Mutations in the acetolactate synthase (ALS) enzyme affect shattercane ( Sorghum bicolor ) response to ALS-inhibiting herbicides

Background Shattercane [Sorghum bicolor (L.) Moench ssp. Arundinaceum (Desv.)] is a competitive weed in North America's corn, soybean, sorghum, and other agronomic crops. Control of shattercane with POST herbicides in corn became possible with the introduction of acetolactate synthase (ALS)-inhibiting herbicides in the 1980s, and their extensive use resulted in the evolution of ALS-inhibitors resistant shattercane. Results Shattercane seeds were collected from 16 south-eastern and south-central Nebraska fields that were treated with primisulfuron for three consecutive years. Three resistant plants were found in greenhouse evaluations of more than 30,000 plants. Results from a greenhouse bioassay conducted to assess the response of each shattercane biotype to ALS-inhibiting herbicides showed a differential response to ALS inhibitors within and between chemical classes. Biotype P8-30 was resistant or partially resistant to all ALS-inhibiting herbicides applied and displayed a unique amino acid sequence substitution (Trp574 to Leu) relative to the other two resistant biotypes, P2-205 and P9-102. Whole plant dose–response studies confirmed a 4- to the 12-fold level of primisulfuron resistance in three shattercane biotypes compared with the known primisulfuron-susceptible shattercane biotype. The ALS gene was sequenced using primers designed from the corn ALS sequence to identify mutations in the ALS gene that confer resistance. A total of seven nucleotide substitutions were detected in the three herbicide-resistant biotypes P2-205, P8-30, and P9-102. These biotypes are being crossed to adapted sorghum lines (grain, sweet, and forage) to broaden germplasm with resistance to ALS-inhibiting herbicides. Conclusion The discovery of these mutants should accelerate the development of sorghum genotypes that tolerate ALS-based herbicides, which provide additional choices for sorghum farmers to control weeds, especially grasses, in their fields.