Resistance of weeds to ALS-inhibiting herbicides: what have we learned?

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.