A Multistate Study of the Association Between Glyphosate Resistance and EPSPS Gene Amplification in Waterhemp (Amaranthus tuberculatus)

Abstract

Waterhemp is an increasingly problematic weed in the U.S. Midwest, having now evolved resistances to herbicides from six different site-of-action groups. Glyphosate-resistant waterhemp in the Midwest is especially concerning given the economic importance of glyphosate in corn and soybean production. Amplification of the target-site gene, 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) was found to be the mechanism of glyphosate resistance in Palmer amaranth, a species closely related to waterhemp. Here, the relationship between glyphosate resistance and EPSPS gene amplification in waterhemp was investigated. Glyphosate dose response studies were performed at field sites with glyphosate-resistant waterhemp in Illinois, Kansas, Kentucky, Missouri, and Nebraska, and relative EPSPS copy number of survivors was determined via quantitative real-time polymerase chain reaction (qPCR). Waterhemp control increased with increasing glyphosate rate at all locations, but no population was completely controlled even at the highest rate (3,360 g ae ha−1). EPSPS gene amplification was present in plants from four of five locations (Illinois, Kansas, Missouri, and Nebraska) and the proportion of plants with elevated copy number was generally higher in survivors from glyphosate-treated plots than in plants from the untreated control plots. Copy number magnitude varied by site, but an overall trend of increasing copy number with increasing rate was observed in populations with gene amplification, suggesting that waterhemp plants with more EPSPS copies are more resistant. Survivors from the Kentucky population did not have elevated EPSPS copy number. Instead, resistance in this population was attributed to the EPSPS Pro106Ser mutation. Results herein show a quantitative relationship between glyphosate resistance and EPSPS gene amplification in some waterhemp populations, while highlighting that other mechanisms also confer glyphosate resistance in waterhemp.

EPSPS Gene Amplification is Present in the Majority of Glyphosate-Resistant Illinois Waterhemp (Amaranthus tuberculatus) Populations

“…This comprised only 9% of the resistant populations studied. These results are consistent with previous findings that gene amplification is associated with glyphosate resistance in multiple geographic locations (Chatham et al 2012(Chatham et al , 2013.…”

Glyphosate-Resistant Palmer Amaranth (Amaranthus palmeri) in Nebraska: Confirmation, EPSPS Gene Amplification, and Response to POST Corn and Soybean Herbicides

“…The evolution of GR Palmer amaranth in south-central Nebraska provides cause for concern, considering that glyphosate is the most common herbicide used for weed control in GR corn-soybean cropping systems. While GR Palmer amaranth has been reported in west-central Nebraska, the evolution of GR Palmer amaranth in south-central Nebraska will add management challenges for growers because GR common ragweed (Ambrosia artemisiifolia L.), common water- A rapid molecular test, EPSPS gene amplification, has been identified to confirm glyphosate resistance in weeds and has been tested in a Palmer amaranth biotype from Georgia (Gaines et al 2010), populations of waterhemp from Illinois (Chatham et al 2015b), and GR waterhemp populations from several states in the Midwest (Chatham et al 2015a). The molecular test confirmed that the putative GR Palmer amaranth from south-central Nebraska acquired resistance by amplifying the EPSPS gene copy number; however, more research is needed to determine whether other mechanisms of resistance are involved.…”

Physical Mapping of Amplified Copies of the 5-Enolpyruvylshikimate-3-Phosphate Synthase Gene in Glyphosate-Resistant Amaranthus tuberculatus

“…Extensive genetic variability, coupled with intense herbicide selection, resulted in evolution of resistance to several commonly used herbicides, including glyphosate in A. tuberculatus populations throughout the US (Nandula et al, 2013;Chatham et al, 2015a;Lorentz et al, 2014;Bell et al, 2013).…”

“…For example, target-site resistance to glyphosate as a result of mutations in the EPSPS gene resulting in amino acid substitution at Pro-106 or Thr-102 residues (corresponding to EPSPS sequence in Arabidopsis [Arabidopsis thaliana]) has been documented in glyphosate-resistant (GR) Eleusine indica (Baerson et al, 2002;Powles and Preston, 2006;Kaundun et al, 2008;Yu et al, 2015), L. rigidum (Kaundun et al, 2011;Powles and Preston, 2006;Preston, 2006a, 2006b), and A. tuberculatus (Nandula et al, 2013). On the other hand, EPSPS gene amplification as a mechanism of glyphosate resistance was first reported by Gaines et al (2010) in Amaranthus palmeri, a closely related species of A. tuberculatus, and later was also found in many other weeds, including Kochia scoparia (Wiersma et al, 2015), A. tuberculatus (Lorentz et al, 2014;Chatham et al, 2015a), A. spinosus (Nandula et al, 2013), L. perenne ssp. multiflorum (Salas et al, 2012), E. indica , and Bromus diandrus (Malone et al, 2016).…”

“…More importantly, although closely related, GR A. tuberculatus and A. palmeri were found to have considerable difference in the number of EPSPS copies amplified. While 10 to 100 EPSPS copies are common in GR A. palmeri (Gaines et al, 2010), GR A. tuberculatus typically has 10 or fewer EPSPS copies (Chatham et al, 2015a(Chatham et al, , 2015bLorentz et al, 2014). Investigation of the distribution of EPSPS copies in the genome of A. tuberculatus will enable better understanding of the mechanisms of EPSPS amplification.…”