Multiple resistance to glyphosate, paraquat and ACCase‐inhibiting herbicides in Italian ryegrass populations from California: confirmation and mechanisms of resistance

BACKGROUND Glyphosate, paraquat and acetyl CoA carboxylase (ACCase)‐inhibiting herbicides are widely used in California annual and perennial cropping systems. Recently, glyphosate, paraquat, and ACCase‐ and acetolactate synthase (ALS)‐inhibitor resistance was confirmed in several Italian ryegrass populations from the Central Valley of California. This research characterized the possible mechanisms of resistance. RESULTS Multiple‐resistant populations (MR1, MR2) are resistant to several herbicides from at least three modes of action. Dose–response experiments revealed that the MR1 population was 45.9‐, 122.7‐ and 20.5‐fold, and the MR2 population was 24.8‐, 93.9‐ and 4.0‐fold less susceptible to glyphosate, sethoxydim and paraquat, respectively, than the susceptible (Sus) population. Accumulation of shikimate in Sus plants was significantly greater than in MR plants 32 h after light pretreatments. Glyphosate resistance in MR plants was at least partially due to Pro106‐to‐Ala and Pro106‐to‐Thr substitutions at site 106 of 5‐enolpyruvylshikimate‐3‐phosphate synthase (EPSPS). EPSPS gene copy number and expression level were similar in plants from the Sus and MR populations. An Ile1781‐to‐Leu substitution in ACCase gene of MR plants conferred a high level of resistance to sethoxydim and cross‐resistance to other ACCase‐inhibitors. Radiolabeled herbicide studies and phosphorimaging indicated that MR plants had restricted translocation of 14C‐paraquat to untreated leaves compared to Sus plants. CONCLUSION This study shows that multiple herbicide resistance in Italian ryegrass populations in California, USA, is due to both target‐site and non‐target‐site resistance mechanisms.