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While herbicides are the most effective and widely adopted weed management approach, the evolution of multiple herbicide resistance in damaging weed species threatens the yield and profitability of many crops. Weeds accumulate multiple resistance mechanisms through sequential selection and/or gene flow, with long‐range and international transport of herbicide‐resistant weeds proving to be a serious issue. Metabolic resistance mechanisms can confer resistance across multiple sites of action and even to herbicides not yet discovered. When a new site of action herbicide is introduced to control a key driver weed, it likely will be one of very few effective available herbicide options for that weed in a specific crop due to the continuous use of herbicides over the years and the resulting accumulation of resistance mechanisms, placing it at even higher risk to be rapidly lost to resistance due to the high selection pressure it will experience. The number of available, effective herbicides for certain driver weeds is decreasing over time because the rate of resistance evolution is faster than the rate of new herbicide discovery. Effective monitoring for species movement and diagnostics for resistance should be deployed to rapidly identify emerging resistance to any new site of action. While innovation in herbicide discovery is urgently needed to combat the pressing issue of resistance in weeds, the rate of selection for herbicide resistance in weeds must be slowed through changes in the patterns of how herbicides are used. © 2021 Society of Chemical Industry. © 2021 Society of Chemical Industry. [ABSTRACT FROM AUTHOR]

BACKGROUND Multiple‐herbicide resistance in Lolium rigidum and other weed species is increasingly exerting pressure on herbicide discovery research for solutions against resistance‐prone weeds. In this study we investigate: (i) the responses of L. rigidum populations and wheat to the new herbicide cinmethylin in comparison with other pre‐emergence herbicides, (ii) the effect of seed burial depths on cinmethylin efficacy and crop selectivity, and (iii) the basis of cinmethylin selectivity in wheat. RESULTS: Cinmethylin at 400 g ha−1 controls herbicide‐susceptible and multiple‐resistant L. rigidum, with a reduction of >85% in plant emergence and 90% in aboveground biomass. Cinmethylin provides effective control of a large number of field populations of L. rigidum with evident resistance to trifluralin. When the wheat seed is buried ≥1 cm below the cinmethylin‐treated soil surface, the emergence of crop seedlings is not different from the untreated control. The organophosphate insecticide phorate synergizes cinmethylin toxicity in wheat, with an LD50 of 682 g ha−1 in the absence of phorate versus 109 g ha−1 in the presence of phorate (84% reduction). The synergistic effect of phorate with cinmethylin on herbicide‐susceptible L. rigidum appears smaller (a 44% reduction in the LD50 of cinmethylin). CONCLUSIONS: Cinmethylin is effective in controlling multiple‐resistant L. rigidum and appears safe for wheat when the seed is separated at depth from the herbicide applied to the soil surface. The basis of this metabolism‐based selectivity is likely regulated by cytochrome P450 monooxygenases. © 2020 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

The evolution of resistant weed populations in response to intensive herbicide selection pressure is a global issue. Resistance to post-emergence herbicides is widespread, whereas soil-applied pre-emergence herbicides can often remain effective. For example, in Australia pyroxasulfone is a new pre-emergence soil-applied herbicide which provides control of Lolium rigidum populations resistant to multiple post-emergence herbicide modes of action. A fundamental knowledge of the genetic basis of metabolic resistance in weeds is important for understanding plant evolution pathways under herbicide selection and sustaining long-term weed resistance management. In this study we define the mechanistic basis of resistance to pyroxasulfone in a L. rigidum population. TLC provides evidence that pyroxasulfone resistance is metabolism-based with approximately 88% of parental [ 14 C]-labelled pyroxasulfone metabolized in resistant plants 24 h after the herbicide treatment. HPLC-MS allowed identification of several metabolites of pyroxasulfone formed via a glutathione (GSH) conjugation pathway in pyroxasulfone-resistant L. rigidum plants . However, the initial pyroxasulfone-glutathione conjugate was not found likely due to its labile nature. The observed constitutive over-expression from six to nine-fold of two putative resistance-endowing GST genes was associated with the pyroxasulfone resistance phenotype. The most logical conclusion, based on the data thus far available, is that rapid detoxification of pyroxasulfone mediates pyroxasulfone resistance in L. rigidum plants. Future research is warranted to confirm the hypothesis advanced by this study of rapid pyroxasulfone metabolism due to GSH conjugation mediated by GST over-expressed in pyroxasulfone-resistant plants which similarly leads to the production of distinctive GSH-pyroxasulfone metabolites in L. rigidum and wheat. [ABSTRACT FROM AUTHOR]

BACKGROUND Organophosphate insecticides can inhibit specific cytochrome P450 enzymes involved in metabolic herbicide resistance mechanisms, leading to synergistic interactions between the insecticide and the herbicide. In this study we report synergistic versus antagonistic interactions between the organophosphate insecticide phorate and five different herbicides observed in a population of multiple herbicide-resistant Lolium rigidum. RESULTS Phorate synergised with three different herbicide modes of action, enhancing the activity of the ALS inhibitor chlorsulfuron (60% LD50 reduction), the VLCFAE inhibitor pyroxasulfone (45% LD50 reduction) and the mitosis inhibitor trifluralin (70% LD50 reduction). Conversely, phorate antagonised the two thiocarbamate herbicides prosulfocarb and triallate with a 12-fold LD50 increase. CONCLUSION We report the selective reversal of P450-mediated metabolic multiple resistance to chlorsulfuron and trifluralin in the grass weed L. rigidum by synergistic interaction with the insecticide phorate, and discuss the putative mechanistic basis. This research should encourage diversity in herbicide use patterns for weed control as part of a long-term integrated management effort to reduce the risk of selection of metabolism-based multiple herbicide resistance in L. rigidum. © 2016 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

BACKGROUND Weeds can be a greater constraint to crop production than animal pests and pathogens. Pre-emergence herbicides are crucial in many cropping systems to control weeds that have evolved resistance to selective post-emergence herbicides. In this study we assessed the potential to evolve resistance to the pre-emergence herbicides prosulfocarb + S-metolachlor or pyroxasulfone in 50 individual field Lolium rigidum populations collected in a random survey in Western Australia prior to commercialisation of these pre-emergence herbicides. RESULTS This study shows for the first time that in randomly collected L. rigidum field populations the selection with either prosulfocarb + S-metolachlor or pyroxasulfone can result in concomitant evolution of resistance to both prosulfocarb + S-metolachlor and pyroxasulfone after three generations. CONCLUSIONS In the major weed L. rigidum, traits conferring resistance to new herbicides can be present before herbicide commercialisation. Proactive and multidisciplinary research (evolutionary ecology, modelling and molecular biology) is required to detect and analyse resistant populations before they can appear in the field. Several studies show that evolved cross-resistance in weeds is complex and often unpredictable. Thus, long-term management of cross-resistant weeds must be achieved through heterogeneity of selection by effective chemical, cultural and physical weed control strategies that can delay herbicide resistance evolution. © 2016 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

Weed control failures due to herbicide resistance are an increasing and worldwide problem that significantly affect crop yields. Metabolism-based herbicide resistance (referred to as metabolic resistance) in weeds is not well characterized at the genetic level. An RNA-Seq transcriptome analysis was used to find candidate genes that conferred metabolic resistance to the herbicide diclofop in a diclofop-resistant population (R) of the major global weed Lolium rigidum. A reference c DNA transcriptome (19 623 contigs) was assembled and assigned putative annotations. Global gene expression was measured using Illumina reads from untreated control, adjuvant-only control, and diclofop treatment of R and susceptible (S). Contigs that showed constitutive expression differences between untreated R and untreated S were selected for further validation analysis, including 11 contigs putatively annotated as cytochrome P450 (CytP450), glutathione transferase ( GST), or glucosyltransferase ( GT), and 17 additional contigs with annotations related to metabolism or signal transduction. In a forward genetics validation experiment, nine contigs had constitutive up-regulation in R individuals from a segregating F2 population, including three CytP450, one nitronate monooxygenase ( NMO), three GST, and one GT. Principal component analysis using these nine contigs differentiated F2-R from F2-S individuals. In a physiological validation experiment in which 2,4-D pre-treatment induced diclofop protection in S individuals due to increased metabolism, seven of the nine genetically validated contigs were induced significantly. Four contigs (two CytP450, NMO, and GT) were consistently highly expressed in nine field-evolved metabolic resistant L. rigidum populations. These four contigs were strongly associated with the resistance phenotype and are major candidates for contributing to metabolic diclofop resistance. [ABSTRACT FROM AUTHOR]

Background Plants can rapidly evolve resistance to herbicide in response to repeated selection. This study focuses on cross-resistance patterns observed in Lolium rigidum following pyroxasulfone recurrent selection. Results The parental MR (multiresistant) population following four generations of pyroxasulfone recurrent selection evolved cross-resistance to prosulfocarb and triallate. At the recommended label rate of prosulfocarb or triallate (2000 g ha−1), the progeny selected four times with pyroxasulfone ( MR4) displayed 58 and 35% plant survival respectively. One additional cycle of prosulfocarb selection increased the resistance level to both prosulfocarb and triallate in the population MR4-P1. Prosulfocarb resistance is yet to be reported in L. rigidum field populations. Conclusions This study suggests that L. rigidum plants can rapidly evolve cross-resistance to several wheat-selective herbicides under recurrent selection of a single mode of action. Weed populations displaying broad-spectrum cross-resistance to several herbicide modes of action are increasing in frequency in intensive world agriculture. Proactive and integrated measures for resistance management need to be developed globally on appropriate herbicide use in crop rotations. © 2013 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

The interaction between environment and genetic traits under selection is the basis of evolution. In this study, we have investigated the genetic basis of herbicide resistance in a highly characterized initially herbicide-susceptible Lolium rigidum population recurrently selected with low (below recommended label) doses of the herbicide diclofop-methyl. We report the variability in herbicide resistance levels observed in F1 families and the segregation of resistance observed in F2 and back-cross (BC) families. The selected herbicide resistance phenotypic trait(s) appear to be under complex polygenic control. The estimation of the effective minimum number of genes (NE), depending on the herbicide dose used, reveals at least three resistance genes had been enriched. A joint scaling test indicates that an additive-dominance model best explains gene interactions in parental, F1, F2 and BC families. The Mendelian study of six F2 and two BC segregating families confirmed involvement of more than one resistance gene. Cross-pollinated L. rigidum under selection at low herbicide dose can rapidly evolve polygenic broad-spectrum herbicide resistance by quantitative accumulation of additive genes of small effect. This can be minimized by using herbicides at the recommended dose which causes high mortality acting outside the normal range of phenotypic variation for herbicide susceptibility. [ABSTRACT FROM AUTHOR]

Abstract: The potential for low rates of diclofop-methyl to result in rapid evolution of herbicide resistance in a herbicide-susceptible Lolium rigidum (annual ryegrass) population was demonstrated in a recent crop-field study. In this present study, the data from the crop-field study was used together with simulation modelling to identify possible genetics of the herbicide resistance that was selected for. This analysis clearly indicated that the herbicide resistance was polygenic. Subsequently, the estimated genetic possibilities were used to parameterise a model of herbicide resistance evolution in a simulated crop-field situation, and the potential of different rates of diclofop-methyl (ACCase herbicide) to cause herbicide-resistance evolution in L. rigidum was explored and compared using the calibrated model. The calibrated model outputs indicated that the evolution of diclofop-methyl resistance would generally be faster at low herbicide rates than at higher rates due to the rapid selection of minor gene herbicide resistance traits at low rates and their subsequent recombination by cross-pollination. The results of the study therefore indicate potential risks in herbicide rate cutting and highlight the need for careful scientific evaluation of any herbicide use rate for its potential to select for minor gene herbicide resistance from a weed population. [Copyright &y& Elsevier]