Uptake, transport and toxicity of engineered nanomaterials (ENMs) into plant cells are complex processes that are currently still not well understood. Parts of this problem are the multifaceted plant anatomy, and analytical challenges to visualize and quantify ENMs in plants. We critically reviewed the currently known ENM uptake, translocation, and accumulation processes in plants. A vast number of studies showed uptake, clogging, or translocation in the apoplast of plants, most notably of nanoparticles with diameters much larger than the commonly assumed size exclusion limit of the cell walls of ∼5-20 nm. Plants that tended to translocate less ENMs were those with low transpiration, drought-tolerance, tough cell wall architecture, and tall growth. In the absence of toxicity, accumulation was often linearly proportional to exposure concentration. Further important factors strongly affecting ENM internalization are the cell wall composition, mucilage, symbiotic microorganisms (mycorrhiza), the absence of a cuticle (submerged plants) and stomata aperture. Mostly unexplored are the roles of root hairs, leaf repellency, pit membrane porosity, xylem segmentation, wounding, lateral roots, nodes, the Casparian band, hydathodes, lenticels and trichomes. The next steps towards a realistic risk assessment of nanoparticles in plants are to measure ENM uptake rates, the size exclusion limit of the apoplast and to unravel plant physiological features favoring uptake.

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Due to growing production, carbon nanotubes (CNT) may soon be found in a broad range of products and thus in the environment. In this work, an algal growth test was developed to determine effects of pristine and oxidized CNT on the green algae Chlorella vulgaris and Pseudokirchneriella subcapitata. CNT suspensions were prepared in algal test medium and characterized taking into account the suspension age, the reduced light transmittance of nanoparticle suspensions defined as shading of CNT and quantified by UV/vis spectroscopy, and the agglomeration of the CNT and of the algal cells. Growth inhibition and photosynthetic activity were investigated as end points. Growth of C. vulgaris was inhibited with effect concentrations of 50% (EC(50)) values of 1.8 mg CNT/L and of 24 mg CNT/L in well dispersed and in agglomerated suspensions, respectively, and 20 mg CNT/L and 36 mg CNT/L for P. subcapitata, respectively. However, the photosynthetic activity was not affected. Growth inhibition was highly correlated with the shading of CNT and the agglomeration of algal cells. This suggests that the reduced algal growth might be caused mainly by indirect effects, i.e. by reduced availability of light and different growth conditions caused by the locally elevated algal concentration inside of CNT agglomerates.

Terrestrial plants can internalize and translocate nanoparticles (NPs). However, direct evidence for the processes driving the NP uptake and distribution in plants is scarce at the cellular level. Here, NP-root interactions were investigated after 10 days of exposure of Arabidopsis thaliana to 10 mg·L of negatively or positively charged gold NPs (∼12 nm) in gels. Two complementary imaging tools were used: X-ray computed nanotomography (nano-CT) and enhanced dark-field microscopy combined with hyperspectral imaging (DF-HSI). The use of these emerging techniques improved our ability to detect and visualize NP in plant tissue: by spectral confirmation via DF-HSI, and in three dimensions via nano-CT. The resulting imaging provides direct evidence that detaching border-like cells (i.e., sheets of border cells detaching from the root) and associated mucilage can accumulate and trap NPs irrespective of particle charge. On the contrary, border cells on the root cap behaved in a charge-specific fashion: positively charged NPs induced a higher mucilage production and adsorbed to it, which prevented translocation into the root tissue. Negatively charged NPs did not adsorb to the mucilage and were able to translocate into the apoplast. These observations provide direct mechanistic insight into NP-plant interactions, and reveal the important function of border cells and mucilage in interactions of plants with charged NPs.

Terrestrial crops are directly exposed to silver nanoparticles (Ag-NPs) and their environmentally transformed analog silver sulfide nanoparticles (Ag2S-NPs) when wastewater treatment biosolids are applied as fertilizer to agricultural soils. This leads to a need to understand their bioavailability to plants. In the present study, the mechanisms of uptake and distribution of silver in alfalfa (Medicago sativa) were quantified and visualized upon hydroponic exposure to Ag-NPs, Ag2S-NPs, and AgNO3 at 3 mg total Ag/L. Total silver uptake was measured in dried roots and shoots, and the spatial distribution of elements was investigated using transmission electron microscopy (TEM) and synchrotron-based X-ray imaging techniques. Despite large differences in release of Ag(+) ions from the particles, Ag-NPs, Ag2S-NPs, and Ag(+) became associated with plant roots to a similar degree, and exhibited similarly limited (<1%) amounts of translocation of silver into the shoot system. X-ray fluorescence (XRF) mapping revealed differences in the distribution of Ag into roots for each treatment. Silver nanoparticles mainly accumulated in the (columella) border cells and elongation zone, whereas Ag(+) accumulated more uniformly throughout the root. In contrast, Ag2S-NPs remained largely adhered to the root exterior, and the presence of cytoplasmic nano-SixOy aggregates was observed. Exclusively in roots exposed to particulate silver, NPs smaller than the originally dosed NPs were identified by TEM in the cell walls. The apparent accumulation of Ag in the root apoplast determined by XRF, and the presence of small NPs in root cell walls suggests uptake of partially dissolved NPs and translocation along the apoplast.

Carbon nanotubes (CNT) are more and more likely to be present in the environment, where they will associate with organic micropollutants due to strong sorption. The toxic effects of these CNT-micropollutant mixtures on aquatic organisms are poorly characterized. Here, we systematically quantified the effects of the herbicide diuron on the photosynthetic activity of the green alga Chlorella vulgaris in presence of different multiwalled CNT (industrial, purified, pristine, and oxidized) or soot. The presence of carbonaceous nanoparticles reduced the adverse effect of diuron maximally by <78% (industrial CNT) and <34% (soot) at 10.0 mg CNT/L, 5.0 mg soot/L, and diuron concentrations in the range 0.73-2990 μg/L. However, taking into account the measured dissolved instead of the nominal diuron concentration, the toxic effect of diuron was equal to or stronger in the presence of CNT by a factor of up to 5. Sorbed diuron consequently remained partially bioavailable. The most pronounced increase in toxicity occurred after a 24 h exposure of algae and CNT. All results point to locally elevated exposure concentration (LEEC) in the proximity of algal cells associated with CNT as the cause for the increase in diuron toxicity.

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Aquatic ecosystems are expected to receive Ag and AgS nanoparticles (NPs) through anthropogenic waste streams. The speciation of silver in Ag-NPs affects their fate in ecosystems, but its influence on interactions with aquatic plants is still unclear. Here, the Ag speciation and distribution was measured in an aquatic plant, duckweed (Landoltia punctata), exposed to Ag or AgS NPs, or to AgNO. The silver distribution in duckweed roots was visualized using synchrotron-based micro X-ray fluorescence (XRF) mapping and Ag speciation was determined using extended X-ray absorption fine structure (EXAFS) spectroscopy. Duckweed exposed to AgS-NPs or Ag-NPs accumulated similar Ag concentrations despite an order of magnitude smaller dissolved Ag fraction measured in the exposure medium for AgS-NPs compared to Ag-NPs. By 24 h after exposure, all three forms of silver had accumulated on and partially in the roots regardless of the form of Ag exposed to the plants. Once associated with duckweed tissue, Ag-NPs had transformed primarily into silver sulfide and silver thiol species. This suggests that plant defenses were active within or at the root surface. The AgS-NPs remained as AgS, while AgNO exposure led to Ag and sulfur-associated Ag species in plant tissue. Thus, regardless of initial speciation, Ag was readily available to duckweed.