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Nanomaterials 2020, 10, 1763 11 of 24 While it is not clear how metal NPs affect the environment, some studies reveal that plants overexposed to them may reveal pathways involved in the cytotoxicity. Proteomic studies on Oryza sativa (Asian rice) with AuNPs and AgNPs have increased protein precursors for oxidative stress tolerance, calcium regulation and signaling, apoptosis, and other kinds of damages [181]. Additionally, Vanini et al. developed research to study the proteomic profile of Eruca sativa exposed to AgNPs and bulk Ag. Seedlings were treated for five days with different concentrations of AgNPs and AgNO3, resulting in changes in proteins involved in redox regulation and the sulfur metabolism for both cases. However, further analysis revealed an altered number of proteins in the endoplasmic reticulum and vacuole of plant cells caused by AgNPs [182]. Kaveh et al. studied the model agro-crop Arabidopsis thaliana and reported the phytoaccumulation of AgNPs [183]. Another approach developed by Taylor et al. described Medicago Sativa L. (alfalfa) tendency to accumulate metal NPs of different sizes [184]. A research conducted by Raliya et al. studied uptake, translocation, and accumulation of different AgNPs with sizes ranging from 30–80 nm delivered by aerosol application to a watermelon plant. The findings indicate that NPs could be taken up by direct penetration and transport through the stomatal opening. Besides, they observed translocation of NPs from leaf to shoots, which suggests they travel by the phloem transport mechanism (i.e., the mechanism of long-distance transport through plant’s sieve tube) [185]. It has been reported that Au is taken up in A. thaliana predominantly in an ionic form, having a significant role in seed germination and antioxidant system. However, other studies suggest that AuNPs’ exposure results in the upregulation of plant genes, causing downregulation of specific-metal transporters to reduce Au uptake [184]. This can be used for studying the limits of NPs in the environment. Moreover, Qian et al. discovered that high concentrations of Ag could be overwhelming to A. thaliana seeds, which should not be exposed to AgNPs during its germination [186]. In contrast, Stampoulis et al. reported no toxic effect on seed germination and root elongation of Cucurbita pepo (zucchini) when exposing to AuNPs and AgNPs. However, in a 15-day hydroponic trial, the biomass and transpiration of the plants exposed to AgNPs were reduced by 75% and 41%, respectively, as compared to control plants and the corresponding bulk Ag powder. Additionally, zucchini shoots exposed to these NPs contained, on average, 4.7 greater Ag concentration than the ones from bulk solutions [187]. Germination studies in Lolium perenne, Hordeum vulgare, and Linum usitatissimum have shown to be affected at low concentrations of AgNPs but never fully inhibited [188]. This suggests that different mechanisms of action might occur across plant species, concerning the effect on germination [189]. Courtois et al. published an important study of the impact of Ag species introduced into the soil via sewage sludge. As mentioned before, AgNPs are incorporated into many conventional and novel products due to their special physicochemical and antimicrobial properties. However, the discharge of these products into wastewater causes an accumulation of AgNPs and Ag2S in sewage sludge. The major concern is related to land application of sewage sludge for agricultural purposes since soils receive a great source of contamination for plants and crops. Soil exposure to metal NPs may lead to changes in microbial biomass that can affect plant growth, causing physiological, biochemical, and molecular effects on them. Nonetheless, much is still unknown about the ecotoxicology of silver species, where several doubts are focused on the possibility of transfer along the trophic chain via accumulation in plants, and for that, research to evaluate the long-term impact of AgNPs on plants is ongoing [190]. 6. AuNPs and AgNPs in Soils Biological indicators are important parameters for evaluating soil quality since soil microbiota directly participates in different processes of this ecosystem, such as decomposition of inorganic matter and nutrient cycling. Therefore, any factor that alters soil microbial biomass will have an impact on soil sustainability [191]. Needless to say, the growing use of AuNPs and AgNPs due to their recognized antimicrobial activity has led to their accumulation in soil ecosystems, affecting its quality [192,193]. Although their environmental impact on the soil microbial community is still under consideration,PDF Image | Green Synthesis of Gold and Silver Nanoparticles from Plant Extracts
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