Physiological and Molecular Basis of Acetolactate Synthase-Inhibiting Herbicide Resistance in Barnyardgrass (Echinochloa crus-galli)

Barnyardgrass biotypes from Arkansas (AR1 and AR2) and Mississippi (MS1) have evolved cross-resistance to imazamox, imazethapyr, and penoxsulam. Additionally, AR1 and MS1 have evolved cross-resistance to bispyribac-sodium. Studies were conducted to determine if resistance to acetolactate synthase (ALS)-inhibiting herbicides in these biotypes is target-site or non-target-site based. Sequencing and analysis of a 1701 base pair ALS coding sequence revealed Ala₁₂₂ to Val and Ala₁₂₂ to Thr substitutions in AR1 and AR2, respectively. The imazamox concentrations required for 50% inhibition of ALS enzyme activity in vitro of AR1 and AR2 were 2.0 and 5.8 times, respectively, greater than the susceptible biotype. Absorption of ¹⁴C-bispyribac-sodium, -imazamox, and -penoxsulam was similar in all biotypes. ¹⁴C-Penoxsulam translocation out of the treated leaf (≤2%) was similar among all biotypes. ¹⁴C-Bispyribac-treated AR1 and MS1 translocated 31- 43% less radioactivity to aboveground tissue below the treated leaf compared to the susceptible biotype. ¹⁴C-Imazamox-treated AR1 plants translocated 39% less radioactivity above the treated leaf and aboveground tissue below the treated leaf, and MS1 translocated 54 and 18% less radioactivity to aboveground tissue above and below the treated leaf, respectively, compared to the susceptible biotype. Phosphorimaging results further corroborated the above results. This study shows that altered target site is a mechanism of resistance to imazamox in AR2 and probably in AR1. Additionally, reduced translocation, which may be a result of metabolism, could contribute to imazamox and bispyribac-sodium resistance in AR1 and MS1.