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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]

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]

A relatively low number of weed species have evolved resistance to auxinic herbicides despite their use for almost 70 years. This inheritance study with two Raphanus raphanistrum populations multiple-resistant 2,4-D and the ALS-inhibiting herbicide chlorsulfuron determined the number of genes and genetic dominance of 2,4-D resistance and investigated the association between traits conferring resistance to the two herbicide modes of action. Levels of 2,4-D phenotypic resistance and resistance segregation patterns were assessed in parental populations, F 1 and F 2 families. [ABSTRACT FROM AUTHOR]

This article is a Commentary on Comont et al.,223: 1584–1594. [ABSTRACT FROM AUTHOR]

Highlights: [•] Semi-dominant alleles segregate at one major locus for pyroxasulfone resistance. [•] Cross-resistance is also conferred by major locus(i). [•] A combination of herbicide options can reduce pyroxasulfone selection pressure. [•] This study will enable more accurate predictions by computational modelling. [•] Inheritance studies of novel herbicides can help sustain their longevity. [ABSTRACT FROM AUTHOR]

Synthetic herbicides have been used globally to control weeds in major field crops. This has imposed a strong selection for any trait that enables plant populations to survive and reproduce in the presence of the herbicide. Herbicide resistance in weeds must be minimized because it is a major limiting factor to food security in global agriculture. This represents a huge challenge that will require great research efforts to develop control strategies as alternatives to the dominant and almost exclusive practice of weed control by herbicides. Weed scientists, plant ecologists and evolutionary biologists should join forces and work towards an improved and more integrated understanding of resistance across all scales. This approach will likely facilitate the design of innovative solutions to the global herbicide resistance challenge. [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: In two different locations of the Western Australian “wheatbelt”, Lolium rigidum (rigid ryegrass) seeds were collected from organic fields (no herbicide use) and neighbouring conventional fields (persistent herbicide use), the latter infested with herbicide-resistant plants, to investigate the occurrence of gene flow among field populations as revealed by herbicide resistance gene transfer. Herbicides targeting acetyl-CoA carboxylase (ACCase) or acetolactate-synthase (ALS) were used to detect herbicide-resistant plants. Overall, the frequency of plants resistant to ACCase- or ALS-inhibiting herbicides was, respectively, 21% and 74% in the conventional fields and 2% and 37% in neighbouring organic fields. Mutant, herbicide-resistant ACCase and ALS alleles were detected in 16% and 38% of plants from conventional fields and in 0.53% and 3.7% of plants from organic fields. Identical mutant, herbicide-resistant ALS haplotypes were detected both in conventional and organic fields, supporting the occurrence of gene flow between L. rigidum populations in different fields. Gene flow can thus substantially increase the frequency of herbicide-resistant plants in unselected L. rigidum populations. Although gene flow cannot be prevented, it can be limited or managed. Hygiene tactics such as clean crop seed, weed seed removal at harvest and seed destruction post-harvest should be considered in order to minimize gene transfer among farms. [Copyright &y& Elsevier]

Gene flow promotes genetic exchange among plant populations mediating evolutionary dynamics; yet, the importance of gene flow at distance via pollen movement is poorly understood. A field experiment at the landscape level was conducted with Lolium rigidum herbicide-susceptible individuals (population VLR1) placed into an otherwise Lolium-free bushland environment at increasing distances from adjacent large commercial crop fields infested with herbicide-resistant L. rigidum. Herbicide resistance was used as a marker to quantify the distance and the rate of pollen-mediated gene flow. About 21,245 seeds were produced on the isolated, susceptible mother plants of which 3,303 seedlings were tested for herbicide resistance and 664 seedlings were found to be resistant. Pollen-mediated gene flow occurred at 3,000 m (maximum tested distance). Both Mendelian and molecular analyses (sequencing and CAPS markers) confirmed the introgression of herbicide resistance genes. This is the first documented case of long-distance gene flow in L. rigidum. The results are important for future modeling simulations of herbicide resistance evolution and subsequent mobility. The adoption of integrated agronomic strategies, the control of potential receptor plants on fields’ margins and conservative use of herbicides can be realistic options to minimize herbicide resistance spread. [ABSTRACT FROM AUTHOR]

• Herbicide resistance gene flow in weeds is under-estimated and under-appreciated. • The consequences are increasing occurrence of multiple-resistant weed populations. and dwindling efficacy of remaining herbicide tools. • The best mitigation strategy is weed seed bank management. Interest in the magnitude and consequences of intra- and inter-specific herbicide resistance gene flow, particularly that mediated by pollen, increased in the mid-1990s with the introduction of herbicide-resistant (HR) transgenic crops. During that time, less attention was paid to the movement of HR alleles via pollen or seed among weed populations. Incidence of HR weeds in a region is often attributed to independent evolution through herbicide selection; the role or contribution of HR allele movement via pollen, seeds and/or vegetative propagules is often under-estimated and under-appreciated. Once a new HR weed biotype has been confirmed in a jurisdiction, how often have we been surprised at its rapid areawide expansion? In genotypic studies of HR weed populations, the contribution of gene flow to incidence of resistance is frequently similar or greater than that of independent evolution. Simulation models have consistently predicted that frequent widespread applications of highly effective herbicides (e.g., acetolactate synthase inhibitors, glyphosate) provide connective high-fitness habitats across the landscape, which facilitate a rapid increase in the frequency and movement of an HR trait within and among populations. Such habitats, characterized by minimal heterogeneity but high selection intensity due to frequent herbicide 'on' vs. 'off' exposure periods, strongly favour the fitness of HR propagules at great disadvantage of herbicide-susceptible individuals. The unanticipated speed of areawide expansion of some HR weed biotypes has spurred numerous calls over the past decade for a collective community or regional response to mitigate this unhindered spread of HR alleles. The best mitigation strategy is minimizing weed population abundance and seed bank replenishment in fields and adjacent ruderal areas. This goal is difficult, but necessary for preserving the remaining public good and common pool resource of herbicide susceptibility. [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]