Survey of the genomic landscape surrounding the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene in glyphosate-resistant Amaranthus palmeri from geographically distant populations in the USA.

Background: Glyphosate resistance in Amaranthus palmeri, one of the most prevalent herbicide-resistant weeds in the USA, is attributable to amplification and increased expression of the gene encoding the target site of glyphosate, 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS). The EPSPS gene and the surrounding 287 kilobases (kb) of amplified sequence are unique to glyphosate-resistant plants and termed the EPSPS cassette. It has only been sequenced in one A. palmeri population from Mississippi. This research compares EPSPS cassettes in seven resistant and five sensitive populations from geographically distant locations within the USA, including Mississippi, Arizona, Kansas, Maryland, Delaware and Georgia.

Results: Polymerase chain reaction (PCR) products from 40 primer pairs specific to the cassette were similar in size and sequence in resistant populations. Several primer pairs failed to generate PCR products in sensitive populations. Regions of the cassette sequenced in the resistant populations were found to be nearly identical to those from Mississippi. Gene expression analysis showed that both EPSPS and another gene in the cassette, a reverse transcriptase, were elevated in all resistant populations tested relative to the sensitive populations.

Conclusion: EPSPS cassettes from distant resistant populations were nearly homologous. Considering the complexity of the cassette, and the degree of similarity among some cassette sequences, the results are consistent with the hypothesis that glyphosate resistance probably evolved once and then rapidly spread across the USA. © 2017 Society of Chemical Industry.