Quantifying the relative contribution of particulate versus dissolved silver to toxicity and uptake kinetics of silver nanowires in lettuce: impact of size and coating

Molecular characterization and transcriptional response of Lactuca sativa seedlings to co-exposure to graphene nanoplatelets and titanium dioxide nanoparticles

The widespread use of nanomaterials in agriculture may introduce multiple engineered nanoparticles (ENPs) into the environment, posing a combined risk to crops. However, the precise molecular mechanisms explaining how plant tissues respond to mixtures of individual ENPs remain unclear, despite indications that their combined toxicity differs from the summed toxicity of the individual ENPs. Here, we used a variety of methods including physicochemical, biochemical, and transcriptional analyses to examine the combined effects of graphene nanoplatelets (GNPs) and titanium dioxide nanoparticles (TiO2 NPs) on hydroponically exposed lettuce (Lactuca sativa) seedlings. Results indicated that the presence of GNPs facilitated the accumulation of Ti as TiO2 NPs in the seedling roots. Combined exposure to GNPs and TiO2 NPs caused less severe oxidative damage in the roots compared to individual exposures. Yet, GNPs and TiO2 NPs alone and in combination did not cause oxidative damage in the shoots. RNA sequencing data showed that the mixture of GNPs and TiO2 NPs led to a higher number of differentially expressed genes (DEGs) in the seedlings compared to exposure to the individual ENPs. Moreover, the majority of the DEGs encoding superoxide dismutase displayed heightened expression levels in the seedlings exposed to the combination of GNPs and TiO2 NPs. The level of gene ontology (GO) enrichment in the seedlings exposed to the mixture of GNPs and TiO2 NPs was found to be greater than the level of GO enrichment observed after exposure to isolated GNPs or TiO2 NPs. Furthermore, the signaling pathways, specifically the "MAPK signaling pathway-plant" and "phenylpropanoid biosynthesis," exhibited a close association with oxidative stress. This study has provided valuable insights into the molecular mechanisms underlying plant resistance against multiple ENPs.

Species-dependent responses of crop plants to polystyrene microplastics

Only recently there has been a strong focus on the impacts of microplastics on terrestrial crop plants. This study aims to examine and compare the effects of microplastics on two monocotyledonous (barley, Hordeum vulgare and wheat, Triticum aestivum), and two dicotyledonous (carrot, Daucus carota and lettuce, Lactuca sativa) plant species through two complimentary experiments. First, we investigated the effects of low, medium, and high (103, 105, 107 particles per mL) concentrations of 500 nm polystyrene microplastics (PS-MPs) on seed germination and early development. We found species-dependent effects on the early development, with microplastics only significantly affecting lettuce and carrot. When acutely exposed during germination, PS-MPs significantly delayed the germination of lettuce by 24%, as well as promoted the shoot growth of carrot by 71% and decreased its biomass by 26%. No effect was recorded on monocot species. Secondly, we performed a chronic (21 d) hydroponic experiment on lettuce and wheat. We observed that PS-MPs significantly reduced the shoot growth of lettuce by up to 35% and increased its biomass by up to 64%, while no record was reported on wheat. In addition, stress level indicators and defence mechanisms were significantly up-regulated in both lettuce and wheat seedlings. Overall, this study shows that PS-MPs affect plant development: impacts were recorded on both germination and growth for dicots, and responses identified by biochemical markers of stress were increased in both lettuce and wheat. This highlights species-dependent effects as the four crops were grown under identical conditions to allow direct comparison. For future research, our study emphasizes the need to focus on crop specific effects, while also working towards knowledge of plastic-induced impacts at environmentally relevant conditions.

Toxicokinetics and Particle Number-Based Trophic Transfer of a Metallic Nanoparticle Mixture in a Terrestrial Food Chain

Herein, we investigated to which extent metallic nanoparticles (MNPs) affect the trophic transfer of other coexisting MNPs from lettuce to terrestrial snails and the associated tissue-specific distribution using toxicokinetic (TK) modeling and single-particle inductively coupled plasma mass spectrometry. During a period of 22 days, snails were fed with lettuce leaves that were root exposed to AgNO3 (0.05 mg/L), AgNPs (0.75 mg/L), TiO2NPs (200 mg/L), and a mixture of AgNPs and TiO2NPs (equivalent doses as for single NPs). The uptake rate constants (ku) were 0.08 and 0.11 kg leaves/kg snail/d for Ag and 1.63 and 1.79 kg leaves/kg snail/d for Ti in snails fed with NPs single- and mixture-exposed lettuce, respectively. The elimination rate constants (ke) of Ag in snails exposed to single AgNPs and mixed AgNPs were comparable to the corresponding ku, while the ke for Ti were lower than the corresponding ku. As a result, single TiO2NP treatments as well as exposure to mixtures containing TiO2NPs induced significant biomagnification from lettuce to snails with kinetic trophic transfer factors (TTFk) of 7.99 and 6.46. The TTFk of Ag in the single AgNPs treatment (1.15 kg leaves/kg snail) was significantly greater than the TTFk in the mixture treatment (0.85 kg leaves/kg snail), while the fraction of Ag remaining in the body of snails after AgNPs exposure (36%) was lower than the Ag fraction remaining after mixture exposure (50%). These results indicated that the presence of TiO2NPs inhibited the trophic transfer of AgNPs from lettuce to snails but enhanced the retention of AgNPs in snails. Biomagnification of AgNPs from lettuce to snails was observed in an AgNPs single treatment using AgNPs number as the dose metric, which was reflected by the particle number-based TTFs of AgNPs in snails (1.67, i.e., higher than 1). The size distribution of AgNPs was shifted across the lettuce-snail food chain. By making use of particle-specific measurements and fitting TK processes, this research provides important implications for potential risks associated with the trophic transfer of MNP mixtures.

Similarity assessment of metallic nanoparticles within a risk assessment framework: A case study on metallic nanoparticles and lettuce

Similarity assessment is one of the means of optimally using scarcely available experimental data on the fate and hazards of nanoforms (NFs) for regulatory purposes. For a set of NFs that are shown to be similar it is allowed in a regulatory context to apply the information available on any of the NFs within the group to the whole set of NFs. Obviously, a proper justification for such a similarity assessment is to be provided. Within the context of exemplifying such a justification, a case study was performed aimed at assessing the similarity of a set of spherical metallic NFs that different with regard to chemical composition (three metals) and particle size (three different sizes). The endpoints of assessment were root elongation and biomass increase of lettuce (Lactuca sativa L.) seedlings and exposure assessment was performed in order to express the actual exposure concentration in terms of time-weighted average particle concentrations. The results of the study show that for the specific endpoints assessed, chemical composition is driving NF toxicity and this is mostly due to impacts on the fate of the NFs. On the other hand, particle size of Cu NFs had a negligible impact on the dose-response relationships for the specific endpoints assessed. It is thus concluded that hazard data available on spherical Cu NF tested in our case can be used to inform on the hazards of any spherical Cu NF within the size range of 25-100 nm, but only applies for the certain endpoints. Also, toxicity data for the Cu2+-ion are suited for such a similarity assessment.

Uptake, translocation, and transformation of silver nanoparticles in plants

This article reviews the plant uptake of silver nanoparticles (AgNPs) that occurred in soil systems and the in planta fate of Ag.

Trophic Transfer and Toxicity of (Mixtures of) Ag and TiO2 Nanoparticles in the Lettuce-Terrestrial Snail Food Chain

The increasing application of biosolids and agrochemicals containing silver nanoparticles (AgNPs) and titanium dioxide nanoparticles (TiO2NPs) results in their inevitable accumulation in soil, with unknown implications along terrestrial food chains. Here, the trophic transfer of single NPs and a mixture of AgNPs and TiO2NPs from lettuce to snails and their associated impacts on snails were investigated. Both AgNPs and TiO2NPs were transferred from lettuce to snails with trophic transfer factors (defined as the ratio of the Ag/Ti concentration in snail tissues to the Ag/Ti concentration in lettuce leaves) of 0.2-1.1 for Ag and 3.8-47 for Ti. Moreover, the majority of Ag captured by snails in the AgNP-containing treatments was excreted via feces, whereas more than 70% of Ti was distributed in the digestive gland of snails in the TiO2NP-containing treatments. Additionally, AgNP-containing treatments significantly inhibited the activity of snails, while TiO2NP-containing treatments significantly reduced feces excretion of snails. Furthermore, the concurrent application of AgNPs and TiO2NPs did not affect the biomagnification and distribution patterns of Ag and Ti in snails, whereas their co-existence exhibited more severe inhibition of the growth and activity of snails than in the case of applying AgNPs or TiO2NPs alone. This highlights the possibility of nanoparticle transfer to organisms of higher trophic levels via food chains and the associated risks to ecosystem health.

Comparative assessment of the fate and toxicity of chemically and biologically synthesized silver nanoparticles to juvenile clams

Nanoparticles (NPs) can be produced via physical, chemical, or biological approaches. Yet, the impact of the synthesis approaches on the environmental fate and effects of NPs is poorly understood. Here, we synthesized AgNPs through chemical and biological approaches (cit-AgNPs and bio-AgNPs), characterized their properties, and toxicities relative to commercially available Ag nanopowder (np-AgNPs) to the clam Mercenaria mercenaria. The chemical synthesis is based on the reduction of ionic silver using sodium borohydride as a reducing agent and trisodium citrate as a capping agent. The biological synthesis is based on the reduction of ionic silver using biomolecules extracted from an atoxigenic strain of a filamentous fungus Aspergillus parasiticus. The properties of AgNPs were determined using UV-vis, dynamic light scattering, laser Doppler electrophoresis, (single particle)-inductively coupled plasma-mass spectroscopy, transmission electron microscopy, and asymmetric flow-field flow fractionation. Both chemical and biological synthesis approaches generated spherical AgNPs. The chemical synthesis produced AgNPs with narrower size distributions than those generated through biological synthesis. The polydispersity of bio-AgNPs decreased with increases in cell free extract (CFE):Ag ratios. The magnitude of the zeta potential of the cit-AgNPs was higher than those of bio-AgNPs. All AgNPs formed aggregates in the test media i.e., natural seawater. Based on the same total Ag concentrations, all AgNPs were less toxic than AgNO₃. The toxicity of AgNPs toward the juvenile clam, Mercenaria mercenaria, decreased following the order np-AgNPs > cit-AgNPs > bio-AgNPs. Expressed as a function of dissolved Ag concentrations, the toxicity of Ag decreased following the order cit-AgNPs > bio-AgNPs > AgNO₃ ~ np-AgNPs. Therefore, the toxicity of AgNP suspensions can be attributed to a combined effect of dissolved and particulate Ag forms. These results indicate that AgNP synthesis methods determine their environmental and biological behaviors and should be considered for a more comprehensive environmental risk assessment of AgNPs.

Particle-Specific Toxicity of Copper Nanoparticles to Soybean (Glycine max L.): Effects of Nanoparticle Concentration and Natural Organic Matter

For the soluble metallic nanoparticles (NPs), which forms (particles [NP(particle) ] vs. dissolved ions [NP(ion) ]) are the main cause of toxicity of the NP suspension (NP(total) ) remains uncertain. In the present study, soybean was exposed to Cu NPs in a hydroponic system to determine how natural organic matter (NOM; 10 mg/l) and concentration of Cu NP(total) (2-50 mg/l) affect the relative contributions of Cu NP(particle) and Cu NP(ion) to the overall toxicity. We found that NOM mitigated the phytotoxicity of Cu NP(particle) more significantly than that of Cu salt. When no NOM was added, Cu NP(particle) rather than Cu NP(ion) was the main contributor to the observed toxicity regardless of the concentration of Cu NP(total) . However, NOM tended to reduce the relative contribution of Cu NP(particle) to the toxicity of Cu NP(total) . Especially at a low concentration of Cu NP(total) (2 mg/l), the toxicity of Cu NP(total) mainly resulted from Cu NP(ion) in the presence of NOM (accounting for ≥70% of the overall toxicity). This might be attributable to the combined effects of increased dissolution of Cu NPs and steric-electrostatic hindrance between Cu NP(particle) and the soybean roots caused by NOM. Fulvic acids (FAs) tended to reduce the role of Cu NP(particle) in the overall toxicity more effectively than humic acids (HAs), which might partially be due to the higher extent of Cu NP dissolution on FA treatment than in HA treatment. Our results suggest that because of the relatively low metallic NP concentration and the presence of NOM in natural water, NP(ion) are likely problematic, which can inform management and mitigation actions. Environ Toxicol Chem 2021;40:2825-2835. © 2021 SETAC.

Nanoimpact in Plants: Lessons from the Transcriptome

Transcriptomics studies are available to evaluate the potential toxicity of nanomaterials in plants, and many highlight their effect on stress-responsive genes. However, a comparative analysis of overall expression changes suggests a low impact on the transcriptome. Environmental challenges like pathogens, saline, or drought stress induce stronger transcriptional responses than nanoparticles. Clearly, plants did not have the chance to evolve specific gene regulation in response to novel nanomaterials; but they use common regulatory circuits with other stress responses. A shared effect with abiotic stress is the inhibition of genes for root development and pathogen response. Other works are reviewed here, which also converge on these results.