First report of Ser653 Asn mutation endowing high-level resistance to imazamox in downy brome (Bromus tectorum L.)

Abstract

BACKGROUND Bromus tectorum L. is one of the most troublesome grass weed species in cropland and non-cropland areas of the northwestern United States. In summer 2016, a B. tectroum accession (R) that survived imazamox at the field-use rate (44 g ha-1 ) in an imidazolinone-tolerant (IMI-tolerant or Clearfield™ ) winter wheat field was collected near Hammond, Carter County, MT, USA. The aim of this study was to determine the resistance profile of the B. tectroum R accession to imazamox and other ALS inhibitors, and investigate the mechanism of resistance to imazamox. RESULTS The R B. tectorum accession had a high-level resistance (110.1-fold) to imazamox (IMI) and low to moderate-levels cross-resistance to pyroxsulam (TP) (4.6-fold) and propoxycarbazone (SCT) (13.9-fold). The R accession was susceptible to sulfosulfuron (SU) and quizalofop and clethodim (ACCase inhibitors), paraquat (PS I inhibitor), glyphosate (EPSPS inhibitor), and glufosinate (GS inhibitor). Sequence analysis of the ALS gene revealed a single, target-site Ser653 Asn mutation in R plants. The pre-treatment of malathion followed by imazamox at 44 or 88 g ha-1 did not reverse the resistance phenotype. CONCLUSION This is the first report of evolution of cross-resistance to ALS-inhibiting herbicides in B. tectorum. A single-point mutation, Ser653 Asn, was identified, conferring the high-level resistance to imazamox.