First report of glyphosate resistance in an Amaranthus palmeri population from Europe

The invasive weed Amaranthus palmeri is spreading throughout Spain, with Catalonia being one of the most affected regions. For this species, acetolactate synthase-inhibiting herbicide-resistant populations have been reported, and now glyphosate resistance is also suspected. Glyphosate targets and inhibits the enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), but A. palmeri has evolved different resistant mechanisms leading to plant survival. One of the most effective is the EPSPS overexpression due to copy number variation (CNV). Gene copies accumulate within the EPSPS cassette that is an extrachromosomal circular DNA displaying unique structural polymorphisms. This study aims to determine the response to glyphosate of a suspected resistant population collected from a roadside and investigate the resistance mechanism involved. The herbicide bioassay confirmed that 40% of the plants survived glyphosate applied at 540 g a.i. ha(-1). No known mutations endowing glyphosate resistance were found at EPSPS amongst confirmed resistant plants, while in most of them (70%) specific molecular markers revealed the presence of the EPSPS cassette. All these results indicate that this population is glyphosate resistant and it is very likely that the EPSPS gene CNV is the main resistance mechanism. This is the first case of glyphosate resistance in A. palmeri in Europe whose introduction is likely due to importation of contaminated seed with glyphosate-resistant Palmer amaranth from the Americas. This introduction poses a significant danger to summer crops in our continent.