A New Pro-197-Ile Mutation in Amaranthus palmeri Associated with Acetolactate Synthase-Inhibiting Herbicide Resistance

Palmer amaranth (Amaranthus palmeri S. Watson), native to North America, is one of the most prominent invasive weed species on agricultural land. Acetolactate synthase (ALS)-resistant A. palmeri (Amaranthus palmeri) is widespread, while the research focus on resistance pattern and molecular basis of A. palmeri to imazethapyr is seldom documented in China. An A. palmeri population that survived the recommended rate of imazethapyr was collected in Shandong Province, China. The resistant mechanism and pattern of A. palmeri to imazethapyr was investigated. Dose–response assay showed that the resistant (R) population displayed a high resistance level (292.5-fold) to imazethapyr compared with the susceptible (S) population. Sequence analysis of the ALS gene revealed that nucleotide mutations resulted in three resistance-conferring amino acid substitutions, Pro-197-Ile, Trp-574-Leu, and Ser-653-Asp, in the individual plants of the R population. An in vitro enzyme assay indicated that the ALS was relatively unsusceptible to imazethapyr in the R population, showing a resistance index of 88.6-fold. ALS gene expression and copy number did not confer resistance to imazethapyr in the R population. Pro-197-Ile is the first reported amino acid substitution conferring ALS resistance to A. palmeri. This is the first case of an imazethapyr-resistant A. palmeri biotype in China.