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BACKGROUND: The evolution of herbicide-resistant weeds is one of the most important concerns of global agriculture. Amaranthus hybridus L. is a competitive weed for summer crops in South America. In this article, we intend to unravel the molecular mechanisms by which an A. hybridus population from Argentina has become resistant to extraordinarily high levels of glyphosate. RESULTS:The glyphosate-resistant population (A) exhibited particularly high parameters of resistance (GR 50 = 20 900 g ai ha −1 , Rf = 314), with all plants completing a normal life cycle even after 32X dose application. No shikimic acid accumulation was detected in the resistant plants at any of the glyphosate concentrations tested. Molecular and genetic analyses revealed a novel triple substitution (TAP-IVS: T102I, A103V, and P106S) in the 5-enol-pyruvylshikimate-3-phosphate synthase (EPSPS) enzyme of population A and an incipient increase on the epsps relative copy number but without effects on the epsps transcription levels. The novel mechanism was prevalent, with 48% and 52% of the individuals being homozygous and heterozygous for the triple substitution, respectively. In silico conformational studies revealed that TAP-IVS triple substitution would generate an EPSPS with a functional active site but with an increased restriction to glyphosate binding. CONCLUSION: The prevalence of the TAP-IVS triple substitution as the sole mechanism detected in the highly glyphosate resistant population suggests the evolution of a new glyphosate resistance mechanism arising in A. hybridus. This is the first report of a naturally occurring EPSPS triple substitution and the first glyphosate target-site resistance mechanism described in A. hybridus. /journal/ps estimation of plant fitness costs associated with herbicide-resistance genes. Weed Sci 63:203-216 (2015). 42 Schönbrunn E, Eschenburg S, Shuttleworth WA, Schloss JV, Amrhein N, Evans JN, et al., Interaction of the herbicide glyphosate with its target enzyme 5-enolpyruvylshikimate 3-phosphate synthase in atomic detail. Proc Natl Acad Sci 98:1376-1380 (2001). 43 Pollegioni L, Schonbrunn E and Siehl D, Molecular basis of glyphosate resistance-different approaches through protein engineering.

This is the first report of als resistance alleles in A. palmeri in Argentina. The data support the involvement of a target-site mechanism of resistance to ALS-inhibiting herbicides. © 2017 Society of Chemical Industry.