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Weed control failures due to herbicide resistance are an increasing and worldwide problem significantly impacting crop yields. Herbicide resistance due to increased herbicide metabolism in weeds is not well characterized at the genetic level. An RNA-Seq transcriptome analysis was used to identify genes conferring metabolism-based herbicide resistance (MBHR) in a population (R) of a major global weed (Lolium rigidum), in which resistance to the herbicide diclofop-methyl was experimentally evolved through recurrent selection from a susceptible (S) progenitor population. A reference transcriptome of 19,623 contigs was assembled using 454 sequencing technology on a cDNA library and annotated using UniProt and Pfam databases. Transcriptomic-level gene expression was measured using Illumina 100 bp reads from untreated control, mock, and diclofop-methyl treatments of R and S. Due to the established importance of cytochrome P450 (CytP450), glutathione-S-transferase (GST), and glucosyltransferase (GT) genes in MBHR, 11 contigs with these annotations and higher constitutive expression in untreated R than in untreated S were selected as candidate genes for hypothesis testing, along with 17 additional differentially expressed contigs with annotations related to metabolism or signal transduction. In a forward genetics validation experiment, higher constitutive expression of nine contigs co-segregated with the resistance phenotype in an F2 population, including 3 CytP450, 3 GST, and 1 GT. At least nine genes with heritable increased constitutive expression are associated with MBHR trait. In a physiological validation experiment where 2, 4-D pre-treatment induced diclofop-methyl protection in S individuals due to increased metabolism, seven of the nine genetically-validated contigs were significantly induced. These data help explain accumulation of resistance-endowing genes and rapid evolution of MBHR, and provide the opportunity to improve diagnostics of MBHR using molecular tools such as transcriptional markers.

BACKGROUND: Brome grass (Bromus diandrus Roth) is prevalent in the southern and western cropping regions of Australia, where it causes significant economic damage. A targeted herbicide resistance survey was conducted in 2020 by collecting brome grass populations from 40 farms in Western Australia and subjecting these samples to comprehensive herbicide screening. One sample (population 172‐20), from a field that had received 12 applications of clethodim over 20 years of continuous cropping, was found to be highly resistant to the acetyl‐CoA carboxylase (ACCase)‐inhibiting herbicides clethodim and quizalofop, and so the molecular basis of resistance was investigated. RESULTS: All 31 individuals examined from population 172‐20 carried the same resistance‐endowing point mutation causing an aspartate‐to‐glycine substitution at position 2078 in the translated ACCase protein sequence. A wild‐type susceptible population and the resistant population had similar expression levels of plastidic ACCase genes. The level of resistance to quizalofop, either standalone or in mixture with clethodim, in population 172‐20 was lower under cooler growing conditions. CONCLUSION: Target‐site resistance to ACCase‐inhibiting herbicides, conferred by one ACCase mutation, was selected in all tested brome plants infesting a field with a history of repeated clethodim use. This mutation appears to have been fixed in the infesting population. Notably, clethodim resistance in this population was not detected by the farmer, and a high future incidence of quizalofop resistance is anticipated. Herbicide resistance testing is essential for the detection of evolving weed resistance issues and to inform effective management strategies. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry. [ABSTRACT FROM AUTHOR]

BACKGROUND Lolium rigidum is the weed of greatest economic impact in Australia due to its formidable capacity to evolve herbicide resistance. In this study, 579 field‐sampled L. rigidum populations were tested for resistance to 21 herbicides applied at the recommended rate. Nine herbicide treatments were binary mixtures. RESULTS: A total of 15 876 individual resistance tests were conducted by screening two million seeds at the recommended label rate. The overall frequency of resistant populations was 31%, 14%, 71%, 6% and 0% in response to the post‐emergence herbicide treatments clethodim, clethodim + butroxydim, imazamox + imazapyr, glyphosate and paraquat, respectively. The resistance frequency to stand‐alone pre‐emergence wheat‐selective herbicides ranged from 10% to 34%. Conversely, the levels of resistance to pre‐emergence mixtures or stand‐alone propyzamide were significantly lower, ranging from 6% to 0%. In winter, the responses to glyphosate, paraquat, cinmethylin, prosulfocarb, pyroxasulfone and trifluralin were reassessed, with 7%, 0%, 0%, 21%, 21% and 28% as the respective resistance frequencies. South Australia and Victoria are identified as epicenters for L. rigidum population resistance to pyroxasulfone, whereas populations in New South Wales have the greatest resistance to glyphosate and in Western Australia to clethodim. CONCLUSIONS: For the first time, resistance levels to stand‐alone herbicides and binary mixtures are geographically ranked across the Australian continent by benchmark statistical analysis of resistance frequencies and distribution. The extension of these results will raise awareness of rapidly emerging patterns of herbicide resistance, encouraging the adoption of cost‐effective modes of action and integration of diverse strategies for weed resistance management. [ABSTRACT FROM AUTHOR]

Lolium rigidum Gaud., a grass weed species infesting winter field crops, has evolved resistance to the largest number of herbicide modes of action. In this study, 140 field populations of L. rigidum were screened with 14 herbicide treatments. Herbicide resistance at the recommended label dosage of pre‐emergence (PRE), post‐emergence (POST) and binary herbicide mixtures was considered present when plant survival was ≥6%. Plant survival to four acetyl‐CoA carboxylase (ACCase) POST herbicides averaged across all populations was approximately 15%, indicating substantial herbicide resistance. In contrast, the mean survival to the PRE treatments was only 2%, reflecting effective control of L. rigidum. Herbicide mixtures were the most effective treatments, with a significantly lower resistance frequency than stand‐alone herbicides. For example, only 12% of the tested samples were resistant to the mixture of clethodim + butroxydim in comparison with 40% and 61% to either butroxydim or clethodim, respectively. Similarly, 8% of the samples were resistant to the mixture of trifluralin + prosulfocarb versus a much greater frequency of 36% and 51% resistance to prosulfocarb and trifluralin, respectively. Surprisingly, the binary mixtures of trifluralin + triallate or pyroxasulfone + triallate are not affected by resistance (presently) due to the greater efficacy than that of either stand‐alone herbicide. Thus, herbicide mixtures can delay the onset of resistance and mitigate the existing levels of herbicide resistance and cross‐resistance in L. rigidum. Systematic screenings of a large number of field populations could identify the most (cost‐) effective herbicide mixtures and foster their informed adoption on farm to mitigate the rapid evolution of weed resistance in lieu of expert assumptions or modelling simulations. [ABSTRACT FROM AUTHOR]

BACKGROUND: Weed resistance to foliar herbicides has dramatically increased worldwide in the last two decades. As a consequence, current practices of weed management have changed, with an increased adoption of soil‐applied herbicides to restore control of herbicide‐resistant weeds. We foresee metabolism‐based resistance and cross‐resistance to soil‐applied herbicides as a potential global consequence to the increased and widespread adoption of new and old soil‐applied herbicides. Thus, the aim of this study is to use computer simulation modelling to quantify and rank the risk of weeds evolving resistance to soil‐applied herbicides under different usage strategies (single herbicide use, rotations and mixtures) and population genetic hypotheses. RESULTS: Simulations indicate that without rotation it takes twice as long to select for resistance to a particular soil‐applied herbicide – trifluralin – than to any other herbicide option considered. Relative to trifluralin‐only use, simple herbicide rotation patterns have no effect in delaying resistance, whereas more complex rotation patterns can delay resistance two‐ or three‐fold. Herbicide mixtures further delay resistance up to six‐fold in comparison to single use or simple herbicide rotations. CONCLUSION: By computer modelling simulations we demonstrate that mixtures maximize herbicide effectiveness and the selection heterogeneity of soil‐applied herbicides, and delay herbicide resistance evolution in weedy plants. Our study is consistent with previous state‐of‐art scientific evidence (i.e. epidemiological and modelling studies across different systems and pests) and extension efforts (i.e. 'rotate herbicide mixtures') to provide insight to manage the selection and evolution of weed resistance. © 2019 Society of Chemical Industry [ABSTRACT FROM AUTHOR]