Investigation of metabolism and spatial distribution of metabolites of metalaxyl and spirotetramat after root uptake in maize using HPLC-HRMS and MALDI-MSI

Large quantities of pesticides are applied globally on agricultural sites, resulting in elevated concentrations of these compounds in soil pore water. A comprehensive understanding of the metabolic detoxification pathways of residual pesticides, following their uptake in crops, as well as the distribution of metabolites, is essential to ensure the safety of food and feed materials and to predict the potential environmental impacts. Therefore, we examined the metabolism of the pesticides metalaxyl (MET) and spirotetramat (SPI) and the distribution of their metabolites in maize plants grown in a hydroponic solution, utilising high-resolution mass spectrometry. While SPI demonstrated extensive biotransformation, MET exhibited a lower degree of metabolism. Significant discrepancies were observed in the distribution of metabolites between roots and shoots. Even stereoisomeric metabolites with nearly identical physicochemical properties showed clearly different translocation factors. The distribution of metabolites in leaves of different growth stages suggests a certain degree of mobility via the phloem of MET acid, hydroxy-MET and hydroxy-MET-glucoside, as well as SPI-enol and desmethyl-SPI-enol. Conversely, the distribution observed for the unmetabolized pesticides MET and SPI indicate that the xylem represents their dominant pathway for transport. MALDI mass spectrometry imaging has enabled to visualize accumulation sites of pesticide metabolites in root and shoot tissues at the micrometer level. The initial translocation from roots to shoots appears to occur mainly via the fast-transporting xylem, with subsequent phloem redistribution in the shoots being a slower process. Direct evidence for different transport and metabolism pathways of various pesticide metabolites has been observed, providing valuable insight into their fate in plants.