Characterization of multiple-herbicide-resistant Italian ryegrass (Lolium perenne spp. multiflorum)

Redox status upon herbicides application in the control of Lolium multiflorum (2n and 4n) as weed

Lolium multiflorum Lam. is a winter weed of difficult control found as diploid (2n) and tetraploid plants (4n). Our study aimed to evaluate the responses of antioxidant enzymes and lipid peroxidation, in both diploid and tetraploid ryegrass varieties. Treatments consisted of control plants (without any herbicide application), and four herbicides with different mechanisms of action. Leaf material was collected 36 h after treatment imposition to determine the lipid peroxidation by ferrous oxidation-xylenol (FOX) content, and the activity of the enzymes superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), guaiacol peroxidase (GPX), glutathione-S-transferase (GST), and δ-aminolevulinic acid dehydratase (ALAD). Both ryegrass varieties showed oxidative stress mainly due to a downregulated decreased (>31%) in SOD activity and an increase (>32%) in lipid peroxidation (FOX), mainly in ryegrass genotypes exposed to haloxyfop, glyphosate, and iodosulfuron. On the other hand, clethodim-treated plants had an increase in SOD and APX activities, associated with a reduced ALAD activity in both 2n (32%) and 4n (11%) genotypes. In general, the 2n genotype was more affected than the 4n genotype.

Cover crops, hormones and herbicides: Priming an integrated weed management strategy

Herbicide weed resistance has been a major issue of conventional global row crop agriculture for decades. Still current strategies and novel technologies available to address weed resistance are mainly herbicide-based. Thus, there is a need for innovative means of integrated weed management strategies. Our approach proposed herein integrates cover crops, plant hormones and pre-emergence (PRE) herbicides as part of weed management programs. Plant hormones such as gibberellic acid (GA₃) and abscisic acid (ABA) have the potential to induce seed germination and seed dormancy, respectively. Prior to crop emergence, plant hormones are tank mixed with PRE herbicides and sprayed to cover crop residue. Two strategies are proposed (1) PRE herbicides + GA₃ and (2) PRE herbicide + ABA. The hormones provide different results; GA₃ is likely to stimulate a more uniform weed seed germination, thus enhancing efficacy of PRE herbicides. Conversely, ABA could promote weed seed dormancy, reducing selection pressure and weed infestations until crop canopy closure. Much research is needed to understand the impact of hormones on weed and crop species, optimize products and rates, and compatibility of hormones with herbicides and cover crops. If successful, this approach could open a new opportunity for agricultural business, enhance farming sustainability by reducing dependence on herbicides and minimizing agronomic, economic and environmental issues related to weed resistance.

Accumulation of Target Gene Mutations Confers Multiple Resistance to ALS, ACCase, and EPSPS Inhibitors in Lolium Species in Chile

Different Lolium species, common weeds in cereal fields and fruit orchards in Chile, were reported showing isolated resistance to the acetyl CoA carboxylase (ACCase), acetolactate synthase (ALS) and 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) inhibiting herbicides in the late 1990s. The first case of multiple resistance to these herbicides was Lolium multiflorum found in spring barley in 2007. We hypothesized that other Lolium species may have evolved multiple resistance. In this study, we characterized the multiple resistance to glyphosate, diclofop-methyl and iodosulfuron-methyl-sodium in Lolium rigidum, Lolium perenne and Lolium multiflorum resistant (R) populations from Chile collected in cereal fields. Lolium spp. populations were confirmed by AFLP analysis to be L. rigidum, L. perenne and L. multiflorum. Dose-response assays confirmed multiple resistance to glyphosate, diclofop-methyl and iodosulfuron methyl-sodium in the three species. Enzyme activity assays (ACCase, ALS and EPSPS) suggested that the multiple resistance of the three Lolium spp. was caused by target site mechanisms, except the resistance to iodosulfuron in the R L. perenne population. The target site genes sequencing revealed that the R L. multiflorum population presented the Pro-106-Ser/Ala (EPSPS), Ile-2041-Asn++Asp-2078-Gly (ACCase), and Trp-574-Leu (ALS) mutations; and the R L. rigidum population had the Pro-106-Ser (EPSPS), Ile-1781-Leu+Asp-2078-Gly (ACCase) and Pro-197-Ser/Gln+Trp-574-Leu (ALS) mutations. Alternatively, the R L. perenne population showed only the Asp-2078-Gly (ACCase) mutation, while glyphosate resistance could be due to EPSPS gene amplification (no mutations but high basal enzyme activity), whereas iodosulfuron resistance presumably could involve non-target site resistance (NTSR) mechanisms. These results support that the accumulation of target site mutations confers multiple resistance to the ACCase, ALS and EPSPS inhibitors in L. multiflorum and L. rigidum from Chile, while in L. perenne, both target and NTSR could be present. Multiple resistance to three herbicide groups in three different species of the genus Lolium in South America represents a significant management challenge.

instaGraminoid, a Novel Colorimetric Method to Assess Herbicide Resistance, Identifies Patterns of Cross-Resistance in Annual Ryegrass

Herbicide resistance in agricultural weeds is a global problem with an increasing understanding that it is caused by multiple genes leading to quantitative resistance. These quantitative patterns of resistance are not easy to decipher with mortality assays alone, and there is a need for straightforward and unbiased protocols to accurately assess quantitative herbicide resistance. instaGraminoid-a computer vision and statistical analysis package-was developed as an automated and scalable method for quantifying herbicide resistance. The package was tested in rigid ryegrass (Lolium rigidum), the most noxious and highly resistant weed in Australia and the Mediterranean region. This method provides quantitative measures of the degree of chlorosis and necrosis of individual plants which was shown to accurately reflect herbicide resistance. We were able to reliably characterise resistance to four herbicides with different sites of action (glyphosate, sulfometuron, terbuthylazine, and trifluralin) in two L. rigidum populations from Southeast Australia. Cross-validation of the method across populations and herbicide treatments showed high repeatability and transferability. Significant positive correlations in resistance of individual plants were observed across herbicides, which suggest either the accumulation of herbicide-specific resistance alleles in single genotypes (multiple stacked resistance) or the presence of general broad-effects resistance alleles (cross-resistance). We used these quantitative estimates of cross-resistance to simulate how resistance development under an herbicide rotation strategy is likely to be higher than expected.

Herbicide-related signaling in plants reveals novel insights for herbicide use strategies, environmental risk assessment and global change assessment challenges

Herbicide impact is usually assessed as the result of a unilinear mode of action on a specific biochemical target with a typical dose-response dynamics. Recent developments in plant molecular signaling and crosstalk between nutritional, hormonal and environmental stress cues are however revealing a more complex picture of inclusive toxicity. Herbicides induce large-scale metabolic and gene-expression effects that go far beyond the expected consequences of unilinear herbicide-target-damage mechanisms. Moreover, groundbreaking studies have revealed that herbicide action and responses strongly interact with hormone signaling pathways, with numerous regulatory protein-kinases and -phosphatases, with metabolic and circadian clock regulators and with oxidative stress signaling pathways. These interactions are likely to result in mechanisms of adjustment that can determine the level of sensitivity or tolerance to a given herbicide or to a mixture of herbicides depending on the environmental and developmental status of the plant. Such regulations can be described as rheostatic and their importance is discussed in relation with herbicide use strategies, environmental risk assessment and global change assessment challenges.

Multiple Herbicide Resistance in Lolium multiflorum and Identification of Conserved Regulatory Elements of Herbicide Resistance Genes

Herbicide resistance is a ubiquitous challenge to herbicide sustainability and a looming threat to control weeds in crops. Recently four genes were found constituently over-expressed in herbicide resistant individuals of Lolium rigidum, a close relative of Lolium multiflorum. These include two cytochrome P450s, one nitronate monooxygenase and one glycosyl-transferase. Higher expressions of these four herbicide metabolism related (HMR) genes were also observed after herbicides exposure in the gene expression databases, indicating them as reliable markers. In order to get an overview of herbicidal resistance status of L. multiflorum L, 19 field populations were collected. Among these populations, four populations were found to be resistant to acetolactate synthase (ALS) inhibitors while three exhibited resistance to acetyl-CoA carboxylase (ACCase) inhibitors in our initial screening and dose response study. The genotyping showed the presence of mutations Trp-574-Leu and Ile-2041-Asn in ALS and ACCase, respectively, and qPCR experiments revealed the enhanced expression of HMR genes in individuals of certain resistant populations. Moreover, co-expression networks and promoter analyses of HMR genes in O. sativa and A. thaliana resulted in the identification of a cis-regulatory motif and zinc finger transcription factors. The identified transcription factors were highly expressed similar to HMR genes in response to xenobiotics whereas the identified motif is known to play a vital role in coping with environmental stresses and maintaining genome stability. Overall, our findings provide an important step forward toward a better understanding of metabolism-based herbicide resistance that can be utilized to devise novel strategies of weed management.

A new amino acid substitution (Ala-205-Phe) in acetolactate synthase (ALS) confers broad spectrum resistance to ALS-inhibiting herbicides

This is a first report of an Ala-205-Phe substitution in acetolactate synthase conferring resistance to imidazolinone, sulfonylurea, triazolopyrimidines, sulfonylamino-carbonyl-triazolinones, and pyrimidinyl (thio) benzoate herbicides. Resistance to acetolactate synthase (ALS) and photosystem II inhibiting herbicides was confirmed in a population of allotetraploid annual bluegrass (Poa annua L.; POAAN-R3) selected from golf course turf in Tennessee. Genetic sequencing revealed that seven of eight POAAN-R3 plants had a point mutation in the psbA gene resulting in a known Ser-264-Gly substitution on the D1 protein. Whole plant testing confirmed that this substitution conferred resistance to simazine in POAAN-R3. Two homeologous forms of the ALS gene (ALSa and ALSb) were detected and expressed in all POAAN-R3 plants sequenced. The seven plants possessing the Ser-264-Gly mutation conferring resistance to simazine also had a homozygous Ala-205-Phe substitution on ALSb, caused by two nucleic acid substitutions in one codon. In vitro ALS activity assays with recombinant protein and whole plant testing confirmed that this Ala-205-Phe substitution conferred resistance to imidazolinone, sulfonylurea, triazolopyrimidines, sulfonylamino-carbonyl- triazolinones, and pyrimidinyl (thio) benzoate herbicides. This is the first report of Ala-205-Phe mutation conferring wide spectrum resistance to ALS inhibiting herbicides.

Genome-Wide Transcriptional Profiling and Metabolic Analysis Uncover Multiple Molecular Responses of the Grass Species Lolium perenne Under Low-Intensity Xenobiotic Stress

Lolium perenne, which is a major component of pastures, lawns, and grass strips, can be exposed to xenobiotic stresses due to diffuse and residual contaminations of soil. L. perenne was recently shown to undergo metabolic adjustments in response to sub-toxic levels of xenobiotics. To gain insight in such chemical stress responses, a de novo transcriptome analysis was carried out on leaves from plants subjected at the root level to low levels of xenobiotics, glyphosate, tebuconazole, and a combination of the two, leading to no adverse physiological effect. Chemical treatments influenced significantly the relative proportions of functional categories and of transcripts related to carbohydrate processes, to signaling, to protein-kinase cascades, such as Serine/Threonine-protein kinases, to transcriptional regulations, to responses to abiotic or biotic stimuli and to responses to phytohormones. Transcriptomics-based expressions of genes encoding different types of SNF1 (sucrose non-fermenting 1)-related kinases involved in sugar and stress signaling or encoding key metabolic enzymes were in line with specific qRT-PCR analysis or with the important metabolic and regulatory changes revealed by metabolomic analysis. The effects of pesticide treatments on metabolites and gene expression strongly suggest that pesticides at low levels, as single molecule or as mixture, affect cell signaling and functioning even in the absence of major physiological impact. This global analysis of L. perenne therefore highlighted the interactions between molecular regulation of responses to xenobiotics, and also carbohydrate dynamics, energy dysfunction, phytohormones and calcium signaling.