Tomato (Solanum lycopersicum L.) is extremely sensitive to inhibitors of acetohydroxyacid synthase (AHAS; also known as acetolactate synthase [ALS]). Utilizing ethyl methanesulfonate mutagenesis of seeds of the commercial tomato line ‘M82’, we developed a tomato mutant, HRT1, that showed high resistance to imidazolinone herbicides (which act by inhibiting AHAS) in the greenhouse and under field conditions. The activity of AHAS extracted from HRT1 was significantly less affected by imidazolinone herbicides than that from the parental line M82. Following imazapic treatment, no differences were found in the content of free branched-chain amino acids in HRT1 tissues as compared to a dramatic decrease in M82 tissues. No differences were found in the susceptibility of AHAS to sulfonylurea herbicides. A single point transition mutation of C to T in the AHAS1 gene located on chromosome 3 was detected. This mutation resulted in substitution of alanine by valine at amino acid position 194, corresponding to 205-Alal in Arabidopsis. Ligand–protein contact analysis showed that replacement of alanine by the larger hydrophobic valine residue results in increased repulsion, hindering herbicide binding. Segregation analysis indicated that the resistance to imidazolinones in line HRT1 is due to a single recessive gene.

Johnsongrass is a common weed of corn in Chile, which is most often controlled by nicosulfuron, an acetohydroxyacid synthase (AHAS)-inhibiting herbicide. Recurrent nicosulfuron use has resulted in selection for resistant johnsongrass biotypes. We conducted studies to determine nicosulfuron resistance levels in two johnsongrass biotypes from Chile and to investigate if this resistance was target-site mediated. Whole-plant resistance to nicosulfuron was 33 and 46 times higher in resistant (R) than in susceptible (S) plants grown from seed and rhizomes, respectively. The nicosulfuron concentrations for 50% inhibition of AHAS enzyme activity in vitro were more than 11 times higher in R than in S plants. Sequencing analysis of the AHAS coding sequence revealed a Trp-574-Leu substitution in both R biotypes. This study shows that resistance to nicosulfuron in the two R biotypes is conferred by an altered target site. We also report the first consensus sequence of the johnsongrass AHAS gene corresponding to the known mutation sites conferring resistance to AHAS-inhibiting herbicides.