Amino acid substitution (Gly-654-Tyr) in acetolactate synthase (ALS) confers broad spectrum resistance to ALS-inhibiting herbicides

BACKGROUND Amaranthus retroflexus L. is a problematic weed in agricultural fields. In China, the repeated use of acetolactate synthase (ALS) inhibiting herbicides has led to the evolution of many A. retroflexus resistant populations. The objective of this research was to investigate the physiological and molecular basis for resistance to ALS-inhibiting herbicides in A. retroflexus. RESULTS Sequence analysis of the ALS revealed a Trp to Leu substitution at amino acid position 574 (Trp-574-Leu), and a previously unreported substitution of Gly to Tyr substitution at 654 (Gly-654-Tyr) in the resistant A. retroflexus population. A purified R-Tyr654 subpopulation homozygous for the Gly-654-Tyr mutation was generated and characterized in response to five different classes of ALS-inhibiting herbicides. Dose-response analysis revealed that the R-Tyr654 population was highly resistant to two imidazolinones (imazethapyr, >42-fold; imazamox, 48.3-fold), one triazolopyrimidine (flumetsulam, 57.4-fold), and one sulfonylamino-carbonyltriazolinone (flucarbazone, 17.3-fold). Moreover, it was moderately resistant to two sulfonylureas (thifensulfuron-methyl, 4.9-fold; nicosulfuron, 9.1-fold), and one pyrimidinylthio-benzoate (bispyribac-sodium, 3.1-fold). An in vitro ALS activity assay showed that ALS with the Gly-654-Tyr substitution was resistant to all the tested ALS-inhibiting herbicides. However, the field rate of herbicides binding other sites of action demonstrated effective control of the R-Tyr654 population. CONCLUSION These results indicate that the molecular basis of ALS-inhibiting herbicide resistance in A. retroflexus was caused by the Trp-574-Leu and Gly-654-Tyr substitutions in the ALS. This is the first report of Gly-654-Tyr substitution in ALS, and this substitution confers broad spectrum resistance to ALS-inhibiting herbicides.

Automated summary

The study revealed the molecular basis for resistance of Amaranthus retroflexus to ALS-inhibiting herbicides, with Trp-574-Leu and Gly-654-Tyr substitutions in ALS contributing to broad-spectrum resistance. The Gly-654-Tyr substitution, previously unreported, conferred high resistance to multiple classes of ALS-inhibiting herbicides in the R-Tyr654 population. Despite this resistance, effective control of the population was achieved with herbicides targeting other sites of action.