Citral induces auxin and ethylene-mediated malformations and arrests cell division in Arabidopsis thaliana roots

Azelaic acid can efficiently compete for the auxin binding site TIR1, altering auxin polar transport, gravitropic response, and root growth and architecture in Arabidopsisthaliana roots

The present study investigates the phytotoxic potential of azelaic acid (AZA) on Arabidopsis thaliana roots. Effects on root morphology, anatomy, auxin content and transport, gravitropic response and molecular docking were analysed. AZA inhibited root growth, stimulated lateral and adventitious roots, and altered the root apical meristem by reducing meristem cell number, length and width. The treatment also slowed down the roots' gravitropic response, likely due to a reduction in statoliths, starch-rich organelles involved in gravity perception. In addition, auxin content, transport and distribution, together with PIN proteins' expression and localisation were altered after AZA treatment, inducing a reduction in auxin transport and its distribution into the meristematic zone. Computational simulations showed that AZA has a high affinity for the auxin receptor TIR1, competing with auxin for the binding site. The AZA binding with TIR1 could interfere with the normal functioning of the TIR1/AFB complex, disrupting the ubiquitin E3 ligase complex and leading to alterations in the response of the plant, which could perceive AZA as an exogenous auxin. Our results suggest that AZA mode of action could involve the modulation of auxin-related processes in Arabidopsis roots. Understanding such mechanisms could lead to find environmentally friendly alternatives to synthetic herbicides.

Allelochemicals: A source for developing economically and environmentally friendly plant growth regulators

Allelochemicals are specific secondary metabolites that can exhibit autotoxicity by inhibiting the growth of the same plant species that produced them. These metabolites have been found to affect various physical processes during plant growth and development, including inhibition of seed germination, photosynthesis, respiration, root growth, and nutrient uptake, with diverse mechanisms involving cell destruction, oxidative homeostasis and photoinhibition. In some cases, allelochemicals can also have positive effects on plant growth and development. In addition to their ecological significance, allelochemicals also possess potential as plant growth regulators (PGRs) due to their extensive physiological effects. However, a comprehensive summary of the development and applications of allelochemicals as PGRs is currently lacking. In this review, we present an overview of the sources and categories of allelochemicals, discuss their effects and the underlying mechanisms on plant growth and development. We showcase numerous instances of key phytohormonal allelochemicals and non-phytohormonal allelochemicals, highlighting their potential as candidates for the development of PGRs. This review aims to provide a theoretical basis for the development of economical, safe and effective PGRs utilizing allelochemicals, and emphasizes the need for further research in this area.

PLDδ, auxin, and H2O2 mediated the allelopathic effect of cycloastragenol on root growth in Arabidopsis

Cycloastragenol (CAG) is a tetra-cyclic triterpenoid allelochemical. It has been widely studied in animals but rarely in plants. Here, we reported that a model allelochemical CAG inhibited primary root elongation of Arabidopsis by reducing the sizes of both the meristem and elongation zones. Phospholipase Dδ(PLDδ), hydrogen peroxide (H₂O₂), and auxin affected this process. After treatment with CAG, the expression of PLDδ and the activity of the Phospholipase D(PLD) enzyme increased in WT. Mutants analysis demonstrated that PLDδ negatively regulated the primary root elongation by CAG treatment. CAG treatment stimulated the accumulation of H₂O₂ in roots. The production of H₂O₂ was derived from cell wall peroxidase. Mutants analysis showed that PLDδ positively regulated the production of H₂O₂ by CAG treatment. CAG also decreased auxin content in the root tip by affecting the expression of auxin synthesis-related genes. PLDδ was involved in the auxin reduction mediated by CAG, but H₂O₂ did not participate in this process. In conclusion, PLDδ, auxin, and H₂O₂ mediated the inhibition of primary root growth by CAG in Arabidopsis.

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.

Is the Phytotoxic Effect of Digestive Fluid of Nepenthes x ventrata on Tomato Related to Reactive Oxygen Species?

The digestive fluid of pitcher plants is a rich source of enzymes and secondary metabolites, but its impact on higher plant growth and development remains unknown. The aim of the study was to determine the phytotoxicity of the digestive fluid of the pitcher plant (Nepenthes x ventrata) on the germination of tomato (Solanum lycopersicum L.) seeds, elongation growth and cell viability of roots of tomato seedlings. The digestive fluid was collected from pitchers before feeding and four days after feeding; the pH and electrical conductivity of the fluid were determined. Undiluted and 50% fluids were used in the study. An inhibition of germination of tomato seeds, by around 30% and 55%, was respectively observed in 50% and 100% digestive fluids collected before and after feeding. Digestive fluid did not affect the root growth of tomato seedlings; a slight (6%) inhibition was only observed after the application of 100% digestive fluid from an unfed trap. The roots of the tomato seedlings treated with undiluted fluid were characterized by reduced cell viability. Reactive oxygen species (H₂O₂ and O₂^•-) were mainly localized in the root apex regardless of the used phytotoxic cocktail, and did not differ in comparison to control plants.

Emulsification-based interfacial synthesis of citral-loaded hollow MIL-88A for the inhibition of potato tuber sprouting

Economic value of the global potato harvest is impacted by sprouting during storage. We examined how sprouting might be reduced or eliminated using citral, a naturally occurring component in citrus fruit peel. The current study integrated both loading and sustained release of citral using emulsification-based interfacial synthesis of hollow MIL-88A. The structural properties and compositions of MIL-88A and hollow MIL-88A were confirmed using SEM, EDS, and XRD. BET analysis showed a surface area of 30.36 m² g^-1, pore volume of 0.21 cm³ g^-1, and an average pore radius of 13.56 nm for hollow MIL-88A. Citral was successfully loaded into 10 g of MIL-88A and hollow MIL-88A, with a total citral load of 0.21 cm³ and 1.82 cm³, respectively. The citral-loaded hollow MIL-88A induced a sustained release of citral, which effectively inhibited the sprouting, leading to higher starch content by 41%, lower weight losses, reducing sugar content, α-Amylase, β-amylase, and starch phosphorylase activities by 75%, 55%, 34%, 31%, and 43%, respectively. The citral-loaded hollow MIL-88A might inhibit sprouting by suppressing gibberellin and indole-3-acetic acid while maintaining abscisic acid.

Ultrastructural and hormonal changes related to harmaline-induced treatment in Arabidopsis thaliana (L.) Heynh. root meristem

Harmaline is an indole alkaloid with demonstrated phytotoxicity and recognized pharmacological applications. However, no information is available concerning its mode of action on plant metabolism. Therefore, the present work evaluated bioherbicide mode of action of harmaline on plant metabolism of Arabidopsis thaliana (L.) Heynh. Harmaline induced a strong inhibitory activity on root growth of treated seedlings, reaching IC₅₀ and IC₈₀ values of 14 and 29 μM, respectively. Treated roots were shorter and thicker than control and were characterized by a shorter root meristem size and an increase of root hairs production. Harmaline induced ultrastructural changes such as increment of cell wall thickness, higher density and condensation of mitochondria and vacuolization, appearance of cell wall deposits, increment of Golgi secretory activity and higher percentage of aberrant nuclei. The ethylene inhibitor AgNO₃ reversed high root hair appearance and increment of root thickness, and pTCSn::GFP transgenic line showed fluorescence cytokinin signal in stele zone after harmaline treatment that was absent in control, whereas the auxin signal in the transgenic line DR5 was significantly reduced by the treatment. All these results suggest that the mode of action of harmaline could be involving auxin, ethylene and cytokinin synergic/antagonistic action.

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).

Coumarin Interferes with Polar Auxin Transport Altering Microtubule Cortical Array Organization in Arabidopsis thaliana (L.) Heynh. Root Apical Meristem

Coumarin is a phytotoxic natural compound able to affect plant growth and development. Previous studies have demonstrated that this molecule at low concentrations (100 µM) can reduce primary root growth and stimulate lateral root formation, suggesting an auxin-like activity. In the present study, we evaluated coumarin’s effects (used at lateral root-stimulating concentrations) on the root apical meristem and polar auxin transport to identify its potential mode of action through a confocal microscopy approach. To achieve this goal, we used several Arabidopsis thaliana GFP transgenic lines (for polar auxin transport evaluation), immunolabeling techniques (for imaging cortical microtubules), and GC-MS analysis (for auxin quantification). The results highlighted that coumarin induced cyclin B accumulation, which altered the microtubule cortical array organization and, consequently, the root apical meristem architecture. Such alterations reduced the basipetal transport of auxin to the apical root apical meristem, inducing its accumulation in the maturation zone and stimulating lateral root formation.

Imaging of Chlorophyll a Fluorescence in Natural Compound-Induced Stress Detection

Imaging of chlorophyll a fluorescence (CFI) represents an easy, precise, fast and non-invasive technique that can be successfully used for discriminating plant response to phytotoxic stress with reproducible results and without damaging the plants. The spatio-temporal analyses of the fluorescence images can give information about damage evolution, secondary effects and plant defense response. In the last years, some studies about plant natural compounds-induced phytotoxicity have introduced imaging techniques to measure fluorescence, although the analysis of the image as a whole is often missed. In this paper we, therefore, evaluated the advantages of monitoring fluorescence images, presenting the physiological interpretation of different possible combinations of the most relevant parameters linked to fluorescence emission and the images obtained.

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.

Potential Effects of Essential Oils Extracted from Mediterranean Aromatic Plants on Target Weeds and Soil Microorganisms

Essential oils (EOs), extracted from aromatic plants, have been proposed as candidates to develop natural herbicides. This study aimed to evaluate the herbicidal potential of Thymbra capitata (L.) Cav., Mentha × piperita L. and Santolina chamaecyparissus L. essential oils (EOs) on Avena fatua L., Echinochloa crus-galli (L.) P. Beauv, Portulaca oleracea L. and Amaranthus retroflexus L. and their effects on soil microorganisms. A pot experiment was set up and three EOs at three doses were applied by irrigation. Efficacy and effects of EOs on weed growth were determined. Soil microbial biomass carbon and nitrogen, microbial respiration, and the main microbial groups were determined at days 7, 28 and 56. EOs demonstrated herbicidal activity, increasing their toxicity with the dose. T. capitata was the most effective against all weeds at the maximum dose. P. oleracea was the most resistant weed. Soil microorganisms, after a transient upheaval period induced by the addition of EOs, recovered their initial function and biomass. T. capitata EO at the highest dose did not allow soil microorganisms to recover their initial functionality. EOs exhibited great potential as natural herbicides but the optimum dose of application must be identified to control weeds and not negatively affect soil microorganisms.

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.

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

Thymol is a natural phenolic monoterpene widely produced by different species belonging to the Labiateae family. Although the thymol phytotoxicity is well known, the knowledge of its potential toxic mechanism is still limited. In this regard, the model species Arabidopsis thaliana was treated for 16 days by sub-irrigation with 300 μM of thymol. The results confirmed the high phytotoxic potential of this phenolic compound, which caused a reduction in plant growth and development. Thymol induced a water status alteration accompanied by an increase in ABA content and stomatal closure. Furthermore, leaves appeared necrotic in the margins and their temperature rinsed. The increase in H₂O₂ content suggested an oxidative stress experienced by treated plants. Both metabolomic and proteomic analysis confirmed this hypothesis showing a strong increase in osmoprotectants content, such as galactinol and proline, and a significant up-accumulation of proteins involved in ROS detoxification. Furthermore, the down-accumulation of proteins and pigments involved in the photosynthetic machinery, the increase in light sensitivity and the lower PSII efficiency well indicated a reduction in photosynthetic activity. Overall, we can postulate that thymol-induced phytotoxicity could be related to a combined osmotic and oxidative stress that resulted in reduced plant development.

Transcriptome and binding data indicate that citral inhibits single strand DNA-binding proteins

The mechanism of phytotoxicity of citral was probed in Arabidopsis thaliana using RNA-Seq and in silico binding analyses. Inhibition of growth by 50% by citral downregulated transcription of 9156 and 5541 genes in roots and shoots, respectively, after 1 h. Only 56 and 62 genes in roots and shoots, respectively, were upregulated. In the shoots, the downregulation increased at 3 h (6239 genes downregulated, vs 66 upregulated). Of all genes affected in roots at 1 h (time of greatest effect), 7.69% of affected genes were for nucleic acid binding functions. Genes for single strand DNA binding proteins (SSBP) WHY1, WHY 2 and WHY3 were strongly downregulated in the shoot up until 12 h after citral exposure. Effects were strong in the root at just 1 h after the treatment and then at 12 and 24 h. Similar effects occurred with the transcription factors MYC-2, ANAC and SCR-SHR, which were also significantly downregulated for the first hour of treatment, and downregulation occurred again after 12 and 24 h treatment. Downregulation of ANAC in the first hour of treatment was significantly (P < 0.0001) decreased more than eight times compared to the control. In silico molecular docking analysis suggests binding of citral isomers to the SSBPs WHY1, WHY2, and WHY3, as well as with other transcription factors such as MYC-2, ANAC and SCR-SHR. Such effects could account for the profound and unusual effects of citral on downregulation of gene transcription.

Targets and pathways involved in the antitumor activity of citral and its stereo-isomers

This review provides a comprehensive analysis of the anticancer potential of the natural product citral (CIT) found in many plants and essential oils, and extensively used in the food and cosmetic industry. CIT is composed of two stereoisomers, the trans-isomer geranial being a more potent anticancer compound than the cis-isomer neral. CIT inhibits cancer cell proliferation and induces cancer cell apoptosis. Its pluri-factorial mechanism of anticancer activity is essentially based on three pillars: (i) a drug-induced accumulation of reactive oxygen species in cancer cells leading to an oxidative burst and DNA damages, (ii) a colchicine-like inhibition of tubulin polymerization and promotion of microtubule depolymerization, associated with an inhibition of the microtubule affinity-regulating kinase MARK4, and (iii) a potent inhibition of the aldehyde dehydrogenase isoform ALDH1A3 which is associated with cancer stem cell proliferation and chemoresistance. This unique combination of targets and pathways confers a significant anticancer potential. However, the intrinsic potency of CIT is limited, mainly because the drug is not very stable and has a low bioavailability and it does not present a high selectivity for cancer cells versus non-tumor cells. Stable formulations of CIT, using cyclodextrins, biodegradable polymers, or various nano-structured particles have been designed to enhance the bioavailability, to increase the effective doses window and to promote the anticancer activity. The lack of tumor cell selectivity is more problematic and limits the use of the drug in cancer therapy. Nevertheless, CIT offers interesting perspectives to design more potent analogues and drug combinations with a reinforced antitumor potential.

Weisiensin B inhibits primary and lateral root development by interfering with polar auxin transport in Arabidopsis thaliana

Weisiensin B, a new ent-kaurene diterpenoid isolated from Isodon weisiensis (C. Y. Wu) H. Hara, exhibited phytotoxic effects on root growth and lateral root development in Arabidopsis thaliana seedlings. Primary root growth and lateral root formation in A. thaliana seedlings were significantly inhibited by 10-20 μM weisiensin B. Additionally, the role of weisiensin B in response to polar auxin transport in A. thaliana roots was investigated using a PIN promoter (PIN::GUS), a green fluorescent protein (GFP) fusion protein reporter (PINs::PINs:GFP), and DR5::GUS and DR5::GFP reporter genes. The results indicated that weisiensin B reduced the expression of PIN2, PIN3, PIN4, PIN7, and AUX1 genes and significantly decreased the abundance of PIN2-GFP, PIN3-GFP, PIN4-GFP, PIN7-GFP, and AUX1-GFP fusion proteins at their respective cellular locations, simultaneously causing auxin accumulation in the root apex. These results suggest that weisiensin B interferes with polar auxin transport in A. thaliana roots, resulting in auxin accumulation in the root meristematic cells and the inhibition of root growth and lateral root development.

Rosmarinic acid induces programmed cell death in Arabidopsis seedlings through reactive oxygen species and mitochondrial dysfunction

Phytotoxic potential of rosmarinic acid (RA), a caffeic acid ester largely found in aromatic species, was evaluated on Arabidopsis through metabolomic and microscopic approaches. In-vitro bioassays pointed out that RA affected root growth and morphology, causing ROS burst, ROS scavengers activity inhibition and consequently, an alteration on cells organization and ultrastructure. In particular, RA-treatment (175 μM) caused strong vacuolization, alteration of mitochondria structure and function and a consistent ROS-induced reduction of their transmembrane potential (ΔΨm). These data suggested a cell energy deficit also confirmed by the metabolomic analysis, which highlighted a strong alteration of both TCA cycle and amino acids metabolism. Moreover, the increase in H2O2 and O2- contents suggested that RA-treated meristems underwent oxidative stress, resulting in apoptotic bodies and necrotic cells. Taken together, these results suggest that RA inhibits two of the main ROS scavengers causing high ROS accumulation, responsible of the alterations on mitochondrial ultrastructure and activity through ΔΨm dissipation, TCA-cycle alteration, cell starvation and consequently cell death on Arabidopsis seedlings. All these effects resulted in a strong inhibition on root growth and development, which convert RA in a promising molecule to be explored for further use in weed management.

Origanum vulgare essential oils inhibit glutamate and aspartate metabolism altering the photorespiratory pathway in Arabidopsis thaliana seedlings

Essential oils (EOs) have been extensively studied as valuable eco-friendly compounds with herbicidal activity for weed management. Phytotoxic potential of EOs, extracted from a wild population of Origanum vulgare ssp. hirtum (Link) Ietswaart, has been here evaluated on plant model Arabidopsis, through a physiological and metabolomic approach. The EOs composition was mainly characterized by monoterpenes and sesquiterpenes, with a strong abundance of two monoterpenic phenols, namely carvacrol and thymol, and the monoterpene o-cymene. The in vitro bioassay confirmed a strong phytotoxic effect of EOs on Arabidopsis rosettes, showing by both a strong growth reduction and highly chlorotic leaves. In well-developed seedlings, EOs firstly caused growth reduction and leaf chlorosis, together with a series of interconnected metabolic alterations: i) impairing the nitrogen assimilation into amino acids, which affects in particular the glutamine metabolism; and as consequence ii) excessive accumulation of toxic ammonia into the leaves, associated with oxidative stress and damage; iii) declining the efficiency of the photosynthetic apparatus, connected to the reduced CO₂ fixation and photooxidation protection; iv) impairing the photorespiratory pathway. Overall, the results highlights that EOs alters principally the ability of Arabidopsis seedlings to incorporate inorganic nitrogen into amino acids, principally glutamine, leading to a dramatic accumulation of ammonia in leaf cells. This primary effect induces, in turn, a cascade of reactions that limits the efficiency of PSII, inducing oxidative stress and finally causing a strong plant growth reduction, leaf necrosis and eventually plant death. These findings suggest that O. vulgare EOs might be proficiently exploited as a potential bioherbicide in an ecofriendly agriculture. Moreover, its multitarget activity could be advantageous in limiting weed resistance phenomenon.

Physiological and cell ultrastructure disturbances in wheat seedlings generated by Chenopodium murale hairy root exudate

Chenopodium murale L. is an invasive weed species significantly interfering with wheat crop. However, the complete nature of its allelopathic influence on crops is not yet fully understood. In the present study, the focus is made on establishing the relation between plant morphophysiological changes and oxidative stress, induced by allelopathic extract. Phytotoxic medium of C. murale hairy root clone R5 reduced the germination rate (24% less than control value) of wheat cv. Nataša seeds, as well as seedling growth, diminishing shoot and root length significantly, decreased total chlorophyll content, and induced abnormal root gravitropism. The R5 treatment caused cellular structural abnormalities, reflecting on the root and leaf cell shape and organization. These abnormalities mostly included the increased number of mitochondria and reorganization of the vacuolar compartment, changes in nucleus shape, and chloroplast organization and distribution. The most significant structural changes were observed in cell wall in the form of amoeboid protrusions and folds leading to its irregular shape. These structural alterations were accompanied by an oxidative stress in tissues of treated wheat seedlings, reflected as increased level of H₂O₂ and other ROS molecules, an increase of radical scavenging capacity and total phenolic content. Accordingly, the retardation of wheat seedling growth by C. murale allelochemicals may represent a consequence of complex activity involving both cell structure alteration and physiological processes.

Bioherbicides: Current knowledge on weed control mechanism

Weed control is a challenging event during crop cultivation. Integrated management, including the application of bioherbicides, is an emerging method for weed control in sustainable agriculture. Plant extracts, allelochemicals and some microbes are utilized as bioherbicides to control weed populations. Bioherbicides based on plants and microbes inhibit the germination and growth of weeds; however,few studies conducted in weed physiology. This review ascribes the current knowledge of the physiological changes in weeds that occur during the exposure to bioherbicides. Plant extracts or metabolites are absorbed by weed seeds, which initiates damage to the cell membrane, DNA, mitosis, amylase activity and other biochemical processes and delays or inhibits seed germination. The growth of weeds is also retarded due to low rates of root-cell division, nutrient uptake, photosynthetic pigment synthesis, and plant growth hormone synthesis, while the productions of reactive oxygen species (ROS) and stress-mediated hormones increase, including irregular antioxidant activity. However, lytic enzymes and toxic substances secreted from microbes degrade the weed seed coat and utilize the endosperm for survival, which inhibits seed germination. The microbes grow through the intercellular spaces to reach the root core, and the deposition of toxins in the cells affects cell division and cellular functions. Some of the metabolites of deleterious microbes cause disease, necrosis and chlorosis,which inhibit the germination and growth of weed seeds by suppressing photosynthesis and gibberellin activities and enhancing ROS, abscisic acid and ethylene. This review explains the effects of bioherbicides (derived from plants and microbes) on weed-plant physiology to elucidate their modes of action.

The allelochemical farnesene affects Arabidopsis thaliana root meristem altering auxin distribution

Farnesene is a sesquiterpene with semiochemical activity involved in interspecies communication. This molecule, known for its phytotoxic potential and its effects on root morphology and anatomy, caused anisotropic growth, bold roots and a "left-handedness" phenotype. These clues suggested an alteration of auxin distribution, and for this reason, the aim of the present study was to evaluate its effects on: i) PIN-FORMED proteins (PIN) distribution, involved in polar auxin transport; ii) PIN genes expression iii) apical meristem anatomy of primary root, in 7 days old Arabidopsis thaliana seedlings treated with farnesene 250 μM. The following GFP constructs: pSCR::SCR-GFP, pDR5::GFP,pPIN1::PIN1-GFP, pPIN2::PIN2-GFP, pPIN3::PIN3-GFP, pPIN4::PIN4-GFP and pPIN7::PIN7-GFP were used to evaluate auxin distribution. Farnesene caused a reduction in meristematic zone size, an advancement in transition zone, suggesting a premature exit of cells from the meristematic zone, a reduction in cell division and an impairment between epidermal and cortex cells. The auxin-responsive reporter pDR5::GFP highlighted that auxin distribution was impaired in farnesene-treated roots, where auxin distribution appeared maximum in the quiescent center and columella initial cells, without extending to mature columella cells. This finding was further confirmed by the analysis on PIN transport proteins distribution, assessed on individual constructs, which showed an extreme alteration mainly dependent on the PIN 3, 4 and 7, involved in pattern specification during root development and auxin redistribution. Finally, farnesene treatment caused a down-regulation of all the auxin transport genes studied. We propose that farnesene affected auxin transport and distribution causing the alteration of root meristem, and consequently the left-handedness phenotype.

Terpenoid trans-caryophyllene inhibits weed germination and induces plant water status alteration and oxidative damage in adult Arabidopsis

trans-Caryophyllene (TC) is a sesquiterpene commonly found as volatile component in many different aromatic plants. Although the phytotoxic effects of trans-caryophyllene on seedling growth are relatively explored, not many information is available regarding the phytotoxicity of this sesquiterpenes on weed germination and on adult plants. The phytotoxic potential of TC was assayed in vitro on weed germination and seedling growth to validate its phytotoxic potential on weed species. Moreover, it was assayed on the metabolism of Arabidopsis thaliana adult plants, through two different application ways, spraying and watering, in order to establish the primary affected organ and to deal with the unknown mobility of the compound. The results clearly indicated that TC inhibited both seed germination and root growth, as demonstrated by comparison of the ED50 values. Moreover, although trans-caryophyllene-sprayed adult Arabidopsis plants did not show any effect, trans-caryophyllene-watered plants became strongly affected. The results suggested that root uptake was a key step for the effectiveness of this natural compound and its phytotoxicity on adult plants was mainly due to the alteration of plant water status accompanied by oxidative damage.

Loss of Gravitropism in Farnesene-Treated Arabidopsis Is Due to Microtubule Malformations Related to Hormonal and ROS Unbalance

Mode of action of farnesene, a volatile sesquiterpene commonly found in the essential oils of several plants, was deeply studied on the model species Arabidopsis thaliana. The effects of farnesene on the Arabidopsis root morphology were evaluated by different microscopic techniques. As well, microtubules immunolabeling, phytohormone measurements and ROS staining helped us to elucidate the single or multi-modes of action of this sesquiterpene on plant metabolism. Farnesene-treated roots showed a strong growth inhibition and marked modifications on morphology, important tissue alterations, cellular damages and anisotropic growth. Left-handed growth of farnesene-treated roots, reverted by taxol (a known microtubule stabilizer), was related to microtubule condensation and disorganization. As well, the inhibition of primary root growth, lateral root number, lateral root length, and both root hairs length and density could be explained by the strong increment in ethylene production and auxin content detected in farnesene-treated seedlings. Microtubule alteration and hormonal unbalance appear as important components in the mode of action of farnesene and confirm the strong phytotoxic potential of this sesquiterpene.

Toxicity of canavanine in tomato (Solanum lycopersicum L.) roots is due to alterations in RNS, ROS and auxin levels

Canavanine (CAN) is non-proteinogenic aminoacid and a structural analog of arginine (Arg). Naturally, CAN occurs in legumes e.g. jack bean and is considered as a strong allelochemical. As a selective inhibitor of inducible nitric oxide synthase in mammalians, it could act as a modifier of nitric oxide (NO) concentration in plants. Modifications in the content of endogenous reactive nitrogen species (RNS) and reactive oxygen species (ROS) influence root structure and architecture, being also under hormonal control. The aim of the work was to investigate regulation of root growth in tomato (Solanum lycopersicum L. cv. Malinowy Ożarowski) seedling by application of CAN at concentration (10 and 50 μM) leading to 50% or 100% restriction of root elongation. CAN at higher concentration led to slight DNA fragmentation, increased total RNA and protein level. Decline in total respiration rate after CAN supplementation was not associated with enhanced membrane permeability. Malformations in root morphology (shorter and thicker roots, limited number of lateral roots) were accompanied by modification in NO and ONOO(-) localization; determined mainly in peridermal cells and some border cells. Although, CAN resulted in low RNS production, addition of exogenous NO by usage of NO donors did not reverse its negative effect, nor recovery effect was detected after roots imbibition in Arg. To build up a comprehensive view on mode of action of CAN as root growth inhibitor, it was shown an elevated level of auxin. To summarize, we demonstrated several secondary mode of action of CAN, indicating its toxicity in plants linked to restriction in RNS formation accompanied by simultaneous overaccumulation of ROS.

The plant secondary metabolite citral alters water status and prevents seed formation in Arabidopsis thaliana

Based on previous results, which showed that the secondary metabolite citral causes disturbances to plant water status, the present study is focused on demonstrating and detailing these effects on the water-related parameters of Arabidopsis thaliana adult plants, and their impact on plant fitness. Clear evidence of effects on water status and fitness were observed: plants treated with 1200 and 2400 μm citral showed decreased RWC, reduced Ψs , increased Ψw and reduced stomatal opening, even 7 days after the beginning of the experiment. Plant protection signals, such as leaf rolling or increased anthocyanin content, were also detected in these plants. In contrast, 14 days after beginning the treatment, treated plants showed signs of citral-related damage. Moreover, the reproductive success of treated plants was critically compromised, with prematurely withered flowers and no silique or seed development. This effect of citral on fitness of adult plants suggests a promising application of this natural compound in weed management by reducing the weed seed bank in the soil.

Narciclasine, a potential allelochemical, affects subcellular trafficking of auxin transporter proteins and actin cytoskeleton dynamics in Arabidopsis roots

The present study documented the action of a potential allelochemical, narciclasine, on auxin transport in Arabidopsis by mainly affecting subcellular trafficking of PIN and AUX1 proteins and through interfering actin cytoskeletal organization. Narciclasine (NCS), an Amaryllidaceae alkaloid isolated from Narcissus tazetta bulbs, has potential allelopathic activity and affects auxin transport. However, little is known about the cellular mechanism of this inhibitory effect of NCS on auxin transport. The present study characterizes the effects of NCS at the cellular level using transgenic Arabidopsis plants harboring the promoters of PIN, in combination with PIN-GFP proteins or AUX1-YFP fusions. NCS treatment caused significant reduction in the abundance of PIN and AUX1 proteins at the plasma membrane (PM). Analysis of the subcellular distribution of PIN and AUX1 proteins in roots revealed that NCS induced the intracellular accumulation of auxin transporters, including PIN2, PIN3, PIN4, PIN7 and AUX1. However, other PM proteins, such as PIP2, BRI1, and low temperature inducible protein 6b (LTI6b), were insensitive to NCS treatment. NCS-induced PIN2 compartments were further defined using endocytic tracer FM 4-64 labeled early endosomes and suggested that this compound affects the endocytosis trafficking of PIN proteins. Furthermore, pharmacological analysis indicated that the brefeldin A (BFA)-insensitive pathway is employed in the cellular effects of NCS on PIN2 trafficking. Although NCS did not alter actin dynamics in vitro, it resulted in the depolymerization of the actin cytoskeleton in vivo. This disruption of actin filaments by NCS subsequently influences the actin-based vesicle motility. Hence, the elucidation of the specific role of NCS is useful for further understanding the mechanisms of allelopathy at the phytohormone levels.

Research Progress on the use of Plant Allelopathy in Agriculture and the Physiological and Ecological Mechanisms of Allelopathy

Allelopathy is a common biological phenomenon by which one organism produces biochemicals that influence the growth, survival, development, and reproduction of other organisms. These biochemicals are known as allelochemicals and have beneficial or detrimental effects on target organisms. Plant allelopathy is one of the modes of interaction between receptor and donor plants and may exert either positive effects (e.g., for agricultural management, such as weed control, crop protection, or crop re-establishment) or negative effects (e.g., autotoxicity, soil sickness, or biological invasion). To ensure sustainable agricultural development, it is important to exploit cultivation systems that take advantage of the stimulatory/inhibitory influence of allelopathic plants to regulate plant growth and development and to avoid allelopathic autotoxicity. Allelochemicals can potentially be used as growth regulators, herbicides, insecticides, and antimicrobial crop protection products. Here, we reviewed the plant allelopathy management practices applied in agriculture and the underlying allelopathic mechanisms described in the literature. The major points addressed are as follows: (1) Description of management practices related to allelopathy and allelochemicals in agriculture. (2) Discussion of the progress regarding the mode of action of allelochemicals and the physiological mechanisms of allelopathy, consisting of the influence on cell micro- and ultra-structure, cell division and elongation, membrane permeability, oxidative and antioxidant systems, growth regulation systems, respiration, enzyme synthesis and metabolism, photosynthesis, mineral ion uptake, protein and nucleic acid synthesis. (3) Evaluation of the effect of ecological mechanisms exerted by allelopathy on microorganisms and the ecological environment. (4) Discussion of existing problems and proposal for future research directions in this field to provide a useful reference for future studies on plant allelopathy.

Phytotoxic cyanamide affects maize (Zea mays) root growth and root tip function: from structure to gene expression

Cyanamide (CA) is a phytotoxic compound produced by four Fabaceae species: hairy vetch, bird vetch, purple vetch and black locust. Its toxicity is due to complex activity that involves the modification of both cellular structures and physiological processes. To date, CA has been investigated mainly in dicot plants. The goal of this study was to investigate the effects of CA in the restriction of the root growth of maize (Zea mays), representing the monocot species. CA (3mM) reduced the number of border cells in the root tips of maize seedlings and degraded their protoplasts. However, CA did not induce any significant changes in the organelle structure of other root cells, apart from increased vacuolization. CA toxicity was also demonstrated by its effect on cell cycle activity, endoreduplication intensity, and modifications of cyclins CycA2, CycD2, and histone HisH3 gene expression. In contrast, the arrangement of microtubules was not altered by CA. Treatment of maize seedlings with CA did not completely arrest mitotic activity, although the frequency of dividing cells was reduced. Furthermore, prolonged CA treatment increased the proportion of endopolyploid cells in the root tip. Cytological malformations were accompanied by an induction of oxidative stress in root cells, which manifested as enhanced accumulation of H2O2. Exposure of maize seedlings to CA resulted in an increased concentration of auxin and stimulated ethylene emission. Taken together, these findings suggested that the inhibition of root growth by CA may be a consequence of stress-induced morphogenic responses.

An improved quantitative analysis method for plant cortical microtubules

The arrangement of plant cortical microtubules can reflect the physiological state of cells. However, little attention has been paid to the image quantitative analysis of plant cortical microtubules so far. In this paper, Bidimensional Empirical Mode Decomposition (BEMD) algorithm was applied in the image preprocessing of the original microtubule image. And then Intrinsic Mode Function 1 (IMF1) image obtained by decomposition was selected to do the texture analysis based on Grey-Level Cooccurrence Matrix (GLCM) algorithm. Meanwhile, in order to further verify its reliability, the proposed texture analysis method was utilized to distinguish different images of Arabidopsis microtubules. The results showed that the effect of BEMD algorithm on edge preserving accompanied with noise reduction was positive, and the geometrical characteristic of the texture was obvious. Four texture parameters extracted by GLCM perfectly reflected the different arrangements between the two images of cortical microtubules. In summary, the results indicate that this method is feasible and effective for the image quantitative analysis of plant cortical microtubules. It not only provides a new quantitative approach for the comprehensive study of the role played by microtubules in cell life activities but also supplies references for other similar studies.