound (levetiracetam) and 3 antibiotic compounds (metronidazole, clarithromycin, lincomycin) was determined. The antibiotic, sulfadiazine, was not detected in sampled plant cells contents. Pharmaceutical transformation products including degradants, metabolites and conjugates were identified where parent compounds were also detected. This includes detection of biologically active metabolites of clarithromycin that were previously undocumented in plant tissues. The results of this work will help reveal the uptake mechanisms and ultimate chemical fate of pharmaceuticals taken up by plants, and thereby inform potential risks to human and ecosystem health associated with the use of reclaimed wastewater for crop irrigation.