Environmental hormesis of non-specific and specific adaptive mechanisms in plants

Hormetic responses to cadmium exposure in wheat seedlings: insights into morphological, physiological, and biochemical adaptations

Cadmium is commonly recognized as toxic to plant growth, low-level Cd has promoting effects on growth performance, which is so-called hormesis. Although Cd toxicity in wheat has been widely investigated, knowledge of growth response to a broad range of Cd concentrations, especially extremely low concentrations, is still unknown. In this study, the morphological, physiological, and biochemical performance of wheat seedlings to a wide range of Cd concentrations (0-100 µΜ) were explored. Low Cd treatment (0.1-0.5 µM) improved wheat biomass and root development by enhancing the photosynthetic system and antioxidant system ability. Photosynthetic rate (Pn) was improved by 5.72% under lower Cd treatment (1 µΜ), but inhibited by 6.05-49.85% from 5 to 100 µΜ. Excessive Cd accumulation induced oxidative injury manifesting higher MDA content, resulting in lower photosynthetic efficiency, stunted growth, and reduction of biomass. Further, the contents of ascorbate, glutathione, non-protein thiols, and phytochelatins were improved under 5-100 µΜ Cd treatment. The ascorbate peroxidase activity in the leaf showed a hormetic dose-response characteristic. Correlation analysis and partial least squares (PLS) results indicated that antioxidant enzymes and metabolites were closely correlated with Cd tolerance and accumulation. The results of the element network, correlation analysis, and PLS showed a crucial role for exogenous Cd levels in K, Fe, Cu, and Mn uptake and accumulation. These results provided a deeper understanding of the hormetic effect of Cd in wheat, which would be beneficial for improving the quality of hazard and risk assessments.

Earthworms Enhance Crop Resistance to Insects Under Microplastic Stress by Mobilizing Physical and Chemical Defenses

To assess the ecological risk of microplastics (MPs) in agricultural systems, it is critical to simultaneously focus on MP-mediated single-organism response and different trophic-level organism interaction. Herein, we placed earthworms in soils contaminated with different concentrations (0.02% and 0.2% w/w) of polyethylene (PE) and polypropylene (PP) MPs to investigate the effect of earthworms on tomato against Helicoverpa armigera (H. armigera) under MPs stress. We found that earthworms alleviated the inhibitory effects of MPs stress on tomato growth and disrupted H. armigera growth. Compared to individual MPs exposure, earthworm incorporation significantly increased the silicon and lignin content in herbivore-damaged tomato leaves by 19.1% and 57.6%, respectively. Metabolites involved in chemical defense (chlorogenic acid) and phytohormones (jasmonic acid) were also activated by earthworm incorporation. Furthermore, earthworms effectively reduced oxidative damage induced by H. armigera via promoting antioxidant metabolism. Overall, our results suggest that utilizing earthworms to regulate above- and below-ground interactions could be a promising strategy for promoting green agriculture.

Ozone risk assessment with free-air controlled exposure (FACE) experiments: A critical revisit

Since risk assessments of tropospheric ozone (O3) are crucial for agricultural and forestry sectors, there is a growing body for realistic assessments by a stomatal flux-based approach in Free-Air Controlled Exposure (FACE) facilities. Ozone risks are normally described as relative risks (RRs), which are calculated by assuming the biomass or yield at zero O3 dose as "reference". However, the estimation of the reference biomass or yield is challenging due to a lack of O3-clean-air treatment at the FACEs and the extrapolation without data in a low O3 range increases the bias for estimating the reference values. Here, we reviewed a current methodology for the risk assessment at FACEs and presented a simple and effective way ("modified Paoletti's approach") of defining RRs just using biomass or yield data with a range of expected impacts under the FACE conditions hypothesizing three possible scenarios based on prediction limits using 95% credible intervals (CI) (1. Best fit using the intercept as reference, 2. Optimistic scenario using a lower CI and 3. Worst scenario using an upper CI). As a result, O3-sensitive species show a relatively narrow effect range (optimistic vs. worst scenario) whereas a wide range of response may be possibly taken in resistant species. Showing a possible effect range allows for a comprehensive understanding of the potential risks and its uncertainties related to a species sensitivity to O3. As a supporting approach, we also recommend to use scientifically relevant tools (i.e., ethylenediurea treatments; mechanistic plant models) for strengthening the obtained results for the RRs against O3. Interestingly, the moderately sensitive or resistant species showed non-linear rather than linear dose-response relationships, suggesting a need for the flexible functional form for the risk assessment to properly describe the complex plant response such as hormesis, which depends on their plasticity to O3 stress.

Exploring behavioural and physiological adaptations in mountain pine beetle in response to elevated ozone concentrations

Elevated ozone (eO3) concentrations pose a threat to insect populations by potentially altering their behaviour and physiology. This study investigates the effects of eO3 concentrations on the mountain pine beetle which is a major tree-killing species of conifers in northwestern North America. We are particularly interested in understanding the effects of eO3 concentrations on beetle behaviour and physiology and possible transgenerational impacts on bark beetle broods. We conducted O3-enrichment experiments in a controlled laboratory setting using different O3 concentrations (100-200 ppb; projected for 2050-2100) and assessed various beetle responses, including CO2 respiration, mating behaviour, survival probability, locomotion, and attraction behaviour. Transgenerational impacts on the first and second generations were also analyzed by studying brood morphology, mating behaviour, survival, and pheromone production. We found that beetles exposed to eO3 concentrations had shorter oviposition galleries and reduced brood production. Beetle pheromones were also degraded by eO3 exposure. However, exposure to eO3 also prompted various adaptive responses in beetles. Despite reduced respiration, eO3 improved locomotor activity and the olfactory response of beetles. Surprisingly, beetle survival probability was also improved both in the parents and their broods. We also observed transgenerational plasticity in the broods of eO3-exposed parents, suggesting potential stress resistance mechanisms. This was evident by similar mating success, oviposition gallery length, and brood numbers produced in both control and eO3 concentration treatments. This study demonstrates the sensitivity of mountain pine beetles to increased O3 concentrations, contributing crucial insights into the ecological implications of eO3 concentrations on their populations. Overall, the outcome of this study contributes to informed climate change mitigation strategies and adaptive management practices for the development of resilient forests in response to emerging forest insect pests worldwide.

Hormetic Response of Photosystem II Function Induced by Nontoxic Calcium Hydroxide Nanoparticles

In recent years, inorganic nanoparticles, including calcium hydroxide nanoparticles [Ca Ca(OH)2 NPs], have attracted significant interest for their ability to impact plant photosynthesis and boost agricultural productivity. In this study, the effects of 15 and 30 mg L-1 oleylamine-coated calcium hydroxide nanoparticles [Ca(OH)2@OAm NPs] on photosystem II (PSII) photochemistry were investigated on tomato plants at their growth irradiance (GI) (580 μmol photons m-2 s-1) and at high irradiance (HI) (1000 μmol photons m-2 s-1). Ca(OH)2@OAm NPs synthesized via a microwave-assisted method revealed a crystallite size of 25 nm with 34% w/w of oleylamine coater, a hydrodynamic size of 145 nm, and a ζ-potential of 4 mV. Compared with the control plants (sprayed with distilled water), PSII efficiency in tomato plants sprayed with Ca(OH)2@OAm NPs declined as soon as 90 min after the spray, accompanied by a higher excess excitation energy at PSII. Nevertheless, after 72 h, the effective quantum yield of PSII electron transport (ΦPSII) in tomato plants sprayed with Ca(OH)2@OAm NPs enhanced due to both an increase in the fraction of open PSII reaction centers (qp) and to the enhancement in the excitation capture efficiency (Fv'/Fm') of these centers. However, the decrease at the same time in non-photochemical quenching (NPQ) resulted in an increased generation of reactive oxygen species (ROS). It can be concluded that Ca(OH)2@OAm NPs, by effectively regulating the non-photochemical quenching (NPQ) mechanism, enhanced the electron transport rate (ETR) and decreased the excess excitation energy in tomato leaves. The delay in the enhancement of PSII photochemistry by the calcium hydroxide NPs was less at the GI than at the HI. The enhancement of PSII function by calcium hydroxide NPs is suggested to be triggered by the NPQ mechanism that intensifies ROS generation, which is considered to be beneficial. Calcium hydroxide nanoparticles, in less than 72 h, activated a ROS regulatory network of light energy partitioning signaling that enhanced PSII function. Therefore, synthesized Ca(OH)2@OAm NPs could potentially be used as photosynthetic biostimulants to enhance crop yields, pending further testing on other plant species.

6PPD-quinone affects the photosynthetic carbon fixation in cyanobacteria by extracting photosynthetic electrons

Photosynthetic carbon fixation by cyanobacteria plays a pivotal role in the global carbon cycle but is threatened by environmental pollutants. To date, the impact of quinones, with electron shuttling properties, on cyanobacterial photosynthesis is unknown. Here, we present the first study investigating the effects of an emerging quinone pollutant, i.e., 6PPD-Q (N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone), on the cyanobacterium Synechocystis sp. over a 400-generation exposure period. Synechocystis sp. exhibited distinct sequential phases, including hormesis, toxicity, and eventual recovery, throughout this exposure. Extensive evidence, including results of thylakoid membrane morphological and photosynthetic responses, carbon fixation rate, and key gene/protein analyses, strongly indicates that 6PPD-Q is a potent disruptor of photosynthesis. 6PPD-Q accepts photosynthetic electrons at the plastoquinone QB site in photosystem II (PSII) and the phylloquinone A1 site in PSI, leading to a sustained decrease in the carbon fixation of cyanobacteria after an ephemeral increase. This work revealed the specific mechanism by which 6PPD-Q interferes with photosynthetic carbon fixation in cyanobacteria, which is highly important for the global carbon cycle.

Glyphosate hormesis stimulates tomato (Solanum lycopersicum L.) plant growth and enhances tolerance against environmental abiotic stress by triggering nonphotochemical quenching

BACKGROUND

Glyphosate is the most widely applied herbicide in the world. Hormesis caused by low glyphosate doses has been widely documented in many plant species. However, the specific adaptative mechanism of plants responding to glyphosate hormesis stimulation remains unclear. This study focused on the biphasic relationship between glyphosate dose and tomato plant growth, and how glyphosate hormesis stimulates plant growth and enhances tolerance to environmental stress.

RESULTS

We constructed a hormesis model to describe the biphasic relationship with a maximal stimulation (MAX) of 162% above control by glyphosate at 0.063 g ha-1. Low-dose glyphosate increased photosynthetic pigment contents and improve photosynthetic efficiency, leading to plant growth stimulation. We also found that glyphosate hormesis enhanced plant tolerance to diuron (DCMU; a representative photosynthesis inhibitor) by triggering the nonphotochemical chlorophyll fluorescence quenching (NPQ) reaction to dissipate excess energy stress from photosystem II (PSII). Transcriptomic analysis and quantitative real-time polymerase chain reaction results revealed that the photosynthesis-antenna proteins pathway was the most sensitive to glyphosate hormesis, and PsbS (encoding photosystem II subunit S), ZEP (encoding zeaxanthin epoxidase) and VDE (encoding violaxanthin de-epoxidase) involved in NPQ played crucial roles in the plant response to glyphosate hormesis.

CONCLUSION

These results provide novel insights into the mechanisms of plant hormesis and is meaningful to the application of glyphosate hormesis in agriculture. © 2024 Society of Chemical Industry.

Multi-biomarker response of cyanobacteria Synechocystis salina and Microcystis aeruginosa to diclofenac

The cyanobacterial response to pharmaceuticals is less frequently investigated compared to green algae. Pharmaceuticals can influence not only the growth rate of cyanobacteria culture, but can also cause changes at the cellular level. The effect of diclofenac (DCF) as one of the for cyanobacteria has been rarely tested, and DCF has never been applied with cellular biomarkers. The aim of this work was to test the response of two unicellular cyanobacteria (Synechocystis salina and Microcystis aeruginosa) toward DCF (100 mg L-1) under photoautotrophic growth conditions. Such endpoints were analyzed as cells number, DCF uptake, the change in concentrations of photosynthetic pigments, the production of toxins, and chlorophyll a in vivo fluorescence. It was noted that during a 96 h exposure, cell proliferation was not impacted. Nevertheless, a biochemical response was observed. The increased production of microcystin was noted for M. aeruginosa. Due to the negligible absorption of DCF into cells, it is possible that the biochemical changes are induced by an external signal. The application of non-standard biomarkers demonstrates the effect of DCF on microorganism metabolism without a corresponding effect on biomass. The high resistance of cyanobacteria to DCF and the stimulating effect of DCF on the secretion of toxins raise concerns for environment biodiversity.

Mechanistic Insights on Salicylic Acid-Induced Enhancement of Photosystem II Function in Basil Plants under Non-Stress or Mild Drought Stress

Photosystem II (PSII) functions were investigated in basil (Ocimum basilicum L.) plants sprayed with 1 mM salicylic acid (SA) under non-stress (NS) or mild drought-stress (MiDS) conditions. Under MiDS, SA-sprayed leaves retained significantly higher (+36%) chlorophyll content compared to NS, SA-sprayed leaves. PSII efficiency in SA-sprayed leaves under NS conditions, evaluated at both low light (LL, 200 μmol photons m-2 s-1) and high light (HL, 900 μmol photons m-2 s-1), increased significantly with a parallel significant decrease in the excitation pressure at PSII (1-qL) and the excess excitation energy (EXC). This enhancement of PSII efficiency under NS conditions was induced by the mechanism of non-photochemical quenching (NPQ) that reduced singlet oxygen (1O2) production, as indicated by the reduced quantum yield of non-regulated energy loss in PSII (ΦNO). Under MiDS, the thylakoid structure of water-sprayed leaves appeared slightly dilated, and the efficiency of PSII declined, compared to NS conditions. In contrast, the thylakoid structure of SA-sprayed leaves did not change under MiDS, while PSII functionality was retained, similar to NS plants at HL. This was due to the photoprotective heat dissipation by NPQ, which was sufficient to retain the same percentage of open PSII reaction centers (qp), as in NS conditions and HL. We suggest that the redox status of the plastoquinone pool (qp) under MiDS and HL initiated the acclimation response to MiDS in SA-sprayed leaves, which retained the same electron transport rate (ETR) with control plants. Foliar spray of SA could be considered as a method to improve PSII efficiency in basil plants under NS conditions, at both LL and HL, while under MiDS and HL conditions, basil plants could retain PSII efficiency similar to control plants.

Plant adaptation to ionizing radiation: Mechanisms and patterns

Adaptation to environmental stressors is an essential property of plants that allows them, despite an immobile lifestyle, to survive in a changeable environment. The chain of successive events culminating in the final radiobiological reaction begins with the absorption of energy of ionizing radiation in the cell. Starting from stochastic acts of molecular injury formation, radiation damage gradually acquires deterministic features, which are expressed in a limited number of phenomena that complete plant radiation damage. As plants undergo specialization, the differences between plants and animals become more pronounced, leading to distinct responses to radiation. Chronic radiation exposure may activate biological mechanisms resulting in increased radioresistance of the population. The higher the level of radiation exposure and the sensitivity of plants to radiation, the more intensive the selection. Depending on the circumstances, enhanced radioresistance of a population can be achieved in different ways or has not evolved at all. High dose rates of chronic irradiation leаd to selection for the efficiency of repair systems, while low dose rates activate epigenetic mechanisms that lead to the maintenance of oxidative balance, additional synthesis of chaperones, and control of TEs transposition. Due to huge differences in the radiosensitivity of organisms that make up the ecosystem, irradiation can result in disruption of connections between components of ecosystems which may lead to consequences that can differ drastically from those expected at the organismal and population levels. Therefore, the use of ecological knowledge is essential for understanding the responses of populations and ecosystems to radiation exposure.

Less is more: The hormetic effect of titanium dioxide nanoparticles on plants

Titanium dioxide nanoparticles have attracted considerable attention due to their extensive applications; however, their multifaceted influence on plant physiology and the broader environment remains a complex subject. This review systematically synthesizes recent studies on the hormetic effects of TiO2 nanoparticles on plants - a phenomenon characterized by dual dose-response behavior that impacts various plant functions. It provides crucial insights into the molecular mechanisms underlying these hormetic effects, encompassing their effects on photosynthesis, oxidative stress response and gene regulation. The significance of this article consists in its emphasis on the necessity to establish clear regulatory frameworks and promote international collaboration to standardize the responsible adoption of nano-TiO2 technology within the agricultural sector. The findings are presented with the intention of stimulating interdisciplinary research and serving as an inspiration for further exploration and investigation within this vital and continually evolving field.

The intrinsic and extrinsic mechanisms regulated by functional carbon nanodots for the phytoremediation of multi-metal pollution in soils

Functional carbon nanodots (FCNs) were currently demonstrated to regulate plant behavior in the agricultural and environmental areas. However, their regulation mechanisms on the interactions of plant-soil system during phytoremediation remain unrevealed. Here, Solanum nigrum L. was employed to explore the intrinsic and extrinsic mechanisms regulated by FCNs in the phytoremediation of Cd-Pb co-contaminated soils. The mediation of FCNs on metal removal and plant growth showed a hormesis manner, wherein the maximum induction effect was contributed by 15 mg kg-1 FCNs. Cd/Pb removal were enhanced by 8.5% and 31.6%, respectively. Moreover, FCNs reallocate metal distribution in plant by immobilized metals in roots and suppressed metal translocation to leaves. Improving plant growth (by 82.8% for root), stimulating plant hormesis, and activating plant detoxification pathways are the intrinsic mechanism for the phytoremediation smartly regulated by FCNs. Notably, FCNs induced soil enzyme activities that associated with soil nutrients recycling, up-regulated the microbial diversity and the soil immune system, and regulated S. nigrum L. to recruit beneficial microbials in the rhizosphere. The above-mentioned comprehensive improvement of soil micro-environment is the extrinsic mechanism regulated by FCNs. This study provides new insights to evaluate the interactions of nanomaterials with plant-soil system under soil contamination.

Environmental risk thresholds and prediction models of Cd in Chinese agricultural soils

Soil environmental risk threshold of cadmium (Cd) is an important index in formulating soil protection policy. Environmental risk threshold refers to the maximal allowable critical concentration of hazardous substances in the environment. Although there is less study on how to determine soil Cd environmental risk threshold, it is a crucial indicator in formulating soil conservation policies and a key factor in assessing soil environmental quality. The main research content of the study is deducing the environmental risk threshold, aiming to provide scientific basis for the study of environmental quality standards of agricultural land and provide technical support for the protection of Cd pollution of agricultural land. The hazard concentration of 5 % species (HC5, which protects 95 % of species) was determined here using different toxicological data of Cd from 23 test endpoints, interspecific extrapolation using the species sensitivity distribution (SSD) method, and a prediction model was created on the basis of several soil parameters. According to the findings, Cd effective concentration (EC10) (Cd concentration which blocks 10 % of an endpoint's bioactivity) varied from 0.109 to 221 mg·kg-1, and the hormetic response induced by Cd reached 118 % displaying in the dose-response curve of Lolium perenne L.. Toxicology data was rectified by the aging factor considering biogeochemical processes of the newly added pollutants prior to SSD curves fitting. After that, the prediction model was created with the equation of LogHC5 = 0.147 pH + 0.067 OC -1.616. The field test properly validated the prediction model, demonstrating its ability to forecast Cd toxicity levels for various soil conditions. This study offers a scientifically sound methodology for determining the environmental risk limitation for Cd and identifies critical paths for the preservation of environmental species.

Ameliorating Effects of Graphene Oxide on Cadmium Accumulation and Eco-Physiological Characteristics in a Greening Hyperaccumulator (Lonicera japonica Thunb.)

Graphene oxide (GO), as a novel carbon-based nanomaterial (CBN), has been widely applied to every respect of social life due to its unique composite properties. The widespread use of GO inevitably promotes its interaction with heavy metal cadmium (Cd), and influences its functional behavior. However, little information is available on the effects of GO on greening hyperaccumulators under co-occurring Cd. In this study, we chose a typical greening hyperaccumulator (Lonicera japonica Thunb.) to show the effect of GO on Cd accumulation, growth, net photosynthesis rate (Pn), carbon sequestration and oxygen release functions of the plant under Cd stress. The different GO-Cd treatments were set up by (0, 10, 50 and 100 mg L-1) GO and (0, 5 and 25 mg L-1) Cd in solution culture. The maximum rate of Cd accumulation in the roots and shoots of the plant were increased by 10 mg L-1 GO (exposed to 5 mg L-1 Cd), indicating that low-concentration GO (10 mg L-1) combined with low-concentration Cd (5 mg L-1) might stimulate the absorption of Cd by L. japonica. Under GO treatments without Cd, the dry weight of root and shoot biomass, Pn value, carbon sequestration per unit leaf area and oxygen release per unit leaf area all increased in various degrees, especially under 10 mg L-1 GO, were 20.67%, 12.04%, 35% and 28.73% higher than the control. Under GO-Cd treatments, it is observed that the cooperation of low-concentration GO (10 mg L-1) and low-concentration Cd (5 mg L-1) could significantly stimulate Cd accumulation, growth, photosynthesis, carbon sequestration and oxygen release functions of the plant. These results indicated that suitable concentrations of GO could significantly alleviate the effects of Cd on L. japonica, which is helpful for expanding the phytoremediation application of greening hyperaccumulators faced with coexistence with environment of nanomaterials and heavy metals.

Mechanistic Approach on Melatonin-Induced Hormesis of Photosystem II Function in the Medicinal Plant Mentha spicata

Melatonin (MT) is considered a new plant hormone having a universal distribution from prokaryotic bacteria to higher plants. It has been characterized as an antistress molecule playing a positive role in the acclimation of plants to stress conditions, but its impact on plants under non-stressed conditions is not well understood. In the current research, we evaluated the impact of MT application (10 and 100 μM) on photosystem II (PSII) function, reactive oxygen species (ROS) generation, and chlorophyll content on mint (Mentha spicata L.) plants in order to elucidate the molecular mechanism of MT action on the photosynthetic electron transport process that under non-stressed conditions is still unclear. Seventy-two hours after the foliar spray of mint plants with 100 μM MT, the improved chlorophyll content imported a higher amount of light energy capture, which caused a 6% increase in the quantum yield of PSII photochemistry (ΦPSII) and electron transport rate (ETR). Nevertheless, the spray with 100 μM MT reduced the efficiency of the oxygen-evolving complex (OEC), causing donor-side photoinhibition, with a simultaneous slight increase in ROS. Even so, the application of 100 μM MT decreased the excess excitation energy at PSII implying superior PSII efficiency. The decreased excitation pressure at PSII, after 100 μM MT foliar spray, suggests that MT induced stomatal closure through ROS production. The response of ΦPSII to MT spray corresponds to a J-shaped hormetic curve, with ΦPSII enhancement by 100 μM MT. It is suggested that the hormetic stimulation of PSII functionality was triggered by the non-photochemical quenching (NPQ) mechanism that stimulated ROS production, which enhanced the photosynthetic function. It is concluded that MT molecules can be used under both stress and non-stressed conditions as photosynthetic biostimulants for enhancing crop yields.

Environmental hormesis in living systems: The role of hormetic trade-offs

Hormesis (low-dose stimulation and high-dose inhibition) can be accompanied by hormetic trade-offs, that is, stimulation of some traits and inhibition (trade-off 1) or invariability (trade-off 2) of others. Currently, trade-off options and their biological significance are insufficiently studied. Therefore, the review analyses trade-off types, their relationship with asynchronous stress responses of indicators, the importance of trade-offs for preconditioning, hormesis transgenerational effects, fitness, and evolution. The analysis has shown that hormetic trade-offs 1 and 2 can be observed in evolutionarily distant groups of organisms and at different biological levels (cells, individuals, populations, and communities) with abiotic and biotic stressors, as well as various pollutants. Trade-offs 1 and 2 are found both between different functional traits (e.g., self-maintenance and reproduction in animals, growth and defense in plants), and between the endpoints of the same functional trait (e.g., seed weight and seed number in plants). Asynchronous responses of indicators to a low-dose stressor can lead to hormetic trade-offs in two cases: 1) these indicators have different responses (hormesis, inhibition or zero reaction) in the same dose range; 2) these indicators have hormetic responses with different hormetic zones. Trade-offs can have a positive, negative or zero effect on preconditioning, offspring, and fitness of the population. Trade-offs can potentially affect evolution in two ways: 1) the creation of trends in genotype selection; 2) participation in the assimilation of phenotypic adaptations in the genotype through the Baldwin effect (selection of mutations copying adaptive phenotypes).

Sublethal chemical stimulation of arthropod parasitoids and parasites of agricultural and environmental importance

An increasing number of studies have reported stimulation of various organisms in the presence of environmental contaminants. This has created a need to critically evaluate sublethal stimulation and hormetic responses of arthropod parasitoids and parasites following exposure to pesticides and other contaminants. Examining this phenomenon with a focus on arthropods of agricultural and environmental importance serves as the framework for this literature review. This review shows that several pesticides, with diverse chemical structures and different modes of action, applied individually or in combination at sublethal doses, commonly stimulate an array of arthropod parasitoids and parasites. Exposure at sublethal doses can enhance responses related to physiology (e.g., respiration, total lipid content, and total protein content), behavior (e.g., locomotor activity, antennal drumming frequency, host location, and parasitization), and fitness (longevity, growth, fecundity, population net and gross reproduction). Concordantly, the parasitic potential (e.g., infestation efficacy, parasitization rate, and parasitoid/parasite emergence) can be increased, and as a result host activities inhibited. There is some evidence illustrating hormetic dose-responses, but the relevant literature commonly included a limited number and range of doses, precluding a robust differentiation between sub- and superNOAEL (no-observed-adverse-effect level) stimulation. These results reveal a potentially significant threat to ecological health, through stimulation of harmful parasitic organisms by environmental contaminants, and highlight the need to include sublethal stimulation and hormetic responses in relevant ecological pesticide risk assessments. Curiously, considering a more utilitarian view, hormesis may also assist in optimizing mass rearing of biological control agents for field use, a possibility that also remains neglected.

Patterns of salt transport and factors affecting typical shrub in desert-oases transition areas

Soil salinization and water deficits are considered the primary factors limiting economic development and environmental improvement in arid areas. However, there remains limited knowledge of the adaptability of typical shrubs to salinization of desert areas in arid zones. This study was conducted in a desert oasis transition zone (Tarim River, China), aiming to investigate: i) the spatial-temporal changes in soil salinity; ii) the interactions between the pedoenvironment vs typical shrub (Calligonum mongolicum). The van Genuchten soil salinity retention ensemble model (TVGSSREM-3D) was developed to simulate variations in soil water-salt transport in the desert-oasis zone and to accurately explain the main factors influencing Calligonum mongolicum desert-oases transition areas. The results showed that monthly average salinity ranged from 2.0 to 8.0 g kg-1, with a peak in August (9.17 g kg-1). The presence of human activities (Salt Drainage Canal) and the distribution of Calligonum mongolicum resulted in a clear spatial salinity zonation. Moreover, analysis of environmental indicators using the TVGSSREM-3D model revealed strong correlations between the distribution of salinity in Calligonum mongolicum desert-oases transition areas and groundwater depth (GD), minimum relative humidity (MRH), and water vapor pressure (WVP). These findings provide a scientific basis for stabilizing, restoring, and reconstructing the ecosystem of the oasis-desert transition zone.

Stimulation of insect vectors of pathogens by sublethal environmental contaminants: A hidden threat to human and environmental health?

Sublethal stimulation and hormetic responses are increasingly identified and acknowledged in scientific research. However, the occurrence and characteristics of such responses in insect vectors of pathogens are little explored and poorly understood. Here, we collate significant evidence from the scientific literature showing that sublethal doses of environmental contaminants, such as pesticides, microplastics, and plasticizers, induce stimulation and hormetic responses in insect vectors of pathogens of agricultural and public health importance, including mosquitoes, other dipterans, psyllids, aphids, and planthoppers. Physiological, behavioral, and demographic traits can be enhanced by exposure to lower subtoxic contaminant doses while being inhibited by higher toxic doses. Energetic trade-offs can also occur, especially at sublethal doses higher than the no-observed-adverse-effect level (NOAEL). The relevant literature is limited and so are the number of doses commonly included in the studies, precluding firm conclusions and enhanced understanding. Nevertheless, these effects are significant and could undermine human and environmental health, and thus sustainability agendas, if ultimately the transmission of pathogens and disease spread and severity are increased. Further research is urgently needed to tackle these phenomena, especially under field conditions. The findings discussed here are relevant to chemical risk assessment and chemical safety evaluations, in which all possible effects from the lowest to higher doses should be considered.

Plant hormesis: Revising of the concepts of biostimulation, elicitation and their application in a sustainable agricultural production

Current research in basic and applied knowledge of plant science has aimed to unravel the role of the interaction between environmental factors and the genome in the physiology of plants to confer the ability to overcome challenges in a climate change scenario. Evidence shows that factors causing environmental stress (stressors), whether of biological, chemical, or physical origin, induce eustressing or distressing effects in plants depending on the dose. The latter suggests the induction of the "hormesis" phenomenon. Sustainable crop production requires a better understanding of hormesis, its basic concepts, and the input variables to make its management feasible. This implies that acknowledging hormesis in plant research could allow specifying beneficial effects to effectively manage environmental stressors according to cultivation goals. Several factors have been useful in this regard, which at low doses show beneficial eustressing effects (biostimulant/elicitor), while at higher doses, they show distressing toxic effects. These insights highlight biostimulants/elicitors as tools to be included in integrated crop management strategies for reaching sustainability in plant science and agricultural studies. In addition, compelling evidence on the inheritance of elicited traits in plants unfolds the possibility of implementing stressors as a tool in plant breeding.

Antioxidant and drought-acclimation responses in UV-B-exposed transgenic Nicotiana tabacum displaying constitutive overproduction of H2O2

Hydrogen peroxide (H2O2) is an important molecule that regulates antioxidant responses that are crucial for plant stress resistance. Exposure to low levels of ultraviolet-B radiation (UV-B, 280-315 nm) can also activate antioxidant defenses and acclimation responses. However, how H2O2 and UV-B interact to promote stress acclimation remains poorly understood. In this work, a transgenic model of Nicotiana tabacum cv Xanthi nc, with elevated Mn-superoxide dismutase (Mn-SOD) activity, was used to study the interaction between the constitutive overproduction of H2O2 and a 14-day UV-B treatment (1.75 kJ m-2 d-1 biologically effective UV-B). Subsequently, these plants were subjected to a 7-day moderate drought treatment to evaluate the impact on drought resistance of H2O2- and UV-dependent stimulation of the plants' antioxidant system. The UV-B treatment enhanced H2O2 levels and altered the antioxidant status by increasing the epidermal flavonol index, Trolox Equivalent Antioxidant Capacity, and catalase, peroxidase and phenylalanine ammonia lyase activities in the leaves. UV-B also retarded growth and suppressed acclimation responses in highly H2O2-overproducing transgenic plants. Plants not exposed to UV-B had a higher drought resistance in the form of higher relative water content of leaves. Our data associate the interaction between Mn-SOD transgene overexpression and the UV-B treatment with a stress response. Finally, we propose a hormetic biphasic drought resistance response curve as a function of leaf H2O2 content in N. tabacum cv Xanthi.

Human milk oligosaccharide lacto-N-tetraose: Physiological functions and synthesis methods

Human milk oligosaccharides (HMOs) have attracted considerable attention due to their unique role in boosting infant health. Among the HMOs, lacto-N-tetraose (LNT) is a significant constituent associated with various health benefits, such as prebiotic effects, antiadhesive antimicrobials, antiviral protection, and immune modulators. LNT has received a "Generally Recognized as Safe" status by the American Food and Drug Administration and was approved as a food ingredient for infant formula. However, the limited availability of LNT poses a major challenge for its application in food and medicine. In this review, we first explored the physiological functions of LNT. Next, we describe several synthesis methods for production of LNT, including chemical, enzymatic, and cell factory approaches, and summarize the pivotal research results. Finally, challenges and opportunities for the large-scale synthesis of LNT were discussed.

The role of the ABF1 gene in regulation of Cd-induced hormesis in Arabidopsis thaliana

Hormesis is important in plant performance in contaminated environments, but the underlying genetic mechanisms are poorly understood. This study aimed at mining key genes in regulating Cd-induced hormesis in Arabidopsis thaliana and verifying their biological function. Hormesis of fresh weight, dry weight, and root length occurred at concentrations of 0.003-2.4, 0.03-0.6, and 0.03-0.6 µM Cd, respectively. Superoxide dismutase and catalase activities, and chlorophyll content displayed inverted U-shaped curves, indicating that the antioxidant defense system and photosynthesis system played roles in hormesis. Based on KEGG pathway analysis with the trend chart of differentially expressed genes and weighted correlation network analysis, the key gene ABF1 in the metabolic pathway of abscisic acid was identified. Subsequently, genetic experiments with wild, overexpressing, and knockdown lines of A. thaliana were conducted to further verify the biological function of ABF1 involving Cd-induced hormesis in A. thaliana. The results revealed that the resistance capability of the overexpressing type to Cd stress was significantly enhanced and implicated that the ABF1 gene is essential for Cd-induced hormesis in A. thaliana. Mining key genes that regulate Cd-induced hormesis in plants and stimulate them could have a transformative impact on the phytoremediation of metal-contaminated environments.

Heterologous codA Gene Expression Leads to Mitigation of Salt Stress Effects and Modulates Developmental Processes

Transgenic tobacco plants overexpressing the choline oxidase gene from A. globiformis showed an increase in resistance at the level of primary and secondary biosynthesis of metabolites, removing the damage characteristic of salinity and stabilizing the condition of plants. We used 200 mM NaCl, which inhibits the growth of tobacco plants at all stages of development. Leaves of transgenic and wild-type (WT) plants Nicotiána tabácum were used for biochemical, cytological and molecular biological analysis. However, for transgenic lines cultivated under normal conditions (without salinity), we noted juvenile characteristics, delay in flowering, and slowing down of development, including the photosynthetic apparatus. This caused changes in the amount of chlorophyll, a delay in the plastid grana development with the preservation of prolamellar bodies. It also caused changes in the amount of sugars and indirectly downstream processes. A significant change in the activity of antioxidant enzymes and a change in metabolism is probably compensated by the regulation of a number of genes, the expression level of which was also changed. Thus, the tolerance of transgenic tobacco plants to salinity, which manifested itself as a result of the constitutive expression of codA, demonstrates an advantage over WT plants, but in the absence of salinity, transgenic plants did not have such advantages due to juvenilization.

Hormetic dose responses induced by nickel oxide nanoparticles (NiONPs) on growth, biochemical, and antioxidant defense systems of Dracocephalum kotschyi

The application of nickel oxide nanoparticles (NiONPs) in various fields leads to their release into soil and water and, consequently, interaction with plants. Unlike its bulk counterpart, the phytotoxic potential of NiONPs is relatively less studied, particularly in a hormesis framework. Hormesis is an interesting phenomenon characterized by low-dose stimulation and high-dose inhibition. Therefore, this study demonstrates the stimulatory and inhibitory effects of NiONPs on Dracocephalum kotschyi Boiss as a medicinal plant cultivated in a pot experiment carried out in a greenhouse for 3 weeks. High bioaccumulation of nickel (Ni) in roots of treated plants relative to shoots indicates higher oxidative damage. NiONPs induced hormetic effects on photosynthetic pigments, as at low concentration of 50 mg/L stimulated chlorophyll (2.8-46.7%), carotenoid (16%), and anthocyanin (5.9%) contents and at higher concentrations inhibited the content of these pigments. A hormetic response was observed in growth parameters, i.e., NiONPs induced shoot height (7.2%) and weight (33%) at 100 mg/L, while inhibited shoot and root length (14.5-16.1% and 28.7-42.7%) and weight (46.8-48.1% and 37-40.6%), respectively, at 1000 and 2500 mg/L. The treated plants declined the toxic effects and oxidative stress caused by NiONPs by activating non-enzymatic antioxidants (phenolic compounds and proline) and enzymatic antioxidants, i.e., increasing the levels of SOD, POD, CAT, and APX. Therefore, the present study investigated for the first time the different mechanisms and responses of D. kotschyi plants to NiONPs in a wide range of concentrations. The results suggest that NiONPs may act as an elicitor at lower concentrations in medicinal plants according to specific conditions. However, these NPs at higher concentrations induce oxidative stress and harmful effects on plants, so their use poses serious risks to human health and the environment.

Effects of Cadmium Stress on Carbon Sequestration and Oxygen Release Characteristics in A Landscaping Hyperaccumulator-Lonicera japonica Thunb

The carbon sequestration and oxygen release of landscape plants are dominant ecological service functions, which can play an important role in reducing greenhouse gases, improving the urban heat island effect and achieving carbon peaking and carbon neutrality. In the present study, we are choosing Lonicera japonica Thunb. as a model plant to show the effects of Cd stress on growth, photosynthesis, carbon sequestration and oxygen release characteristics. Under 5 mg kg^-1 of Cd treatment, the dry weight of roots and shoots biomass and the net photosynthetic rate (P_N) in L. japonica had a significant increase, and with the increase in Cd treatment concentration, the dry weight of roots and shoots biomass and P_N in the plant began to decrease. When the Cd treatment concentration was up to 125 mg kg^-1, the dry weight of root and shoots biomass and P_N in the plant decreased by 5.29%, 1.94% and 2.06%, and they had no significant decrease compared with the control, indicating that the plant still had a good ability for growth and photoenergy utilization even under high concentrations of Cd stress. The carbon sequestration and oxygen release functions in terms of diurnal assimilation amounts (P), carbon sequestration per unit leaf area (W_CO2), oxygen release per unit leaf area (W_O2), carbon sequestration per unit land area (P_CO2) and oxygen release per unit land area (P_O2) in L. japonica had a similar change trend with the photosynthesis responses under different concentrations of Cd treatments, which indicated that L. japonica as a landscaping Cd-hyperaccumulator, has a good ability for carbon sequestration and oxygen release even under high concentrations of Cd stress. The present study will provide a useful guideline for effectively developing the ecological service functions of landscaping hyperaccumulators under urban Cd-contaminated environment.

Combination of slurry-bioreactors and actinobacteria consortia as strategy to bioremediate chlordane-contaminated soils

Soil contamination caused by pesticides poses a significant environmental challenge, and addressing it requires effective solutions. Bioremediation, combining the utilization of slurry-bioreactors and microbial consortia, emerges as an appropiated strategy to tackle this issue. Therefore, this research evaluated the chlordane (CLD) removal efficiency by a Streptomyces consortium through bioaugmentation of polluted soils, and slurry-bioreactors. For that, a Streptomyces defined consortium with CLD removal abilities was inoculated in soil microcosms and soil-slurry bioreactors (SB), with (SB-TSB) and without stimulation (SB-water). In soil, CLD presence has no negative effect on consortium growth. This was supported by comparing its duplication time (7.48 ± 0.14 h) with the obtained in the biotic control (7.45 ± 0.04 h). Furthermore, 17% of pesticide removal by microbial action was detected in the treated microcosms. In SB, the microbial development was not affected by the pesticide presence. In SB-TSB, the microbial growth was higher than in SB-water. This was supported by its lesser duplication time (7.27 ± 0.17 h) with respect to the non-stimulated systems (10.88 ± 0.29 h). However, SB-water showed the highest CLD removal ability (34.8%), with a concomitant increase in the chloride ion release. In the phytotoxicity test, the vigor index showed that the bioremediation in SB-water did not exert adverse effects greater than those generated by the CLD. Indeed, the root length increased after the treatment. These findings demonstrate the versatility of the Streptomyces consortium to remediate solid and semi-solid matrices impacted with pesticides, and the advantage of using bioaugmented SB to enhance the pollutants removal and accelerating the clean-up time required.

Hormesis effects in tomato plant growth and photosynthesis due to acephate exposure based on physiology and transcriptomic analysis

BACKGROUND

Hormesis is a common phenomenon in toxicology described as low-dose stimulation due to a toxin which causes inhibition at a high dose. Pesticide hormesis in plants has attracted considerable research interest in recent years; however, the specific mechanism has not yet been clarified. Acephate is an organophosphorus insecticide that is used worldwide. Here, hormesis in tomato (Solanum lycopersicum L.) plant growth and photosynthesis after acephate exposure is confirmed, as stimulation occurred at low stress levels, whereas inhibition occurred after exposure to high concentrations.

RESULTS

We found that low acephate concentration (5-fold lower than recommended application dosage) could enhance chlorophyll biosynthesis and stimulate photosynthesis effects, and thus improve S. lycopersicum growth. A high level of acephate (5-fold higher than recommended application dosage) stress inhibited chlorophyll accumulation, decreased photosystem II efficiency and blocked antioxidant reactions in leaves, increasing reactive oxygen species levels and damaging plant growth. Transcriptomic analysis and quantitative real-time PCR results revealed that the photosynthesis - antenna proteins pathway played a crucial role in the hormesis effect, and that LHCB7 as well as LHCP from the pathway were the most sensitive to acephate hormesis.

CONCLUSION

Our results showed that acephate could induce hormesis in tomato plant growth and photosynthesis, and that photosystem II and the photosynthesis - antenna proteins pathway played important roles in hormesis. These results provide novel insights into the scientific and safe application of chemical pesticides, and new guidance for investigation into utilizing pesticide hormesis in agriculture. © 2023 Society of Chemical Industry.

Ethylenediurea protects against ozone phytotoxicity not by adding nitrogen or controlling stomata in a stomata-unresponsive hybrid poplar

Ozone (O₃) pollution is a persistent environmental issue worldwide, which causes widespread damage to vegetation, deteriorating plant health and reducing plant productivity. Ethylenediurea (EDU) is a synthetic chemical that has been widely applied in scientific studies as a protectant against O₃ phytotoxicities. Despite four decades of active research, the exact mechanisms to explain its mode of action remain unclear. Here, we aimed to reveal whether EDU's phytoprotective property is due to its control over stomatal regulation and/or its action as a nitrogen (N) fertilizer, utilizing stomatal-unresponsive plants of a hybrid poplar (Populus koreana × trichocarpa cv. Peace) grown in a free-air O₃-concenctration enrichment (FACE) facility. Plants were treated with water (WAT), EDU (400 mg L^-1), or EDU's constitutive amount of N every nine days, and exposed to ambient (AOZ) or elevated (EOZ) O₃ during a growing season (June-September). EOZ led to extensive foliar injuries (but protected against rust disease), lower photosynthetic rate (A), impaired dynamics of responses of A to changes in light intensity, and smaller total plant leaf area. EDU protected against common phytotoxicities caused by EOZ without inducing stomatal closure, since stomatal conductance (g_s) was generally unresponsive to the experimental treatments. EDU also modulated the dynamic response of A to light fluctuations under O₃ stress. N addition acted as a fertilizer but did not satisfactorily protect plants against O₃ phytotoxicities. The results suggest that EDU protects against O₃ phytotoxicity not by adding N or controlling stomata, which provides a new insight into our understanding of the mode of action of EDU as a protectant against O₃ phytotoxicity.

Do changes in Lactuca sativa metabolic performance, induced by mycorrhizal symbionts and leaf UV-B irradiation, play a role towards tolerance to a polyphagous insect pest?

The increased ultraviolet radiation (UV) due to the altered stratospheric ozone leads to multiple plant physiological and biochemical adaptations, likely affecting their interaction with other organisms, such as pests and pathogens. Arbuscular mycorrhizal fungi (AMF) and UV-B treatment can be used as eco-friendly techniques to protect crops from pests by activating plant mechanisms of resistance. In this study, we investigated plant (Lactuca sativa) response to UV-B exposure and Funneliformis mosseae (IMA1) inoculation as well as the role of a major insect pest, Spodoptera littoralis. Lettuce plants exposed to UV-B were heavier and taller than non-irradiated ones. A considerable enrichment in phenolic, flavonoid, anthocyanin, and carotenoid contents and antioxidant capacity, along with redder and more homogenous leaf color, were also observed in UV-B-treated but not in AMF-inoculated plants. Biometric and biochemical data did not differ between AMF and non-AMF plants. AMF-inoculated plants showed hyphae, arbuscules, vesicles, and spores in their roots. AMF colonization levels were not affected by UV-B irradiation. No changes in S. littoralis-feeding behavior towards treated and untreated plants were observed, suggesting the ability of this generalist herbivore to overcome the plant chemical defenses boosted by UV-B exposure. The results of this multi-factorial study shed light on how polyphagous insect pests can cope with multiple plant physiological and biochemical adaptations following biotic and abiotic preconditioning.

Hormesis Responses of Growth and Photosynthetic Characteristics in Lonicera japonica Thunb. to Cadmium Stress: Whether Electric Field Can Improve or Not?

"Hormesis" is considered a dose-response phenomenon mainly observed at hyperaccumulator plants under heavy metals stress. In this study, the effects of electric fields on hormesis responses in Lonicera japonica Thunb. under cadmium (Cd) treatments were investigated by assessing the plant growth and photosynthetic characteristics. Under Cd treatments without electric fields, the parameters of plant growth and photosynthetic characteristics increased significantly when exposed to 5 mg L^-1 Cd, and decreased slightly when exposed to 25 mg L^-1 Cd, showing an inverted U-shaped trend, which confirmed that low concentration Cd has a hormesis effect on L. japonica. Under electric fields, different voltages significantly promoted the inverted U-shaped trend of the hormesis effect on the plant, especially by 2 V cm^-1 voltage. Under 2 V cm^-1 voltage, the dry weight of the root and leaf biomass exposed to 5 mg L^-1 Cd increased significantly by 38.38% and 42.14%, and the photosynthetic pigment contents and photosynthetic parameters were also increased significantly relative to the control, indicating that a suitable electric field provides better improvements for the hormesis responses of the plant under Cd treatments. The synergistic benefits of the 5 mg L^-1 Cd and 2 V cm^-1 electric field in terms of the enhanced hormesis responses of growth and photosynthetic characteristics could contribute to the promoted application of electro-phytotechnology.

Ecological risk threshold for Pb in Chinese soils

Derivation of ecological risk threshold (the threshold concentration value that protect a certain proportion of species within the acceptable hazard level) of lead (Pb) is a yardstick and plays a key role in formulating soil protection policies, while the research about deducing soil Pb ecological risk threshold is still limited. In this study, toxicological data of Pb based on 30 different test endpoints was collected from our experiment and literature, and applied into interspecific extrapolation by species sensitivity distribution (SSD) method to derive the hazard concentration for 5% of species (HC₅, that can protect 95% of species), the prediction models according to different soil properties were established. The results showed that EC₁₀ (the effective concentrations of Pb that inhibit 10% of endpoint bioactivity) ranged from 205.6 to 1596.3 mg kg¹, and hormesis induced by Pb were up to 118%. Toxicity data were corrected by leaching and aging process before SSD curves fitting. HC₅ was then derived and prediction model was developed, as LogHC₅ = 0.134 pH + 0.315 LogOC + 0.324 LogCEC + 1.077. The prediction model was well verified in the field test, indicating that can correctly estimate Pb ecotoxicity thresholds in different soils. This study provides a scientific frame for deriving the ecological risk threshold of Pb and is of great significance for ecological species protection.

Thiamethoxam in soybean seed treatment: Plant bioactivation and hormesis, besides whitefly control?

Amid concerns on the myriad of existing chemical stressors in agroecosystems, pesticides and particularly neonicotinoid insecticides are in the forefront. Despite that, these neurotoxic compounds remain the dominant group of insecticides in worldwide use with the added versatility of use in seed coatings. Such use sparks environmental concerns counterbalanced by their reported insecticidal efficacy and potential plant bioactivation. Nonetheless, this alleged double benefit and interconnection expected with neonicotinoids has been little explored particularly when the whole plant phenology is considered. Regardless of the expected efficacy against targeted insect pest species, like whiteflies, neonicotinoids may spark dual effect on plants - negative at higher concentrations, positive at low concentrations, which is consistent with the hormesis phenomenon that may be expressed as a plant bioactivation. This effect may also cascade to the targeted insect species, what deserves attention. Therefore, soybean seeds treated with increasing concentrations of the neonicotinoid thiamethoxam were followed throughout their development in greenhouse, recording the plant response and yield, besides their effect in whiteflies (Bemisia tabaci MEAM1). Thiamethoxam application was correlated to leaf contents of thiamethoxam and its metabolite clothianidin. Plant hormesis was found for leaf area and root growth, but not for other plant morphological or physiological parameters, nor plant yield. The insecticide concentration-dependency compromised whitefly population growth without evidence of cascading any plant-mediated hormesis to the insects. Thus, although plant hormesis was recognized with thiamethoxam in treated soybean seeds in relevant parameters, no evidence of plant bioactivation was observed to justify its use with such a secondary objective, nor did this hormesis impair whitefly control.

The Hormetic Response of Soil P Extraction Induced by Low-Molecular-Weight Organic Acids

The hormetic effect is a toxicological phenomenon in the soil ecosystem. The influence of low-molecular-weight organic acids (LMWOAs) on the release and activation of soil phosphorus (P) has become the focus of toxicological research. To what extent LMWOAs can regulate the hormetic effect of P release and then influence soil P nutrients is worth attention. This study aimed to investigate the effects of different types and concentrations of exogenous LMWOAs on P extraction, establish the relationship between the concentration of LMWOAs and P extraction efficiency, and calculate the hormetic parameters to understand the mechanism of types and concentrations of LMWOAs in P extraction efficiency. Four organic acids, i.e., citric, oxalic, tartaric, and malic acids, induced hormetic effects on P extraction that were concentration dependent. The relationship between LMWOAs and P extraction efficiency was explained by a quadratic polynomial equation. The critical threshold of citric acid concentration was similar to that of oxalic acid, whereas that of tartaric acid was similar to that of malic acid. The critical thresholds of the P concentration extracted by malic acid and citric acid were higher than those extracted by oxalic acid and tartaric acid due to the differences in the structure and properties of LMWOAs. The critical thresholds of P extraction efficiency of oxalic acid were lower than those of the other three organic acid types. These results provide evidence for the use of citric acid and malic acid to increase soil P.

Transfer pattern of hormesis into personal care product mixtures from typical hormesis-inducing compounds

Some personal care products (PCPs) and their chemical components showed a hormetic effect in the freshwater photobacterium Vibrio qinghaiensis sp. -Q67 (Q67) after long-term exposure. However, how hormesis transfers between chemical components and PCP mixture, and which chemical component plays a major role remain unknown. To this end, according to the seven compounds detected in one skin lotion (SK5) and their concentration ratios, many mixture rays were constructed to simulate the SK5. Of these seven compounds, three presented monotonic concentration-response curves (CRC) to Q67 at 0.25 and 12 h (called a S-shaped compound). The other four compounds showed hormetic CRCs after 12 h and monotonic CRCs at 0.25 h (called a J-shaped compound). Based on their mixture ratios, we designed one ternary mixture ray of all S-shaped compounds, one quaternary mixture ray of all J-shaped compounds, and four quaternary mixture rays of one J-shaped and three S-shaped compounds. It was shown that SK5 could be approximately simulated by the mixture ray of the seven compounds detected in SK5 and only the mixture rays containing at least one hormesis-inducing compound produced hormesis to Q67 at 12 h. Based on the concentration ratios of various compounds and comparison of four hormetic characteristic parameters to those of various mixture rays, it was found that the compound betaine (BET) is a key compound affecting the hormesis of mixtures. Additionally, we studied the hormesis mechanism of BET on Q67 via quorum sensing (QS). This preliminarily indicated that the autoinducer-2 triggered the QS pathway. This study elucidated the transfer pattern of hormesis into mixtures, which would be an efficient method to identifying the potential components that affect hormesis transfer in mixtures. We expect that this study will provide new insights into hormesis and its mixtures.

Combined effects of increased water temperature and cyanobacterial compounds exert heterogeneous effects on survival and ecological processes in key freshwater species

Climate change is increasing water temperature and intensifying the incidence of cyanobacterial blooms worldwide. However, the combined effects of increased temperature and microcystin concentrations as co-stressors on survival and ecological processes in freshwater species are unclear. Here, using purified MC-LR and crude extract of toxigenic Microcystis aeruginosa, we tested the individual and combined effects of three water temperatures (15, 20, 25 °C) and a range of environmentally relevant concentrations of dissolved microcystin and crude extract (0.01-10 µg·L-1) on survival, growth inhibition, grazing and predation rates in three freshwater species: phytoplankton (Scenedesmus quadricauda), zooplankton (Daphnia pulex), and an invertebrate predator (Ischnura elegans). Purified MC-LR exerted a higher growth inhibitory effect on S. quadricauda compared to crude extract with the same concentration of MC-LR, while neither treatment affected its chlorophyll-a content or survival of D. pulex. Crude extract reduced grazing and survival of D. pulex and I. elegans, respectively. The combined effect of higher temperature and crude extract reduced I. elegans survival by 50%. Increased temperature reduced prey handing time in I. elegans by 49%, suggesting a higher predation rate. However, warming together with higher concentrations of crude extract jointly increased zooplankton grazing and reduced damselfly predation. Taken together, these results suggest crude extract, and not necessarily microcystin, can affect survival and productivity in freshwater species, although these effects may vary unevenly across trophic levels. Our findings highlight the importance of complex ecological mechanisms by which warming can exacerbate toxic effects of cyanobacterial bloom extracts on survival and functions among species in eutrophic freshwaters.

Interplay of an array of salt-responding mechanisms in Iranian borage: Evidence from physiological, biochemical, and histochemical examinations

In order to address the lacuna of data on the response of physiological and biochemical attributes and chemical compounds of glandular trichomes of Iranian borage (Echium amoenum Fisch. & C.A.Mey.) to saline water (0, 25, 50, 75, and 100 mM NaCl) an experiment was conducted on 13 genotypes. Genotypic differences and salt-induced modifications in chlorophyll concentration and fluorescence, plant growth, relative water content, proline concentration, antioxidant defense, and chemical compounds of glandular trichomes upon exposure to salt stress were observed. Chlorophyll and carotenoids concentrations and catalase (EC 1.11.1.6) and ascorbate peroxidase (EC 1.11.1.11) activities were either enhanced or remained unchanged in the presence of moderate salt concentrations (i.e. 25 and 50 mM NaCl) in a majority of the genotypes. Though, 75 and 100 mM NaCl were modestly and severely detrimental, respectively, to the majority of the genotypes. The 75 and 100 mM NaCl led to substantial increases and decreases in the Na+ and K+, respectively, resulting in notable increase in the Na+/K+. Increases in proline, total phenolic compounds, and alkaloids concentrations, essential oils, alkaloids, and phenolic compounds of the glandular trichomes were concomitant to decreases in the relative water content, leaf area, maximum quantum efficiency of photosystem II, shoot and root dry masses. This study revealed, for the first time, that Iranian borage tolerates 25 and 50 mM NaCl and antioxidative enzymes as well as secondary metabolites such as alkaloids and phenolic compounds accumulated mainly in the trichomes play key role in this regard.

Quantitative Analysis of Plant Cytosolic Calcium Signals in Response to Water Activated by Low-Power Non-Thermal Plasma

Non-thermal plasma technology is increasingly being applied in the plant biology field. Despite the variety of beneficial effects of plasma-activated water (PAW) on plants, information about the mechanisms of PAW sensing by plants is still limited. In this study, in order to link PAW perception to the positive downstream responses of plants, transgenic Arabidopsis thaliana seedlings expressing the Ca²⁺-sensitive photoprotein aequorin in the cytosol were challenged with water activated by low-power non-thermal plasma generated by a dielectric barrier discharge (DBD) source. PAW sensing by plants resulted in the occurrence of cytosolic Ca²⁺ signals, whose kinetic parameters were found to strictly depend on the operational conditions of the plasma device and thus on the corresponding mixture of chemical species contained in the PAW. In particular, we highlighted the effect on the intracellular Ca²⁺ signals of low doses of DBD-PAW chemicals and also presented the effects of consecutive plant treatments. The results were discussed in terms of the possibility of using PAW-triggered Ca²⁺ signatures as benchmarks to accurately modulate the chemical composition of PAW in order to induce environmental stress resilience in plants, thus paving the way for further applications in agriculture.

Low Levels of Contaminants Stimulate Harmful Algal Organisms and Enrich Their Toxins

A widespread increase in intense phytoplankton blooms has been noted in lakes worldwide since the 1980s, with the summertime peak intensity amplifying in most lakes. Such blooms cause annual economic losses of multibillion USD and present a major challenge, affecting 11 out of the 17 United Nations Sustainable Development Goals. Here, we evaluate recent scientific evidence for hormetic effects of emerging contaminants and regulated pollutants on Microcystis sp., the most notorious cyanobacteria forming harmful algal blooms and releasing phycotoxins in eutrophic freshwater systems. This new evidence leads to the conclusion that pollution is linked to algal bloom intensification. Concentrations of contaminants that are considerably smaller than the threshold for toxicity enhance the formation of harmful colonies, increase the production of phycotoxins and their release into the environment, and lower the efficacy of algaecides to control algal blooms. The low-dose enhancement of microcystins is attributed to the up-regulation of a protein controlling microcystin release (McyH) and various microcystin synthetases in tandem with the global nitrogen regulator Ycf28, nonribosomal peptide synthetases, and several ATP-binding cassette transport proteins. Given that colony formation and phycotoxin production and release are enhanced by contaminant concentrations smaller than the toxicological threshold and are widely occurring in the environment, the effect of contaminants on harmful algal blooms is more prevalent than previously thought. Climate change and nutrient enrichment, known mechanisms underpinning algal blooms, are thus joined by low-level pollutants as another causal mechanism.

A Conserved Mechanism for Hormesis in Molecular Systems

Hormesis refers to dose-response phenomena where low dose treatments elicit a response that is opposite the response observed at higher doses. Hormetic dose-response relationships have been observed throughout all of biology, but the underlying determinants of many reported hormetic dose-responses have not been identified. In this report, we describe a conserved mechanism for hormesis on the molecular level where low dose treatments enhance a response that becomes reduced at higher doses. The hormetic mechanism relies on the ability of protein homo-multimers to simultaneously interact with a substrate and a competitor on different subunits at low doses of competitor. In this case, hormesis can be observed if simultaneous binding of substrate and competitor enhances a response of the homo-multimer. We characterized this mechanism of hormesis in binding experiments that analyzed the interaction of homotrimeric proliferating cell nuclear antigen (PCNA) with uracil DNA glycosylase (UNG2) and a fluorescein-labeled peptide. Additionally, the basic features of this molecular mechanism appear to be conserved with at least two enzymes that are stimulated by low doses of inhibitor: dimeric BRAF and octameric glutamine synthetase 2 (GS2). Identifying such molecular mechanisms of hormesis may help explain specific hormetic responses of cells and organisms treated with exogenous compounds.

Hormesis induced by silver iodide, hydrocarbons, microplastics, pesticides, and pharmaceuticals: Implications for agroforestry ecosystems health

Increasing amounts of silver iodide (AgI) in the environment are expected because of the recent massive expansion of weather modification programs. Concurrently, pharmaceuticals, microplastics, hydrocarbons, and pesticides in terrestrial ecosystems continue contaminating forests and agroforests. Our review supports that AgI induces hormesis, a biphasic dose response characterized by often beneficial low-dose responses and toxic high-dose effects, which adds to the evidence for pharmaceuticals, microplastics, hydrocarbons, and pesticides induced hormesis in numerous species. Doses smaller than the no-observed-adverse-effect-level (NOAEL) positively affect defense physiology, growth, biomass, yields, survival, lifespan, and reproduction. They also lead to negative or undesirable outcomes, including stimulation of pathogenic microbes, pest insects, and weeds with enhanced resistance to drugs and potential negative multi- or trans-generational effects. Such sub-NOAEL effects perplex terrestrial ecosystems managements and may compromise combating outbreaks of disease vectors that can threaten not only forest and agroforestry health but also sensitive human subpopulations living in remote forested areas.

Hormetic dose responses induced by organic flame retardants in aquatic animals: Occurrence and quantification

The organic flame retardants (OFRs) have attracted global concerns due to their potential toxicity and ubiquitous presence in the aquatic environment. Hormesis refers to a biphasic dose response, characterized by low-dose stimulation and high-dose inhibition. The present study provided substantial evidence for the widespread occurrence of OFRs-induced hormesis in aquatic animals, including 202 hormetic dose response relationships. The maximum stimulatory response (MAX) was commonly lower than 160% of the control response, with a combined value of 134%. Furthermore, the magnitude of MAX varied significantly among multiple factors and their interactions, such as chemical types and taxonomic groups. Moreover, the distance from the dose of MAX to the no-observed-adverse-effect-level (NOAEL) (NOAEL: MAX) was typically below 10-fold (median = 6-fold), while the width of the hormetic zone (from the lowest dose inducing hormesis to the NOAEL) was approximately 20-fold. Collectively, the quantitative features of OFRs-induced hormesis in aquatic animals were in accordance with the broader hormetic literature. In addition, the implications of hormetic dose response model for the risk assessment of OFRs were discussed. This study offered a novel insight for understanding the biological effects of low-to-high doses of OFRs on aquatic animals and assessing the potential risks of OFRs in the aquatic environment.

Identification of the hormetic dose-response and regulatory network of multiple metals co-exposure-related hypertension via integration of metallomics and adverse outcome pathways

Environmental stressors, including heavy metals, can be associated with hypertension development. However, little information regarding the dose-response relationship and toxicity mechanisms of metal mixtures with hypertension development is currently available. Therefore, we recruited 940 participants from six factories in northeastern China and measured the urinary concentrations of 19 metals. Then, we used Bayesian kernel machine regression (BKMR) to explore associations between metals co-exposure and hypertension. The BKMR model indicated a hermetic dose-response relationship between eight urinary metals (Co, Cr, Ni, Cd, As, Fe, Zn, and Pb) and hypertension risk. Moreover, heterogeneous and non-linear association patterns were detected across different metals/metalloids concentrations. Next, for the first time, we analyzed data of chemicals containing specific metal elements in the Comparative Toxicogenomics Database (CTD) from a disease perspective and provided insights from various biological levels to explain heavy metal co-exposure-related hypertension. On the molecular scale, 43 chemical components and 112 potential target genes were detected for metal exposure-related hypertension. Further, the network topology analysis indicated that target genes such as insulin (INS, degree = 78), albumin (ALB, degree = 74), renin (REN, degree = 71), interleukin-6 (IL6, degree = 70), endothelin 1 (EDN1, degree = 70), and endothelial nitric oxide synthase (NOS3, degree = 69) have a strong correlation with heavy metals co-exposure. Finally, we used integrative analyses in the adverse outcome pathway (AOP) wiki to analyze the co-exposure of heavy metals and hypertension and support an integrated metallomics approach. We selected the AOP 149 as the framework and found that the molecular initiating events (MIEs) of hypertension stems from the oxidation of AA residues on critical peptides of the NO pathway. The NOS3 was particularly promising since its subunit has three metal ion cross-linking domains with Zn²⁺, Fe²⁺, and Ga³⁺, which might serve as a binding site for heavy metal ions.

The hormetic response of heart rate of fish embryos to contaminants - Implications for research and policy

Evidence of contaminant-induced hormesis is rapidly accumulating, while the underlying mechanisms of hormesis are becoming increasingly understood. Recent developments in this research area, and especially the emergence of the nuclear factor-erythroid factor 2-related factor 2 (Nrf2) as the master mechanism, suggest that contaminants can induce cardiac hormetic responses. This paper collates significant evidence of hormetic response of the heart rate of fish embryos to contaminants, in particular antibiotics, microplastics, and herbicides, characterized by a low-dose increase (tachycardia) and a high-dose decrease (bradycardia). The increase often occurs at doses about 100-800 times smaller than the no-observed-adverse-effect-level (NOAEL). There are also indications for even triphasic responses, which include a sub-hormetic decrease of the heart rate by doses over 10⁶ times smaller than the NOAEL. Such sub-NOAEL effects cannot be captured by linear-no-threshold (LNT) and threshold models, raising concerns about environmental health and highlighting the pressing need to consider hormetic responses in the ecological risk assessment. A visionary way forward is proposed, but addressing this research bottleneck would require improved research designs with enhanced ability and statistical power to study diphasic and triphasic responses of heart rate.

Atmospheric Pb induced hormesis in the accumulator plant Tillandsia usneoides

While numerous studies reported hormesis in plants exposed to heavy metals, metals were commonly added in the growth substrate (e.g. soil or solution). The potential of heavy metals in the atmosphere to induce hormesis in plants, however, remains unknown. In this study, we exposed the widely-used accumulator plant Tillandsia usneoides to 10 atmospheric Pb concentrations (0-25.6 μg·m^-3) for 6 or 12 h. Three types of dose-response relationships between different response endpoints (biomarkers) and Pb concentrations were found for T. usneoides. The first was a monophasic dose response, in which the response increased linearly with increasing Pb concentrations, as seen for metallothionein (MT) content after a 6-h exposure. The second and dominating type was a biphasic-hormetic dose response, exhibited by malondialdehyde (MDA), superoxide anion radical (O₂^-), and superoxide dismutase (SOD) after 6 or 12 h of exposure and by glutathione (GSH) and MT content after 12 h of treatment. The third type was a triphasic dose response, as seen for leaf electric conductivity after 6 or 12 h of exposure and GSH after 6 h of exposure. This finding suggests that Pb inhibited the response of T. usneoides at very low concentrations, stimulated it at low-to-moderate concentrations, and inhibited it at higher concentrations. Our results demonstrate diverse adaptation mechanisms of plants to stress, in the framework of which alternating between up- and down-regulation of biomarkers is at play when responding to different levels of toxicants. The emergence of the triphasic dose response will further enhance the understanding of time-dependent hormesis.

Ethylenediurea offers moderate protection against ozone-induced rice yield loss under high ozone pollution

Tropospheric ozone (O₃) is the main phytotoxic air pollutant threatening food security, while ethylenediurea (EDU) can effectively mitigate O₃-induced crop yield loss. EDU's mode of action, however, remains unclear, and the underlying physiological mechanisms of mitigating O₃-induced crop yield loss are poorly understood. We cultivated hybrid rice seedlings under two O₃ treatments (NF, nonfiltered ambient air; and NF60, ambient air plus 60 ppb O₃) and sprayed foliage with 0 or 450 ppm EDU every ten days and determine photosynthesis-related traits, biomass indicators, and yield components. We found that EDU significantly increased the leaf nitrogen (N) allocation to photosynthesis (N_P) and the grain N accumulation, while the grain N accumulation was positively correlated with N_P and root biomass. EDU significantly increased the rice yield mainly by increasing the individual grain weight rather than the number of panicles and grains. While EDU protected from yield loss, the degree of protection was only 31% under NF60 treatment, thus EDU was unable to offer complete protection under high O₃ pollution. These results will be conducive to a better understanding of the EDU protection mechanism and better application of EDU under high O₃ pollution in the future.