Resistance to acetolactate synthase inhibitors is due to a W 574 to L amino acid substitution in the ALS gene of redroot pigweed and tall waterhemp

Several Amaranthus spp. around the world have evolved resistance (and cross resistance) to various herbicide mechanisms of action. Populations of redroot pigweed (RRPW-R) and tall waterhemp (TW-R) in Mississippi, USA have been suspected to be resistant to one or more acetolactate synthase (ALS) inhibiting herbicides. Whole plant dose-response experiments with multiple ALS inhibitors, ALS enzyme assays with pyrithiobac, and molecular sequence analysis of ALS gene constructs were conducted to confirm and characterize the resistance profile and nature of the mechanism in the RRPW-R and TW-R populations. Two susceptible populations, RRPW-S and TW-S were included for comparison with RRPW-R and TW-R, correspondingly. The resistance index (R/S; the herbicide dose required to reduce plant growth by 50% of resistant population compared to the respective susceptible population) values of the RRPW-R population were 1476, 3500, and 900 for pyrithiobac, imazaquin, and trifloxysulfuron, respectively. The R/S values of the TW-R population for pyrithiobac, imazaquin, and trifloxysulfuron were 51, 950, and 2600, respectively. I(50) values of RRPW-S and RRPW-R populations for pyrithiobac were 0.062 and 208.33 μM, indicating that the ALS enzyme of the RRPW-R population is 3360-fold more resistant to pyrithiobac than the RRPW-S population under our experimental conditions. The ALS enzyme of the TW-R population was 1214-fold resistant to pyrithiobac compared to the TW-S population, with the I(50) values for pyrithiobac of ALS from TW-R and TW-S populations being 87.4 and 0.072 μM, correspondingly. Sequencing of the ALS gene identified a point mutation at position 574 of the ALS gene leading to substitution of tryptophan (W) residue with a leucine (L) residue in both RRPW-R and TW-R populations. Thus, the RRPW-R and TW-R populations are resistant to several ALS-inhibiting herbicides belonging to different chemical classes due to an altered target site, i.e., ALS. Resistance in Amaranthus spp. to commonly used ALS-inhibiting herbicides warrants an integrated weed management scheme incorporating chemical, mechanical, and cultural strategies by growers.