Research over the last three decades showed that chromium, particularly the oxyanion chromate Cr(VI) behaves as a toxic environmental pollutant that strongly damages plants due to oxidative stress, disruption of nutrient uptake, photosynthesis and metabolism, and ultimately, represses growth and development. However, mild Cr(VI) concentrations promote growth, induce adventitious root formation, reinforce the root cap, and produce twin roots from single root meristems under conditions that compromise cell viability, indicating its important role as a driver for root organogenesis. In recent years, considerable advance has been made towards deciphering the molecular mechanisms for root sensing of chromate, including the identification of regulatory proteins such as SOLITARY ROOT and MEDIATOR 18 that orchestrate the multilevel dynamics of the oxyanion. Cr(VI) decreases the expression of several glutamate receptors, whereas amino acids such as glutamate, cysteine and proline confer protection to plants from hexavalent chromium stress. The crosstalk between plant hormones, including auxin, ethylene, and jasmonic acid enables tissues to balance growth and defense under Cr(VI)-induced oxidative damage, which may be useful to better adapt crops to biotic and abiotic challenges. The highly contrasting responses of plants manifested at the transcriptional and translational levels depend on the concentration of chromate in the media, and fit well with the concept of hormesis, an adaptive mechanism that primes plants for resistance to environmental challenges, toxins or pollutants. Here, we review the contrasting facets of Cr(VI) in plants including the cellular, hormonal and molecular aspects that mechanistically separate its toxic effects from biostimulant outputs.

Mecoprop-P (MCPP-P) is an auxin herbicide which has been used against dicotyledonous weed plants since the 1980s. While fate and monitoring data of MCPP-P in the aquatic environment revealing concentrations up to 103 μg/L in freshwaters are well documented, only very few toxicity data and no studies with dicotyledonous macrophytes have been published in open literature so far. To fill up this essential data gap, a microcosm study was conducted in order to test the sensitivity of nine dicotyledonous and one Ceratophyllales macrophyte species. The plant species were exposed to seven MCPP-P concentrations ranging from 8 to 512 μg/L for 21/22 days in one microcosm per concentration, and two further microcosms served as controls. Plant preparation was adapted to each species and endpoints were measured to calculate growth rates. Data were generated to obtain effect concentrations (ECX) which then were used to construct species sensitivity distribution curves (SSD). Eight species proved to be sensitive to MCPP-P in the tested concentration range with EC₅₀ values ranging from 46.9 μg/L for Ranunculus aquatilis to 656.4 μg/L MCPP-P for Ludwigia repens. Taking the EC₅₀ values of this study and published data for autotrophic organisms into account, a hazard concentration (HC₅) of 2.7 μg/L was derived from the SSD curve, while an SSD curve without dicotyledonous macrophytes resulted in an about 100 times higher HC₅ (360.8 μg/L MCCP-P). This confirms that a re-evaluation for old auxin herbicides by including dicotyledonous test species into the environmental risk assessment may be indicated. Furthermore, the use of MCPP-P in bitumen felts as protection against rooting by plants is not in the focus of any risk regulation so far. This application, however, can lead to high run-off concentrations that can enter surface waters easily, exceeding the new regulatory acceptable concentration values.

Plants have the ability to adapt themselves under stressed conditions through reprogramming their growth and development. Understanding the mechanisms regulating overall growth of stressed plant is an important issue for plant and environmental biology research. Although the role of NO in modulating arsenic (As) toxicity is known, nitric oxide (NO) induced alteration in auxin and nutrient related transporters during As stress in rice is poorly understood. Experimental results showed that As exposure decreased gene expression level of polar auxin transporter (PIN proteins), and nutrient transporter related genes (AMT, NRT, NiR, PHT, KTP). The improved tolerance induced by As + NO combination is attributed to reduced As accumulation in rice seedlings, improved root architectural changes, overall growth of plant, chlorophyll, protein content, and accumulation of mineral nutrients by reducing the ROS generation. Further, enhanced transcript levels of PIN proteins and mineral nutrition related genes were also observed under As + NO treatment. Additional biochemical data revealed enhanced oxidative stress by increasing the level of antioxidant enzymes, and stress-related parameters. Overall, the study provides an integrated view of plant response during As + NO interaction to change the plant metabolism through different cellular processes.

Polycyclic aromatic hydrocarbons (PAHs), ubiquitous organic pollutants in the environment, can accumulate in humans via the food chain and then harm human health. MiRNAs (microRNAs), a kind of non-coding small RNAs with a length of 18–30 nucleotides, regulate plant growth and development and respond to environmental stress. In this study, it is demonstrated that miR164 can regulate root growth and adventitious root generation of wheat under phenanthrene exposure by targeting NAC (NAM/ATAF/CUC) transcription factor. We observed that phenanthrene treatment accelerated the senescence and death of wheat roots, and stimulated the occurrence of new roots. However, it is difficult to compensate for the loss caused by old root senescence and death, due to the slower growth of new roots under phenanthrene exposure. Phenanthrene accumulation in wheat roots caused to generate a lot of reactive oxygen species, and enhanced lipoxygenase activity and malonaldehyde concentration, meaning that lipid peroxidation is the main reason for root damage. MiR164 was up-regulated by phenanthrene, enhancing the silence of NAC1, weakening the association with auxin signal, and inhibiting the occurrence of adventitious roots. Phenanthrene also affected the expression of CDK (the coding gene of cyclin-dependent kinase) and CDC2 (a gene regulating cell division cycle), the key genes in the cell cycle of pericycle cells, thereby affecting the occurrence and growth of lateral roots. In addition, NAM (a gene regulating no apical meristem) and NAC23 may also be related to the root growth and development in wheat exposed to phenanthrene. These results provide not only theoretical basis for understanding the molecular mechanism of crop response to PAHs accumulation, but also knowledge support for improving phytoremediation of soil or water contaminated by PAHs.

Copper nanoparticles (nCu) are widely used in industry and in daily life, due to their unique physical, chemical, and biological properties. Few studies have focused on nCu phytotoxicity, especially with regard to toxicity mechanisms in crop plants. The present study examined the effect of 15.6 μM nCu exposure on the root morphology, physiology, and gene transcription levels of wheat (Triticum aestivum L.), a major crop cultivated worldwide. The results obtained were compared with the effects of exposing wheat to an equivalent molar concentration of ionic Cu (Cu²⁺ released from CuSO₄) and to control plants. The relative growth rate of roots decreased to approximately 60% and the formation of lateral roots was stimulated under nCu exposure, possibly due to the enhancement of nitrogen uptake and accumulation of auxin in lateral roots. The expression of four of the genes involved in the positive regulation of cell proliferation and negative regulation of programmed cell death decreased to 50% in the Cu²⁺ treatment compared to that of the control, while only one gene was down-regulated to about half of the control in nCu treatment. This explained the decreased root cell proliferation and higher extent of induced cell death in Cu²⁺- than in nCu-exposed plants. The increased methane dicarboxylic aldehyde accumulation (2.17-fold increase compared with the control) and decreased antioxidant enzyme activities (more than 50% decrease compared with the control) observed in the Cu²⁺ treatment in relation to the nCu treatment indicated higher oxidative stress in Cu²⁺- than in nCu-exposed plants. Antioxidant (e.g., proline) synthesis was pronouncedly induced by nCu to scavenge excess reactive oxygen species, alleviating phytotoxicity to wheat exposed to this form of Cu. Overall, oxidative stress and root growth inhibition were the main causes of nCu toxicity.

Bisphenol A (BPA) is a harmful environmental contaminant acting as an endocrine disruptor in animals, but it also affects growth and development in plants. Here, we have elucidated the functional mechanism of root growth inhibition by BPA in Arabidopsis thaliana using mutants, reporter lines and a pharmacological approach. In response to 10 ppm BPA, fresh weight and main root length were reduced, while auxin levels increased. BPA inhibited root growth by reducing root cell length in the elongation zone by suppressing expansin expression and by decreasing the length of the meristem zone by repressing cell division. The inhibition of cell elongation and cell division was attributed to the enhanced accumulation/redistribution of auxin in the elongation zone and meristem zone in response to BPA. Correspondingly, the expressions of most auxin biosynthesis and transporter genes were enhanced in roots by BPA. Taken together, it is assumed that the endocrine disruptor BPA inhibits primary root growth by inhibiting cell elongation and division through auxin accumulation/redistribution in Arabidopsis. This study will contribute to understanding how BPA affects growth and development in plants.

Treated wastewater is increasingly used to meet agriculture's water needs; however, treated wastewater contains numerous contaminants of emerging concern (CECs). With exposure and uptake of CECs, phytotoxicity and health of crop plants is of concern, but is poorly understood. This study evaluated the effect of low-dose, chronic exposure to a mixture of 10 CECs, including 4 antibiotics, 3 anti-inflammatory drugs, 1 antiepileptic, 1 beta-blocker, and 1 antimicrobial, on lettuce (Lactuca sativa) and cucumber (Cucumis sativa L.) plants. The CEC mixture was added in nutrient media at 1 to 20X of their typical levels in treated wastewater effluents. Biological endpoints including germination, growth, phytohormone homeostasis, and CEC bioaccumulation were determined. Exposure to the CEC mixture did not affect the germination rate of lettuce seeds, but stimulated root elongation and increased the root-to-shoot biomass ratio during a 7 d cultivation. A dose-dependent decrease in biomass was observed in cucumber seedling after a 30 d exposure, with the highest rate CEC treatment resulting in decreases of 51.2 ± 20.9, 26.3 ± 34.1, and 33.2 ± 41.7% in the below-ground, above-ground, and total biomass, respectively. Levels of abscisic acid were significantly elevated (p < 0.05) in the leaves, but decreased (p < 0.05) in the roots. The dose-response of auxin was characterized by a hormesis effect. A significant 6-fold increase in the stem auxin level was observed at the 1X CEC rate, followed by a decrease to 2-fold the control at the 20X rate. Leaf auxin concentrations also significantly increased at the 1X CEC rate to 16-fold, followed by a decrease at the highest CEC rate. The results of this study suggeste that chronic exposure to low levels of CEC mixtures may compromise the fitness of plants, and the impairments are underlined by alterations in hormone balances.

Clopyralid is one of the synthetic pyridine-carboxylate auxin herbicides and used to control perennial and annual broadleaf weeds in wheat, sugar beets, canola, etc. In this study, dose-dependent cytotoxicity and genotoxicity of clopyralid at different concentrations (25, 50, and 100 μg/mL) have been evaluated on the Allium cepa roots. The evaluation has been performed at macroscopic (root growth) and microscopic levels [mitotic index (MI), chromosome aberrations (CAs) in ana-telophase cells, and DNA damage] using root growth inhibition, Allium ana-telophase, and comet tests. The percentage of root growth inhibition and concentration of reducing root growth by 50% (EC₅₀) of clopyralid were determined compared with the negative control by using various concentrations of clopyralid (6.25–1000 μg/L). The 96 h EC₅₀ of clopyralid was recorded as 50 μg/L. The gradual decrease in root growth and the MI reveals the cytotoxic effects of clopyralid. All the tested concentrations of clopyralid induced total CAs (polyploidy, stickiness, anaphase bridges, chromosome laggards, and disturbed ana-telophase) and DNA damage dose and time dependently. These results confirm the cytotoxic and genotoxic effects of clopyralid on non-target organism.

The accumulation of heavy metal in the soil is a serious concern for sustainable food production due to their toxic effects on plants and other living things. The strategies are required on urgent bases for the management of metal-contaminated soils. Thus, the microbes from the genus Pseudomonas were characterized for different traits and lead (Pb)-resistant ability and their effects were assessed on growth, photosynthesis, antioxidant capacity, and Pb uptake by dill (Anethum graveolens L.). Furthermore, soil basal respiration and induced respiration in soil were also assessed under microbes and Pb stress. Among the tested three strains, Pseudomonas P159 and P150 were more tolerant to Pb stress than Pseudomonas P10, whereas P159 showed the highest values for phosphorus (P), siderophore, auxin, and hydrogen cyanide production. The bacterial inoculation increased the plant shoot dry weights, carbohydrates, proline, and chlorophyll contents under Pb stress. The catalase (CAT) and peroxidase (POD) activities of the plants were higher in bacterial-treated plants than control. The bacterial inoculation decreased Pb concentration in plants, and the response varied with the type of microbes. The bacterial strains enhanced the soil basal and induced respiration than respective Pb treatments alone. Overall, Pseudomonas P159 is potentially suitable for the remediation of Pb-contaminated soils. Graphical abstract

Sodium (Na) and arsenic (As) toxicity were monitored by hyperaccumulation of metals in Salvinia natans L. with 2,4-dichlorophenoxyacetic acid (2,4-D) induction. Salvinia was recorded with significant bioaccumulation of those metals with de-folding of cellular attributes in sustenance under toxic environment. 2,4-D priming has revised the growth components like net assimilation rate and relative water content to register initial plants’ survival against Na and As. Proline biosynthesis supported in the maintenance of osmotic adjustment and plants sustained better activity through subdued electrolytic leakage. Oxidative stress due to both Na and As exposure is responsible for induction under significant moderation of lipid peroxidation and protein carbonization by 2,4-D application was evident to release the stress from metal and metalloids. Reactive oxygen species (ROS) like superoxide and hydrogen peroxide accumulation were monitored with activity of NADP(H)-oxidase. However, it was downregulated by 2,4-D to check the oxidative damages. Superoxide dismutase and peroxidases were significantly moderated to reduce the oxidative degradation for both metals with 2,4-D induction. Glutathione metabolism and recycling of ascorbate with monodehydroascorbate activity were other features to maintain the redox homeostasis for metal toxicity. At the molecular level, polymorphic variations of concern genes in redox cascades demarked significantly for those two metals and established the biomarker for those metals, respectively. As a whole, the biocompatibility of auxin herbicide in Salvinia may raise the possibility for auxin metabolism and thereby, the bioaccumulation to Na and As vis-à-vis tolerance for ecological safety is established.