Novel bioassay for the discovery of inhibitors of the 2-C-methyl-D-erythritol 4-phosphate (MEP) and terpenoid pathways leading to carotenoid biosynthesis

Pesticides impacts on human health and the environment with their mechanisms of action and possible countermeasures

Pesticides are chemical constituents used to prevent or control pests, including insects, rodents, fungi, weeds, and other unwanted organisms. Despite their advantages in crop production and disease management, the use of pesticides poses significant hazards to the environment and public health. Pesticide elements have now perpetually entered our atmosphere and subsequently contaminated water, food, and soil, leading to health threats ranging from acute to chronic toxicities. Pesticides can cause acute toxicity if a high dose is inhaled, ingested, or comes into contact with the skin or eyes, while prolonged or recurrent exposure to pesticides leads to chronic toxicity. Pesticides produce different types of toxicity, for instance, neurotoxicity, mutagenicity, carcinogenicity, teratogenicity, and endocrine disruption. The toxicity of a pesticide formulation may depend on the specific active ingredient and the presence of synergistic or inert compounds that can enhance or modify its toxicity. Safety concerns are the need of the hour to control contemporary pesticide-induced health hazards. The effectiveness and implementation of the current legislature in providing ample protection for human health and the environment are key concerns. This review explored a comprehensive summary of pesticides regarding their updated impacts on human health and advanced safety concerns with legislation. Implementing regulations, proper training, and education can help mitigate the negative impacts of pesticide use and promote safer and more sustainable agricultural practices.

The search for new herbicide mechanisms of action: Is there a 'holy grail'?

New herbicide modes of action (MOAs) are in great demand because of the burgeoning evolution of resistance of weeds to existing commercial herbicides. This need has been exacerbated by the almost complete lack of introduction of herbicides with new MOAs for almost 40 years. There are many highly phytotoxic compounds with MOAs not represented by commercial herbicides, but neither these compounds nor structural analogues have been developed as herbicides for a variety of reasons. Natural products provide knowledge of many MOAs that are not being utilized by commercial herbicides. Other means of identifying new herbicide targets are discussed, including pharmaceutical target sites and metabolomic and proteomic information, as well as the use of artificial intelligence and machine learning to predict herbicidal compounds with new MOAs. Information about several newly discovered herbicidal compounds with new MOAs is summarized. The currently increased efforts of both established companies and start-up companies are likely to result in herbicides with new MOAs that can be used in herbicide resistance management within the next decade. © 2021 Society of Chemical Industry.

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.

Synthesis and evaluation of hydroxyazolopyrimidines as herbicides; the generation of amitrole in planta

BACKGROUND

Exploiting novel herbicidal modes of action is an important method to overcome the challenges faced by increasing resistance and regulatory pressure on existing commercial herbicides. Recent reports of inhibitors of enzymes in the non-mevalonate pathway of isoprenoid biosynthesis led to the design of a novel class of azolopyrimidines which were assessed for their herbicidal activity. Studies were also undertaken to determine the mode of action responsible for the observed herbicidal activity.

RESULTS

In total, 30 novel azolopyrimidines were synthesised and their structures were unambiguously determined by 1 H NMR, mass spectroscopy and X-ray crystallographic analysis. The herbicidal activity of this new chemical class was assessed against six common weed species, with compounds from this series displaying bleaching symptomology in post-emergence tests. A structure-activity relationship for the novel compounds was determined, which showed that only those belonging to the hydroxytriazolopyrimidine subclass displayed significant herbicidal activity. Observed similarities between the bleaching symptomology displayed by these herbicides and amitrole suggested that hydroxytriazolopyrimidines could be acting as elaborate propesticides of amitrole, and this was subsequently demonstrated in plant metabolism studies using Amaranthus retroflexus. It was shown that selected hydroxytriazolopyrimidines that displayed promising herbicidal activity generated amitrole, with peak concentrations of amitrole generally being observed 1 day after application. Additionally, the herbicidal activity of selected compounds was profiled against tobacco plants engineered to overexpress 4-diphosphocytidyl-2C-methyl-d-erythritol synthase (IspD) or lycopene β-cyclase, and the results suggested that, where significant herbicidal activity was observed, inhibition of IspD was not responsible for the activity. Tobacco plants overexpressing lycopene β-cyclase showed tolerance to amitrole and the two most herbicidally active triazolopyrimidines.

CONCLUSIONS

Inhibition of IspD leading to herbicidal activity has been ruled out as the mode of action for the hydroxytriazolopyrimidine class of herbicides. Additionally, tobacco plants overexpressing lycopene β-cyclase showed tolerance to amitrole, which indicates that this is the main herbicidal mode of action for amitrole. Results from the metabolic fate study of selected hydroxytriazolopyrimidines suggested that the herbicidal activity displayed by these compounds is due to amitrole production, which was confirmed when tobacco plants overexpressing lycopene β-cyclase also showed tolerance towards two triazolopyrimidines from this study. © 2016 Society of Chemical Industry.

Differential effects of glyphosate and aminomethylphosphonic acid (AMPA) on photosynthesis and chlorophyll metabolism in willow plants

We used a willow species (Salix miyabeana cultivar SX64) to examine the differential secondary-effects of glyphosate and aminomethylphosphonic acid (AMPA), the principal glyphosate by-product, on chlorophyll metabolism and photosynthesis. Willow plants were treated with different concentrations of glyphosate (equivalent to 0, 1.4, 2.1 and 2.8kgha(-1)) and AMPA (equivalent to 0, 0.28, 1.4 and 2.8kgha(-1)) and evaluations of pigment contents, chlorophyll fluorescence, and oxidative stress markers (hydrogen peroxide content and antioxidant enzyme activities) in leaves were performed after 12h of exposure. We observed that AMPA and glyphosate trigger different mechanisms leading to decreases in chlorophyll content and photosynthesis rates in willow plants. Both chemicals induced ROS accumulation in willow leaves although only glyphosate-induced oxidative damage through lipid peroxidation. By disturbing chlorophyll biosynthesis, AMPA induced decreases in chlorophyll contents, with consequent effects on photosynthesis. With glyphosate, ROS increases were higher than the ROS-sensitive threshold, provoking chlorophyll degradation (as seen by pheophytin accumulation) and invariable decreases in photosynthesis. Peroxide accumulation in both AMPA and glyphosate-treated plants was due to the inhibition of antioxidant enzyme activities. The different effects of glyphosate on chlorophyll contents and photosynthesis as described in the literature may be due to various glyphosate:AMPA ratios in those plants.

Isoprenoid generating systems in plants - A handy toolbox how to assess contribution of the mevalonate and methylerythritol phosphate pathways to the biosynthetic process

Isoprenoids comprise an astonishingly diverse group of metabolites with numerous potential and actual applications in medicine, agriculture and the chemical industry. Generation of efficient platforms producing isoprenoids is a target of numerous laboratories. Such efforts are generally enhanced if the native biosynthetic routes can be identified, and if the regulatory mechanisms responsible for the biosynthesis of the compound(s) of interest can be determined. In this review a critical summary of the techniques applied to establish the contribution of the two alternative routes of isoprenoid production operating in plant cells, the mevalonate and methylerythritol pathways, with a focus on their co-operation (cross-talk) is presented. Special attention has been paid to methodological aspects of the referred studies, in order to give the reader a deeper understanding for the nuances of these powerful techniques. This review has been designed as an organized toolbox, which might offer the researchers comments useful both for project design and for interpretation of results obtained.

Biosynthesis of Carotenoids in Plants: Enzymes and Color

Carotenoids are the most important biocolor isoprenoids responsible for yellow, orange and red colors found in nature. In plants, they are synthesized in plastids of photosynthetic and sink organs and are essential molecules for photosynthesis, photo-oxidative damage protection and phytohormone synthesis. Carotenoids also play important roles in human health and nutrition acting as vitamin A precursors and antioxidants. Biochemical and biophysical approaches in different plants models have provided significant advances in understanding the structural and functional roles of carotenoids in plants as well as the key points of regulation in their biosynthesis. To date, different plant models have been used to characterize the key genes and their regulation, which has increased the knowledge of the carotenoid metabolic pathway in plants. In this chapter a description of each step in the carotenoid synthesis pathway is presented and discussed.

Development of an image-based screening system for inhibitors of the plastidial MEP pathway and of protein geranylgeranylation

In a preceding study we have recently established an in vivo visualization system for the geranylgeranylation of proteins in a stably transformed tobacco BY-2 cell line, which involves expressing a dexamethasone-inducible GFP fused to the prenylable, carboxy-terminal basic domain of the rice calmodulin CaM61, which naturally bears a CaaL geranylgeranylation motif (GFP-BD-CVIL). By using pathway-specific inhibitors it was there demonstrated that inhibition of the methylerythritol phosphate (MEP) pathway with oxoclomazone and fosmidomycin, as well as inhibition of protein geranylgeranyl transferase type 1 (PGGT-1), shifted the localization of the GFP-BD-CVIL protein from the membrane to the nucleus. In contrast, the inhibition of the mevalonate (MVA) pathway with mevinolin did not affect this localization. Furthermore, in this initial study complementation assays with pathway-specific intermediates confirmed that the precursors for the cytosolic isoprenylation of this fusion protein are predominantly provided by the MEP pathway. In order to optimize this visualization system from a more qualitative assay to a statistically trustable medium or a high-throughput screening system, we established now new conditions that permit culture and analysis in 96-well microtiter plates, followed by fluorescence microscopy. For further refinement, the existing GFP-BD-CVIL cell line was transformed with an estradiol-inducible vector driving the expression of a RFP protein, C-terminally fused to a nuclear localization signal (NLS-RFP). We are thus able to quantify the total number of viable cells versus the number of inhibited cells after various treatments. This approach also includes a semi-automatic counting system, based on the freely available image processing software. As a result, the time of image analysis as well as the risk of user-generated bias is reduced to a minimum. Moreover, there is no cross-induction of gene expression by dexamethasone and estradiol, which is an important prerequisite for this test system.

Development of an image-based screening system for inhibitors of the plastidial MEP pathway and of protein geranylgeranylation

We have recently established an in vivo visualization system for the geranylgeranylation of proteins in a stably transformed tobacco BY-2 cell line, which involves expressing a dexamethasone-inducible GFP fused to the prenylable, carboxy-terminal basic domain of the rice calmodulin CaM61, which naturally bears a CaaL geranylgeranylation motif (GFP-BD-CVIL). By using pathway-specific inhibitors it was demonstrated that inhibition of the methylerythritol phosphate (MEP) pathway with oxoclomazone and fosmidomycin, as well as inhibition of protein geranylgeranyl transferase type 1 (PGGT-1), shifted the localization of the GFP-BD-CVIL protein from the membrane to the nucleus. In contrast, the inhibition of the mevalonate (MVA) pathway with mevinolin did not affect this localization. Furthermore, complementation assays with pathway-specific intermediates confirmed that the precursors for the cytosolic isoprenylation of this fusion protein are predominantly provided by the MEP pathway. In order to optimize this visualization system from a more qualitative assay to a statistically trustable medium or a high-throughput screening system, we established new conditions that permit culture and analysis in 96-well microtiter plates, followed by fluorescence microscopy. For further refinement, the existing GFP-BD-CVIL cell line was transformed with an estradiol-inducible vector driving the expression of a RFP protein, C-terminally fused to a nuclear localization signal (NLS-RFP). We are thus able to quantify the total number of viable cells versus the number of inhibited cells after various treatments. This approach also includes a semi-automatic counting system, based on the freely available image processing software. As a result, the time of image analysis as well as the risk of user-generated bias is reduced to a minimum. Moreover, there is no cross-induction of gene expression by dexamethasone and estradiol, which is an important prerequisite for this test system.