Acetolactate synthase (ALS), a common enzyme in the biosynthesis of branched-chain amino acids, is a target for the sulfonylurea, imidazolinone, triazolopyrimidine, pyrimidinylthiobenzoate, and the sulfonylaminocarbonyltriazolinone classes of herbicides. Widespread resistance to the ALS-inhibiting herbicides has been attributed to single-base mutations in the ALS gene. The objective of this study was to investigate the feasibility of using denaturing high-performance liquid chromatography (DHPLC), a recently developed method of mutation analysis, for the detection of three ALS mutations, Ala122Thr, Leu574Trp, and Ser653Thr, which confer herbicide resistance. The mutated variants of the ALS gene were isolated from herbicide-resistant biotypes of smooth pigweed and Powell amaranth using polymerase chain reaction (PCR). The PCR products were hybridized with a wild-type sample and subjected to DHPLC analysis. All three mutations could be detected using a modified High performance liquid chromatography system; however, the sensitivity of the method was strongly dependent on the melting temperature profile of the analyzed DNA fragment. Once the primers and the DHPLC conditions are optimized, the procedure is economical, rapid, and requires little sample preparation. Because of these favorable features, DHPLC can be used as an alternative to other commonly used mutation detection methods.

Experiments were conducted to evaluate a biotype of smooth pigweed that had survived applications of sulfonylurea (SU) and imidazolinone (IMI) herbicides in a single season. The source field had a history of repeated acetolactate synthase (ALS)-inhibiting herbicide use over several years. Whole-plant response experiments evaluated the resistant (R11) biotype and an ALS-inhibitor susceptible (S) smooth pigweed biotype to herbicides from the SU, IMI, pyrimidinylthiobenzoate (PTB), and triazolopyrimidine sulfonanilide (TP) chemical families. The R11 biotype exhibited 60- to 3,200-fold resistance to all four ALS-Inhibiting herbicide chemistries compared with the S biotype. Nucleotide sequence comparison of ALS genes from R11 and S biotypes revealed a single nucleotide difference that resulted in R11 having an amino acid substitution of aspartate to glutamate at position 376, as numbered relative to the protein sequence of mouseearcress. This is the first report of an amino acid substitution at this position of an ALS gene isolated from a field-selected weed biotype. To verify the role of this mutation in herbicide resistance, the ALS gene was cloned and expressed in Arabidopsis. Transgenic Arabidopsis expressing this ALS gene exhibited resistance to SU, IMI, PTB, TP, and sulfonylaminocarbonyltriazolinone ALS-Inhibiting herbicide classes.

Greenhouse studies determined dose responses of winter annual weeds and winter wheat to preemergence (PRE) and postemergence (POST) treatments of MKH 6561 and the residual effects on kochia. MKH 6561 at 11 to 45 g ha−1 did not affect wheat. MKH 6561 at 11, 34, and 45 g ha−1 reduced winter annual weed densities compared to an untreated control. Weed growth was inhibited as the rate of MKH 6561 was increased, but downy brome, cheat, and Japanese brome were three to six times more susceptible than jointed goatgrass. The GR70 values for cheat, downy brome, Japanese brome, and jointed goatgrass were 7, 11, 4, and >45 g ha−1, respectively. Dry weights of kochia seeded after removal of the winter annual grasses decreased as MKH 6561 rate increased, with average control being 77% compared to the nontreated control. Regression analysis indicated that kochia was controlled 80% with MKH 6561 at 28 g ha−1.