Plant response to silver nanoparticles: a critical review

Small particles, big effects: How nanoparticles can enhance plant growth in favorable and harsh conditions

By 2050, the global population is projected to reach 9 billion, underscoring the imperative for innovative solutions to increase grain yield and enhance food security. Nanotechnology has emerged as a powerful tool, providing unique solutions to this challenge. Nanoparticles (NPs) can improve plant growth and nutrition under normal conditions through their high surface-to-volume ratio and unique physical and chemical properties. Moreover, they can be used to monitor crop health status and augment plant resilience against abiotic stresses (such as salinity, drought, heavy metals, and extreme temperatures) that endanger global agriculture. Application of NPs can enhance stress tolerance mechanisms in plants, minimizing potential yield losses and underscoring the potential of NPs to raise crop yield and quality. This review highlights the need for a comprehensive exploration of the environmental implications and safety of nanomaterials and provides valuable guidelines for researchers, policymakers, and agricultural practitioners. With thoughtful stewardship, nanotechnology holds immense promise in shaping environmentally sustainable agriculture amid escalating environmental challenges.

Review on interactions between nanomaterials and phytohormones: Novel perspectives and opportunities for mitigating environmental challenges

Nanotechnology offers the potential to provide innovative solutions for sustainable crop production as plants are exposed to a combination of climate change factors (CO2, temperature, UV radiation, ozone), abiotic (heavy metals, salinity, drought), and biotic (virus, bacteria, fungi, nematode, and insects) stresses. The application of particular sizes, shapes, and concentration of nanomaterials (NMs) potentially mitigate the negative impacts in plants by modulation of photosynthetic rate, redox homeostasis, hormonal balance, and nutrient assimilation through upregulation of anti-stress metabolites, antioxidant defense pathways, and genes and genes network. The present review inculcates recent advances in uptake, translocation, and accumulation mechanisms of NMs in plants. The critical theme of this review provides detailed insights into different physiological, biochemical, molecular, and stress tolerance mechanism(s) of NMs action and their cross-talk with different phytohormones. The role of NMs as a double-edged sword for climate change factors, abiotic, and biotic stresses for nutrients uptake, hormones synthesis, cytotoxic, and genotoxic effects including chromosomal aberration, and micronuclei synthesis have been extensively studied. Importantly, this review aims to provide an in-depth understanding of the hormesis effect at low and toxicity at higher doses of NMs under different stressors to develop innovative approaches and design smart NMs for sustainable crop production.

Comparative physiological and metabolomics analyses using Ag⎯NPs and HAS31 (PGPR) to alleviate Cr stress in barley (Hordeum vulgare L.)

In the current industrial scenario, chromium (Cr) as a metal is of great importance but poses a major threat to the ecosystem because of its toxicity, but fewer studies have been conducted on its effects and alleviation strategies by using nanoparticles (NPs) and plant growth promoting rhizobacteria (PGPR). Taking into consideration the positive effects of silver⎯nanoparticles (Ag⎯NPs) and (HAS31) rhizobacteria in reducing Cr toxicity in plants, the present study was conducted. A pot experiment was conducted to determine the effects of single and/or combined application of different levels [0 (no Ag⎯NPS), 15 and 30 mM] of Ag⎯NPs and HAS31 [0 (no HAS31), 50 g and 100 g] on Cr accumulation, morpho-physiological and antioxidative defense attributes of barley (Hordeum vulgare L.) exposed to severe Cr stress [0 (without Cr stress), 50 and 100 μM)]. Results from the present study showed that the increasing levels of Cr in the soil significantly (P < 0.05) decreased plant growth and biomass, photosynthetic pigments, gas exchange attributes, sugars, and nutritional contents from the roots and shoots of the plants. In contrast, increasing levels of Cr in the soil significantly (P < 0.05) increased oxidative stress indicators in term of malondialdehyde, hydrogen peroxide, and electrolyte leakage, and also increased organic acid exudation patter in the roots of H. vulgare. Although, the activities of enzymatic antioxidants and the response of their gene expressions in the roots and shoots of the plants and non-enzymatic such as phenolic, flavonoid, ascorbic acid, and anthocyanin contents were increased by increasing the Cr concentration in the soil. The negative impacts of Cr injury were reduced by the application of PGPR (HAS31) and Ag⎯NPs, which increased plant growth and biomass, improved photosynthetic apparatus, antioxidant enzymes, and mineral uptake, as well as diminished the exudation of organic acids and oxidative stress indicators in roots of H. vulgare by decreasing Cr toxicity. Research findings, therefore, suggest that the application of PGPR (HAS31) and Ag⎯NPs can ameliorate Cr toxicity in H. vulgare, resulting in improved plant growth and composition under metal stress, as depicted by balanced exudation of organic acids.

Silver and salicylic acid-chitosan nanoparticles alter indole alkaloid production and gene expression in root and shoot cultures of Isatis tinctoria and Isatis ermenekensis

Isatis spp. are well-known for their industrial significance due to natural sources of indigotin and indirubin, important indole alkaloids, used in the dye and pharmaceutical industries. In this study, silver nanoparticles (AgNP) and salicylic acid-chitosan nanoparticles (SA-CNP) were synthesized and applied to enhance the production of indigotin and indirubin in shoot and root cultures of Isatis tinctoria and Isatis ermenekensis. Different doses of AgNP and SA-CNP were administered to three-week-old shoot and root cultures, and the effects were assessed at 12, 24, and 48 h. The harvested samples were analyzed to quantify indigotin and indirubin levels. Furthermore, the expression levels of It-TSA and CYP79B2 genes, known to be involved in indole alkaloid biosynthesis, were determined. In I. tinctoria roots, the highest levels of indigotin and indirubin were observed after applying 150 mg L-1 of SA-CNP for 48 h while in I. ermenekensis shoots, indigotin and indirubin reached the maximum levels with the application of 8 mg L-1 AgNP for 48 h. NP application had no remarkable effects on the accumulation of indigotin and indirubin in I. tinctoria shoots and I. ermenekensis roots compared to controls. Additionally, shoot cultures demonstrated superior indirubin production, which significantly increased with AgNP applications. The gene expression analysis also exhibited significant correlations with the changes in indigotin and indirubin levels. The findings of this study lay the groundwork for enhancing in vitro production of indigotin and indirubin in Isatis species through NP applications, and for developing high-capacity production strategies by determining optimal dosages in scale-up studies.

Determination of the Particle Number Concentration, Size Distribution, and Species of Dominant Silver-Containing Nanoparticles in Soils by Single-Particle ICP-MS

The potential risk of various silver-containing nanoparticles (AgCNPs) in soils is related to the concentration, size, and speciation, but their determination remains a great challenge. Herein, we developed an effective method for determining the particle number, size, and species of dominant AgCNPs in soils, including nanoparticles of silver (Ag NPs), silver chloride (AgCl NPs), and silver sulfide (Ag₂S NPs). By ultrasonication wand-assisted tetrasodium pyrophosphate extraction, these AgCNPs were extracted efficiently from soils. Then, multistep selective dissolution of Ag NPs, AgCl NPs, and whole Ag NPs/AgCl NPs/Ag₂S NPs was achieved by 1% (v/v) H₂O₂, 5% (v/v) NH₃·H₂O, and 10 mM thiourea in 2% (v/v) acetic acid, respectively. Finally, the particle number concentration and size distribution of AgCNPs in the extracts and the remaining AgCNP particle number concentration after each dissolution were determined by single-particle inductively coupled plasma mass spectroscopy for speciation of the dominant AgCNPs. AgCNPs were detected in all five soil samples with the concentrations of 0.23-8.00 × 10⁷ particles/g and sizes of 16-110 nm. Ag₂S NPs were the main form of AgCNPs in the examined soils with the percentage range of 53.98-69.19%, followed by AgCl NPs (11.42-23.31%) and Ag NPs (7.78-16.19%). Our method offers a new approach for evaluating the occurrence and potential risk of AgCNPs in environmental soils.

Exogenously Applied Cyclitols and Biosynthesized Silver Nanoparticles Affect the Soluble Carbohydrate Profiles of Wheat (Triticum aestivum L.) Seedling

Cyclitols, such as myo-inositol and its isomers and methyl derivatives (i.e., d-chiro-inositol and d-pinitol (3-O-methyl-chiro-inositol)), are classified as osmolytes and osmoprotectants and are significantly involved in plant responses to abiotic stresses, such as drought, salinity and cold. Moreover, d-pinitol demonstrates a synergistic effect with glutathione (GSH), increasing its antioxidant properties. However, the role of cyclitols in plant protection against stresses caused by metal nanoparticles is not yet known. Therefore, the present study examined the effects of myo-inositol, d-chiro-inositol and d-pinitol on wheat germination, seedling growth and changes in the profile of soluble carbohydrates in response to biologically synthesized silver nanoparticles ((Bio)Ag NPs). It was found that cyclitols were absorbed by germinating grains and transported within the growing seedlings but this process was disrupted by (Bio)Ag NPs. Cyclitols applied alone induced sucrose and 1-kestose accumulation in seedlings slightly, while (Bio)Ag NP doubled the concentrations of both sugars. This coincided with a decrease in monosaccharides; i.e., fructose and glucose. Cyclitols and (Bio)Ag NPs present in the endosperm resulted in reductions in monosaccharides, maltose and maltotriose, with no effect on sucrose and 1-kestose. Similar changes occurred in seedlings developing from primed grains. Cyclitols that accumulated in grain and seedlings during grain priming with d-pinitol and glutathione did not prevent the phytotoxic effects of (Bio)Ag NPs.

Silver Inhibits Lemna minor Growth at High Initial Frond Densities

Silver nanoparticles (AgNPs) are the most popular engineered nanomaterials in consumer products due to their antimicrobial properties. They enter aquatic ecosystems via insufficient purified wastewaters from manufacturers or consumers. AgNPs inhibit growth of aquatic plants, including duckweeds. Growth media nutrient concentration and initial duckweed frond density can affect growth. However, it is not well understood how frond density affects nanoparticle toxicity. We investigated the toxicity of 500 µg/L AgNPs and AgNO₃ on Lemna minor at different initial frond densities (20, 40, and 80 fronds per 28.5 cm²) over 14 days. Plants were more sensitive to silver at high initial frond densities. Growth rates based on frond number and area were lower for plants at 40 and 80 initial frond density in both silver treatments. AgNPs had no effect on frond number, biomass, and frond area at 20 initial frond density. However, AgNO₃ plants had lower biomass than control and AgNP plants at 20 initial frond density. Competition and crowding at high frond densities resulted in reduced growth when silver was present, therefore plant density and crowding effects should be considered in toxicity studies.

Biosynthesized Ag nanoparticles on urea-based periodic mesoporous organosilica enhance galegine content in Galega

The biological approach for synthesizing nanoparticles (NPs) using plant extracts is an efficient alternative to conventional physicochemical methods. Galegine, isolated from Galega (Galega officinalis L.), has anti-diabetic properties. In the present study, silver nanoparticles (AgNPs) loaded onto urea-based periodic mesoporous organosilica (AgNPs/Ur-PMO) were bio-synthesized using G. officinalis leaf extract. The synthesized NPs were characterized and confirmed via analysis methods. Different concentrations of biosynthesized AgNPs/Ur-PMO nanoparticles (0, 1, 5, 10, and 20 mg L^-1) were used as elicitors in cell suspension culture (CSC) of G. officinalis. The callus cells from hypocotyl explants were treated at their logarithmic growth phase (8^th d) and were collected at time intervals of 24, 72, 120, and 168 h. The viability and growth of cells were reduced (by 17% and 35%, respectively) at higher concentrations and longer treatments of AgNPs/Ur-PMO; however, the contents of hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) were increased (1.23 and 3.01 fold, respectively in comparison with the control average). The highest total phenolic (2.43 mg g^-1 dry weight) and flavonoid (2.22 mg g^-1 dry weight) contents were obtained 168 h after treatment with 10 mg L^-1 AgNPs/Ur-PMO. An increasing tendency in the antioxidant enzyme activities was also observed in all the elicitor concentrations. Treatment with AgNPs/Ur-PMO (in particular 5 mg L^-1 for 120 h) significantly enhanced the galegine content (up to 17.42 mg g^-1) about 1.80 fold compared with the control. The results suggest that AgNPs/Ur-PMO can be used as an effective elicitor for enhancing galegine production in the CSC of G. officinalis. KEY POINTS: • The green biosynthesis of AgNPs/Ur-PMO was done using G. officinalis leaf extract • Its toxicity as an elicitor increased with increasing concentration and treatment time • AgNPs/Ur-PMO significantly increased the antioxidant capacity and galegine content.

Biosynthesis of silver nanoparticles using actinomycetes, phytotoxicity on rice seeds, and potential application in the biocontrol of phytopathogens

To find effective silver nanoparticles (AgNPs) for control of phytopathogens, in this study, two strains of actinomycetes isolated from the soil of the Brazilian biome Caatinga (Caat5-35) and from mangrove sediment (Canv1-58) were utilized. The strains were identified by using the 16S rRNA gene sequencing as Streptomyces sp., related to Streptomyces mimosus species. The obtained AgNPs were coded as AgNPs ₃₅ and AgNPs₅₈ and characterized by size and morphology using dynamic light scattering, zeta potential, transmission electron microscopy, and Fourier transformed infrared (FTIR). The antifungal activity of the AgNPs₃₅ and AgNPs₅₈ was evaluated in vitro by the minimal inhibitory concentration (MIC) assay on the phytopathogens, Alternaria solani, Alternaria alternata, and Colletotrichum gloeosporioides. The phytotoxic effect was evaluated by the germination rate and seedling growth of rice (Oryza sativa). AgNPs₃₅ and AgNPs₅₈ showed surface plasmon resonance and average sizes of 30 and 60 nm, respectively. Both AgNPs presented spherical shape and the FTIR analysis confirmed the presence of functional groups such as free amines and hydroxyls of biomolecules bounded to the external layer of the nanoparticles. Both AgNPs inhibited the growth of the three phytopathogens tested, and A. alternate was the most sensible (MIC ≤ 4 µM). Moreover, the AgNPs₃₅ and AgNPs₅₈ did not induce phytotoxic effects on the germination and development of rice seedlings. In conclusion, these AgNPs are promising candidates to biocontrol of these phytopathogens without endangering rice plants.

The Size-Dependent Effects of Silver Nanoparticles on Germination, Early Seedling Development and Polar Metabolite Profile of Wheat (Triticum aestivum L.)

The phytotoxicity of silver nanoparticles (Ag NPs) to plant seeds germination and seedlings development depends on nanoparticles properties and concentration, as well as plant species and stress tolerance degrees. In the present study, the effect of citrate-stabilized spherical Ag NPs (20 mg/L) in sizes of 10, 20, 40, 60, and 100 nm, on wheat grain germination, early seedlings development, and polar metabolite profile in 3-day-old seedlings were analyzed. Ag NPs, regardless of their sizes, did not affect the germination of wheat grains. However, the smaller nanoparticles (10 and 20 nm in size) decreased the growth of seedling roots. Although the concentrations of total polar metabolites in roots, coleoptile, and endosperm of seedlings were not affected by Ag NPs, significant re-arrangements of carbohydrates profiles in seedlings were noted. In roots and coleoptile of 3-day-old seedlings, the concentration of sucrose increased, which was accompanied by a decrease in glucose and fructose. The concentrations of most other polar metabolites (amino acids, organic acids, and phosphate) were not affected by Ag NPs. Thus, an unknown signal is released by small-sized Ag NPs that triggers affection of sugars metabolism and/or distribution.

Synthesis, characterization, antimicrobial activity, and toxicity evaluation of aminolevulinic acid-silver and silver-iron nanoparticles for potential applications in agriculture

Aminolevulinic acid (ALA) is considered one of the most critical plants growth regulators and essential precursors for chlorophyll biosynthesis; besides, its photodynamic activity can be used to exterminate larvae and microorganisms in plants and soil. Silver nanoparticles (AgNPs) have unique physicochemical properties and potent antimicrobial, antiviral, and antifungal activities, and in agriculture, their application as nanopesticides has been proposed. In this study, silver and silver–iron nanoparticles capped/stabilized with aminolevulinic acid (ALAAgNPs and ALAAgFeNPs) were synthesized by the photoreduction method and characterized by UV-vis spectroscopy, transmission electron microscopy, and zeta potential analysis. The kinetics of ¹O₂ generation from ALAAgFeNPs were obtained. The ALANP toxicity was evaluated on stalks of E. densa by observing cell morphology changes and measuring chlorophyll content compared with water-treated plants. Antimicrobial activity was tested against E. coli, P. aeruginosa, and Candida albicans. The results suggested that ALANPs (prepared with [AgNO₃] ≤ 0.2 mM and [ALA] ≤ 0.4 mM) could be suitable for applications in the agricultural sector. The presence of ∼0.3 mmol of iron in ALAAgNPs synthesis increased cell uptake and chlorophyll synthesis.

ALA is a natural metabolite in all living cells and possesses low toxicity. ALANPs exhibit high antimicrobial activity, promote plant growth and have the potential to show photodynamic herbicidal properties under solar illumination.

Toxicity Assessment and Control of Early Blight and Stem Rot of Solanum tuberosum L. by Mancozeb-Loaded Chitosan–Gum Acacia Nanocomposites

Biopolymers such as chitosan and gum acacia are used for nanotechnological applications due to their biosafety and ecofriendly nature. The commercial fungicide mancozeb (M) was loaded into chitosan–gum acacia (CSGA) polymers to form nanocomposite (NC) CSGA-M (mancozeb-loaded) measuring 363.6 nm via the ionic gelation and polyelectrolyte complexation method. The physico-chemical study of nano CSGA-M was accomplished using dynamic light scattering (DLS), scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). Nano CSGA-M-1.0 (containing 1.0 mg/mL mancozeb) at 1.5 ppm demonstrated a maximum inhibition (83.8 ± 0.7%) against Alternaria solani, while Sclerotinia sclerotiorum exhibited a 100% inhibition at 1.0 and 1.5 ppm through the mycelium inhibition method. Commercial mancozeb showed an inhibition of 84.6 ± 0% and 100%, respectively, for both fungi. In pot house conditions, NCs were found to exhibit good antimicrobial activity. Disease control efficiency (DCE, in %) in pathogen-treated plants for CSGA-M-1.0 was 64.6 ± 5.0 and 60.2 ± 1.4% against early blight and stem rot diseases, respectively. NCs showed lower cytotoxicity than commercial mancozeb at the given concentration. In conclusion, both in vitro and in vivo antifungal efficacy for nano CSGA-M was found to be quite comparable but less toxic than mancozeb to Vero cell lines; thus, in the future, this formulation may be used for sustainable agriculture.

The Effect of Bio-Synthesized Silver Nanoparticles on Germination, Early Seedling Development, and Metabolome of Wheat (Triticum aestivum L.)

Changes in the metabolome of germinating seeds and seedlings caused by metal nanoparticles are poorly understood. In the present study, the effects of bio-synthesized silver nanoparticles ((Bio)Ag NPs) on grains germination, early seedlings development, and metabolic profiles of roots, coleoptile, and endosperm of wheat were analyzed. Grains germinated well in (Bio)Ag NPs suspensions at the concentration in the range 10–40 mg/L. However, the growth of coleoptile was inhibited by 25%, regardless of (Bio)Ag NPs concentration tested, whereas the growth of roots gradually slowed down along with the increasing concentration of (Bio)Ag NPs. The deleterious effect of Ag NPs on roots was manifested by their shortening, thickening, browning of roots tips, epidermal cell death, progression from apical meristem up to root hairs zone, and the inhibition of root hair development. (Bio)Ag NPs stimulated ROS production in roots and affected the metabolic profiles of all tissues. Roots accumulated sucrose, maltose, 1-kestose, phosphoric acid, and some amino acids (i.e., proline, aspartate/asparagine, hydroxyproline, and branched-chain amino acids). In coleoptile and endosperm, contrary to roots, the concentration of most metabolites decreased. Moreover, coleoptile accumulated galactose. Changes in the concentration of polar metabolites in seedlings revealed the affection of primary metabolism, disturbances in the mobilization of storage materials, and a translocation of sugars and amino acids from the endosperm to growing seedlings.