Environmental Fate of RNA Interference Pesticides: Adsorption and Degradation of Double-Stranded RNA Molecules in Agricultural Soils

Double-stranded RNA (dsRNA) pesticides are a new generation of crop protectants that interfere with protein expression in targeted pest insects by a cellular mechanism called RNA interference (RNAi). The ecological risk assessment of these emerging pesticides necessitates an understanding of the fate of dsRNA molecules in receiving environments, among which agricultural soils are most important. We herein present an experimental approach using phosphorus-32 (P-32)-radiolabeled dsRNA that allows studying key fate processes of dsRNA in soils with unprecedented sensitivity. This approach resolves previous analytical challenges in quantifying unlabeled dsRNA and its degradation products in soils. We demonstrate that P-32-dsRNA and its degradation products are quantifiable at concentrations as low as a few nanograms of dsRNA per gram of soil by both Cerenkov counting (to quantify total P-32-activity) and by polyacrylamide gel electrophoresis followed by phosphorimaging (to detect intact P-32-dsRNA and its P-32-containing degradation products). We show that dsRNA molecules added to soil suspensions undergo adsorption to soil particle surfaces, degradation in solution, and potential uptake by soil microorganisms. The results of this work on dsRNA adsorption and degradation advance a process-based understanding of the fate of dsRNA in soils and will inform ecological risk assessments of emerging dsRNA pesticides.