Target-site is the main mechanism of resistance to ALS-inhibitor herbicides in a rice flatsedge population from Southern Brazil

Background: Overuse of ALS-inhibiting herbicides in rice paddy fields has resulted in the selection of Cyperus iria L. (rice flatsedge) with a high level of resistance to this herbicide group. Objective: This study aimed to identify mutations endowing ALS resistance (target-site resistance) and the involvement of a metabolicmediated resistance mechanism of C. iria to ALS herbicides. Methods: Dose-response experiments were performed to estimate GR(50) values (GR(50) denotes the rate at with a herbicide reduces growth by 50%). Experiments I and II were conducted in a greenhouse, with a factorial arrangement comprised of two C. iriapopulations, CYPIR-S and CYPIR-R (with and without malathion treatment), which were susceptible and resistant to ALS-inhibiting herbicides, respectively; ALS-inhibiting herbicides (bispyribac-sodium, imazapyr + imazapic, penoxsulam, and pyrazosulfuron-ethyl); and herbicide doses above and below the maximum field rates. Genomic DNA was extracted from CYPIR- S and CYPIR-R for partial sequencing of the ALS gene. Results: The GR(50) values of CYPIR-R were 400 to > 4,000 times higher than those ofCYPIR-S, indicating ahighlevel of resistance to all herbicides evaluated. Tests with bispyribac-sodium plus malathion showed that metabolism might be involved. However, dose-response curves demonstrated that a specific mutation in the ALS gene is the main resistance mechanism. DNA sequencing electropherogram analysis for CYPIR-R showed different nucleotide changes at Trp(574) in the first [thymine (T) to adenine (A)] and second bases [guanine (G) to T], which can result in changes from tryptophan (TGG) to arginine (AGG), to leucine (TTG), and/or to methionine (ATG), respectively. Conclusions: Target-site resistance was involved in the high level of resistance to ALS-inhibiting herbicides in the rice flatsedge, due to a nucleotide change resulting in an amino acid substitution at the Trp(574) ALS gene.