Glyphosate Resistance of C3 and C4 Weeds under Rising Atmospheric CO2

Enhanced β-glucosidase in Western flower thrips affects its interaction with the redox-based strategies of kidney beans under elevated CO2

β-Glucosidase is validated as an elicitor for early immune responses in plants and it was detected in the salivary glands of Frankliniella occidentalis in previous research. Seven differentially expressed genes encoding β-glucosidase were obtained by comparing the transcriptomes of F. occidentalis adults grown under two different CO₂ concentrations (800 vs. 400 ppm), which might be associated with the differences in the interaction between F. occidentalis adults and its host plant, Phaseolus vulgaris under different CO₂ levels. To verify this speculation, changes in defense responses based on the production and elimination of reactive oxygen species (ROS) in P. vulgaris leaves treated with three levels of β-glucosidase activity under ambient CO₂ (aCO₂ ) and elevated CO₂ (eCO₂ ) were measured in this study. According to the results, significantly higher levels of ROS were noticed under eCO₂ compared to aCO₂ , which was caused by the increased β-glucosidase activity in thrips due to increased cellulose content in P. vulgaris leaves under eCO₂ . Together with the lower activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) in injured leaves under eCO₂ , P. vulgaris leaves would be negatively affected on redox-based defense by eCO₂ , thus facilitating thrips damage under climate change.

Deciphering the Structure of Arabidopsis thaliana 5-enol-Pyruvyl-Shikimate-3-Phosphate Synthase: an Essential Step toward the Discovery of Novel Inhibitors to Supersede Glyphosate

Glyphosate interferes with plant aromatic metabolism through the inhibition of 5-enol-pyruvyl-shikimate-3-phosphate (EPSP) synthase [EPSPS, EC 2.5.1.19]. For this reason, EPSPS has been extensively studied in a vast array of organisms. This notwithstanding, up to date, the crystal structure of the protein has been solved exclusively in a few prokaryotes, while that of the plant enzyme has been only deduced in silico by similarity. This study aimed at determining the structure of EPSPS from the plant model species Arabidopsis thaliana, which has been cloned, heterologously expressed and affinity-purified. The kinetic properties of the enzyme have been determined, as well as its susceptibility to the inhibition brought about by glyphosate. The crystal structure of the protein has been resolved at high resolution (1.4 Å), showing open conformation of the enzyme, which is the state ready for substrate/inhibitor binding. This provides a framework for the structure-based design of novel EPSPS inhibitors. Surface regions near the active-site cleft entrance or at the interdomain hinge appear promising for inhibitor selectivity, while bound chloride near the active site is a potential placeholder for anionic moieties of future herbicides.

Elevated CO2 aggravates invasive thrip damage by altering its host plant nutrient and secondary metabolism

As atmospheric CO₂ concentration continues to increase, plants using CO₂ as raw materials for photosynthesis will inevitably be affected, which in turn affects the life history and behavior of herbivorous insects. Our previous research has shown increased food intake and aggravated damage of western flower thrips, Frankliniella occidentalis to kidney bean (Phaseolus vulgaris) caused by elevated CO₂ (eCO₂), however the molecular mechanism of this phenomenon is unclear. In this study, the comparative transcriptome analysis combined with corresponding phenotypic changes were studied to reveal the molecular mechanism of interaction between F. occidentalis and P. vulgaris under eCO₂. Inferred from the results, eCO₂ had different degrees of inhibition to the defense responses caused by thrips infestation in P. vulgaris leaf sap based on nutrients, plant hormones and secondary metabolites, making P. vulgaris leaves less resistant to thrips under eCO₂ compared to ambient CO₂ (aCO₂). Besides, the contents of glucose, trehalose, triglycerides and free fatty acids in F. occidentalis adults increased significantly after feeding on the P. vulgaris leaf sap with significantly increased soluble sugars content under eCO₂, which might lead to glucolipid metabolic disorders and increased food intake of F. occidentalis adults. The results indicated that decreased plant defense of P. vulgaris and increased food intake of F. occidentalis adults were combined to aggravate the thrips damage under eCO₂, providing a theoretical basis for future occurrence trend of thrips under eCO₂.

Tolerance of Hymenaea courbaril L. to glyphosate

The objective was to evaluate the effect of the glyphosate on Hymenaea courbaril L. A randomized block design with five replications was implemented. Each experimental unit was composed of one plant in a 5 L container. The treatments were 0 "control"; 96; 240; 480; and 960 g ha-1 "corresponding to 10, 25, 50, and 100% of the commercial dose of glyphosate recommended for Caryocar brasiliense crop, respectively". The evaluations were performed at 24 h and 60 days after application. Visual and anatomical evaluations did not change regardless of the dose, while the histochemical evaluation showed an accumulation of starch grains in leaf tissues. There was an increase in the photosynthetic rate, in the electron transport rate, and in the effective quantum yield of photosystem II at 24 h after application. At 60 days after the application of the treatments, the photosynthetic rate showed a slight decrease and the transpiratory rate showed quadratic behavior. An increase in plant height was observed up to the dose of 480 g ha-1, a linear increase in stem diameter and a decrease in the number of leaves with increasing glyphosate doses. These results show that the cuticle protected the plant, and that the little absorbed glyphosate increased photosynthesis and transpiration to favor the plants. We can conclude that the H. courbaril species is able to survive after contact with glyphosate during the evaluated time, with no visual and/or anatomical damage, showing increases in growth and physiological characteristics for the tested doses.

Glyphosate: Uses Other Than in Glyphosate-Resistant Crops, Mode of Action, Degradation in Plants, and Effects on Non-target Plants and Agricultural Microbes

Glyphosate is the most used herbicide globally. It is a unique non-selective herbicide with a mode of action that is ideal for vegetation management in both agricultural and non-agricultural settings. Its use was more than doubled by the introduction of transgenic, glyphosate-resistant (GR) crops. All of its phytotoxic effects are the result of inhibition of only 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), but inhibition of this single enzyme of the shikimate pathway results in multiple phytotoxicity effects, both upstream and downstream from EPSPS, including loss of plant defenses against pathogens. Degradation of glyphosate in plants and microbes is predominantly by a glyphosate oxidoreductase to produce aminomethylphosphonic acid and glyoxylate and to a lesser extent by a C-P lyase to produce sarcosine and phosphate. Its effects on non-target plant species are generally less than that of many other herbicides, as it is not volatile and is generally sprayed in larger droplet sizes with a relatively low propensity to drift and is inactivated by tight binding to most soils. Some microbes, including fungal plant pathogens, have glyphosate-sensitive EPSPS. Thus, glyphosate can benefit GR crops by its activity on some plant pathogens. On the other hand, glyphosate can adversely affect some microbes that are beneficial to agriculture, such as Bradyrhizobium species, although GR crop yield data indicate that such an effect has been minor. Effects of glyphosate on microbes of agricultural soils are generally minor and transient, with other agricultural practices having much stronger effects.

Implications of elevated carbon dioxide on the susceptibility of the globally invasive weed, Parthenium hysterophorus, to glyphosate herbicide

BACKGROUND

The noxious annual herb, Parthenium hysterophorus L. (Asteraceae), is an invasive weed of global significance, threatening food security, biodiversity and human health. In South Africa, chemical control is frequently used to manage P. hysterophorus, however, concern surrounds increasing atmospheric CO2 levels, which may reduce the efficacy of glyphosate against the weed. Therefore, this study aimed to determine the susceptibility of P. hysterophorus to glyphosate (1L/ha: recommended) after being grown for five generations in Convirons under ambient (400 ppm) and elevated (600 and 800 ppm) CO2 .

RESULTS

Glyphosate efficacy decreased with increasing CO2 , with mortalities of 100, 83 and 75% recorded at 400, 600 and 800 ppm, respectively. Parthenium hysterophorus experienced enhanced growth and reproduction under elevated CO2, however, glyphosate application was highly damaging, reducing the growth and flowering of plants across all CO2 treatments. Physiologically, glyphosate-treated plants, in all CO2 treatments, suffered severe declines of >90% in chlorophyll content, maximum quantum efficiency (F v /Fm ), photon absorption (ABS/RC), electron transport (ET 0 /RC) and performance index (PI ABS ), albeit at slower rates for plants grown under elevated CO2 . Low levels of recovery from glyphosate were documented only for plants grown under elevated CO2 and was attributed to their increased biomass.

CONCLUSION

These results suggest that increasing CO2 levels may hinder chemical control efforts used against P. hysterophorus in the future, advocating for further investigation using multigenerational CO2 studies and the maintenance of effective spraying programs at present. © 2020 Society of Chemical Industry.

Response of Chloris truncata to moisture stress, elevated carbon dioxide and herbicide application

Herbicide resistance has been observed in Chloris truncata, an Australian native C4 grass and a summer-fallow weed, which is common in no-till agriculture situations where herbicides are involved in crop management. To investigate the role of drought and increased atmospheric carbon dioxide (CO2) in determining weed growth, three trials were conducted using a 'glyphosate-resistant' and a 'glyphosate-susceptible' biotype. The first two trials tested the effect of herbicide (glyphosate) application on plant survival and growth under moisture stress and elevated CO2 respectively. A third trial investigated the effect on plant growth and reproduction under conditions of moisture stress and elevated CO2 in the absence of herbicide. In the first trial, water was withheld from half of the plants prior to application of glyphosate to all plants, and in the second trial plants were grown in either ambient (450 ppm) or elevated CO2 levels (750 ppm) prior to, and following, herbicide application. In both biotypes, herbicide effectiveness was reduced when plants were subjected to moisture stress or if grown in elevated CO2. Plant productivity, as measured by dry biomass per plant, was reduced with moisture stress, but increased with elevated CO2. In the third trial, growth rate, biomass and seed production were higher in the susceptible biotype compared to the resistant biotype. This suggests that a superior ability to resist herbicides may come at a cost to overall plant fitness. The results indicate that control of this weed may become difficult in the future as climatic conditions change.

Efficient In Vivo Screening Method for the Identification of C4 Photosynthesis Inhibitors Based on Cell Suspensions of the Single-Cell C4 Plant Bienertia sinuspersici

The identification of novel herbicides is of crucial importance to modern agriculture. We developed an efficient in vivo assay based on oxygen evolution measurements using suspensions of chlorenchyma cells isolated from the single-cell C₄ plant Bienertia sinuspersici to identify and characterize inhibitors of C₄ photosynthesis. This novel approach fills the gap between conventional in vitro assays for inhibitors targeting C₄ key enzymes and in vivo experiments on whole plants. The assay addresses inhibition of the target enzymes in a plant context thereby taking care of any reduced target inhibition due to metabolization or inadequate uptake of small molecule inhibitors across plant cell walls and membranes. Known small molecule inhibitors targeting C₄ photosynthesis were used to validate the approach. To this end, we tested pyruvate phosphate dikinase inhibitor bisindolylmaleimide IV and phosphoenolpyruvate carboxylase inhibitor okanin. Both inhibitors show inhibition of plant photosynthesis at half-maximal inhibitory concentrations in the sub-mM range and confirm their potential to act as a new class of C₄ selective inhibitors.

Imaging of glyphosate uptake and identification of early microscopic markers in leaves of C3 and C4 glyphosate-resistant and -susceptible species

Details of glyphosate uptake are not fully elucidated, and although this herbicide promotes important alterations in the plant phenotype few hours after its application (early responses), a detailed description of the presumable changes in plant anatomy is still poorly assessed by now. Due to glyphosate effects over leaf permeability, the use of an inert fluorescent tracer may allow the observation of the uptake event in situ. In addition, microscopic markers might put a light on the recognition of glyphosate-resistant (GR) and -susceptible (GS) species or varieties, which may vary in C3 and C4 species due to their putative distinct leaf anatomy. Here we aimed (i) to provide a new technique to track the route of glyphosate formulation towards leaf tissues using a fluorescent tracer, and (ii) to describe the early specific microscopic alterations in GR and GS -C3 or -C4 caused by the glyphosate formulation. Roundup Transorb® was applied in seedlings cultivated in a greenhouse and response alterations in leaf anatomy were described. Lucifer Yellow CH (LYCH) was applied over the same region where glyphosate formulation was previously applied to track the alterations in leaf permeability caused by this herbicide. LYCH successfully tracked the glyphosate formulation uptake, reaching the vascular bundles of GS species, and becoming retained in leaf tissues of GR species. All species exhibited a decrease in chlorophyll content at the site of glyphosate application regardless of their photosynthetic metabolism or susceptibility. GS species showed alterations in chloroplast morphology and activity of non-enzymatic antioxidants (carotenoids and flavonoids), in addition to symptoms indicating a process of accelerated cell senescence. A specific type of cell necrosis (hypersensitive response) was observed in GR-C4 species as a way to prevent the translocation of this herbicide, while GR-C3 species accumulated phenolic compounds inside the vacuole, probably sequestrating and inactivating the glyphosate action. This study provides a reliable tool to track glyphosate formulation uptake in situ and is the first attempt to the identification of early specific microscopic markers caused by glyphosate formulation.

Herbicide effectiveness in controlling invasive plants under elevated CO2: Sufficient evidence to rethink weeds management

Previous studies have reported that chemical weed control will be less effective for some weed species under future atmospheric CO₂ concentrations. Such reductions in plant sensitivity to herbicides under elevated CO₂ may be due to greater biomass accumulation and differences among growth types. However, these studies have been limited to few growth types (herbaceous and grass species) and to a single herbicide (glyphosate). This study tested a more extensive range of weed species (both in number and growth form) and herbicides to assess general patterns of plant response. We grew 14 environmental weed species representing four different growth forms (grasses, herbs, shrubs and vines), that are commonly found in south-eastern Australia, under ambient (380 ppm) and elevated (550 ppm) CO₂ concentrations. We then applied the recommended and double-recommended concentrations of two herbicides: glyphosate and fluroxypyr-meptyl. We found that responses of the weed species to herbicide under elevated CO₂ were species-specific. However, the C₃ grasses tended to be the most sensitive to herbicide application followed by the herbs and C₄ grasses while shrubs and vines demonstrated the highest resistance. Our results highlight the need for broader testing to determine the species most likely to exhibit increased tolerance to herbicide in the future in order to improve management options beforehand and thus offset a future liability.

Guarding the gateway to histidine biosynthesis in plants: Medicago truncatula ATP-phosphoribosyltransferase in relaxed and tense states

In the first committed step of histidine biosynthesis, adenosine 5'-triphosphate (ATP) and 5-phosphoribosyl-α1-pyrophosphate (PRPP), in the presence of ATP phosphoribosyltransferase (ATP-PRT, EC 2.4.2.17), yield phosphoribosyl-ATP. ATP-PRTs are subject to feedback inhibition by histidine that allosterically binds between the regulatory domains. Histidine biosynthetic pathways of bacteria, lower eukaryotes, and plants are considered promising targets for the design of antibiotics, antifungal agents, and herbicides because higher organisms are histidine heterotrophs. Plant ATP-PRTs are similar to one of the two types of their bacterial counterparts, the long-type ATP-PRTs. A biochemical and structural study of ATP-PRT from the model legume plant, Medicago truncatula (MedtrATP-PRT1) is reported herein. Two crystal structures, presenting homohexameric MedtrATP-PRT1 in its relaxed (R-) and histidine-bound, tense (T-) states allowed to observe key features of the enzyme and provided the first structural insights into an ATP-PRT from a eukaryotic organism. In particular, they show pronounced conformational reorganizations during R-state to T-state transition that involves substantial movements of domains. This rearrangement requires a trans- to cis- switch of a peptide backbone within the hinge region of MedtrATP-PRT1. A C-terminal α-helix, absent in bacteria, reinforces the hinge that is constituted by two peptide strands. As a result, conformations of the R- and T-states are significantly different from the corresponding states of prokaryotic enzymes with known 3-D structures. Finally, adenosine 5'-monophosphate (AMP) bound at the active site is consistent with a competitive (and synergistic with histidine) nature of AMP inhibition.

Gene Duplication and Aneuploidy Trigger Rapid Evolution of Herbicide Resistance in Common Waterhemp

An increase in gene copy number is often associated with changes in the number and structure of chromosomes, as has been widely observed in yeast and eukaryotic tumors, yet little is known about stress-induced chromosomal changes in plants. Previously, we reported that the EPSPS (5-enolpyruvylshikimate-3-phosphate synthase) gene, the molecular target of glyphosate, was amplified at the native locus and on an extra chromosome in glyphosate-resistant Amaranthus tuberculatus Here, we report that the extra chromosome is a ring chromosome termed extra circular chromosome carrying amplified EPSPS (ECCAE). The ECCAE is heterochromatic, harbors four major EPSPS amplified foci, and is sexually transmitted to 35% of the progeny. Two highly glyphosate resistant (HGR) A. tuberculatus plants with a chromosome constitution of 2n = 32+1 ECCAE displayed soma cell heterogeneity. Some cells had secondary ECCAEs, which displayed size polymorphisms and produced novel chromosomal variants with multiple gene amplification foci. We hypothesize that the ECCAE in the soma cells of HGR A. tuberculatus plants underwent breakage-fusion-bridge cycles to generate the observed soma cell heterogeneity, including de novo EPSPS gene integration into chromosomes. Resistant soma cells with stable EPSPS amplification events as de novo insertions into chromosomes may survive glyphosate selection pressure during the sporophytic phase and are plausibly transmitted to germ cells leading to durable glyphosate resistance in A. tuberculatus This is the first report of early events in aneuploidy-triggered de novo chromosome integration by an as yet unknown mechanism, which may drive rapid adaptive evolution of herbicide resistance in common waterhemp.