A triple target-site mutation confers high levels of resistance to glyphosate in an Amaranthus quitensis population from Argentina   Sarah-Jane Hutchings^a, Eddie McIndoe^a, Ryan Carlin^b, Wenjin Yu^b, Anushka Howell^a, Weining Gu^b, Wenling Wang^b, Mike Langford^a, Jo Mattocks^a and Shiv-Shankar Kaundun^{a   a} Syngenta, Jealott’s Hill International Research Centre, RG42 6EY Bracknell, UK ^b Syngenta, Research Triangle Park, NC USA   E:mail: sarah-jane.hutchings@syngenta.com

Resistance to glyphosate has evolved and is quickly spreading in Amaranthus quitensis due to excessive use of the EPSPS-inhibiting herbicide in Argentinian soybean production systems.  Here, we confirmed resistance to glyphosate and determined the mechanism involved in an A. quitensis population (AMAQU-R) from Santa Fe Province, Argentina. AMAQU-R plants survived glyphosate rates as high as 48 kg ai/ha, whereas individuals from the sensitive populations (AMAQU-S1 and AMAQU-S2) were killed at 750 g ai/ha or below. Differential glyphosate uptake, movement or metabolism was not associated with resistance in AMAQU-R and only a small average relative increase (1.94) in EPSPS gene copy number/expression was detected in AMAQU-R.  Three linked EPSPS mutations were identified around the binding site of glyphosate in AMAQU-R. These comprised T102I and P106S amino acid substitutions documented to confer high levels of glyphosate resistance, as well as a novel A103V target-site mutation. A 3.5-fold more tolerance to glyphosate was observed for the TIAVPS mutant compared to the double TIPS strain following heterologous gene expression and enzyme analysis. The triple glyphosate resistance mutations may explain the rapid spread of A. quitensis in Argentina and could be a real threat for neighbouring countries under similar glyphosate selection pressure.