Effects of resistance mutations of Pro197, Asp376 and Trp574 on the characteristics of acetohydroxyacid synthase (AHAS) isozymes.

Abstract

Descurainia sophia L., a problematic weed in winter wheat fields in China, has developed high resistance to tribenuron-methyl. Amino acid substitutions at sites Pro197, Asp376 and Trp574 in the target acetohydroxyacid synthase (AHAS) were primarily responsible for D. sophia resistance to tribenuron-methyl. In this study, purified subpopulations of D. sophia plants individually homozygous for a specific resistance mutation (Pro197Leu, Pro197His, Pro197Ser, Pro197Thr, Asp376Glu or Trp574Leu) in AHAS were generated, and the effects of resistance mutations on D. sophia resistance and AHAS characteristics were investigated. All resistance mutations in this study not only caused D. sophia to evolve 152- to 811-fold resistance to tribenuron-methyl but also greatly reduced AHAS sensitivity to tribenuron-methyl and increased AHAS binding affinity for the substrate pyruvate, which was primarily responsible for D. sophia resistance. The molecular docking results indicated that these resistance mutations altered AHAS binding affinity for tribenuron-methyl by reducing the hydrogen bonds and changing hydrophobic interactions. Compared with the wild-type AHAS, these resistance mutations exhibited no significant effects on AHAS feedback inhibition by branched-chain amino acids (BCAAs) at concentrations <0.08 mm. The altered AHAS sensitivity to feedback inhibition by BCAAs did not necessarily increase or decrease the free BCAAs in resistant D. sophia plants. The AHAS resistance mutations conferred D. sophia resistance to tribenuron-methyl by decreasing the binding affinity for tribenuron-methyl and/or increasing the binding affinity for pyruvate, but the mutations did not necessarily affect the biosynthesis of BCAAs in plants. © 2018 Society of Chemical Industry.