Resistance to aryloxyphenoxypropionate herbicides in Amazon sprangletop: Confirmation, control, and molecular basis of resistance.

Amazon sprangletop is problematic weed of rice in the midsouthern USA. Two biotypes of this species from rice fields approximately 100km apart in Louisiana were unaffected when sprayed with the labeled field rate of cyhalofop-butyl (314g ai ha-1) in 2008. Dose response studies were conducted to confirm the level of resistance to cyhalofop-butyl over a range of doses. Cross-resistance to acetyl-CoA carboxylase (ACCase)-inhibiting herbicides from two different chemical families and multiple herbicide resistance to other mechanisms of action were evaluated. Sequencing using the Illumina Hiseq platform and ACCase gene sequencing revealed two different amino acid substitutions, Trp2027-to-Cys in the first resistant biotype and Asp2078-to-Gly in the second resistant biotype, within the CT domain of the ACCase gene. Two known amino acid substitutions confirmed resistance to cyhalofop-butyl and fenoxaprop-P-ethyl in resistant Amazon sprangletop biotypes. Asp2078-to-Gly amino acid substitution that was detected in one of the resistant biotypes did not result in cross-resistance to clethodim, an ACCase-inhibiting cyclohexandione herbicide which has endowed clethodim resistance in other weed species. Based on this research, both resistant Amazon sprangletop biotypes have evolved target-site resistance to the APP herbicides; yet, alternative herbicides are still active on these plants.

Increased temperatures and elevated CO2 levels reduce the sensitivity of Conyza canadensis and Chenopodium album to glyphosate.

Herbicides are the most commonly used means of controlling weeds. Recently, there has been growing concern over the potential impacts of global climate change, specifically, increasing temperatures and elevated carbon dioxide (CO2) concentrations, on the sensitivity of weeds to herbicides. Here, glyphosate response of both Conyza canadensis and Chenopodium album was evaluated under different environmental conditions. Reduced glyphosate sensitivity was observed in both species in response to increased temperature, elevated CO2 level, and the combination of both factors. Increased temperature had greater effect on plant survival than elevated CO2 level. In combination, high temperature and elevated CO2 level resulted in loss of apical dominance and rapid necrosis in glyphosate-treated plants. To investigate the mechanistic basis of reduced glyphosate sensitivity, translocation was examined using 14C-glyphosate. In plants that were subjected to high temperatures and elevated CO2 level, glyphosate was more rapidly translocated out of the treated leaf to shoot meristems and roots than in plants grown under control conditions. These results suggest that altered glyphosate translocation and tissue-specific sequestration may be the basis of reduced plant sensitivity. Therefore, overreliance on glyphosate for weed control under changing climatic conditions may result in more weed control failures.

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 14 C-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. © 2017 Society of Chemical Industry.

First report of resistance to acetolactate-synthase-inhibiting herbicides in yellow nutsedge (Cyperus esculentus): confirmation and characterization.

BACKGROUND: Yellow nutsedge is one of the most problematic sedges in Arkansas rice, requiring the frequent use of halosulfuron (sulfonylurea) for its control. In the summer of 2012, halosulfuron at 53 g ha(-1) (labeled field rate) failed to control yellow nutsedge. The level of resistance to halosulfuron was determined in the putative resistant biotype, and its cross-resistance to other acetolactate synthase (ALS) inhibitors from four different herbicide families. ALS enzyme assays and analysis of the ALS gene were used to ascertain the resistance mechanism. RESULTS: None of the resistant plants was killed by halosulfuron at a dose of 13 568 g ha(-1) (256× the field dose), indicating a high level of resistance. Based on the whole-plant bioassay, the resistant biotype was not controlled by any of the ALS-inhibiting herbicides (imazamox, imazethapyr, penoxsulam, bispyribac, pyrithiobac-sodium, bensulfuron and halosulfuron) tested at the labeled field rate. The ALS enzyme from the resistant biotype was 2540 times less responsive to halosulfuron than the susceptible biotype, and a Trp574 -to-Leu substitution was detected by ALS gene sequencing using the Illumina HiSeq. CONCLUSION: The results suggest a target-site alteration as the mechanism of resistance in yellow nutsedge, which accounts for the cross-resistance to other ALS-inhibiting herbicide families.