Effects of nano-TiO₂ on the agronomically-relevant Rhizobium-legume symbiosis

Metallic nanoparticles and photosynthesis organisms: Comprehensive review from the ecological perspective

This review presents a comprehensive analysis of the ecological implications of metallic nanoparticles (MNPs) on photosynthetic organisms, particularly plants and algae. We delve into the toxicological impacts of various MNPs, including gold, silver, copper-based, zinc oxide, and titanium dioxide nanoparticles, elucidating their effects on the growth and health of these organisms. The article also summarizes the toxicity mechanisms of these nanoparticles in plants and algae from previous research, providing insight into the cellular and molecular interactions that underpin these effects. Furthermore, it discusses the reciprocal interactions between different types of MNPs, their combined effects with other metal contaminants, and compares the toxicity between MNPs with their counterpart. This review highlights the urgent need for a deeper understanding of the environmental impact, considering their escalating use and the potential risks they pose to ecological systems, especially in the context of photosynthetic organisms that are vital to ecosystem health and stability.

Effects of the sunscreen ultraviolet filter oxybenzone (benzophenone-3) on the seagrass Posidonia oceanica (L.) Delile and its associated N2 fixers

Oxybenzone/benzophenone-3 (BP-3) is one of the most detrimental organic ultraviolet filters for marine biota, leading to legislative measures banning its presence in commercial sunscreen formulations of several countries. It remains poorly explored how this contaminant is currently threatening the persistence of critical ecosystems for conservation in the Mediterranean, such as Posidonia oceanica meadows, but it is essential for promoting sustainable coastal tourism. Our investigation aimed to determine the effects of BP-3 on P. oceanica under a short-term laboratory setup, recreating summer conditions while testing three environmentally relevant concentrations for Mallorca, Spain (minimum: 53.6 ng L-1, maximum: 557.5 ng L-1 and increased: 1115 ng L-1) and a control (0 ng L-1). Primary productivity was unaffected by the treatments, however, a reduction in leaf chlorophyll content and nitrogen fixation activity associated with rhizomes was evidenced under BP-3 addition. This may be related with oxidative damage, as reactive oxygen species production and catalase activity in P. oceanica leaves were the highest even at minimum BP-3 concentrations. Alkaline phosphatase rates showed inverted trends between old leaves and rhizomes, being enhanced in the former under BP-3 addition and reduced in the latter. These results are of great relevance for the future management of P. oceanica meadows, elucidating that even minimum concentrations of BP-3 reported in coastal waters of Mallorca can induce elevated levels of oxidative stress in the seagrass, that lead to impairments in its photosynthetic pigments production and supply of essential nutrients through belowground tissues.

Nano-enabled agrochemicals: mitigating heavy metal toxicity and enhancing crop adaptability for sustainable crop production

The primary factors that restrict agricultural productivity and jeopardize human and food safety are heavy metals (HMs), including arsenic, cadmium, lead, and aluminum, which adversely impact crop yields and quality. Plants, in their adaptability, proactively engage in a multitude of intricate processes to counteract the impacts of HM toxicity. These processes orchestrate profound transformations at biomolecular levels, showing the plant's ability to adapt and thrive in adversity. In the past few decades, HM stress tolerance in crops has been successfully addressed through a combination of traditional breeding techniques, cutting-edge genetic engineering methods, and the strategic implementation of marker-dependent breeding approaches. Given the remarkable progress achieved in this domain, it has become imperative to adopt integrated methods that mitigate potential risks and impacts arising from environmental contamination on yields, which is crucial as we endeavor to forge ahead with the establishment of enduring agricultural systems. In this manner, nanotechnology has emerged as a viable field in agricultural sciences. The potential applications are extensive, encompassing the regulation of environmental stressors like toxic metals, improving the efficiency of nutrient consumption and alleviating climate change effects. Integrating nanotechnology and nanomaterials in agrochemicals has successfully mitigated the drawbacks associated with traditional agrochemicals, including challenges like organic solvent pollution, susceptibility to photolysis, and restricted bioavailability. Numerous studies clearly show the immense potential of nanomaterials and nanofertilizers in tackling the acute crisis of HM toxicity in crop production. This review seeks to delve into using NPs as agrochemicals to effectively mitigate HM toxicity and enhance crop resilience, thereby fostering an environmentally friendly and economically viable approach toward sustainable agricultural advancement in the foreseeable future.

Lithium-induced alterations in soybean nodulation and nitrogen fixation through multifunctional mechanisms

The increasing footprints of lithium (Li) in agroecosystems combined with limited recycling options have raised uncertain consequences for important crops. Nitrogen (N2)-fixation by legumes is an important biological response process, but the cause and effect of Li exposure on plant root-nodule symbiosis and biological N2-fixation (BNF) potential are still unclear. Soybean as a model plant was exposed to Li at low (25 mg kg-1), medium (50 mg kg-1), and high (100 mg kg-1) concentrations. We found that soybean growth and nodulation capacity had a concentration-dependent response to Li. Li at 100 mg kg-1 reduced the nodule numbers, weight, and BNF potential of soybean in comparison to the low and medium levels. Significant shift in soybean growth and BNF after exposure to Li were associated with alteration in the nodule metabolic pathways involved in nitrogen uptake and metabolism (urea, glutamine and glutamate). Importantly, poor soybean nodulation after high Li exposure was due in part to a decreased abundance of bacterium Ensifer in the nodule bacterial community. Also, the dominant N2-fixing bacterium Ensifer was significantly correlated with carbon and nitrogen metabolic pathways. The findings of our study offer mechanistic insights into the environmental and biological impacts of Li on soybean root-nodule symbiosis and N2-acquisition and provide a pathway to develop strategies to mitigate the challenges posed by Li in agroecosystems.

Graphene oxide affects the symbiosis of legume-rhizobium and associated rhizosphere rhizobial communities

The large-scale production and utilization of graphene oxide (GO) have raised concerns regarding its environmental exposure and potential risks. However, existing research on GO toxicity has primarily focused on individual organisms. Little attention has been given to the interaction between GO and the nitrogen-fixing symbiosis of legume-rhizobium. In this study, we focused on alfalfa (Medicago sativa L.), a typical leguminous nitrogen-fixing plant, to investigate the effects of GO on various aspects of this symbiotic relationship, including root nodulation, rhizobial viability, nodule nitrogen fixation, DNA damage, and the composition of the rhizobial community in the rhizosphere. As the dosage of GO increased, a significant inhibition in nodulation development was observed. Exposure to GO resulted in decreased growth and viability of rhizobia, as well as induced DNA damage in nodule cells. Furthermore, with increasing GO dosage, there were significant reductions in nitrogenase activity, leghemoglobin level, and cytoplasmic ammonia content within the root nodules. Additionally, the presence of GO led to notable changes in the rhizobial community in the rhizosphere. Our findings support the existence of the damage promoted by GO in the symbiosis of nitrogen fixing rhizobia with legumes. This underscores the importance of careful soil GO management.

Biosynthesis of ZnO and TiO2 nanoparticles using Ipomoea carnea leaf extract and its effect on black carrot (Daucus carota L.) cv. Pusa Asita

Nano fertilizers (NFs) are now becoming an important tool for plant nutrient management having capabilities to improve soil fertility, crop productivity and quality of agricultural products. Since, they are needed in very small amount, thus, reduces cost of crop production. Among different essential or beneficial plant nutrients, Zn and Ti are important micro nutrients having number of beneficial effect on crop growth, yield, quality and post harvest life. Present experiment was carried out to prepare ZnO and TiO2 nanoparticles (NPs) through green technology by using aqueous extract of Ipomoea carnea (morning glory) leaves. In order to investigate size, morphology, composition, and stability of selected NPs, the detailed characterization was carried out using UV-visible spectroscopy, FTIR, HRTEM, EDX, BET, X-ray diffraction, XPS and particle size distribution studies. Subsequently, the effect of foliar spray of ZnO and TiO2 NPs was evaluated in respect of vegetative growth, yield and quality of black carrot (Daucus carota L.) cv. Pusa Asita in presence of 50% Recommended dose of fertilizer (RDF) to assess their effect on fertilizer use efficiency also. There were 8 treatments viz. Control (no fertilizer), recommended dose of fertilizer (RDF), TiO2 (5, 10 and 15 ppm along with 50% RDF), ZnO (50, 75 and 100 ppm along with 50% RDF)] with 3 replications following Randomised Block Design having 24 plots (1 m × 1 m). The observations were taken for vegetative growth, edible root yield and root quality parameters. Although, the growth, yield and quality parameters were found superior (root yield 43.84 g/plant) under conventional system of recommended dose of fertilizers (RDF) of NPK, however, TiO2 NPs also showed very promising result close to RDF as compared to ZnO NPs. Among them, 5 ppm TiO2 foliar application along with 50% NPK was found to be the best in terms of vegetative growth, root yield (38.73 g/plant) and quality of black carrot. It was also found that higher concentration of TiO2 and ZnO NPs had adverse effect on the plant performance. Therefore, it can be concluded that 5 ppm TiO2 NPs along with 50% RDF was good for black carrot production.

Metal oxide nanoparticles inhibit nitrogen fixation and rhizosphere colonization by inducing ROS in associative nitrogen-fixing bacteria Pseudomonas stutzeri A1501

The potential effects of engineered metal oxide nanoparticles (MONPs) on bacterial nitrogen fixation are of great concern. Herein, the impact and mechanism of the increasing-used MONPs, including TiO₂, Al₂O₃, and ZnO nanoparticles (TiO₂NP, Al₂O₃NP, and ZnONP, respectively), on nitrogenase activity was studied at the concentrations ranging from 0 to 10 mg L^-1 using associative rhizosphere nitrogen-fixing bacteria Pseudomonas stutzeri A1501. Nitrogen fixation capacity was inhibited by MONPs in an increasing degree of TiO₂NP < Al₂O₃NP < ZnONP. Realtime qPCR analysis showed that the expressions of nitrogenase synthesis-related genes, including nifA and nifH, were inhibited significantly when MONPs were added. MONPs could cause the explosion of intracellular ROS, and ROS not only changed the permeability of the membrane but also inhibited the expression of nifA and biofilm formation on the root surface. The repressed nifA gene could inhibit transcriptional activation of nif-specific genes, and ROS reduced the biofilm formation on the root surface which had a negative effect on resisting environmental stress. This study demonstrated that MONPs, including TiO₂NP, Al₂O₃NP, and ZnONP, inhibited bacterial biofilm formation and nitrogen fixation in the rice rhizosphere, which might have a negative effect on the nitrogen cycle in bacteria-rice system.

Sunscreen exposure interferes with physiological processes while inducing oxidative stress in seagrass Posidonia oceanica (L.) Delile

The effects of a commercial sunscreen mixture on the Mediterranean seagrass Posidonia oceanica were investigated, evaluating its response in physiological processes and biochemical indicators of oxidative stress. Short-term laboratory experiments were conducted recreating summer conditions, and two sunscreen concentrations were tested in whole P. oceanica plants placed inside aquaria. Although primary productivity of leaf segments seemed to benefit from sunscreen addition, probably due to inorganic nutrients released, the rest of the biological parameters reflected possible impairments in the overall functioning of P. oceanica as a result of oxidative damages. Chlorophyll production and nitrogen fixation associated with old leaves were inhibited under high sunscreen concentrations, which concurred with elevated reactive oxygen species production, catalase activity and polyphenols content in the seagrass leaves. These results emphasize the importance of directing future investigations on determining which specific components of sunscreen products are likely threatening the wellbeing of critical species, such as P. oceanica.

Beneficial effects of magnetite nanoparticles on soybean-Bradyrhizobium japonicum and alfalfa-Sinorhizobium meliloti associations

Nanoparticles (NPs)-based growth stimulators have promising usage in agriculture. This research analyzed the impact of citric acid-coated magnetite nanoparticles (Fe3O4-NPs; 50 mg Fe L-1) added once at pre-sowing on soybean and alfalfa seedlings growing in association with their corresponding microsymbiont partners, Bradyrhizobium japonicum and Sinorhizobium meliloti; also on the in vitro growth rate of these microorganisms. Fe-EDTA (50 mg Fe L-1) was used as a comparator. Fe3O4-NPs significantly augmented the growth rate constant (7-17%) and extracellular polysaccharides production of both microsymbionts (B. japonicum: 2-fold; S. meliloti: 43%), which probably favored bacterial adhesion to the root hairs. In both legumes, Fe3O4-NPs increased chlorophyll content (up to 56% in soybean) and improved plant growth, evidenced by a greater root biomass system (80-90% higher than the control), and increased shoot biomass (30-40%). Besides, Fe3O4-NPs addition resulted in earlier nodule formation and enhanced nodule biomass (about 2.5-fold in both species). Nodules were mainly located in the crown of the root in the NP50 treatment, while they were evenly distributed along lateral roots in the control and the comparator. Fe3O4-NPs also augmented significantly nodule leghemoglobin content (∼50-70%) and total N in legumes' shoots (ca. 20%). CAT activity increased only under NP50 treatment and no symptoms of oxidative damage were evidenced. In this work, we found that besides not being toxic neither to soybean and alfalfa plants nor to their microsymbiont partners, Fe3O4-NPs do not exert adverse effects on the symbioses establishment; oppositely, a more efficient nodulation pattern was verified in both plant species.

A review on transcriptomic and metabolomic responses of plants to nanopollution

Nanoparticles (NPs) are tiny substances that can exist in the soil with different forms at different concentrations. In general, they present enormous effects on the growth, physiology, and molecular responses in plants. Indeed, they can penetrate the roots, stem, and leaves via different ways like stomata, plasmodesmata, xylem, and phloem and through transporter proteins like aquaporins. Once entered the plants, NPs induce reactive oxygen species (ROS) formation, and the plants respond to ROS by stimulates the production of antioxidants and antioxidant enzymes as well as the production of various primary and secondary metabolites like flavonoids and phenolic compounds. In addition, NPs have significantly affected the distribution of mineral profiles in plants. NPs considerably affect plant growth and yield in a dose-dependent fashion. At higher concentrations, they induced potent cytotoxicity and genotoxicity and thus reduced the growth and development of plants in turn decrease the yield. NPs exert potent changes in the transcriptome and metabolome pattern of plants to counteract the ROS imposed by NPs. This review depicts the overview of transcriptomic and metabolomic responses of plants towards nanopollution.

Fate, bioaccumulation and toxicity of engineered nanomaterials in plants: Current challenges and future prospects

The main focus of this review is to discuss the current advancement in nano-metallic caused phytotoxicity on living organisms and current challenges in crops. Nanostructured materials provide new tools in agriculture to boost sustainable food production, but the main concern is that large-scale production and release of nanomaterials (NMs) into the ecosystem is a rising threat to the surrounding environment that is an urgent challenge to be addressed. The usage of NMs directly influences the transport pathways within plants, which directly relates to their stimulatory/ inhibitory effects. Because of the unregulated nanoparticles (NMs) exposure to soil, they are adsorbed at the root surface, followed by uptake and inter/intracellular mobility within the plant tissue, while the aerial exposure is taken up by foliage, mostly through cuticles, hydathodes, stigma, stomata, and trichomes, but the actual mode of NMs absorption into plants is still unclear. NMs-plant interactions may have stimulatory or inhibitory effects throughout their life cycle depending on their composition, size, concentration, and plant species. Although many publications on NMs interactions with plants have been reported, the knowledge on their uptake, translocation, and bioaccumulation is still a question to be addressed by the scientific community. One of the critical aspects that must be discovered and understood is detecting NMs in soil and the uptake mechanism in plants. Therefore, the nanopollution in plants has yet to be completely understood regarding its impact on plant health, making it yet another artificial environmental influence of unknown long-term consequences. The present review summarizes the uptake, translocation, and bioaccumulation of NMs in plants, focusing on their inhibitory effects and mechanisms involved within plants.

Integrated Proteomics and Metabolomics Analysis of Nitrogen System Regulation on Soybean Plant Nodulation and Nitrogen Fixation

The specific mechanisms by which nitrogen affects nodulation and nitrogen fixation in leguminous crops are still unclear. To study the relationship between nitrogen, nodulation and nitrogen fixation in soybeans, dual-root soybean plants with unilateral nodulation were prepared by grafting. At the third trifoliate leaf (V3) to fourth trifoliate leaf (V4) growth stages (for 5 days), nitrogen nutrient solution was added to the non-nodulated side, while nitrogen-free nutrient solution was added to the nodulated side. The experiment was designed to study the effects of exogenous nitrogen on proteins and metabolites in root nodules and provide a theoretical reference for analyzing the physiological mechanisms of the interaction between nitrogen application and nitrogen fixation in soybean root nodules. Compared with no nitrogen treatment, exogenous nitrogen regulated the metabolic pathways of starch and sucrose metabolism, organic acid metabolism, nitrogen metabolism, and amino acid metabolism, among others. Additionally, exogenous nitrogen promoted the synthesis of signaling molecules, including putrescine, nitric oxide, and asparagine in root nodules, and inhibited the transformation of sucrose to malic acid; consequently, the rhizobia lacked energy for nitrogen fixation. In addition, exogenous nitrogen reduced cell wall synthesis in the root nodules, thus inhibiting root nodule growth and nitrogen fixation.

A review on metal-based nanoparticles and their toxicity to beneficial soil bacteria and fungi

The unregulated deposition of metal-based nanoparticles in terrestrial ecosystems particularly in agricultural systems has alarmingly threatened the sustainability of the environment and diversity of beneficial microbial populations such as soil bacteria and fungi. This occurs due to the poor treatment of biosolids during wastewater treatment and their application in agricultural fields to enhance the fertility of soils. Continuous deposition, low biodegradability, and longer persistence of metal nanoparticles in soils adversely impact the population of soil beneficial bacteria and fungi. The current literature suggests the toxic outcome of nanoparticle-fungi and nanoparticle-bacteria interactions based on various toxicity endpoints. Therefore, due to the extreme importance of beneficial soil bacteria and fungi for soil fertility and plant growth, this review summarizes the production, application, release of metal nanoparticles in the soil system and their impact on various soil microbes specifically plant growth-promoting rhizobacteria, cellular toxicity and impact of nanoparticles on bioactive molecule production by microbes, destructive nanoparticle impact on unicellular, mycorrhizal, and cellulose/lignin degrading fungi. This review also highlights the molecular alterations in fungi and bacteria-induced by nanoparticles and suggests a plausible toxicity mechanism. This review advances the understanding of the nano-toxicity aspect as a common outcome of nanoparticles and fungi/bacteria interactions.

Effects of metal nanoparticle-mediated treatment on seed quality parameters of different crops

The increasing population of the world requires novel techniques to feed everyone, which can replace or work along with traditional methods to increase production of agricultural crops. In recent times, nanotechnology is considered as a promising and emerging approach to be incorporated in agriculture to improve productivity of different crops by the administration of nanoparticles through seed treatment, foliar spray on plants, nano-fertilizers for balanced crop nutrition, nano-herbicides for effective weed control, nanoinsecticides for plant protection, early detection of plant diseases and nutrient deficiencies using diagnostics kits, and nano-pheromones for effective monitoring of pests. Further, distinct nanoparticles with unique physicochemical and biological properties are used in agriculture to increase the percentage of seed germination, which is the initial step to increase the crop yield. In the context of agricultural crops, nanoparticles have both positive effects on seed quality parameters, such as germination percentage, seedling length, seedling dry weight and vigor indices, as well as negative impacts of causing toxicity toward the environment. Thus, the aim of this review article is to provide a comprehensive overview on the effects of super-dispersive metal powders, such as zinc, silver, and titanium nanoparticles on the seed quality parameters of different crops. In addition, the drawback of conventional seed growth enhancers, impact of metal nanoparticles toward seeds, and mechanism of nanoparticles to increase seed germination were also discussed.

Ecological Drawbacks of Nanomaterials Produced on an Industrial Scale: Collateral Effect on Human and Environmental Health

Currently, hundreds of different nanomaterials with a broad application in products that make daily lives a little bit easier, in every aspect, are being produced on an industrial scale at thousands of tons per year. However, several scientists, researchers, politics, and ordinary citizens have stated their concern regarding the life cycle, collateral effects, and final disposal of these cutting-edge materials. This review summarizes, describes, and discusses all manuscripts published in the Journal Citation Reports during the last 10 years, which studied the toxicity or the effects of nanomaterials on human and environmental health. It was observed that 23.62% of the manuscripts analyzed found no ecological or human risks; 54.39% showed that several nanomaterials have toxicological effects on the ecosystems, human, or environmental health. In comparison, only 21.97% stated the nanomaterials had a beneficial impact on those. Although only 54.39% of the manuscripts reported unfavorable effects of nanomaterials on ecosystems, human, or environmental health, it is relevant because the potential damage is invaluable. Therefore, it is imperative to make toxicological studies of nanomaterials with holistic focus under strictly controlled real conditions before their commercialization, to deliver to the market only innocuous and environmentally friendly products.

Knowledge domain and emerging trends in nanoparticles and plants interaction research: A scientometric analysis

The potential releases of nanoparticles (NPs) into soil medium have drawn considerable attention due to the increasing production and application of NPs worldwide. Understanding the interactions between NPs and plants is particularly important to assess the risks of NPs in the soil ecosystem. Although important knowledge has been gained about the NPs-plants interactions, current results of numerous published articles are still scattered. Therefore, this paper reviews the scientific progress in the NPs-plants interactions via a scientometric analysis to identify the main gaps and to provide future perspectives. Scientific documents on the interaction of nanoparticles and plant research during the period January 2000-July 2020 have been collected from Web of Science core collection and analyzed using CiteSpace. Overall, 9 scientometric indicators, i.e. literature quantity and growth trend, contributing countries, authors, institutions, keywords, cited journals, cited authors, and cited articles, are employed to understand the results retrieved from the 961 documents collected. The number of studies on nano-plant interaction research has been growing at an average annual rate of 56%. 71 countries and around 3380 authors have contributed to this field. Among the cited journals, Environmental Science and Technology stands out as the most-cited journal followed by Science of the Total Environment and Environmental Pollution, respectively. Moreover, the keyword citation burst, an indicator of the most active area of research or emerging trend, indicates that the beneficial side of nanoparticles and the trophic transfer require further exploration. This paper will be beneficial for fully understanding the salient research themes and the research trends of nano-plant interaction in future.

Influence of Funneliformis mosseae enhanced with titanium dioxide nanoparticles (TiO2NPs) on Phaseolus vulgaris L. under salinity stress

The Arbuscular mycorrhizal fungi (AMF) (Funneliformis mosseae), are the most widely distributed symbiont assisting plants to overcome counteractive environmental conditions. In order to improve the sustainability and the activity of AMF, the use of nanotechnology was important. The main objective of this study was to investigate the effect of titanium dioxide nanoparticles (TiO2NPs) on the activity of AMF in common bean roots as well as its activity under salinity stress using morphological and molecular methods. The activity of AMF colonization has increased in the presence of TiO2NPs especially for arbuscule activity (A%), which increased three times with the presence of TiO2NPs. The improvement rate of Funneliformis mosseae on plant growth increased from 180% to 224% of control at the lowest level of salinity and increased from 48% to 130% at higher salinity level, respectively. The AMF dependencies for plant dry biomass increased in the presence of TiO2NPs from 277% in the absence of salinity to 465 and 883% % at low and high salinity levels, respectively. The presence of AMF co-inoculated with TiO2NPs resulted in increasing the salinity tolerance of plants at all levels and reached 110% at salinity level of 100 mM NaCl. Quantitative colonization methods showed that the molecular intensity ratio and the relative density of paired inocula AMF Nest (NS) or chitin synthases gene (Chs) with TiO2NPs were higher significantly P.>0.05 than single inoculants of AMF gene in roots under the presence or the absence of salinity by about two folds and about 40%. Hence, the positive effect of TiO2NPs was confined to its effect on AMF not on bean plants itself.

Fluorine and white clover: Assessing fluorine's impact on Rhizobium leguminosarum

The soil fluorine (F) concentration in New Zealand agricultural soils has increased with time as a direct result of the widespread application of phosphate fertilizer to land. Elevated soil F concentrations may potentially harm soil microorganisms, which are important for nutrient cycling and soil formation. Rhizobium leguminosarum is a N₂ -fixing soil bacterium that is a fundamental component in New Zealand legume-based pastoral farming. Any impact of F on Rhizobium leguminosarum would have an adverse effect on New Zealand pasture production. In this study, F toxicity to Rhizobium leguminosarum was examined as a first step to develop F guideline values for New Zealand agricultural soils. Bottle-based experiments were conducted to examine the effect of the F^- ion on Rhizobium-white clover (Trifolium repens L.) symbiosis by observing nodule morphology and growth. Results indicate that the F^- concentration that causes 10% inhibition of Rhizobium respiration (IC₁₀ ) for F^- toxicity to Rhizobium leguminosarum was >100 mg F^-  L^-1 . Significant morphological changes occurred when Rhizobium was exposed to F concentrations of 500 and 1000 mg L^-1 . Both light and transmission electron micrographs confirmed that the Rhizobium leguminosarum-white clover interaction was not influenced by F^- concentrations >100 mg L^-1 . The toxic F^- concentration for Rhizobium leguminosarum determined in this study is orders of magnitude higher than the F^- concentration in New Zealand agriculture soils under "normal conditions." There appears to be no indication of imminent risk of soil F to Rhizobium leguminosarum.

Physical Properties of Carbon Nanomaterials and Nanoceria Affect Pathways Important to the Nodulation Competitiveness of the Symbiotic N2 -Fixing Bacterium Bradyrhizobium diazoefficiens

The pathogenicity and antimicrobial properties of engineered nanomaterials (ENMs) are relatively well studied. However, less is known regarding the interactions of ENMs and agriculturally beneficial microorganisms that affect food security. Nanoceria (CeO2 nanoparticles (NPs)), multiwall carbon nanotubes (MWCNTs), graphene nanoplatelets (GNPs), and carbon black (CB) have been previously shown to inhibit symbiotic N2 fixation in soybeans, but direct rhizobial susceptibility is uncertain. Here, Bradyrhizobium diazoefficiens associated with symbiotic N2 fixation in soybeans is assessed, evaluating the role of soybean root exudates (RE) on ENM-bacterial interactions and the effects of CeO2 NPs, MWCNTs, GNPs, and CB on bacterial growth and gene expression. Although bacterial growth is inhibited by 50 mg L-1 CeO2 NPs, MWCNTs, and CB, all ENMs at 0.1 and 10 mg L-1 cause a global transcriptomic response that is mitigated by RE. ENMs may interfere with plant-bacterial signaling, as evidenced by suppressed upregulation of genes induced by RE, and downregulation of genes encoding transport RNA, which facilitates nodulation signaling. MWCNTs and CeO2 NPs inhibit the expression of genes conferring B. diazoefficiens nodulation competitiveness. Surprisingly, the transcriptomic effects on B. diazoefficiens are similar for these two ENMs, indicating that physical, not chemical, ENM properties explain the observed effects.

Metabolomic analyses of the bio-corona formed on TiO2 nanoparticles incubated with plant leaf tissues

BACKGROUND

The surface of a nanoparticle adsorbs molecules from its surroundings with a specific affinity determined by the chemical and physical properties of the nanomaterial. When a nanoparticle is exposed to a biological system, the adsorbed molecules form a dynamic and specific surface layer called a bio-corona. The present study aimed to identify the metabolites that form the bio-corona around anatase TiO₂ nanoparticles incubated with leaves of the model plant Arabidopsis thaliana.

RESULTS

We used an untargeted metabolomics approach and compared the metabolites isolated from wild-type plants with plants deficient in a class of polyphenolic compounds called flavonoids.

CONCLUSIONS

These analyses showed that TiO₂ nanoparticle coronas are enriched for flavonoids and lipids and that these metabolite classes compete with each other for binding the nanoparticle surface.

Nanomaterials modify the growth of crops and some characteristics of organisms from agricultural or forest soils: An experimental study at laboratory, greenhouse and land level

Currently, some concerns regarding the potential toxicity of nanoparticles (NP) on the environment have emerged. The effect of ZnO, TiO2, and Fe2O3 NP on corn (Zea mays L.), common beans (Phaseolus vulgaris L.), nanobioremediation of polycyclic aromatic hydrocarbons (PAH), and soil organisms from agricultural or forest soils was studied at laboratory, greenhouse, and land level. The samples were analyzed by X-ray diffraction (XRD), field emission scanning electron microscopy with X-ray energy dispersion spectrometry (FESEM-EDS), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS) gas chromatography (GC), ultra-performance liquid chromatography coupled to mass spectrometry (UPLC-MS) and Fourier transform infrared spectrometry with attenuated total reflectance (FTIR-ATR). ZnO-NP did not harm the mycorrhizal root colonization but, the presence of ZnO-NP decreased the degradation of PAH. The synthesis of metabolites from corn was more affected by the PAH than by ZnONP. FTIR spectra showed that NP affected the synthesis of compounds from specific functional groups in common bean plants. Fe2O3-NP were attached to the body of forestsoil organisms and significantly increased the concentration of Fe in their body, while TiO2-NP changed the morphological tissue of roots and stems of common bean as witnessed by micrographs of longitudinal and cross-sections. The NP used in this research significantly changed some response variables on the experiments carried-out at laboratory, greenhouse, and land level.

Are Nanoparticles a Threat to Mycorrhizal and Rhizobial Symbioses? A Critical Review

Soil microorganisms can be exposed to, and affected by, nanoparticles (NPs) that are either purposely released into the environment (e.g., nanoagrochemicals and NP-containing amendments) or reach soil as nanomaterial contaminants. It is crucial to evaluate the potential impact of NPs on key plant-microbe symbioses such as mycorrhizas and rhizobia, which are vital for health, functioning and sustainability of both natural and agricultural ecosystems. Our critical review of the literature indicates that NPs may have neutral, negative, or positive effects on development of mycorrhizal and rhizobial symbioses. The net effect of NPs on mycorrhizal development is driven by various factors including NPs type, speciation, size, concentration, fungal species, and soil physicochemical properties. As expected for potentially toxic substances, NPs concentration was found to be the most critical factor determining the toxicity of NPs against mycorrhizas, as even less toxic NPs such as ZnO NPs can be inhibitory at high concentrations, and highly toxic NPs such as Ag NPs can be stimulatory at low concentrations. Likewise, rhizobia show differential responses to NPs depending on the NPs concentration and the properties of NPs, rhizobia, and growth substrate, however, most rhizobial studies have been conducted in soil-less media, and the documented effects cannot be simply interpreted within soil systems in which complex interactions occur. Overall, most studies indicating adverse effects of NPs on mycorrhizas and rhizobia have been performed using either unrealistically high NP concentrations that are unlikely to occur in soil, or simple soil-less media (e.g., hydroponic cultures) that provide limited information about the processes occurring in the real environment/agrosystems. To safeguard these ecologically paramount associations, along with other ecotoxicological considerations, large-scale application of NPs in farming systems should be preceded by long-term field trials and requires an appropriate application rate and comprehensive (preferably case-specific) assessment of the context parameters i.e., the properties of NPs, microbial symbionts, and soil. Directions and priorities for future research are proposed based on the gaps and experimental restrictions identified.

Assessing the impacts of sewage sludge amendment containing nano-TiO2 on tomato plants: A life cycle study

Increasing evidence indicates the presence of engineered nanoparticles (ENPs) in sewage sludge derived from wastewater treatment. Land application of sewage sludge is, therefore, considered as an important pathway for ENP transfer to the environment. The aim of this work was to understand the effects of sewage sludge containing nano-TiO₂ on plants (tomato) when used as an amendment in agricultural soil. We assessed developmental parameters for the entire plant life cycle along with metabolic and bio-macromolecule changes and titanium accumulation in plants. The results suggest that the sewage sludge amendment containing nano-TiO₂ increased plant growth (142% leaf biomass, 102% fruit yield), without causing changes in biochemical responses, except for a 43% decrease in leaf tannin concentration. Changes in elemental concentrations (mainly Fe, B, P, Na, and Mn) of plant stem, leaves and, to a lesser extent fruits were observed. Fourier-transformed infrared analysis showed maximum changes in plant leaves (decrease in tannins and lignins and increase in carbohydrates) but no change in fruits. No significant Ti enrichment was detected in tomato fruits. In conclusion, we evidenced no acute toxicity to plants and no major implication for food safety after one plant life cycle exposure.

Nanotoxicity of engineered nanomaterials (ENMs) to environmentally relevant beneficial soil bacteria - a critical review

Deposition of engineered nanomaterials (ENMs) in various environmental compartments is projected to continue rising exponentially. Terrestrial environments are expected to be the largest repository for environmentally released ENMs. Because ENMs are enriched in biosolids during wastewater treatment, agriculturally applied biosolids facilitate ENM exposure of key soil micro-organisms, such as plant growth-promoting rhizobacteria (PGPR). The ecological ramifications of increasing levels of ENM exposure of terrestrial micro-organisms are not clearly understood, but a growing body of research has investigated the toxicity of ENMs to various soil bacteria using a myriad of toxicity end-points and experimental procedures. This review explores what is known regarding ENM toxicity to important soil bacteria, with a focus on ENMs which are expected to accumulate in terrestrial ecosystems at the highest concentrations and pose the greatest potential threat to soil micro-organisms having potential indirect detrimental effects on plant growth. Knowledge gaps in the fundamental understanding of nanotoxicity to bacteria are identified, including the role of physicochemical properties of ENMs in toxicity responses, particularly in agriculturally relevant micro-organisms. Strategies for improving the impact of future research through the implementation of in-depth ENM characterization and use of necessary experimental controls are proposed. The future of nanotoxicological research employing microbial ecoreceptors is also explored, highlighting the need for continued research utilizing bacterial isolates while concurrently expanding efforts to study ENM-bacteria interactions in more complex environmentally relevant media, e.g. soil. Additionally, the particular importance of future work to extensively examine nanotoxicity in the context of bacterial ecosystem function, especially of plant growth-promoting agents, is proposed.

A seasonal observation on the distribution of engineered nanoparticles in municipal wastewater treatment systems exemplified by TiO2 and ZnO

The present research attempted to assess the seasonal variation of engineered nanoparticles (ENPs) in a major municipal wastewater treatment system. A monthly survey over a 12-month period was conducted to monitor the concentration of TiO₂ and ZnO nanoparticles throughout the treatment process. Results showed inflow concentrations in the range of 21.6±5.0-391.0±43.0μg/L and 20.0±12.0-212.0±53.0μg/L for TiO₂ and ZnO, respectively. Seasonal pattern of the inflow ENPs concentration showed elevated value in the summer and winter periods for both TiO₂ and ZnO. Based on the concentration profile, the hydraulic flow rate, and the concentration of mixed liquid suspended solid (MLSS), the daily mass loading (DML) or mass flow rate of nanoparticles and the mass ratio of engineered nanoparticle to MLSS were calculated. DML provided a real-time estimate of temporal distribution of ENPs in the treatment processes. Results indicated a daily mass loading of 50.1±12.7 and 44.7±14.1kg/day (yearly average) for TiO₂ and ZnO, respectively. The amount of ENPs captured by sludge particulates were, yearly average, of 7.1kg-ZnO/d and 39.8kg-TiO₂/d, and 8.9kg-ZnO/d and 25.1kg-TiO₂/d, by the primary and the secondary sludge particulates, respectively. ENPs to MLSS mass ratio also showed a seasonal patter similar to the inflow ENPs concentration, where summer and winter periods showed elevated values. Additionally, loss of ENPs throughout the treatment plant that was not accounted for, also can be estimated from the daily mass loading rate and the mass ratio of ENPs to MLSS. Based on the seasonal distribution of ENPs in wastewater treatment systems, especially the daily mass loading rate, it is possible to estimate the uses of nanoparticle-related commercial and personal care products in the urban areas and enable decision-making on the strategy of sludge disposal management.

Effects of biosolids from a wastewater treatment plant receiving manufactured nanomaterials on Medicago truncatula and associated soil microbial communities at low nanomaterial concentrations

Concern has grown regarding engineered nanomaterials (ENMs) entering agricultural soils through the application of biosolids and their possible effects on agroecosystems, even though the ENMs are extensively transformed. The effects of exposure to biosolids containing transformation products of these ENMs at low concentrations remain largely unexplored. We examined the responses of Medicago truncatula and its symbiotic rhizobia Sinorhizobium meliloti exposed to soil amended with biosolids from WWTP containing low added concentrations of ENMs (ENM Low), bulk/dissolved metals (bulk/dissolved Low), or no metal additions (control). We targeted adding approximately 5mg/kg of Ag and 50mg/kg of Zn, and Ti. Measured endpoints included M. truncatula growth, nodulation, changes in the expression of stress response genes, uptake of metals (Ag, Zn and Ti) into shoots, and quantification of S. meliloti populations and soil microbial communities. After 30days exposure, no effects on root or shoot biomass were observed in ENM Low and bulk/dissolved Low treatments, whereas both treatments had a larger average number of nodules (5.7 and 5.57, respectively) compared to controls (0.33). There were no significant differences in either total accumulated metal or metal concentrations in shoots among the treatments. Expression of five stress-related genes (metal tolerance protein (MTP), metal transporter (MTR), peroxidase (PEROX), NADPH oxidase (NADPH) and 1-aminocyclopropane-1-carboxylate oxidase-like protein (ACC_Oxidase)) was significantly down-regulated in both bulk/dissolved Low and ENM Low treatments. However, a change in soil microbial community composition and a significant increase in total microbial biomass were observed in ENM Low relative to control. The ENM Low treatment had increased abundance of Gram-negative and anaerobic bacteria and reduced abundance of eukaryotes compared to control. The study demonstrated that although there were some subtle shifts in microbial community composition, plant health was minimally impacted by ENMs within the time frame and at the low exposure concentrations used in this study.

Salinity-Based Toxicity of CuO Nanoparticles, CuO-Bulk and Cu Ion to Vibrio anguillarum

Bacteria are used in ecotoxicology for their important role in marine ecosystems and their quick, reproducible responses. Here we applied a recently proposed method to assess the ecotoxicity of nanomaterials on the ubiquitous marine bacterium Vibrio anguillarum, as representative of brackish and marine ecosystems. The test allows the determination of 6-h EC₅₀ in a wide range of salinity, by assessing the reduction of bacteria actively replicating and forming colonies. The toxicity of copper oxide nanoparticles (CuO NPs) at different salinities (5-20-35 ‰) was evaluated. CuSO₄ 5H₂O and CuO bulk were used as reference toxicants (solubility and size control, respectively). Aggregation and stability of CuO NP in final testing dispersions were characterized; Cu²⁺ dissolution and the physical interactions between Vibrio and CuO NPs were also investigated. All the chemical forms of copper showed a clear dose-response relationship, although their toxicity was different. The order of decreasing toxicity was: CuSO₄ 5H₂O > CuO NP > CuO bulk. As expected, the size of CuO NP aggregates increased with salinity and, concurrently, their toxicity decreased. Results confirmed the intrinsic toxicity of CuO NPs, showing modest Cu²⁺ dissolution and no evidence of CuO NP internalization or induction of bacterial morphological alterations. This study showed the V. anguillarum bioassay as an effective tool for the risk assessment of nanomaterials in marine and brackish environments.

David JF, Nelson BC, Xing B. Multiple Method Analysis of TiO2 Nanoparticle Uptake in Rice (Oryza sativa L.) Plants

Understanding the translocation of nanoparticles (NPs) into plants is challenging because qualitative and quantitative methods are still being developed and the comparability of results among different methods is unclear. In this study, uptake of titanium dioxide NPs and larger bulk particles (BPs) in rice plant (Oryza sativa L.) tissues was evaluated using three orthogonal techniques: electron microscopy, single-particle inductively coupled plasma mass spectroscopy (spICP-MS) with two different plant digestion approaches, and total elemental analysis using ICP optical emission spectroscopy. In agreement with electron microscopy results, total elemental analysis of plants exposed to TiO₂ NPs and BPs at 5 and 50 mg/L concentrations revealed that TiO₂ NPs penetrated into the plant root and resulted in Ti accumulation in above ground tissues at a higher level compared to BPs. spICP-MS analyses revealed that the size distributions of internalized particles differed between the NPs and BPs with the NPs showing a distribution with smaller particles. Acid digestion resulted in higher particle numbers and the detection of a broader range of particle sizes than the enzymatic digestion approach, highlighting the need for development of robust plant digestion procedures for NP analysis. Overall, there was agreement among the three techniques regarding NP and BP penetration into rice plant roots and spICP-MS showed its unique contribution to provide size distribution information.

Novel impacts of functionalized multi-walled carbon nanotubes in plants: promotion of nodulation and nitrogenase activity in the rhizobium-legume system

The rhizobium-legume symbiosis system is critical for nitrogen-cycle balance in agriculture. However, the potential effects of carbon nanomaterials (CNMs) on this system remain largely unknown. Herein, we studied the effects of four carbon-based materials (activated carbon (AC), single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs) and graphene oxide (GO)) on the rhizobium-legume symbiosis system consisting of Lotus japonicus and Mesorhizobium loti MAFF303099. Under non-symbiotic conditions, the bacterial growth and root development of plants were both clearly inhibited by SWCNTs and GO, while the elongation of plant stems was enhanced by MWCNTs to a certain degree. More importantly, only MWCNTs could increase the number of nodules and enhance the activity of nitrogenase in the rhizobium-plant interaction. Further analyses showed that the average number of nodules in plants treated with 100 μg mL^-1 MWCNTs was significantly increased by 39% at 14 days post inoculation (dpi) and by 41% at 28 dpi. Meanwhile, the biological nitrogen fixation of the nodules was promoted by more than 10% under 100 μg mL^-1 MWCNT treatment, which enhanced the above- and below-ground fresh biomass by 14% and 25% respectively at 28 dpi. Transmission electron microscopy images further indicated that MWCNTs penetrated the cell wall, and pierced through the cell membrane to be transmitted into the cytoplasm. In addition, gene expression analysis showed that the promotion of nodulation by MWCNTs was correlated with the up-regulation of certain genes involved in this signaling pathway. In particular, the expression of NIN, a crucial gene regulating the development of nodules, was significantly elevated 2-fold by MWCNTs at an early stage of nodulation. These findings are expected to facilitate the understanding and future utilization of MWCNTs in agriculture.

Agglomeration Determines Effects of Carbonaceous Nanomaterials on Soybean Nodulation, Dinitrogen Fixation Potential, and Growth in Soil

The potential effects of carbonaceous nanomaterials (CNMs) on agricultural plants are of concern. However, little research has been performed using plants cultivated to maturity in soils contaminated with various CNMs at different concentrations. Here, we grew soybean for 39 days to seed production in soil amended with 0.1, 100, or 1000 mg kg-1 of either multiwalled carbon nanotubes (MWCNTs), graphene nanoplatelets (GNPs), or carbon black (CB) and studied plant growth, nodulation, and dinitrogen (N2) fixation potential. Plants in all CNM treatments flowered earlier (producing 60% to 372% more flowers when reproduction started) than the unamended controls. The low MWCNT-treated plants were shorter (by 15%) with slower leaf cover expansion (by 26%) and less final leaf area (by 24%) than the controls. Nodulation and N2 fixation potential appeared negatively impacted by CNMs, with stronger effects at lower CNM concentrations. All CNM treatments reduced the whole-plant N2 fixation potential, with the highest reductions (by over 91%) in the low and medium CB and the low MWCNT treatments. CB and GNPs appeared to accumulate inside nodules as observed by transmission electron microscopy. CNM dispersal in aqueous soil extracts was studied to explain the inverse dose-response relationships, showing that CNMs at higher concentrations were more agglomerated (over 90% CNMs settled as agglomerates >3 μm after 12 h) and therefore proportionally less bioavailable. Overall, our findings suggest that lower concentrations of CNMs in soils could be more impactful to leguminous N2 fixation, owing to greater CNM dispersal and therefore increased bioavailability at lower concentrations.

Surface coating changes the physiological and biochemical impacts of nano-TiO2 in basil (Ocimum basilicum) plants

Little is known about the effects of surface coating on the interaction of engineered nanoparticles (ENPs) with plants. In this study, basil (Ocimum basilicum) was cultivated for 65 days in soil amended with unmodified, hydrophobic (coated with aluminum oxide and dimethicone), and hydrophilic (coated with aluminum oxide and glycerol) titanium dioxide nanoparticles (nano-TiO₂) at 125, 250, 500, and 750 mg nano-TiO₂ kg^-1 soil. ICP-OES/MS, SPAD meter, and UV/Vis spectrometry were used to determine Ti and essential elements in tissues, relative chlorophyll content, carbohydrates, and antioxidant response, respectively. Compared with control, hydrophobic and hydrophilic nano-TiO₂ significantly reduced seed germination by 41% and 59%, respectively, while unmodified and hydrophobic nano-TiO₂ significantly decreased shoot biomass by 31% and 37%, respectively (p ≤ 0.05). Roots exposed to hydrophobic particles at 750 mg kg^-1 had 87% and 40% more Ti than the pristine and hydrophilic nano-TiO₂; however, no differences were found in shoots. The three types of particles affected the homeostasis of essential elements: at 500 mg kg^-1, unmodified particles increased Cu (104%) and Fe (90%); hydrophilic increased Fe (90%); while hydrophobic increased Mn (339%) but reduced Ca (71%), Cu (58%), and P (40%). However, only hydrophobic particles significantly reduced root elongation by 53%. Unmodified, hydrophobic, and hydrophilic particles significantly reduced total sugar by 39%, 38%, and 66%, respectively, compared with control. Moreover, unmodified particles significantly decreased reducing sugar (34%), while hydrophobic particles significantly reduced starch (35%). Although the three particles affected basil plants, coated particles impacted the most its nutritional quality, since they altered more essential elements, starch, and reducing sugars.

Effect of metal and metal oxide nanoparticles on growth and physiology of globally important food crops: A critical review

The concentrations of engineered metal and metal oxide nanoparticles (NPs) have increased in the environment due to increasing demand of NPs based products. This is causing a major concern for sustainable agriculture. This review presents the effects of NPs on agricultural crops at biochemical, physiological and molecular levels. Numerous studies showed that metal and metal oxide NPs affected the growth, yield and quality of important agricultural crops. The NPs altered mineral nutrition, photosynthesis and caused oxidative stress and induced genotoxicity in crops. The activities of antioxidant enzymes increased at low NPs toxicity while decreased at higher NPs toxicity in crops. Due to exposure of crop plants to NPs, the concentration of NPs increased in different plant parts including fruits and grains which could transfer to the food chain and pose a threat to human health. In conclusion, most of the NPs have both positive and negative effects on crops at physiological, morphological, biochemical and molecular levels. The effects of NPs on crop plants vary greatly with plant species, growth stages, growth conditions, method, dose, and duration of NPs exposure along with other factors. Further research orientation is also discussed in this review article.

Jointed toxicity of TiO2 NPs and Cd to rice seedlings: NPs alleviated Cd toxicity and Cd promoted NPs uptake

Previous studies have reported that nanoparticles (NPs) and heavy metals are toxic to the environment. However, the jointed toxicity is not yet well understood. This study was aimed to investigate the combined toxicity of TiO₂ NPs and the heavy metal cadmium (Cd) to plants. Rice (Oryzasativa L.) was selected as the target plant. The rice seedlings were randomly separated into 12 groups and treated with CdCl₂ (0, 10 and 20 mg/L) and TiO₂ NPs (0, 10, 100 and 1000 mg/L). The plant height, biomass and root length indicated significant toxicity of Cd to the growth, but TiO₂ NPs exhibited the potential ability to alleviate the Cd toxicity. Transmission electron microscopy (TEM) and energy dispersive spectrometer (EDS) confirmed the existence of TiO₂ NPs in plants. Elemental analysis of Ti and Cd suggested that the presences of Cd significantly decreased the Ti accumulation in the rice roots in the co-exposure treatments. Interestingly, TiO₂ NPs could lower the Cd uptake and distribution in rice roots and leaves. The results of antioxidant enzyme activity, lipid peroxide as well as phytohormones varied in the different treatments. Comparing with the Cd alone treatment, the net photosynthetic rate and chlorophyll content were significantly increased in the co-exposure treatments, suggesting that TiO₂ NPs could tremendously reduce the Cd toxicity.

An overview on manufactured nanoparticles in plants: Uptake, translocation, accumulation and phytotoxicity

The unprecedented capability to control and characterize materials on the nanometer scale has led to the rapid expansion of nanostructured materials. The expansion of nanotechnology, resulting into myriads of consumer and industrial products, causes a concern among the scientific community regarding risk associated with the release of nanomaterials in the environment. Bioavailability of excess nanomaterials ultimately threatens ecosystem and human health. Over the past few years, the field of nanotoxicology dealing with adverse effects and the probable risk associated with particulate structures <100 nm in size has emerged from the recognized understanding of toxic effects of fibrous and non-fibrous particles and their interactions with plants. The present review summarizes uptake, translocation and accumulation of nanomaterials and their recognized ways of phytotoxicity on morpho-anatomical, physiological, biochemical and molecular traits of plants. Besides this, the present review also examines the intrinsic detoxification mechanisms in plants in light of nanomaterial accumulation within plant cells or parts.

Saltwater ecotoxicology of Ag, Au, CuO, TiO2, ZnO and C60 engineered nanoparticles: An overview

This review paper examined 529 papers reporting experimental nanoecotoxicological original data. Only 126 papers referred to saltwater environments (water column and sediment) including a huge variety of species (n=51), their relative endpoints and engineered nanoparticles (ENPs) (n=38). We tried to provide a synthetic overview of the ecotoxicological effects of ENPs from existing data, refining papers on the basis of cross-cutting selection criteria and supporting a "mind the gap" approach stressing on missing data for hazard and risk assessment. After a codified selection procedure, attention was paid to Ag, Au, CuO, TiO2, ZnO and C60 ENPs, evidencing and comparing the observed nanoecotoxicity range of effect. Several criticisms were evidenced: i) some model organisms are overexploited like microalgae and molluscs compared to annelids, echinoderms and fish; ii) underexploited model organisms: mainly bacteria and fish; iii) exposure scenario variability: high species-specific and ENP scenarios including organism life stage and way of administration/spiking of toxicants; iv) scarce comparability between results due to exposure scenario variability; v) micro- and mesocosms substantially unexplored; vi) mixture effects: few examples are available only for ENPs and traditional pollutants; mixtures of ENPs have not been investigated yet; vii) effects of ions and ENPs: nAg, nCuO and nZnO toxicity aetiology is still a matter of discussion; viii) size and morphology effects of ENPs: scarcely investigated, justified and understood. Toxicity results evidenced that: nAu>nZnO>nAg>nCuO>nTiO2>C60.

Effects of Titanium Dioxide Nanoparticles on Red Clover and Its Rhizobial Symbiont

Titanium dioxide nanoparticles (TiO₂ NPs) are in consideration to be used in plant protection products. Before these products can be placed on the market, ecotoxicological tests have to be performed. In this study, the nitrogen fixing bacterium Rhizobium trifolii and red clover were exposed to two TiO₂ NPs, i.e., P25, E171 and a non-nanomaterial TiO₂. Growth of both organisms individually and their symbiotic root nodulation were investigated in liquid and hydroponic systems. While 23 and 18 mg l^-1 of E171 and non-nanomaterial TiO₂ decreased the growth rate of R. trifolii by 43 and 23% respectively, P25 did not cause effects. Shoot length of red clover decreased between 41 and 62% for all tested TiO₂ NPs. In 21% of the TiO₂ NP treated plants, no nodules were found. At high concentrations certain TiO₂ NPs impaired R. trifolii as well as red clover growth and their symbiosis in the hydroponic systems.

Effect of nanoparticles on red clover and its symbiotic microorganisms

BACKGROUND

Nanoparticles are produced and used worldwide and are released to the environment, e.g., into soil systems. Titanium dioxide (TiO2) nanoparticles (NPs), carbon nanotubes (CNTs) and cerium dioxide (CeO2) NPs are among the ten most produced NPs and it is therefore important to test, whether these NPs affect plants and symbiotic microorganisms that help plants to acquire nutrients. In this part of a joint companion study, we spiked an agricultural soil with TiO2 NPs, multi walled CNTs (MWCNTs), and CeO2 NPs and we examined effects of these NP on red clover, biological nitrogen fixation by rhizobia and on root colonization of arbuscular mycorrhizal fungi (AMF). We also tested whether effects depended on the concentrations of the applied NPs.

RESULTS

Plant biomass and AMF root colonization were not negatively affected by NP exposure. The number of flowers was statistically lower in pots treated with 3 mg kg(-1) MWCNT, and nitrogen fixation slightly increased at 3000 mg kg(-1) MWCNT.

CONCLUSIONS

This study revealed that red clover was more sensitive to MWCNTs than TiO2 and CeO2 NPs. Further studies are necessary for finding general patterns and investigating mechanisms behind the effects of NPs on plants and plant symbionts.

Zinc oxide induces the stringent response and major reorientations in the central metabolism of Bacillus subtilis

Microorganisms, such as bacteria, are one of the first targets of nanoparticles in the environment. In this study, we tested the effect of two nanoparticles, ZnO and TiO2, with the salt ZnSO4 as the control, on the Gram-positive bacterium Bacillus subtilis by 2D gel electrophoresis-based proteomics. Despite a significant effect on viability (LD50), TiO2 NPs had no detectable effect on the proteomic pattern, while ZnO NPs and ZnSO4 significantly modified B. subtilis metabolism. These results allowed us to conclude that the effects of ZnO observed in this work were mainly attributable to Zn dissolution in the culture media. Proteomic analysis highlighted twelve modulated proteins related to central metabolism: MetE and MccB (cysteine metabolism), OdhA, AspB, IolD, AnsB, PdhB and YtsJ (Krebs cycle) and XylA, YqjI, Drm and Tal (pentose phosphate pathway). Biochemical assays, such as free sulfhydryl, CoA-SH and malate dehydrogenase assays corroborated the observed central metabolism reorientation and showed that Zn stress induced oxidative stress, probably as a consequence of thiol chelation stress by Zn ions. The other patterns affected by ZnO and ZnSO4 were the stringent response and the general stress response. Nine proteins involved in or controlled by the stringent response showed a modified expression profile in the presence of ZnO NPs or ZnSO4: YwaC, SigH, YtxH, YtzB, TufA, RplJ, RpsB, PdhB and Mbl. An increase in the ppGpp concentration confirmed the involvement of the stringent response during a Zn stress. All these metabolic reorientations in response to Zn stress were probably the result of complex regulatory mechanisms including at least the stringent response via YwaC.

Toxicogenomic Responses of the Model Legume Medicago truncatula to Aged Biosolids Containing a Mixture of Nanomaterials (TiO₂, Ag, and ZnO) from a Pilot Wastewater Treatment Plant

Toxicogenomic responses in Medicago truncatula A17 were monitored following exposure to biosolids-amended soils. Treatments included biosolids produced using a pilot wastewater treatment plant with either no metal introduced into the influent (control); bulk/ionic TiO2, ZnO, and AgNO3 added to influent (bulk/dissolved treatment); or Ag, ZnO, and TiO2 engineered nanomaterials added to influent (ENM treatment) and then added to soil, which was aged in the field for 6 months. In our companion study, we found inhibition of nodulation in the ENM but not in the bulk/dissolved treatment. Gene expression profiling revealed highly distinct profiles with more than 10-fold down-regulation in 239 genes in M. truncatula roots from the ENM treatment, while gene expression patterns were similar between bulk/dissolved and control treatments. In response to ENM exposure, many of the identified biological pathways, gene ontologies, and individual genes are associated with nitrogen metabolism, nodulation, metal homeostasis, and stress responses. Expression levels of nine genes were independently confirmed with qRT-PCR. Exposure to ENMs induced unique shifts in expression profiles and biological pathways compared with bulk/dissolved treatment, despite the lack of difference in bioavailable metal fractions, metal oxidation state, and coordination environment between ENM and bulk/dissolved biosolids. As populations of Sinorhizobium meliloti Rm2011 were similar in bulk/dissolved and ENM treatments, our results suggest that inhibition of nodulation in the ENM treatment was primarily due to phytotoxicity, likely caused by enhanced bioavailability of Zn ions.

Evidence of Phytotoxicity and Genotoxicity in Hordeum vulgare L. Exposed to CeO2 and TiO2 Nanoparticles

Engineered nanoscale materials (ENMs) are considered emerging contaminants since they are perceived as a potential threat to the environment and the human health. The reactions of living organisms when exposed to metal nanoparticles (NPs) or NPs of different size are not well known. Very few studies on NPs-plant interactions have been published, so far. For this reason there is also great concern regarding the potential NPs impact to food safety. Early genotoxic and phytotoxic effects of cerium oxide NPs (nCeO2) and titanium dioxide NPs (nTiO2) were investigated in seedlings of Hordeum vulgare L. Caryopses were exposed to an aqueous dispersion of nCeO2 and nTiO2 at, respectively 0, 500, 1000, and 2000 mg l(-1) for 7 days. Genotoxicity was studied by Randomly Amplified Polymorphism DNA (RAPDs) and mitotic index on root tip cells. Differences between treated and control plants were observed in RAPD banding patterns as well as at the chromosomal level with a reduction of cell divisions. At cellular level we monitored the oxidative stress of treated plants in terms of reactive oxygen species (ROS) generation and ATP content. Again nCeO2 influenced clearly these two physiological parameters, while nTiO2 were ineffective. In particular, the dose 500 mg l(-1) showed the highest increase regarding both ROS generation and ATP content; the phenomenon were detectable, at different extent, both at root and shoot level. Total Ce and Ti concentration in seedlings was detected by ICP-OES. TEM EDSX microanalysis demonstrated the presence of aggregates of nCeO2 and nTiO2 within root cells of barley. nCeO2 induced modifications in the chromatin aggregation mode in the nuclei of both root and shoot cells.

Effects of nano-ZnO on the agronomically relevant Rhizobium-legume symbiosis

The impact of nano-ZnO (nZnO) on Rhizobium-legume symbiosis was studied with garden pea and its compatible bacterial partner Rhizobium leguminosarum bv. viciae 3841. Exposure of peas to nZnO had no impact on germination, but significantly affected root length. Chronic exposure of plant to nZnO impacted its development by decreasing the number of the first- and the second-order lateral roots, stem length, leaf surface area, and transpiration. The effect of nZnO dissolution on phytotoxicity was also examined. Results showed that Zn(2+) had negative impact on plant development. Exposure of R. leguminosarum bv. viciae 3841 to nZnO brought about morphological changes by rendering the microbial cells toward round shape and damaging the bacterial surface. Furthermore, the presence of nZnO in the rhizosphere affected root nodulation, delayed the onset of nitrogen fixation, and caused early senescence of nodules. Attachment of nanoparticles on the root surface and dissolution of Zn(2+) are important factors affecting the phytotocity of nZnO. Hence, the presence of nZnO in the environment is potentially hazardous to the Rhizobium-legume symbiosis system.

Trophic transfer, transformation, and impact of engineered nanomaterials in terrestrial environments

Engineered nanomaterials (ENMs) are released into the environment with unknown implications in the food chain. Recent findings demonstrate that ENMs may accumulate and/or increase concentrations of the component metal or carbon nanomaterials in the fruits/grains of agricultural crops, have detrimental or beneficial effects on the agronomic traits, yield, and productivity of plants, induce modifications in the nutritional value of food crops, and transfer within trophic levels. Given this information, important questions needed to be resolved include a determination of actual or predicted concentrations of ENMs through the development of new and perhaps hybridized analytical tools, assessment of the nutritional content modifications and/or accumulation of ENMs, component metal, and cocontaminants in edible plants and their implications on human diet, nutrition, and health, assessment of the consequences of ENM-induced changes in soil health, physiological process, and yield on agricultural production and food security, and transfer of ENMs in trophic levels. Given the significant implications of ENMs exposure and the rather large knowledge gaps that exist, it will be prudent to observe judicious and targeted use of ENMs so as to minimize environmental release until a comprehensive environmental fate and effects assessment can be undertaken.