Induction of hormesis in plants by urban trace metal pollution

Comparative assessment of pantothenic, aspartic, ascorbic and tartaric acids assisted Pb-phytoextraction by sunflower (Helianthus annuus L.)

Phytoextraction of lead (Pb) is a challenging task due to its extremely low mobility within soil and plant systems. In this study, we tested the influence of some novel chelating agents for Pb-phytoextraction using sunflower. The Pb was applied at control (0.0278 mM) and 4.826 mM Pb as Pb(NO3)2 through soil-spiking. After 10 days of Pb addition, four different organic ligands (aspartic, ascorbic, tartaric, and pantothenic acids) were added to the soil at 1 mM concentration each. respectively. In the absence of any chelate, sunflower plants grown at 4.826 mM Pb level accumulated Pb concentrations up to 104 µg g-1 DW in roots, whereas 64 µg g-1 DW in shoot. By contrast, tartaric acid promoted significantly Pb accumulation in roots (191 µg g-1 DW; + 45.5%) and shoot (131.6 µg g-1 DW; + 51.3%). Pantothenic acid also resulted in a significant Pb-uptake in the sunflower shoots (123 µg g-1 DW; + 47.9%) and in roots (177.3 µg g-1 DW; + 41.3%). The least effective amongst the chelates tested was aspartic acid, but it still contributed to + 40.1% more Pb accumulation in the sunflower root and shoots. In addition, plant growth, biochemical, and ionomic parameters were positively regulated by the organic chelates used. Especially, an increase in leaf Ca, P, and S was evident in Pb-stressed plants in response to chelates. These results highlight that the use of biocompatible organic chelates positively alters plant physio-biochemical traits contributing to higher Pb-sequestration in sunflower plant parts.

The promoting effects of soil microplastics on alien plant invasion depend on microplastic shape and concentration

Both alien plant invasions and soil microplastic pollution have become a concerning threat for terrestrial ecosystems, with consequences on the human well-being. However, our current knowledge of microplastic effects on the successful invasion of plants remains limited, despite numerous studies demonstrating the direct and indirect impacts of microplastics on plant performance. To address this knowledge gap, we conducted a greenhouse experiment involving the mixtures of soil and low-density polyethylene (LDPE) microplastic pellets and fragments at the concentrations of 0, 0.5 % and 2.0 %. Additionally, we included Solidago decurrens (native plant) and S. canadensis (alien invasive plant) as the target plants. Each pot contained an individual of either species, after six-month cultivation, plant biomass and antioxidant enzymes, as well as soil properties including soil moisture, pH, available nutrient, and microbial biomass were measured. Our results indicated that microplastic effects on soil properties and plant growth indices depended on the Solidago species, microplastic shapes and concentrations. For example, microplastics exerted positive effects on soil moisture of the soil with native species but negative effects with invasive species, which were impacted by microplastic shapes and concentrations, respectively. Microplastics significantly impacted catalase (P < 0.05) and superoxide dismutase (P < 0.01), aboveground biomass (P < 0.01), and belowground/aboveground biomass (P < 0.01) of the native species depending on microplastic shapes, but no significant effects on those of the invasive species. Furthermore, microplastics effects on soil properties, nutrient, nutrient ratio, and plant antioxidant enzyme activities contributed to plant biomass differently among these two species. These results suggested that the microplastics exerted a more pronounced impact on native Solidago plants than the invasive ones. This implies that the alien invasive species displays greater resistance to microplastic pollution, potentially promoting their invasion. Overall, our study contributes to a better understanding of the promoting effects of microplastic pollution on plant invasion.

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.

Toxic effects of lead on plants: integrating multi-omics with bioinformatics to develop Pb-tolerant crops

MAIN CONCLUSION

Lead disrupts plant metabolic homeostasis and key structural elements. Utilizing modern biotechnology tools, it's feasible to develop Pb-tolerant varieties by discovering biological players regulating plant metabolic pathways under stress. Lead (Pb) has been used for a variety of purposes since antiquity despite its toxic nature. After arsenic, lead is the most hazardous heavy metal without any known beneficial role in the biological system. It is a crucial inorganic pollutant that affects plant biochemical and morpho-physiological attributes. Lead toxicity harms plants throughout their life cycle and the extent of damage depends on the concentration and duration of exposure. Higher levels of lead exposure disrupt numerous key metabolic activities of plants including oxygen-evolving complex, organelles integrity, photosystem II connectivity, and electron transport chain. This review summarizes the detrimental effects of lead toxicity on seed germination, crop growth, and yield, oxidative and ultra-structural alterations, as well as nutrient absorption, transport, and assimilation. Further, it discusses the Pb-induced toxic modulation of stomatal conductance, photosynthesis, respiration, metabolic-enzymatic activity, osmolytes accumulation, and antioxidant activity. It is a comprehensive review that reports on omics-based studies along with morpho-physiological and biochemical modifications caused by lead stress. With advances in DNA sequencing technologies, genomics and transcriptomics are gradually becoming popular for studying Pb stress effects in plants. Proteomics and metabolomics are still underrated and there is a scarcity of published data, and this review highlights both their technical and research gaps. Besides, there is also a discussion on how the integration of omics with bioinformatics and the use of the latest biotechnological tools can aid in developing Pb-tolerant crops. The review concludes with core challenges and research directions that need to be addressed soon.

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.

Investigation of the role of TmMYB16/123 and their targets (TmMTP1/11) in the tolerance of Taxus media to cadmium

The toxicity and stress caused by heavy metal contamination has become an important constraint to the growth and flourishing of trees. In particular, species belonging to the genus Taxus, which are the only natural source for the anti-tumor medicine paclitaxel, are known to be highly sensitive to environmental changes. To investigate the response of Taxus spp. to heavy metal stress, we analyzed the transcriptomic profiles of Taxus media trees exposed to cadmium (Cd2+). In total, six putative genes from the metal tolerance protein (MTP) family were identified in T. media, including two Cd2+ stress inducible TMP genes (TmMTP1, TmMTP11 and Taxus media). Secondary structure analyses predicted that TmMTP1 and TmMTP11, which are members of the Zn-CDF and Mn-CDF subfamily proteins, respectively, contained six and four classic transmembrane domains, respectively. The introduction of TmMTP1/11 into the ∆ycf1 yeast cadmium-sensitive mutant strain showed that TmMTP1/11 might regulate the accumulation of Cd2+ to yeast cells. To screen the upstream regulators, partial promoter sequences of the TmMTP1/11 genes were isolated using the chromosome walking method. Several myeloblastosis (MYB) recognition elements were identified in the promoters of these genes. Furthermore, two Cd2+-induced R2R3-MYB TFs, TmMYB16 and TmMYB123, were identified. Both in vitro and in vivo assays confirmed that TmMTB16/123 play a role in Cd2+ tolerance by activating and repressing the expression of TmMTP1/11 genes. The present study elucidated new regulatory mechanisms underlying the response to Cd stress and can contribute to the breeding of Taxus species with high environmental adaptability.

Effect of Thallium(I) on Growth, Nutrient Absorption, Photosynthetic Pigments, and Antioxidant Response of Dittrichia Plants

Dittrichia plants were exposed to thallium (Tl) stress (10, 50, and 100 µM) for 7 days. The Tl toxicity altered the absorption and accumulation of other nutrients. In both the roots and the leaves, there was a decline in K, Mg, and Fe content, but an increase in Ca, Mn, and Zn. Chlorophylls decreased, as did the photosynthetic efficiency, while carotenoids increased. Oxidative stress in the roots was reflected in increased lipid peroxidation. There was more production of superoxide (O2.−), hydrogen peroxide (H2O2), and nitric oxide (NO) in the roots than in the leaves, with increases in both organs in response to Tl toxicity, except for O2.− production in the roots, which fluctuated. There was increased hydrogen sulfide (H2S) production, especially in the leaves. Superoxide dismutase (SOD), ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR), monodehydroascorbate reductase (MDHAR), and glutathione reductase (GR) showed increased activities, except for APX and MDHAR in the roots and GR in the leaves. The components of the ascorbate–glutathione cycle were affected. Thus, ascorbate (AsA) increased, while dehydroascorbate (DHA), reduced glutathione (GSH), and oxidized glutathione (GSSG) decreased, except for in the roots at 100 µM Tl, which showed increased GSH. These Tl toxicity-induced alterations modify the AsA/DHA and GSH/GSSG redox status. The NO and H2S interaction may act by activating the antioxidant system. The effects of Tl could be related to its strong affinity for binding with -SH groups, thus altering the functionality of proteins and the cellular redox state.

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.

Ecological toxicity (ECx) of Pb and its prediction models in Chinese soils with different physiochemical properties

The lack of toxicological data becomes the main bottleneck of ecological risk assessment of lead (Pb) in Chinese soils. The present study assessed Pb toxicity on three underground test endpoints (barley root elongation, earthworm avoidance response, and substrate-induced respiration (SIR) of microorganism) in 10 different soils. Hormetic dose-response induced by Pb was >118 % for earthworm avoidance response. EC₁₀ and EC₅₀ (the effective concentrations of Pb that inhibit 10 % or 50 % of endpoint bioactivity and also represents the toxicity threshold of Pb) after leaching increased by 0.32-8.73 times, and 1.02-3.75 times, respectively. Leaching factor (LF) prediction models indicated pH and cation exchange capacity (CEC) were the vital predictors for LF₁₀ and LF₅₀, explaining 60.6 % and 73.1 % of variations, respectively. SIR was one sensitive test endpoint for Pb toxicity, with the lowest of EC₁₀ and EC₅₀ values (from 373.7 to 1008.5 mg·kg^-1, and from 837.1 to 2869.0 mg·kg^-1, respectively). The best prediction models between EC_x and soil properties is LogEC₅₀ = 1.324Log(pH) + 0.423Log(CEC) + 1.742 (R² = 0.761, p < 0.01). The results displayed significant implications for deriving EC_x of Pb, and provided a scientific basis for soil ecological risk assessment of Pb.

Assessment of the Hormetic Effect of Arsenic on Growth and Physiology of Two Cultivars of Maize (Zea mays L.)

Although growth stimulation at low arsenic doses was observed in several plants, few studies have focused on this phenomenon in more detail. The effects of different concentrations of arsenic (0-50 mg kg-1 of soil: As0-As50) on the growth and selected physiological parameters of two maize cultivars (Zea mays L. cvs. Chapalu and MvNK 333) were tested. Cultivar MvNK 333 manifested a generally higher tolerance to As than cv. Chapalu, which may be related to the lower content of As in the tissues. The highest stimulatory effect of As was recorded at doses of As1 and As2 (cv. Chapalu), and at the As5 dose (MvNK 333), there was an increase in shoot elongation, biomass, and relative water content (RWC), as well as the content of photosynthetic pigments. The stimulatory effect of lower doses of As apparently represents an adaptation mechanism that is associated with water content regulation in the given conditions. The stomata of the studied cultivars were involved in this regulation in different ways. While cv. Chapalu exhibited increased numbers of stomata on both sides of leaves, cv. MvNK 333 instead responded to the given conditions with decreased stomata size. Although hormetic manifestations closely related to changes in stomatal number and size were observed, a typical stomatal hormetic response was not observed in the given range of As doses.

Aging factor and its prediction models of chromium ecotoxicity in soils with various properties

Aging of pollutants determines bioavailability and toxicity thresholds of environmental pollutants in soil. However, the ecotoxicity of chromium (Cr) rarely considers the effect of aging as well as soil properties. In order to explore the aging characteristics and establish their quantitative relationship with different soil properties, this study selected 7 soils with different properties through exogenous addition of Cr and determined its toxicity on barley root elongation. From 14d to 540d, EC₁₀ and EC₅₀ of barley root elongation ranged from 21.40 to 312.52 (mg·kg^-1) and 50.15 to 883.88 (mg·kg^-1) respectively. The hormesis appeared in the dose-response curve of acid soil as relative barley root elongation reached >110 % compared with the control. Extended aging time of Cr from 14d to 540d was associated with the attenuation of the toxicity of Cr, as the aging factor increased from 1.26 to 6.09 for EC₅₀, from 0.88 to 4.98 for EC₁₀. The prediction model of AF_EC50 and soil properties is lg (AF_360d) = 0.306lg Clay+0.026lg CEC + 0.240 (R² = 0.872, P < 0.01). The results demonstrated that with the extension of aging time, the toxicity of Cr decreased at 360d and reached a slow reaction stage, after that soil OC, Clay and CEC could well explain the aging procedure of Cr (VI). These results are beneficial for risk assessment of Cr contaminated soils and establishment of a soil environmental quality criteria for Cr.

Comparative Analysis of Cell Metabolic Activity Sensing by Escherichia coli rrnB P1-lux and Cd Responsive-Lux Biosensors: Time-Resolved Experiments and Mechanistic Modelling

Whole-cell bacterial sensors are used in medical/environmental applications to detect chemicals, and to assess medium toxicity or stress. Non-specific constitutive biosensors generally serve the latter purpose, whereas chemical detection is performed with biosensors involving a specific chemical-inducible promoter. Herein, we show that functioning principles of specific and non-specific whole-cell biosensors are not exclusive as both can probe modulations of cell metabolic activity under stressing conditions. The demonstration is based on (i) time-resolved measurements of bioluminescence produced by constitutive rrnB P1-luxCDABE Escherichia coli biosensor in media differing with respect to carbon source, (ii) theoretical reconstruction of the measured signals using a here-reported theory for bioluminescence generated by constitutive cells, (iii) comparison between time-dependent cell photoactivity (reflecting metabolic activity) retrieved by theory with that we reported recently for cadmium-inducible PzntA-luxCDABE E. coli in media of similar compositions. Whereas signals of constitutive and non-constitutive biosensors differ in terms of shape, amplitude and peak number depending on nutritional medium conditions, analysis highlights the features shared by their respective cell photoactivity patterns mediated by the interplay between stringent response and catabolite repressions. The work advocates for the benefits of a theoretical interpretation for the time-dependent response of biosensors to unravel metabolic and physicochemical contributions to the bioluminescence signal.

Comprehensive insights in thallium ecophysiology in the hyperaccumulator Biscutella laevigata

Biscutella laevigata is the strongest known thallium (Tl) hyperaccumulator plant species. However, little is known about the ecophysiological processes leading to root uptake and translocation of Tl in this species, and the interactions between Tl and its chemical analogue potassium (K). Biscutella laevigata was subjected to hydroponics experimentation in which it was exposed to Tl and K, and it was investigated in a rhizobox experiment. Laboratory-based micro-X-ray fluorescence spectroscopy (μ-XRF) was used to reveal the Tl distribution in the roots and leaves, while synchrotron-based μ-XRF was utilised to reveal elemental distribution in the seed. The results show that in the seed Tl was mainly localised in the endosperm and cotyledons. In mature plants, Tl was highest in the intermediate leaves (16,100 μg g^-1), while it was one order of magnitude lower in the stem and roots. Potassium did not inhibit or enhance Tl uptake in B.laevigata. At the organ level, Tl was localised in the blade and margins of the leaves. Roots foraged for Tl and cycled Tl across roots growing in the control soils. Biscutella laevigata has ostensibly evolved specialised mechanisms to tolerate high Tl concentrations in its shoots. The lack of interactions and competition between Tl and K suggests that it is unlikely that Tl is taken up via K channels, but high affinity Tl transporters remain to be identified in this species. Thallium is not only highly toxic but also a valuable metal and Tl phytoextraction using B. laevigata should be explored.

Tartaric acid soil-amendment increases phytoextraction potential through root to shoot transfer of lead in turnip

The phytoextraction potential of turnip and comparative effectiveness of three different organic ligands towards removal of lead (Pb) was investigated under field conditions. The 20 d old turnip seedlings were exposed to different Pb levels (0.0218, 2.42 and 4.83 mM Pb) spiked in the soil. After 10 d of Pb application, the soil was spiked with 2.4 mM concentration of different chelates viz. ethylenediaminetetraacetic acid (EDTA), citric acid (CA) and tartaric acid (TA). The 60 d old plants were harvested for growth analyses and determination of photosynthetic pigments, while Pb-concentration in different plant parts was determined from 60 and 90 d old plants. Yield attributes were recorded at the harvesting stage (HS, 90 d old plants). No suppression (rather a stimulation) in the root and shoot growth was evident upon Pb exposure whereas, a reduction in the chlorophyll content occurred at 4.83 mM Pb level. Soil amendment with TA improved chlorophyll contents irrespective of Pb levels while the effect of CA and EDTA was differential. A reduction in the root length while an increase in its diameter was recorded particularly at 4.83 mM Pb stress in 90 d old plants. The turnip retained maximum Pb-fraction in the roots at early growth stages, while EDTA application further increased its retention in root at 4.83 mM Pb regime. Nonetheless, only TA amendment promoted the transfer of Pb to shoot (∼30%) irrespective of Pb regimes. At the HS, application of both TA and EDTA caused substantial uptake of Pb in the root while the maximum shoot Pb-fraction was recorded again due to TA application, particularly at 4.83 mM Pb level. Above all, TA was identified as the most effective chelate that mobilized Pb from root to shoot leading to better growth possibly due to dilution effect, and thus enhanced phytoextraction efficiency in turnip.

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.