Mutation at residue 376 of ALS confers tribenuron-methyl resistance in flixweed (Descurainia sophia) populations from Hebei Province, China

• Two ALS genes were found in flixweed and only one of them had mutations related to resistance to tribenuron.
• There were two mutation sites Pro197Ser and Asp376Glu of ALS conferring the resistance and the latter mutation was firstly found in flixweed.
• Mutation Asp376Glu of ALS induced slight cross resistance to florasulam, but mutation Pro197Ser did not.

The acetolactate synthase (ALS) inhibitor tribenuron has been used continuously for approximately twenty years as an herbicide in winter wheat fields in China. Flixweed (Descurainia sophia) has evolved resistance to tribenuron, due to multiple amino acid mutations at the 197th residue of ALS. In this study, the molecular basis of tribenuron resistance was investigated using two resistant populations, Xingtai (XT) and Shijiazhuang (SJ), and two susceptible populations, Cangzhou (CZ) and Handan (HD). Whole-plant tests and ALS activity assays showed that the two resistant populations were highly resistant to tribenuron. Targeted amplification of ALS genes from the four populations showed that there were two ALS genes in each population, and both of them were expressed in flixweed; the full coding lengths of the two ALS genes were 1998 bp and 2004 bp. Mutations related to tribenuron resistance in flixweed were located in only the 1998 bp paralog. An ALS activity assay showed that the resistant population SJ displayed slight cross-resistance to florasulam, with a resistance factor of 4.81, but the resistant population XT did not have cross-resistance to florasulam. The resistant population XT was found to carry the previously reported mutation Pro197Ser, but the resistant population SJ carried a different mutation, Asp376Glu, known from other weeds but novel in flixweed. Our results demonstrated that multiple versions of ALS genes exist in flixweed and that mutations at multiple sites may result in ALS-inhibitor resistance in this weed.

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