Metabolomic, proteomic and physiological insights into the potential mode of action of thymol, a phytotoxic natural monoterpenoid phenol

Allelopathic inhibitory of thymol on Arabidopsis thaliana primary root growth is mediated by ABA signaling pathway

Abscisic acid (ABA) is a sesquiterpenoid phytohormone involved in controlling plant root growth and development. Thymol, a monoterpene allelochemical, showed a potent phytotoxic effect in plants. It can rapidly inhibit seed germination and seedling growth. In this study, we employed a combination of transcriptome sequencing and validation methods from plant genetics and physiology to investigate the allelopathic inhibitory effects of thymol on the primary roots of Arabidopsis. We found that thymol affected the growth of Arabidopsis thaliana primary root in a dose-dependent manner, low concentration (10 μM) generally enhances, and high concentration (150 μM) inhibits. RNA sequencing analysis showed that a high concentration of thymol affected a series of biological processes and signaling transduction, including ABA biosynthesis, auxin polar transport, oxidative stress, root growth, and development. Exogenous ABA (10 μM) enhanced the inhibitory effect of thymol on the primary root and the application of the ABA biosynthesis inhibitor Na2WO4 rescued this inhibitory effect. During this process, the content and distribution of auxin in the roots were significantly altered. The lengths of primary root and meristem of mutant abi1, abi2, and abi1 abi2, showed that ABI1 and ABI2 positively regulate the process of thymol inhibition of root growth. In summary, the allelopathic inhibitory of thymol on Arabidopsis thaliana primary root growth is mediated by ABA signaling pathway.

Label free quantitative proteomic analysis reveals the physiological and biochemical responses of Arabidopsis thaliana to cinnamon essential oil

The increasing use of synthetic chemical herbicides has resulted in environmental, human and animal health issues. This has also led to the development of herbicide resistance in weed populations. The use of essential oils (EOs) can contribute to the development of effective, eco-friendly and nature-based alternatives to these chemical products due to their phytotoxicity and multisite action. Our study aimed to evaluate the proteomic response of Arabidopsis thaliana (A. thaliana) leaves to the application of a cinnamon essential oil (CEO) emulsion. The results showed that the application of CEO emulsion at a concentration of 6% severely impacted the proteomic profile of A. thaliana, especially for membrane proteins and those involved in the photosynthesis process. Interestingly, 40 proteins were identified and listed as the most differentially accumulated proteins in the leaves of A. thaliana. CEO decreased the expression of all the proteins associated with catabolism and anabolism processes while simultaneously increasing the expression levels of proteins involved in the response to oxidative stress. Overall, these findings allowed us to obtain a global view of the proteome response to CEO, opening promising perspectives for the development of natural herbicides, especially given the low probability of developing resistant weed populations.

The combination of hyperspectral imaging, untargeted metabolomics and lipidomics highlights a coordinated stress-related biochemical reprogramming triggered by polyethylene nanoparticles in lettuce

Polyethylene nanoplastics (NPs) are widely diffused in terrestrial environments, including soil ecosystems, but the stress mechanisms in plants are not well understood. This study aimed to investigate the effects of two increasing concentrations of NPs (20 and 200 mg kg-1 of soil) in lettuce. To this aim, high-throughput hyperspectral imaging was combined with metabolomics, covering both primary (using NMR) and secondary metabolism (using LC-HRMS), along with lipidomics profiling (using ion-mobility-LC-HRMS) and plant performance. Hyperspectral imaging highlighted a reduced plant growth pattern. Several vegetative indexes indicated plant toxicity, with 20 mg kg-1 NPs significantly decreasing lettuce density and vegetation health (as indicated by NDVI and plant senescence reflectance indexes). Consistently, photosynthetic activity also decreased. At the biochemical level, metabolomics and lipidomics pointed out a multi-layered broad biochemical reprogramming of primary and secondary metabolism involving a decrease in sterols, sphingolipids, glycolipids, and glycerophospholipids in response to NPs. The reduction in phosphatidylinositol coincided with an accumulation of diacylglycerols (DAG), suggesting the activation of the phospholipase C lipid signaling pathway. Moreover, nanoplastic treatments down-modulated different biosynthetic pathways, particularly those involved in N-containing compounds and phenylpropanoids. Our mechanistic basis of NPs stress in plants will contribute to a better understanding of their environmental impact.

Protecting anthocyanins of postharvest blueberries through konjac glucomannan/low-acyl gellan gum coatings contained thymol microcapsule during low-temperature storage

In order to protect the anthocyanins in blueberries during low-temperature storage, TMs/KGM/LAG (TKL) coatings were developed by composite thymol/β-cyclodextrin (β-CD) microcapsules (TMs), konjac glucomannan (KGM), and low acyl gellan gum (LAG). The results showed that the TMs prolonged the release of thymol for at least 30 d. The TKL was effective in maintaining the color of blueberry skin by regulating the activities of key enzymes for anthocyanin synthesis and degradation. Among the different treatment groups, TKL60 (thymol concentration of 60 mg/L) was the most effective in protecting anthocyanin. At 42 d of storage, the TKL60 group showed the highest anthocyanin levels of malvidin-3-O-galactoside (718.38 μg/g), delphinidin-3-O-galactoside (343.75 μg/g) and cyanidin-3-O-galactoside (40.67 μg/g). In addition, TKL60 treatment still showed good maintain the qualities of blueberries (weight loss, decay, hardness and TSS). Thus, this study provides a new approach to protect anthocyanin in blueberries after harvest during low-temperature storage.

Use of natural extract from HOP, thyme, orange and algae to protect potatoes against potato blight (Phytophthora infestans)

The growing demand for alternative plant protection products (PPP) that are more environmentally friendly leads to the investigation of novel approaches in agriculture. The use of plant extracts as PPP belongs to one of the most intensively investigated areas. This study aimed to evaluate if the partial substitution of conventional pesticides with selected botanicals (seaweed extract, orange essential oil, thyme essential oil, and a hop extract) will have the same protective effect against Phytophthora infestans as conventional potato treatment. The research was carried out in field conditions in two locations between 2019 and 2021. The impact of treatments on plant health status, relative chlorophyll content, and yield parameters was determined. The protective efficiency of selected natural substances (in concentrations from 0.125 to 1 %) was comparable to that of conventional fungicides. Moreover, their application reduced the use of traditional fungicides by 14-64 %, depending on the locality. In addition, the potato plants treated with the natural substances showed a higher overall and marketable tuber yield.

A multi-omics approach to unravel the interaction between heat and drought stress in the Arabidopsis thaliana holobiont

The impact of combined heat and drought stress was investigated in Arabidopsis thaliana and compared to individual stresses to reveal additive effects and interactions. A combination of plant metabolomics and root and rhizosphere bacterial metabarcoding were used to unravel effects at the plant holobiont level. Hierarchical cluster analysis of metabolomics signatures pointed out two main clusters, one including heat and combined heat and drought, and the second cluster that included the control and drought treatments. Overall, phenylpropanoids and nitrogen-containing compounds, hormones and amino acids showed the highest discriminant potential. A decrease in alpha-diversity of Bacteria was observed upon stress, with stress-dependent differences in bacterial microbiota composition. The shift in beta-diversity highlighted the pivotal enrichment of Proteobacteria, including Rhizobiales, Enterobacteriales and Azospirillales. The results corroborate the concept of stress interaction, where the combined heat and drought stress is not the mere combination of the single stresses. Intriguingly, multi-omics interpretations evidenced a good correlation between root metabolomics and root bacterial microbiota, indicating an orchestrated modulation of the whole holobiont.

Biochemical and Physiological Responses of Weeds to the Application of a Botanical Herbicide Based on Cinnamon Essential Oil

The use of chemical herbicides induces negative impacts on the environment, animals, and human health. It also leads to the development of herbicide-resistant weeds. In this context, natural and efficacious herbicides are highly sought after. Essential oils are natural compounds with antibacterial, fungicidal, and phytotoxic properties. For this reason, we studied the post-emergence phytotoxic effect of cinnamon essential oil (cinnamon EO) from Cinnamomum cassia under greenhouse conditions, testing it against Trifolium incarnatum (T. incarnatum) and Lolium perenne (L. perenne). The content of malondialdehyde (MDA), percentage of water loss, electrolyte leakage, and the fluorescence of treated leaves by cinnamon EO were determined in order to understand the physiological and biochemical responses. In addition, transmission electron microscopy (TEM) was used to study the effect of cinnamon EO on cellular organelles in different tissues of T. incarnatum leaves. Results showed that cinnamon EO quickly induced oxidative stress in treated leaves by increasing MDA content, impacting membrane integrity and causing water loss. TEM observations confirmed the cell desiccation by cellular plasmolysis and showed an alteration of the membrane integrity and chloroplast damages. Moreover, Raman analysis confirms the disturbance of the plant metabolism by the disappearance of some scattering bands which correspond to primary metabolites. Through our finding, we confirm that cinnamon essential oil (EO) could be proposed in the future as a potential bioherbicide and a suitable source of natural phytotoxic compounds with a multisite action on weeds.

Comparative ecotoxicity assessment of highly bioactive isomeric monoterpenes carvacrol and thymol on aquatic and edaphic indicators and communities

The growing demand for sustainable natural products to replace harmful synthetic ones requires comprehensive ecotoxicity assessments to ensure their eco-friendly nature. This study explored for the first time the changes in microbial community growth and metabolic profiles from river and natural soil samples exposed to the two structural isomers, thymol (THY) and carvacrol (CARV), utilizing Biolog EcoPlate™ assays and 16S rRNA gene sequencing for taxonomic analysis. In addition, we addressed existing ecotoxicity data gaps for these two compounds by using aquatic (Daphnia magna and Vibrio fischeri) and soil (Eisenia fetida and Allium cepa) indicators. Results show acute toxicity of both CARV and THY on all indicators. V. fischeri (LC50 = 0.59 mg/L) > D. magna (4.75 mg/L) > A. cepa (6.47 mg/L) for CARV, and V. fischeri (LC50 = 1.71 mg/L) > A. cepa (4.05 mg/L) > D. magna (8.13 mg/L) for THY. E. fetida showed LC50 = 7.68 mg/kg for THY and 1.04 for CARV. River and soil microbial communities showed resilience, likely because they contain taxa capable of biodegrading these products. No significant growth inhibition effects were observed up to 100 mg/L, though substrate utilization decreased at higher concentrations, particularly for polymers and amines in soil microorganisms and polymers in aquatic communities. Soil microorganisms were more affected than aquatic ones, with CARV being more toxic than THY (EC50120h = THY 94.13 and CARV 29.79 mg/L in soil microorganisms). These findings suggest that an increase in the consumption of these products and their subsequent ecotoxicity effects from environmental discharge should still be monitored before being ruled out. However, long-term effects are unlikely due to microbial degradation of these natural products, potentially reducing risks to other target species and opening the way for their use as substitutes for commercial antibiotics.

Phytotoxic Activity of Sesquiterpene Lactones-Enriched Fractions from Cynara cardunculus L. Leaves on Pre-Emergent and Post-Emergent Weed Species and Putative Mode of Action

Sesquiterpene lactones (SLs) are compounds that are highly produced in Cynara cardunculus leaves, known for their phytotoxic activity. This study aims to assess SL-enriched fractions' (cynaropicrin, aguerin B, and grosheimin) phytotoxic potentials and putative modes of action, compared to an initial extract, using two approaches: first, against a panel of nine weed species in pre-emergence, and then on Portulaca oleracea L.'s post-emergency stage. The SL-enriched fractions demonstrated greater phytotoxic activity when compared with the C. cardunculus leaf initial extract. The SL-enriched fractions had higher activity at root growth inhibition over the panel tested, doubling the activity in five of them at 800 ppm. Regarding the post-emergence bioassay, the SL-enriched fractions had a higher influence on the plants' growth inhibition (67% at 800 ppm). The SL-effects on the plants' metabolisms were evidenced. The total chlorophyll content was reduced by 65% at 800 ppm. Oxidative stress induction was observed because of the enhancement in MDA levels at 800 ppm compared to control (52%) and the decrease in SOD-specific activity from 4.20 U/mg protein (400 ppm) to 1.74 U/mg protein (800 ppm). The phytotoxic effects of the SL-enriched fractions suggest that they could be used for a future bioherbicide development.

Synthesis and evaluation of esters obtained from phenols and phenoxyacetic acid with significant phytotoxic and cytogenotoxic activities

There is a growing demand for herbicides that are more effective than conventional ones yet less harmful to ecosystems. In light of this, this study aimed to synthesize esters from phenols and phenoxyacetic acid, using compounds with known phytotoxic potential as starting materials. Phenoxyacetic acid was first synthesized and then utilized in the synthesis of seven esters through Steglich esterification, employing N,N'-dicyclohexylcarboimide and N,N-dimethylpyridin-4-amine in the presence of phenols (thymol, vanillin, eugenol, carvacrol, guaiacol, p-cresol, and β-naphthol), yielding esters 1-7. All synthesized compounds were characterized using mass spectrometry, 1H, and 13C NMR. These compounds were tested for phytotoxicity to evaluate their effects on the germination and root development of Sorghum bicolor and Lactuca sativa seeds, and for the induction of alterations in the mitotic cycle of meristematic cells of L. sativa roots. Esters 1, 3, 4, and 5 exhibited the most significant phytotoxic activity in both L. sativa and S. bicolor. Alterations in the mitotic index and frequency of chromosomal alterations in L. sativa roots revealed the cytotoxic, genotoxic effects, and the aneugenic mode of action of the tested molecules. These findings suggest that these compounds could serve as inspiration for the synthesis of new semi-synthetic herbicides.

Evaluating the effects of azelaic acid in the metabolism of Arabidopsis thaliana seedlings through untargeted metabolomics and ionomics approaches

The present study demonstrates that low concentrations of azelaic acid (AZA) significantly impact the metabolism of Arabidopsis thaliana seedlings, leading to imbalances in numerous minerals and metabolites due to AZA-induced stress. Untargeted metabolomic analyses were conducted on untreated and AZA-treated seedlings at two time points: 7 and 14 days after treatment initiation. The results revealed a general accumulation of sugars (e.g., glucose, mannose, xylose), amino acids (e.g., lysine, GABA, threonine, glutamine), and organic acids (e.g., glutaric acid, shikimic acid, succinic acid) in AZA treated-seedlings, suggesting that AZA triggers stress responses in Arabidopsis. Ionomic analysis revealed that AZA induces phosphorus deficiency, which plants compensate by increasing malate content in the roots. Additionally, AZA treatment induced putrescine accumulation within the root, a metabolic biomarker of potassium deficiency and plant stress. The metabolomic profile showed elevated levels of different specialized metabolites, such as nitrogen- and sulphur-containing compounds, and altered levels of various phytohormones, including jasmonates and brassinosteroids, implicated in plant protection under biotic and/or abiotic stresses. These findings support the hypothesis that AZA's mode of action is associated with an auxin imbalance, suggesting its function as an auxinic herbicide. The observed increases in starch and jasmonates, coupled with the disruptions in potassium homeostasis, are linked to the previously reported alterations in the auxin transport, root architecture and gravitropic root response. Statistical analyses were applied, including Kruskal-Wallis tests for ionomic data, as well as multifactor analysis, Principal Component Analysis, Orthogonal Partial Least Squares-Discriminant Analysis, and enrichment pathway analysis for metabolomic data, ensuring the robustness and validity of these findings.

The Effects of Natural Insecticides on the Green Peach Aphid Myzus persicae (Sulzer) and Its Natural Enemies Propylea quatuordecimpunctata (L.) and Aphidius colemani Viereck

Botanical insecticides and soaps are frequently proposed as environmentally safer alternatives to synthetic insecticides. However, the efficacy and selectivity of these products are often only partially supported by empirical evidence. Here, we tested the effectiveness of five botanical insecticides, belonging to different categories, on the green peach aphid Myzus persicae (Sulzer) and their selectivity towards two natural enemies, the ladybird beetle Propylea quatuordecimpunctata (L.) and the parasitoid Aphidius colemani (Dalman). White thyme essential oil (EO), sweet orange EO, crude garlic extract and Marseille soap were tested and compared with a pyrethrin-based commercial product. Both direct spray assays and residual contact assays on treated cabbage leaf disks were carried out. The tested products had low efficacy against aphids when compared to pyrethrins but were in general less detrimental to ladybird beetle larvae, meaning that if applied against other pests, they have a lower chance of harming this agent of aphid biocontrol. Some of the products (soap, orange EO) did, however, show direct exposure toxicity toward ladybird larvae, and thyme EO had extensive phytotoxic effects on cabbage leaves, possibly indirectly leading to higher mortality in ladybird adults. These results underline the necessity for case-by-case evaluations of botanical insecticides.

In vitro and in vivo antibacterial activity of selected essential oil components against Pectobacterium carotovorum subsp. carotovorum and Pectobacterium atrosepticum causing bacterial soft rot of potato tubers

Pectinolytic bacteria cause bacterial soft rot of potato tubers. The most significant losses occur during storage. The efficacy of essential oil (EO) components carvacrol, cinnamaldehyde, d-carvone, l-menthone, R-(+)-limonene and thymol was tested against Pectobacterium carotovorum subsp. carotovorum (Pcc) and Pectobacterium atrosepticum (Pa). Disc diffusion, minimum inhibitory concentration (MIC), and minimum bactericidal concentration (MBC) tests were performed in vitro, as well as potato disc and whole tuber maceration tests in vivo. Under in vitro conditions, cinnamaldehyde was the most effective against both bacteria (MIC 0.5 μL/mL, MBC 1.5 μL/mL). Both bacteria were found to be more susceptible to d-carvone (MIC 1.5-2.5 μL/mL, MBC 2.5 μL/mL) and thymol (MIC 2.5-5 μL/mL, MBC 3-5 μL/mL). R-(+)-limonene was the least effective. Results from the potato tuber disc maceration test confirmed a significant antibacterial effect of cinnamaldehyde at a concentration of 1.5 μL/mL. No rotted area was observed on potato tuber discs after treatment with l-menthone at concentrations of 2.5 μL/mL and 10 μL/mL against Pcc. A more pronounced effect was obtained when carvacrol was used at concentrations of 5 μL/mL against Pcc and 10 μL/mL against Pa. Disease severity tests on potato tubers after soaking for 20 min at MIC concentration of the EO components followed by 7 days of incubation at room temperature and 15 °C confirmed the antibacterial activity of cinnamaldehyde (0.5 μl/ml), l-menthone (2.5 μl/ml) and carvacrol (5-10 μl/ml). Cinnamaldehyde, l-menthone, and carvacrol may be recommended for further testing to treat stored potato tubers.

Effects of konjac glucan-nan/low-acyl gellan edible coatings loaded thymol-β-cyclodextrin microcapsules on postharvest blueberry

This study developed an edible antimicrobial coating using a blend of konjac glucomannan (KGM) and low acyl gellan gum (LAG) hydrogel to incorporate thymol nanoparticles (TKL). The optimized TKL formulation (TKL60) comprised 0.22% thymol microcapsules (TMs), 0.075% total polysaccharide content (KGM:LAG = 1:2), and 99.63% distilled water. When applied to blueberries, TKL60 significantly extended their shelf life to 42 d at 2 ± 0.5 °C, tripling that of control fruit. TKL60 reduced decay rate, weight loss, and respiration rate, delayed softening and senescence during cold storage. It preserved the outer epidermis by retaining cuticular waxes, curbing lipid oxidation, and sustaining defense-related enzyme activities. Flavor analysis revealed altered volatile compound concentrations in TKL60-treated berries, including decreased terpenes and benzaldehyde, and increased esters and aldehydes like 2-methylbutanol, 3-methylbutanol, and linalool. Discriminant Analysis highlighted TKL60's efficacy in delaying aroma deterioration by over 21 d. TKL60 exhibits potential as a substitute for synthetic coatings and chemical insecticides.

A Pseudomonas Plant Growth Promoting Rhizobacterium and Arbuscular Mycorrhiza differentially modulate the growth, photosynthetic performance, nutrients allocation, and stress response mechanisms triggered by a mild Zinc and Cadmium stress in tomato

This study aimed to assess the effectiveness of plant growth-promoting rhizobacteria (PGPR; Pseudomonas strain So_08) and arbuscular mycorrhizal fungi (AMF; Rhizoglomus irregulare BEG72 and Funneliformis mosseae BEG234) in mitigating the detrimental effects of cadmium (Cd) and zinc (Zn) stress in tomato plants. Plant biomass, root morphology, leaf relative water content, membrane stability, photosynthetic performance, chlorophyll content, and heavy metals (HMs) accumulation were determined. Furthermore, an ionomic profile was conducted to investigate whether microbial inoculants affected the uptake and allocation of macro- and micronutrients. Metabolomics with pathway analysis of both roots and leaves was performed to unravel the mechanisms underlying the differential responses to HMs stress. The findings revealed that the levels of HMs did not significantly affect plant growth parameters; however, they affected membrane stability, photosynthetic performance, nutrient allocation, and chlorophyll content. Cadmium was mainly accumulated in roots, whilst Zn exhibited accumulation in various plant organs. Our findings demonstrate the beneficial effects of PGPR and AMF in mitigating Cd and Zn stress in tomato plants. The microbial inoculations improved physiological parameters and induced differential accumulation of macro- and micronutrients, modulating nutrient uptake balance. These results provide insights into the mechanisms underlying the plant-microbe interactions and highlight the differential modulation of the biosynthetic pathways of secondary metabolites related to oxidative stress response, membrane lipids stability, and phytohormone crosstalk.

Carvacrol Treatment Reduces Decay and Maintains the Postharvest Quality of Red Grape Fruits (Vitis vinifera L.) Inoculated with Alternaria alternata

In this study, we isolated and identified pathogenic fungi from the naturally occurring fruits of red grapes, studied their biological characteristics, screened fifteen essential oil components to find the best natural antibacterial agent with the strongest inhibitory effect, and then compared the incidence of postharvest diseases and storage potential of red grapes treated with two concentrations (0.5 EC50/EC50) of essential oil components (inoculated with pathogenic fungi) during storage for 12 d at room temperature. In our research, Alternaria alternata was the primary pathogenic fungus of red grapes. Specifically, red grapes became infected which caused diseases, regardless of whether they were inoculated with Alternaria alternata in an injured or uninjured state. Our findings demonstrated that the following conditions were ideal for Alternaria alternata mycelial development and spore germination: BSA medium, D-maltose, ammonium nitrate, 28 °C, pH 6, and exposure to light. For the best Alternaria alternata spore production, OA medium, mannitol, urea, 34 °C, pH 9, and dark conditions were advised. Furthermore, with an EC50 value of 36.71 μg/mL, carvacrol demonstrated the highest inhibitory impact on Alternaria alternata among the 15 components of essential oils. In the meantime, treatment with EC50 concentration of carvacrol was found to be more effective than 0.5 EC50 concentration for controlling Alternaria alternata-induced decay disease of red grapes. The fruits exhibited remarkable improvements in the activity of defense-related enzymes, preservation of the greatest hardness and total soluble solids content, reduction in membrane lipid peroxidation in the peel, and preservation of the structural integrity of peel cells. Consequently, carvacrol was able to prevent the Alternaria alternata infestation disease that affects red grapes, and its EC50 concentration produced the greatest outcomes.

Leaf Proteomic Analysis in Seedlings of Two Maize Landraces with Different Tolerance to Boron Toxicity

Boron (B) toxicity is an important stressor that negatively affects maize yield and the quality of the produce. The excessive B content in agricultural lands is a growing problem due to the increase in arid and semi-arid areas because of climate change. Recently, two Peruvian maize landraces, Sama and Pachía, were physiologically characterized based on their tolerance to B toxicity, the former being more tolerant to B excess than Pachía. However, many aspects regarding the molecular mechanisms of these two maize landraces against B toxicity are still unknown. In this study, a leaf proteomic analysis of Sama and Pachía was performed. Out of a total of 2793 proteins identified, only 303 proteins were differentially accumulated. Functional analysis indicated that many of these proteins are involved in transcription and translation processes, amino acid metabolism, photosynthesis, carbohydrate metabolism, protein degradation, and protein stabilization and folding. Compared to Sama, Pachía had a higher number of differentially expressed proteins related to protein degradation, and transcription and translation processes under B toxicity conditions, which might reflect the greater protein damage caused by B toxicity in Pachía. Our results suggest that the higher tolerance to B toxicity of Sama can be attributed to more stable photosynthesis, which can prevent damage caused by stromal over-reduction under this stress condition.

Application of Indole-Alkaloid Harmaline Induces Physical Damage to Photosystem II Antenna Complexes in Adult Plants of Arabidopsis thaliana (L.) Heynh

Finding herbicides with new and multiple modes of action is a solution to stop the increase in resistant weed species. Harmaline, a natural alkaloid with proven phytotoxic potential, was tested on Arabidopsis adult plants by watering and spraying; watering resulted as the more effective treatment. Harmaline altered several photosynthetic parameters, reducing the efficiency of the light- (Φ_II) and dark-adapted (F_v/F_m) PSII, suggesting physical damages in photosystem II, although dissipation of the energy in excess under the form of heat was not compromised as demonstrated by the significant increase in Φ_NPQ. Metabolomic alterations, such as osmoprotectant accumulation and reduction in sugars' content, also indicate a reduction of photosynthetic efficiency and suggest early senescence and water status alteration induced by harmaline. Data suggest that harmaline might be considered a new phytotoxic molecule interesting for further studies.

Similarities on the mode of action of the terpenoids citral and farnesene in Arabidopsis seedlings involve interactions with DNA binding proteins

The sesquiterpene farnesene and the monoterpene citral are phytotoxic natural compounds characterized by a high similarity in macroscopic effects, suggesting an equal or similar mechanism of action when assayed at IC₅₀ concentration. In the present study, a short-time experiment (24 and 48 h) using an imaging spectrofluorometer allowed us to monitor the in-vivo effects of the two molecules, highlighting that both terpenoids were similarly affecting all PSII parameters, even when the effects of citral were quicker in appearing than those of farnesene. The multivariate, univariate, and pathway analyses, carried out on untargeted-metabolomic data, confirmed a clear separation of the plant metabolome in response to the two treatments, whereas similarity in the affected pathways was observed. The main metabolites affected were amino acids and polyamine, which significantly accumulated in response to both treatments. On the contrary, a reduction in sugar content (i.e. glucose and sucrose) was observed. Finally, the in-silico studies demonstrated a similar mechanism of action for both molecules by interacting with DNA binding proteins, although differences concerning the affinity with the proteins with which they could potentially interact were also highlighted. Despite the similarities in macroscopic effects of these two molecules, the metabolomic and in-silico data suggest that both terpenoids share a similar but not equal mechanism of action and that the similar effects observed on the photosynthetic machinery are more imputable to a side effect of molecules-induced oxidative stress.

Phytogenotoxicity of thymol and semisynthetic thymoxyacetic acid in pre/post emergence of model plants and weeds

Herbicides are commonly used to control weed. However, some plants are resistant to such products. To identify less harmful herbicides, it is crucial to search for different mechanisms of action. Thymol is an easily acquired allelopathic compound, capable of producing its respective semisynthetic derivative, thymoxyacetic acid. The aim of this study was to determine the effects of thymol and thymoxyacetic acid molecules as bioherbicides in greenhouse at the concentration of 3 mmol L^-1 in pre- and postemergence applications in five species: Amaranthus viridis L., Cucumis sativus L., Lactuca sativa L., Eleusine indica L., and Sorghum bicolor L. The initial seedling development and DNA changes were analyzed. These molecules were contrasting with the solvent, in the negative control, and with the glyphosate, in the positive control, promoting phytogenotoxic activities. The toxic effect of thymoxyacetic acid was more effective in preemergence and thymol's in postemergence. We also observed a reduction in the germination speed index and root growth with a negative correlation to the increase in potassium leaching. Damage to the root and shoot of the seedlings was verified at the DNA level, and the phytotoxicity of the plants treated with the herbicide glyphosate was similar to the plants treated with the natural molecules tested. The bioherbicidal effect of thymol and thymoxyacetic acid exacerbates the reduction of the environmental impact caused by the disordered and increased use of residual pesticides.

Interference of Dihydrocoumarin with Hormone Transduction and Phenylpropanoid Biosynthesis Inhibits Barnyardgrass (Echinochloa crus-galli) Root Growth

Botanical compounds with herbicidal activity exhibit safety, low toxicity, and low chances of herbicide resistance development in plants. They have widespread applications in green agricultural production and the development of organic agriculture. In the present study, dihydrocoumarin showed potential as a botanical herbicide, and its phenotypic characteristics and mechanism of action were studied in barnyardgrass [Echinochloa crus-galli (L.) P.Beauv.] seedlings. The results indicated that dihydrocoumarin inhibited the growth of barnyardgrass without causing significant inhibition of rice seedling growth at concentrations ranging between 0.5 and 1.0 g/L. Additionally, dihydrocoumarin treatment could cause oxidative stress in barnyardgrass, disrupt the cell membrane, and reduce the root cell activity, resulting in root cell death. Transcriptomic analyses revealed that dihydrocoumarin could inhibit barnyardgrass normal growth by affecting the signal transduction of plant hormones. The results showed significant differential expression of plant hormone signal transduction genes in barnyardgrass. Additionally, dihydrocoumarin interfered with the expression of numerous phenylpropanoid biosynthesis genes in barnyardgrass that affect the production of various vital metabolites. We speculate that the barnyardgrass growth was suppressed by the interaction among hormones and phenylpropanoid biosynthesis genes, indicating that dihydrocoumarin can be applied as a bioherbicide to control barnyardgrass growth in rice transplanting fields.

iTRAQ proteomic analysis of the inhibitory effect of 1,6-O,O-diacetylbritannilactone on the plant pathogenic oomycete Phytophthora capsici

Phytophthora capsici is a highly destructive oomycete of vegetables; its management is challenging due to its broad host range, rapid dispersion, resilient spores and severe fungicide resistance. Identifying an effective alternative fungicide is important for the control of P. capsici. 1,6-O,O-diacetylbritannilactone (ABLOO), one of the secondary metabolites of Inula Britannica, showed a favorable inhibitory activity against P. capsici at different developmental stages, with a sensitivity order as follows: sporangia formation (30.45 mg/L) > zoospore discharge (77.69 mg/L) > mycelial growth (93.18 mg/L) > cystospore germination (591.48 mg/L). To investigate the mode of action of ABLOO in P. capsici, iTRAQ-based quantitative proteomic analysis was performed by comparing the expression levels of proteins in the control and ABLOO-treated (400 mg/L, inhibition rate of 80%) mycelial groups. A total of 65 downregulated and 75 upregulated proteins were identified in the proteomic analysis. Functional enrichment analyses showed that proteins with transmembrane transport activity were significantly inhibited, while proteins involved in energy production were significantly increased, including proteins involved in ubiquinone and other terpenoid-quinone biosynthesis, oxidative phosphorylation, and glycolysis/gluconeogenesis. The morphological results indicated that ABLOO treatment could decrease the thickness of the cell walls of P. capsici mycelia. Correspondingly, biochemical results showed that ABLOO treatment reduced the β-1,3-glucan contents (the key component of the cell wall of P. capsici) and increased the cell membrane permeability of P. capsici. ABLOO may exhibit antioomycete activity by destroying the cell membrane of P. capsici. This study provides new evidence regarding the inhibitory mechanisms of ABLOO against P. capsici.

Transcriptomics Reveals the Effect of Thymol on the Growth and Toxin Production of Fusarium graminearum

Fusarium graminearum is a harmful pathogen causing head blight in cereals such as wheat and barley, and thymol has been proven to inhibit the growth of many pathogens. This study aims to explore the fungistatic effect of thymol on F. graminearum and its mechanism. Different concentrations of thymol were used to treat F. graminearum. The results showed that the EC50 concentration of thymol against F. graminearum was 40 μg/mL. Compared with the control group, 40 μg/mL of thymol reduced the production of Deoxynivalenol (DON) and 3-Ac-DON by 70.1% and 78.2%, respectively. Our results indicate that thymol can effectively inhibit the growth and toxin production of F. graminearum and cause an extensive transcriptome response. Transcriptome identified 16,727 non-redundant unigenes and 1653 unigenes that COG did not annotate. The correlation coefficients between samples were all >0.941. When FC was 2.0 times, a total of 3230 differential unigenes were identified, of which 1223 were up-regulated, and 2007 were down-regulated. Through the transcriptome, we confirmed that the expression of many genes involved in F. graminearum growth and synthesis of DON and other secondary metabolites were also changed. The gluconeogenesis/glycolysis pathway may be a potential and important way for thymol to affect the growth of F. graminearum hyphae and the production of DON simultaneously.

ROS Metabolism Perturbation as an Element of Mode of Action of Allelochemicals

The allelopathic interaction between plants is one of the elements that influences plant communities. It has been commonly studied by applying tissue extracts onto the acceptors or by treating them with isolated allelotoxins. Despite descriptive observations useful for agricultural practice, data describing the molecular mode of action of allelotoxins cannot be found. Due to the development of -omic techniques, we have an opportunity to investigate specific reactive oxygen species (ROS)-dependent changes in proteome or transcriptome that are induced by allelochemicals. The aim of our review is to summarize data on the ROS-induced modification in acceptor plants in response to allelopathic plants or isolated allelochemicals. We present the idea of how ROS are involved in the hormesis and plant autotoxicity phenomena. As an example of an -omic approach in studies of the mode of action of allelopatic compounds, we describe the influence of meta-tyrosine, an allelochemical exudated from roots of fescues, on nitration-one of nitro-oxidative posttranslational protein modification in the roots of tomato plants. We conclude that ROS overproduction and an induction of oxidative stress are general plants' responses to various allelochemicals, thus modification in ROS metabolisms is regarded as an indirect mode of action of allelochemicals.

The Steroid Saponin Protodioscin Modulates Arabidopsis thaliana Root Morphology Altering Auxin Homeostasis, Transport and Distribution

To date, synthetic herbicides are the main tools used for weed control, with consequent damage to both the environment and human health. In this respect, searching for new natural molecules and understanding their mode of action could represent an alternative strategy or support to traditional management methods for sustainable agriculture. Protodioscin is a natural molecule belonging to the class of steroid saponins, mainly produced by monocotyledons. In the present paper, protodioscin's phytotoxic potential was assessed to identify its target and the potential mode of action in the model plant Arabidopsis thaliana. The results highlighted that the root system was the main target of protodioscin, which caused a high inhibitory effect on the primary root length (ED₅₀ 50 μM) with morphological alteration, accompanied by a significant increase in the lateral root number and root hair density. Through a pharmacological and microscopic approach, it was underlined that this saponin modified both auxin distribution and transport, causing an auxin accumulation in the region of root maturation and an alteration of proteins responsible for the auxin efflux (PIN2). In conclusion, the saponin protodioscin can modulate the root system of A. thaliana by interfering with the auxin transport (PAT).

Studying the Ability of Thymol to Improve Fungicidal Effects of Tebuconazole and Difenoconazole Against Some Plant Pathogenic Fungi in Seed or Foliar Treatments

Thymol, a secondary plant metabolite possessing antifungal and chemosensitizing activities, disrupts cell wall or membrane integrity and interferes with ergosterol biosynthesis. Thymol also functions as a redox-active compound inducing generation of reactive oxygen species and lipid peroxidation in fungal cells. Previously, we showed thymol significantly enhanced the in vitro growth inhibitory effect of difenoconazole against Bipolaris sorokiniana and Parastagonospora nodorum. More recently, we demonstrated a possibility to use thymol to overcome the resistance of a P. nodorum strain able to grow on difenoconazole-containing media. However, potential for thymol to serve as a chemosensitizing agent in seed or plant treatments, to provide an effective suppression of the above-mentioned plant pathogens by triazole fungicides applied in lowered dosages, had yet to be tested. In the work presented here, we showed combined treatments of naturally infected barley seeds with thymol and difenoconazole (Dividend® 030 FS) synergistically exacerbated the protective effect against common root rot agent, B. sorokiniana, and other fungi (Fusarium spp. and Alternaria spp.). Similarly, co-applied treatment of wheat seeds, artificially inoculated with Fusarium culmorum, resulted in equivalent reduction of disease incidence on barley seedlings as application of Dividend®, alone, at a ten-fold higher dosage. In foliar treatments of wheat seedlings, thymol combined with Folicur® 250 EC (a.i. tebuconazole) enhanced sensitivity of P. nodorum, a glume/leaf blotch pathogen, to the fungicide and provided a significant mitigation of disease severity on treated seedlings, compared to controls, without increasing Folicur® dosages. Folicur® co-applied with thymol was also significantly more effective against a strain of P. nodorum tolerant to Folicur® alone. No additional deoxynivalenol or zearalenone production was found when a toxigenic F. culmorum was cultured in a nutrient medium containing thymol at a concentration used for chemosensitization of root rot agents. Accordingly, F. culmorum exposure to thymol at the sensitizing concentration did not up-regulate key genes associated with the biosynthesis of trichothecene or polyketide mycotoxins in this pathogen. Further studies using field trials are necessary to determine if thymol-triazole co-applications result in sensitization of seed- and foliar-associated plant pathogenic fungi, and if thymol affects production of fusarial toxins under field conditions.

Metabolomics analysis reveals potential mechanisms of phenolic accumulation in lettuce (Lactuca sativa L.) induced by low nitrogen supply

The roles of nitrogen availability in determining the phenolic accumulation of vegetables have been widely studied, but the underlying mechanism involved remains unknown. Thus, primary and secondary metabolites profiling of lettuce leaves were performed using non-targeted metabolomics analysis. The results showed that carbon metabolism, amino acid metabolism, and phenolic biosynthesis metabolism in lettuce were significantly affected by low nitrogen supply (LN). The phenolic content was significantly increased in LN-treated lettuce, indicating that the activated phenolic biosynthesis was triggered by the LN treatment. The reduced citrate cycle and enhanced glucose and sucrose content suggested there is a relative excess of carbon resources in LN-treated lettuce. In addition, the decreased nitrogen-rich amino acids (glutamine and aspartate acid) and the maintained phenylalanine content indicated the redirection of nitrogen resources to phenylalanine biosynthesis. Meanwhile, no significant changes of chlorophyll content were observed in LN-treated lettuce leaves. The LN-treated lettuce showed lower glutamine synthetase activity but higher glutamate synthase activity compared to control. These findings together suggest that LN treatment may increase the phenolic accumulation in lettuce by effectively redirecting more carbon and nitrogen resources to the phenolic biosynthesis pathway.

Proving the Mode of Action of Phytotoxic Phytochemicals

Knowledge of the mode of action of an allelochemical can be valuable for several reasons, such as proving and elucidating the role of the compound in nature and evaluating its potential utility as a pesticide. However, discovery of the molecular target site of a natural phytotoxin can be challenging. Because of this, we know little about the molecular targets of relatively few allelochemicals. It is much simpler to describe the secondary effects of these compounds, and, as a result, there is much information about these effects, which usually tell us little about the mode of action. This review describes the many approaches to molecular target site discovery, with an attempt to point out the pitfalls of each approach. Clues from molecular structure, phenotypic effects, physiological effects, omics studies, genetic approaches, and use of artificial intelligence are discussed. All these approaches can be confounded if the phytotoxin has more than one molecular target at similar concentrations or is a prophytotoxin, requiring structural alteration to create an active compound. Unequivocal determination of the molecular target site requires proof of activity on the function of the target protein and proof that a resistant form of the target protein confers resistance to the target organism.

Integrated proteomics, metabolomics and physiological analyses for dissecting the toxic effects of halosulfuron-methyl on soybean seedlings (Glycine max merr.)

Halosulfuron methyl (HSM) is a herbicide widely used to control sedge and broad-leaved weeds during crop production, but its environmental residue may damage non-target crops. Here, proteomics and metabolomics methods were used to explore the phytotoxicity mechanisms of HSM against soybean (Glycine max Merr.). Soybean seedlings were exposed to 0.01, 0.05 and 0.5 mg/L HSM for 8 d. The HSM applications significantly reduced chlorophyll and carotenoid contents in HSM-treated seedlings. Additionally, chlorophyll a fluorescence was seriously affected. The glutathione, hydrogen peroxide and malondialdehyde contents, as well as antioxidant enzyme activities, significantly increased in seedlings exposed to HSM. Furthermore, five enzymes involved in the tricarboxylic acid (TCA) cycle, α-ketoglutarate dehydrogenase, isocitrate dehydrogenase, aconitase, malic dehydrogenase and succinate dehydrogenase, were inhibited to varying degrees in HSM-treated seedlings compared with controls. Proteomics results showed multiple differentially abundant proteins involved in chlorophyll synthesis, photosystem processes and chloroplast ATP synthetase were down-regulated. Metabolomics analyses revealed that metabolites involved in the TCA cycle decreased significantly. Moreover, metabolites and proteins related to reactive oxygen species detoxification accumulated. In conclusion, the phytotoxicity mechanisms of HSM against soybean mainly act by damaging the photosynthetic machinery, inhibiting chlorophyll synthesis, interrupting the TCA cycle and causing oxidative stress. These results provide new insights into the toxicity mechanisms of sulfonylurea herbicides against non-target crops.

Phytotoxic Effects and Mechanism of Action of Essential Oils and Terpenoids

Weeds are one of the major constraints in crop production affecting both yield and quality. The excessive and exclusive use of synthetic herbicides for their management is increasing the development of herbicide-resistant weeds and is provoking risks for the environment and human health. Therefore, the development of new herbicides with multitarget-site activity, new modes of action and low impact on the environment and health are badly needed. The study of plant–plant interactions through the release of secondary metabolites could be a starting point for the identification of new molecules with herbicidal activity. Essential oils (EOs) and their components, mainly terpenoids, as pure natural compounds or in mixtures, because of their structural diversity and strong phytotoxic activity, could be good candidates for the development of new bioherbicides or could serve as a basis for the development of new natural-like low impact synthetic herbicides. EOs and terpenoids have been largely studied for their phytotoxicity and several evidences on their modes of action have been highlighted in the last decades through the use of integrated approaches. The review is focused on the knowledge concerning the phytotoxicity of these molecules, their putative target, as well as their potential mode of action.

Phytotoxicity, Morphological, and Metabolic Effects of the Sesquiterpenoid Nerolidol on Arabidopsis thaliana Seedling Roots

Natural herbicides that are based on allelopathy of compounds, can offer effective alternatives to chemical herbicides towards sustainable agricultural practices. Nerolidol, a sesquiterpenoid alcohol synthesized by many plant families, was shown to be the most effective allelopathic compound in a preliminary screening performed with several other sesquiterpenoids. In the present study, Arabidopsis thaliana seedlings were treated for 14 d with various cis-nerolidol concentrations (0, 50, 100, 200, 400, and 800 µM) to investigate its effects on root growth and morphology. To probe the underlying changes in root metabolome, we conducted untargeted gas chromatography mass spectrometry (GC-MS) based metabolomics to find out the specificity or multi-target action of this sesquiterpenoid alcohol. Oxidative stress (measured as levels of H₂O₂ and malondialdehyde (MDA) by-product) and antioxidant enzyme activities, i.e., superoxide dismutase (SOD) and catalase (CAT) were also evaluated in the roots. Nerolidol showed an IC₅₀ (120 µM), which can be considered low for natural products. Nerolidol caused alterations in root morphology, brought changes in auxin balance, induced changes in sugar, amino acid, and carboxylic acid profiles, and increased the levels of H₂O₂ and MDA in root tissues in a dose-dependent manner. Several metabolomic-scale changes induced by nerolidol support the multi-target action of nerolidol, which is a positive feature for a botanical herbicide. Though it warrants further mechanistic investigation, nerolidol is a promising compound for developing a new natural herbicide.

Phytotoxicity of Essential Oils: Opportunities and Constraints for the Development of Biopesticides. A Review

The extensive use of chemical pesticides leads to risks for both the environment and human health due to the toxicity and poor biodegradability that they may present. Farmers therefore need alternative agricultural practices including the use of natural molecules to achieve more sustainable production methods to meet consumer and societal expectations. Numerous studies have reported the potential of essential oils as biopesticides for integrated weed or pest management. However, their phytotoxic properties have long been a major drawback for their potential applicability (apart from herbicidal application). Therefore, deciphering the mode of action of essential oils exogenously applied in regards to their potential phytotoxicity will help in the development of biopesticides for sustainable agriculture. Nowadays, plant physiologists are attempting to understand the mechanisms underlying their phytotoxicity at both cellular and molecular levels using transcriptomic and metabolomic tools. This review systematically discusses the functional and cellular impacts of essential oils applied in the agronomic context. Putative molecular targets and resulting physiological disturbances are described. New opportunities regarding the development of biopesticides are discussed including biostimulation and defense elicitation or priming properties of essential oils.