Failure to control Xanthium strumarium with acetolactate synthase (ALS) inhibitor herbicides has been reported in Iowa and surrounding states. Single-seed descent techniques were used to isolate three X. strumarium biotypes: CAM-10 from near Cambridge, Iowa; Colo-25 from near Colo, Iowa; and Ohio-1 from Fulton County, Ohio. Ohio-1 and Colo-25 were selected because of apparent resistance to imazethapyr, whereas CAM-10 was selected for observed sensitivity to imazethapyr. The biotypes were assayed in vitro with three different ALS inhibitor herbicides, and ALS activity was measured. The 50% inhibition values (I50) of ALS for imazethapyr were determined to be ninefold or higher for the Ohio-1 and Colo-25 biotypes compared to the CAM-10 biotype. The I50 for imazaquin was determined to be about ninefold higher for the Colo-25 biotype and sixfold higher for the Ohio-1 biotype when compared to the CAM-10 biotype. All biotypes were equally sensitive to chlorimuron ethyl. The resistance was due to a single dominant nuclear gene.

Weed management is a perennial challenge for growers, and continual innovation is essential to maintain the effectiveness of management technologies. The first generation of herbicide-resistant crops revolutionized weed control. However, weeds are adapting to crop systems that rely on a single mode of herbicide action. Crops with resistance to multiple modes of herbicide action could help maintain weed management. GAT/HRA is a new multiple herbicide–resistance technology for corn, soybean, and other crops. GAT/HRA combines metabolic glyphosate inactivation with an acetolactate synthase (ALS) enzyme that is insensitive to ALS-inhibiting herbicides. The mechanism to inactivate glyphosate is the glyphosate N-acetyltransferase enzyme, which transforms glyphosate into a nonphytotoxic metabolite. The gat gene is derived from a naturally occurring soil bacterium and optimized by repetitive gene shuffling and screening. The resistance mechanism to ALS-inhibiting herbicides is a double-mutant, highly resistant ALS (HRA) that is insensitive to all five classes of ALS herbicides. GAT/HRA crops will maintain natural tolerance to selective herbicides and thus provide more weed management options for growers to help deter weed spectrum shifts and delay the evolution of herbicide-resistant weeds.

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.

Herbicides that target the enzyme acetolactate synthase (ALS) are among the most widely used in the world. Unfortunately, these herbicides are also notorious for their ability to select resistant (R) weed populations. Now, there are more weed species that are resistant to ALS-inhibiting herbicides than to any other herbicide group. In most cases, resistance to ALS-inhibiting herbicides is caused by an altered ALS enzyme. The frequent occurrence of weed populations resistant to ALS inhibitors can be attributed to the widespread usage of these herbicides, how they have been used, the strong selection pressure they exert, and the resistance mechanism. In several cropping systems, ALS-inhibiting herbicides were used repeatedly as the primary mechanism of weed control. These herbicides exert strong selection pressure because of their high activity on sensitive biotypes at the rates used and because of their soil residual activity. Several point mutations within the gene encoding ALS can result in a herbicide-resistant ALS. From investigations of numerous R weed biotypes, five conserved amino acids have been identified in ALS that, on substitution, can confer resistance to ALS inhibitors. Substitutions of at least 12 additional ALS amino acids can also confer herbicide resistance in plants and other organisms but, to date, have not been found in R weed populations. Mutations in ALS conferring herbicide resistance are at least partially dominant, and because the gene is nuclear inherited, it is transmitted by both seed and pollen. Furthermore, in many cases there is apparently a negligible fitness cost of the resistance gene in the absence of herbicide selection. Although resistance to ALS-inhibiting herbicides has been a bane to weed management, it has spurred many advances within and beyond the weed science discipline. As examples, resistance to ALS-inhibiting herbicides has been exploited in the development of herbicide-resistant crops, studies of weed population dynamics, and in developing protocols for targeted gene modification. Resistance to ALS-inhibiting herbicides has greatly affected weed science by influencing how we view the sustainability of our weed management practices, what we consider when developing and marketing new herbicides, and how we train new weed scientists.

The transgenic corn line 98140 has a high level of resistance to glyphosate and all five chemical classes of herbicides that inhibit acetolactate synthase (ALS). The dual herbicide resistance is due to a molecular stack of two constitutively expressed genes: gat4621, which produces a glyphosate acetyltransferase that rapidly inactivates glyphosate, and hra, which produces a highly resistant ALS. On a rate basis, the positive 98140 isoline with a single copy of the gat4621 gene is over 1,000-fold more resistant to glyphosate than a negative isoline without the transgene. Similarly, the positive 98140 isoline with the hra gene is over 1,000-fold more resistant to ALS-inhibiting herbicides such as chlorimuron and sulfometuron at the whole-plant and enzyme level. The gat4621 and hra genes do not change the natural tolerance of corn to selective herbicides, so new corn hybrids based on 98140 will give growers more options to manage weeds and delay the evolution of herbicide-resistant weeds.

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.

The cross-tolerance of imidazolinone-tolerant (IMI-tolerant) rice to various acetolactate synthase (ALS)-inhibiting herbicides at one and two times labeled rates was studied. The IMI-tolerant rice is cross-tolerant to imazaquin, imazapyr, nicosulfuron, pyrithiobac, thifensulfuron plus tribenuron, and triasulfuron; is partially tolerant to imazamethabenz and metsulfuron; and is susceptible to chlorimuron, flumetsulam, imazamox, imazapic, primisulfuron, and rimsulfuron. In the greenhouse, IMI-tolerant rice injury with 70 and 140 g ai ha−1 imazethapyr was 17 and 34%, respectively, 28 DAT. Both rates of imazapyr, imazaquin, rimsulfuron, nicosulfuron, thifensulfuron plus tribenuron, and pyrithiobac, and 25 g ai ha−1 triasulfuron, injured rice the same as imazethapyr. Red rice control with 70 and 140 g ha−1 imazethapyr was 97 and 98%, respectively, 28 DAT. At label and two times the label rate, all imidazolinones, nicosulfuron, and primisulfuron controlled red rice equivalent to imazethapyr. Red rice control with 28 g ai ha−1 rimsulfuron was similar to control with 70 and 140 g ha−1 imazethapyr 28 DAT. In the field, barnyardgrass control with two times the labeled rate of imazamox, imazapic, imazapyr, imazaquin, imazamethabenz, rimsulfuron, and nicosulfuron was equal or greater than control with imazethapyr 30 DAT; however, at two times the labeled rate of imazamox, imazapic, and rimsulfuron, injury was greater than imazethapyr. Of all the herbicides tested, only nicosulfuron, imazaquin, and imazapyr offer a combination of low rice injury and high red rice control compared with imazethapyr.