A205V, D376E, W574L, S653T, and S653N Substitutions in Acetohydroxy Acid Synthase from Amaranthus retroflexus L. Show Different Functional Impacts on Herbicide Resistance

Amaranthus retroflexus L. is a troublesome dicot weed in crop fields and has developed high resistance to nicosulfuron in China. The objective of this study was to determine the effects of specific resistance mutations (A205V, D376E, W574L, S653T, and S653N) of the acetohydroxy acid synthase enzyme (AHAS) on the resistance of A. retroflexus to nicosulfuron. The resistance mutations in A. retroflexus not only conferred 17.17- to 31.70-fold resistance to nicosulfuron but also greatly decreased AHAS sensitivity and increased AHAS binding affinity to substrate pyruvate, which mechanisms were primarily responsible for the observed A. retroflexus resistance. Molecular docking results indicated that these resistance mutations altered AHAS binding free energy with nicosulfuron. All the resistance mutations showed less sensitivity to feedback inhibition by branched-chain amino acids, but the mutations did not necessarily affect biosynthesis in A. retroflexus. This report to compares the various mutations of ArAHAS in vitro and contributes to understanding herbicide resistance in this field weed.