ORIGINAL ABSTRACT

BACKGROUND: Glyphosate-resistant invasive plants, including Amaranthus palmeri S. Watson, have greatly challenged management of new invasions. Elucidating their glyphosate resistance levels rapidly and accurately will better inform management strategies. Quantitative real-time PCR (qPCR) has been used to identify glyphosate resistance in A. palmeri by detecting gene copy numbers of 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), an enzyme inhibited by glyphosate. However, qPCR can only indirectly determine copy numbers because it requires a calibrator sample; it also lacks standardization, thus limiting its usefulness. Droplet digital PCR (ddPCR) is a new method to detect copy number directly and precisely. We evaluated ddPCR as a tool to determine glyphosate-resistance level while using qPCR and glyphosate dose response (GDR) assays as reference technologies to compare performance and efficiency between methods. RESULTS: We identified seven susceptible and seven resistant populations of A. palmeri using the GDR assay. Resistant levels detected by qPCR and ddPCR were generally consistent with the GDR results. Although detected values obtained by qPCR and ddPCR were highly correlated (R2 = 0.94), ddPCR results had a lower proportion of non-ideal values (36%) with better accuracy (100%) and specificity (100%) than those of qPCR results. CONCLUSIONS: Our findings demonstrate that ddPCR offers improved accuracy and specificity in detecting EPSPS gene copy numbers and is a robust and rapid method for glyphosate-resistance identification in A. palmeri. Our research is the first to measure glyphosate resistance in A. palmeri by ddPCR assay and will shed light on future applications of ddPCR in identifying herbicide resistance in other invasive weeds. © 2022 Society of Chemical Industry.