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

Authors: Nandula, Vijay; GIACOMINI, DARCI - University Of Illinois; Ray, Jeffery - Jeff

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/15/2020
Publication Date: 6/29/2020
Citation: Nandula, V.K., Giacomini, D.A., Ray, J.D. 2020. Resistance to acetolactate synthase inhibitors is due to a TRP 574 to LEU amino acid substitution in the ALS gene of redroot pigweed and tall waterhemp from Mississippi. PLoS ONE. 15:6. https://doi.org/10.1371/journal.pone.0235394.
DOI: https://doi.org/10.1371/journal.pone.0235394

Interpretive Summary: Several Amaranthus spp. around the world have developed resistance (and cross resistance) to various herbicide modes of action. Populations of A. retroflexus (RRPW-R) and A. tuberculatus (TW-R) in Mississippi have been suspected to be resistant to one or more acetolactate synthase (ALS) inhibiting herbicides. Scientists from the Crop Production Systems Research Unit and Crop Genetics Research Unit of USDA-ARS, and University of Illinois conducted whole plant dose-response experiments with multiple ALS inhibitors, ALS enzyme assays with pyrithiobac, and molecular sequence analysis of ALS gene constructs to confirm and characterize the resistance profile and nature of 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 RRPW-R and TW-R population were highly resistant to pyrithiobac, imazaquin, and trifloxysulfuron. The mechanism of resistance in the above two resistant populations was concluded to be due to an altered target site, modified ALS, that provides resistance to a broad array of ALS-inhibiting herbicides. Public and private land managers must implement a combination of chemical, mechanical, and cultural weed management strategies wherever and whenever feasible to manage herbicide resistant populations such as RRPW-R and TW-R.

Technical Abstract: Several Amaranthus spp. around the world have developed resistance (and cross resistance) to various herbicide modes of action. Populations of A. retroflexus (RRPW-R) and A. tuberculatus (TW-R) in Mississippi 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 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 R/S values of the RRPW-R population were >283, >448, and >120 for pyrithiobac, imazaquin, and trifloxysulfuron, respectively. The R/S values of the TW-R population for pyrithiobac, imazaquin, and trifloxysulfuron were >10, >448, and >240, respectively. I50 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 TW-R population was 1214-fold resistant to pyrithiobac compared to the TW-S population, with the I50 values for pyrithiobac of 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 severely limits postemergence management options for growers.