2.1 Carbon-based nanoparticles

Carbon is integral to plant compounds like carbohydrates, proteins, and lipids, with plants converting carbon dioxide into food and oxygen using sunlight (Singh et al., 2022). Explorations into carbon-based NPs, such as single-walled (SWCNTs), double-walled, multi-walled carbon nanotubes (MWCNTs), graphene oxide (GO), reduced GO, and fullerene (C60), have assessed their impacts on seed germination and plant growth (Wang et al., 2016). These NPs exhibit unique characteristics, enabling them to penetrate plant cell walls and membranes, and transport chemicals within cells (Kundu et al., 2023). SWCNTs can carry DNA and dyes into plant cells, with studies suggesting that MWCNTs positively affect seed germination and plant development by acting as channels for DNA and other chemicals (Siddiqui et al., 2014). MWCNTs, at concentrations of 100 µg/mL applied via seed priming to barley, soybean, and maize for 24 h, have been shown to enhance germination and seedling growth. Similarly, at 50 µg/mL for wheat, maize, peanut, and garlic overnight, MWCNTs led to improved germination, increased biomass, and greater water absorption in seeds (Lahiani et al., 2013; Sharma et al., 2023). Additionally, carbon-based NPs exhibit antifungal properties against pathogens like Fusarium graminearum and Fusarium poae, and C60 alleviates oxidative stress in plants by scavenging over-accumulated reactive oxygen species (ROS) under abiotic stress (Wu et al., 2020; Zaytseva & Neumann, 2016).

2.2 Silver-based nanoparticles

Ag NPs combat the increasing resistance of pests and fungi to chemical pesticides, displaying broad-spectrum antimicrobial activity against various phytopathogens and confronting agricultural challenges (Aćimović et al., 2015). They have shown potential in combating salt stress, restoring ionic balance, and enhancing nutrient availability, notably improving crop growth in salt-affected regions (Abasi et al., 2022), and in mitigating drought stress in plants like lentils, maintaining water balance, and bolstering growth parameters (Hojjat & Kamyab, 2017). Ag NPs have also enhanced the salt-stress tolerance of crops like Quinoa's Q6 line, nearly doubling growth parameters under saline conditions (Shibli et al., 2022).

2.3 Silicon-based nanoparticles (Si NPs)

Si-based NPs improve plant resistance to biotic and abiotic stresses. SiO₂ NPs have been shown to enhance seed germination-related parameters, and nano-aluminosilicate formulations are used as effective pesticides, resisting Aspergillus species and suppressing Fusarium oxysporum (Francesconi et al., 2022; Siddiqui et al., 2014). Si NPs, through seed priming at 20 ppm, showed upregulated growth and enhanced photosynthetic efficiency in common bean, combated cadmium (Cd) stress in wheat by increasing biomass and reducing oxidative stress and Cd uptake, and improved grain productivity and quality in rice-facing lead (Pb) and Cd stress by downregulating Cd and Pb uptake (Ali et al., 2021; Hussain et al., 2019). Furthermore, Si NPs enhanced germination rate, photosynthetic efficiency, and antioxidant defense in coriander through foliar application, increased SOD, CAT, GR, and PAL efficiency in bamboo under Pb-induced stress conditions, and mitigated arsenic (As) toxicity while enhancing the ascorbate-glutathione cycle in maize (Etesami et al., 2021).

2.4 Copper-based nanoparticles (Cu NPs)

Cu is known for its antimicrobial capabilities, underpinning the development of Cu NPs through both conventional and eco-friendly methods. These NPs boost antimicrobial effectiveness in agriculture, offering a safer fungicide alternative for protecting crops and managing pests with reduced toxicity (Gomes et al., 2022). Cu (OH)₂ and CuO NPs are used in fungicides, insecticides, and nanofertilizers, effective against bacteria like Escherichia coli L. and Bacillus subtilis L., and various plant fungal diseases (He et al., 2023). Cu NPs enhance plant disease resistance by promoting defense enzyme production, such as polyphenol oxidase and phenylalanine ammonia-lyase (Mosa et al., 2018). However, their hydroponic exposure to plants like lettuce and alfalfa alters growth, nutrient concentrations, and enzyme activities, highlighting potential ecological risks, including genotoxic and oxidative stress responses, and changes in Cu–Zn SOD gene expression in crops like cucumber (Simonin et al., 2018).

2.5 Magnesium-based nanoparticles (Mg NPs)

MgO NPs stand out as potent antibacterial agents, particularly against bacteria like Staphylococcus aureus, and combat plant pathogens responsible for diseases like tobacco bacterial wilt caused by Ralstonia solanacearum (Bindhu et al., 2016; Oluoch et al., 2022). Research indicates that MgO NPs positively affect tobacco plant growth and physiological attributes, enhancing chlorophyll content, enzyme activity, and magnesium uptake without phytotoxic effects (Cai et al., 2020). Further studies reveal that MgO NPs influence the growth, chlorophyll content, and gene/miRNA expression in ornamental pineapples with findings that although certain concentrations offer growth benefits, higher doses may inhibit these processes (Owusu Adjei et al., 2021).

2.6 Zinc-based nanoparticles (Zn NPs)

Zinc-based NPs, especially ZnO, play a pivotal role in promoting plant growth and resilience against abiotic stresses such as salinity, Cd stress, and drought. Zinc is crucial for the structure and function of numerous enzymes and essential for the healthy growth of agricultural crops (Saxena et al., 2016; Nandal & Solanki, 2021). ZnO NPs have been shown to significantly improve rice germination rates under salinity stress and enhance drought tolerance in Kotschy's dragonhead plant by positively modifying physiological and biochemical attributes (Karimian & Samiei, 2023; Shoukat et al., 2024). Their foliar application has proven to increase chickpea yields more effectively than bulk ZnSO₄ applications, underscoring their potential to boost plant growth and biomass accumulation and enhance Zn content in grains (Burman et al., 2013).

2.7 Iron-based nanoparticles (Fe NPs)

Fe NPs, particularly Fe₂O₃, are essential for plant metabolic processes, including photosynthesis, respiration, DNA synthesis, and the synthesis of photosynthetic pigments. These NPs have been demonstrated to reduce oxidative stress in plants grown under drought conditions by decreasing ROS levels (Palmqvist et al., 2017; Rout & Sahoo, 2015). Green-synthesized Fe₂O₃ NPs, derived from marine algae, have been shown to enhance drought stress resilience in foxtail millet, suggesting their utility as eco-friendly nanofertilizers (Sreelakshmi et al., 2021). The application of Fe NPs has been observed to promote plant growth under stress conditions, increase chlorophyll content in soybeans, and enhance photosynthesis and yield in wheat grown in cadmium-contaminated soil, though further extensive field testing is recommended to confirm their widespread agricultural applicability (Adrees et al., 2020; Ghafariyan et al., 2013).

2.8 Biologically synthesized nanoparticles

NPs synthesized through biological means using bacteria, fungi, plants, and algae have attracted attention for their environment-friendly, nontoxic nature, and compatibility with biological systems (Mohd Yusof et al., 2019). These NPs are pivotal in agriculture for enhancing plant growth, disease management, and nutrient uptake, providing a green alternative to traditional chemicals (Nayana et al., 2020). Silver NPs from plant extracts improve wheat germination and growth, whereas ZnO NPs from algae aid tomato growth under saline conditions, demonstrating how these NPs can make agriculture more sustainable (Iqbal et al., 2019; Seleiman et al., 2023).

Roots, rich in metabolites, are crucial for the green synthesis of NPs, with ginger and cherry roots being used to create silver, gold, and TiO₂ NPs, showcasing plant-based materials’ versatility in NP synthesis (Al-Shabib et al., 2020). These NPs, especially Ag and Au from ginger, have applications in biomedicine, suggesting their utility beyond agriculture (Kumari et al., 2022). Plant stems have also been used for NP biosynthesis, generating Ag, Fe₃O₄–Ag, Se, and ZnO NPs from grape, lavender Leucas grass, and dhobi tree stems, respectively. These NPs have unique properties beneficial for agriculture and medical applications, like magnetic characteristics and potential in diabetes treatment (Kirupagaran et al., 2016; Saha et al., 2021). This highlights the broad applicability of biologically synthesized NPs, underscoring their role in advancing eco-friendly agricultural practices and beyond.

3.1 Foliar application

NPs can enter plant cells via foliar application through the stomatal (hydrophilic) and cuticular (lipophilic) pathways, with smaller NPs (<4.8 nm) entering directly through the cuticle and larger NPs (>5 nm) through stomata as illustrated in Figure 1. Foliar application of NPs is used to deliver herbicides, fertilizers, and nutritional supplements directly to leaves, bypassing soil to prevent nutrient loss and offering advantages over soil-applied fertilizers, which have low utilization rates due to leaching and adsorption, leading to environmental issues like eutrophication (Sun et al., 2020; Tighe-Neira et al., 2018). Slow-release nanofertilizers from biodegradable polymers like chitosan and mesoporous silica have been developed to enhance nutrient availability and effectiveness (Achari & Kowshik, 2018). Foliar-applied nanofertilizers provide rapid absorption by plants, supplying essential vitamins and elements not present in soil, and have been shown to enhance plant uptake of nitrogen, phosphorus, and potassium, with mesoporous silica NPs (MSNs) reducing nutrient volatilization and environmental impact (Meier et al., 2020; Salem et al., 2016).

Foliar NPs also address nutrient deficiencies quickly and can biofortify crops with essential nutrients, with studies showing ZnO NPs promoting Zn augmentation in plants and fullerene NPs improving drought response by enhancing water storage and alleviating oxidative stress (Budke et al., 2020; Zhang et al., 2017). Silver NPs exhibit antibacterial activity, used in fungicides, and biologically synthesized silver NPs show high antimicrobial efficiency, with bimetallic NPs like Cu and Zn serving as both insecticides and antibacterial agents (Li et al., 2017; Mishra et al., 2017). The absorption of foliar nanofertilizers depends on the NPs’ residence time on the leaf, penetration of the epidermis, and being in an assimilable form for the plant, influenced by plant species, environmental conditions, and NP properties (Alshaal & El-Ramady, 2017; Clarke et al., 2020). Dicotyledons tend to accumulate more NPs than monocotyledons, and plants with large leaf surface areas or certain physical features can accumulate more atmospheric NPs (Lv et al., 2019; Shahid et al., 2017). NP penetration through stomata is a primary absorption pathway, with younger leaves having higher nutrient absorption capacity due to thinner wax layers. Atmospheric NPs can settle on leaves, with fine particles penetrating through leaf trichomes, and smaller particles (<50 nm) are more efficiently transported through the plant. Endocytosis facilitates NP entry into plant cells, with surface modifications of NPs affecting their absorption and internal transport (Larue et al., 2014; Shahid et al., 2017).

NPs initially interact with plants through root application, adhering to the root surface mainly via adsorption, influenced by negative charges from substances like mucus or organic acids secreted by root hairs. This process targets the root surface for the accumulation and uptake of positively charged NPs (Lv et al., 2019). The growth of lateral roots provides new adsorption sites, facilitating NPs’ interaction and entry into the plant (Peng et al., 2015). NPs encounter the root tip's less-developed epidermis and penetrate through semi-permeable walls with small pores, acting as a selective barrier against larger NPs (Pérez-de-Luque, 2017).

3.2 Root application

Once inside, NPs navigate through the plant via pathways like ion channels, endocytosis, or interaction with cell membrane proteins, besides entering through physical damage (Lv et al., 2019), as explained in Figure 1. Endocytosis allows for size-independent NP absorption, with examples including the uptake of carbon NPs in Madagascar periwinkle through this method (Miao et al., 2024; Zhang & Su, 2024). Additionally, NPs can bind to transport proteins on the root's external epidermis, offering another absorption route (Grillo et al., 2021).

Particle size significantly affects root absorption; smaller NPs demonstrate better efficiency. Gold (3.5 nm) and CeO₂ (8 ± 1 nm) NPs show effective root uptake in plants like broad bean and maize. However, NPs larger than 140 nm face absorption challenges, though exceptions exist for certain types, such as silicon-based NPs or natural polymer-derived NPs, which can be absorbed despite being larger than 100 nm (Banerjee et al., 2019; Slomberg & Schoenfisch, 2012; Yan et al., 2024). The root cell wall's negative charge significantly influences the interaction with NPs of various charges, affecting uptake efficiency. Although uncharged and negatively charged NPs can be absorbed, positively charged ones may remain on the root surface without deeper penetration (Sun et al., 2020) (Figure 1).

3.3 Stem injection or stem feeding

Stem injection, known as stem feeding, utilizes the plant's vascular system for efficient NPs distribution, offering an advantage by giving NPs direct access to the plant's vascular network (Figure 1). This method overcomes external barriers like cuticles, ensuring immediate NP availability, minimizing loss, and improving distribution efficiency (Su et al., 2019). External barriers typically challenge NP uptake in plants; however, stem injection circumvents these, allowing for higher NP concentration delivery to intended targets within the plant. NPs, once inside, rapidly move to various tissues via the xylem and phloem, ensuring uniform distribution (Su et al., 2019). An application of stem injection is in disease control, such as using copper NPs to manage citrus canker in citrus trees, effectively addressing plant health issues (Atiq et al., 2022).

4.1 Mechanisms underlying abiotic stress mitigation

Abiotic stress, including drought, salinity, and temperature extremes, activates plant signaling cascades, enhancing stress hormone accumulation like abscisic acid (ABA), which is critical in stress response regulation. Ag NPs increase ABA synthesis under drought stress, improving water use efficiency and drought tolerance (Heikal et al., 2023). NPs also elevate antioxidant enzyme expressions, reducing ROS levels and oxidative damage, with TiO₂ NPs in spinach under salt stress enhancing SOD and CAT activities (El-Saadony et al., 2022; Liu et al., 2021).

NP application stimulates osmolyte biosynthesis, aiding osmotic adjustment and cellular structure protection. Si NPs in wheat under heat stress significantly increase proline accumulation, supporting better growth (Al-Khayri et al., 2023; Younis et al., 2020). NPs modulate the Halliwell–Asada pathway for detoxifying H₂O₂, with SeNPs in mustard plants facing heavy metal stress upregulating pathway enzymes, reducing H₂O₂ levels (Rajput et al., 2021). They also influence the salt overly sensitive (SOS) pathway, improving salt tolerance by stabilizing SOS gene expression, with SiO₂ NPs upregulating SOS1 expression in salt-stressed plants (Cui & Smith, 2022; Mahmoud et al., 2022).

NPs activate the mitogen-activated protein kinase (MAPK) signaling pathway for stress-responsive gene transcription. AuNPs in Arabidopsis activate MAPK signaling, enhancing oxidative stress resistance (Christen et al., 2014; Joshi & Joshi, 2024). They affect calcium signaling, altering Ca²⁺ influx or efflux and activating stress response genes, as seen with Ca₃(PO₄)₂ NPs in rice enhancing drought tolerance (Lee & Hong, 2019; Nasrallah et al., 2022). ZnO NPs in soybean under salt stress fine-tune ROS levels, improving signaling for enhanced tolerance (Gaafar et al., 2020; Huang et al., 2021). CuO NPs in wheat exposed to high temperatures activate heat shock factors signaling, increasing HSPs accumulation for cellular protection against heat damage (Yang et al., 2020).

4.2 Mechanisms underlying abiotic stress mitigation

Biotic stress triggers plant immunity through pattern recognition receptors, initiating PAMP-triggered immunity (PTI). Ag NPs enhance plant immunity by stimulating the innate immune system for a stronger PTI response (Rai et al., 2012). ZnO NPs induce systematic resistance against tobacco mosaic virus, boosting the production of defensive enzymes and compounds (Abdelkhalek & Al-Askar, 2020). Similarly, CuO NPs have been shown to activate induced systemic resistance, leading to increased fungal pathogen resistance (Kamel et al., 2022). ZnO NPs elevate salicylic acid levels, essential for defending against biotrophic pathogens (Hembade et al., 2022), whereas TiO₂ NPs affect the jasmonic acid pathway, enhancing resistance to necrotrophic pathogens and pests (González-García et al., 2021). Ag NPs also modify ethylene signaling, improving resistance to Botrytis cinerea in tomatoes (Ullah et al., 2022).

5.1 Apoplastic transport

The apoplast comprises intercellular spaces and cell walls, excluding the Casparian strip in the root endodermis, regulating vascular system entry (Figure 2) (Läuchli, 1976). This pathway supports the passive movement of water, solutes, and NPs, facilitated by transpiration. It is significant for NPs due to its capacity to accommodate larger particles, leveraging natural transpiration pull to move from higher to lower water potential areas, enhancing plants’ antioxidative capacity. A study on wheat showed that AuNPs sized 20–50 nm transported via this pathway increased the activity of enzymes like superoxide dismutase and catalase, boosting stress resilience (Mondal et al., 2021).

5.2 Targeted delivery via symplastic transport

Engineering NPs to exploit plasmodesmata's natural signaling and gate-opening mechanisms enhances targeted delivery (Figure 2). NPs mimicking molecules that modulate plasmodesmata permeability could enable controlled release in response to physiological signals. Stimuli-responsive NPs changing in response to environmental triggers could effectively navigate symplastic barriers (Mittal et al., 2020). A study on rice with carbon dots highlighted enhanced photosynthesis and biomass, showcasing symplastic transport's potential (Tripathi & Sarkar, 2022).

5.3 Xylem/phloem transport

NPs move through xylem and phloem, with transpiration driving mass flow in the xylem. Physicochemical properties like size and surface functionalization significantly influence NP mobility, with smaller NPs (<50 nm) moving more efficiently (Figure 2) (Avellan et al., 2021; Yue et al., 2019). In phloem, pressure-driven flow distributes NPs from source to sink tissues, requiring specific characteristics or surface modifications for NP entry (Miao et al., 2024). Studies show that AuNPs can traverse phloem after foliar application, indicating successful distribution across the plant (de la Rosa et al., 2021; Khan et al., 2021).

5.4 Ion channels and transporters

Ion channels and transporters regulate ion movement across plant cell membranes, which is essential for nutrient uptake, signaling, and homeostasis (Figure 2) (Dubyak, 2004). These mechanisms are interaction points for NPs uptake into cells. Research indicates that Ag NPs can enter cells by mimicking potassium ions, affecting K⁺ transport channels and altering potassium ion homeostasis, impacting stress responses and growth (Yin et al., 2015). Fe₃O₄ NPs interact with iron transporters like the iron-regulated transporter in Arabidopsis, whereas ZnO NPs utilize ZIP transporters in rice for Zn ion uptake (Afzal et al., 2022; Quintana et al., 2022; Shirsat & K, 2024). CuO NPs engage with copper transporters in lettuce, showcasing potential beneficial and adverse effects (Xiong et al., 2021).

6.1 Nanoparticle-based strategies for biotic stress management in agriculture

NPs present targeted, efficient, and environment-friendly approaches to managing biotic stress, with the potential to transform agriculture, environmental conservation, and health practices (Rodríguez-Serrano et al., 2020). Ag NPs, known for their antibacterial and antifungal properties, effectively combat a range of fungi, such as Alternaria alternata and B. cinerea, and inhibit pathogens, including Erwinia sp. and F. graminearum, demonstrating their fungicidal and bactericidal capabilities (Gautam et al., 2020; Hussein et al., 2019). Their broad-spectrum activity extends to both gram-positive and gram-negative bacteria, including foodborne pathogens (Mahanta et al., 2019; Shehzad et al., 2018).

Beyond silver, other nanomaterials like chitosan NPs and AuNPs have shown antifungal activity against pathogens, such as Macrophomina phaseolina and A. alternata, and antibacterial effects against E. coli (Dang et al., 2019; Divya et al., 2017; Jayaseelan et al., 2013). Cu NPs are noted for their antimicrobial efficacy, along with AgNPs and silver-chitosan composites’ action against Pseudomonas syringae pv. syringae. The antimicrobial potential of MgO and SiO₂ NPs against bacterial and fungal pathogens further underscores the versatility of NPs in combating biotic stress (Bhadra et al., 2019; Singh & Kalia, 2019).

6.2 Physiological and biochemical responses

The application of NPs induces significant physiological and biochemical responses in plants, contributing to their improved tolerance against abiotic stresses. FeO NPs mitigate the toxic effects of heavy metals like Cd and Pb by enhancing plant biomass, chlorophyll content, and activating antioxidant enzyme systems (Hussain et al., 2019; Manzoor et al., 2021). Si NPs alleviate arsenic stress in maize, preserving chlorophyll, carotenoids, and protein levels, and improving photosystem II efficiency (Tripathi et al., 2016), also reducing toxic ion accumulation in wheat, rice, and pea from heavy metals such as Cd and Pb (Gao et al., 2018; ur Rehman et al., 2017). Improving drought tolerance, Si and chitosan NPs increase relative water content, photosynthetic rate, CAT, and SOD activities, enhancing yield and biomass, demonstrating their role in mitigating oxidative stress and improving water use efficiency (Ashkavand et al., 2015; Behboudi et al., 2019).

Under salinity, Si NPs enhance seed germination, carbon assimilation, leaf turgor, and antioxidant defense in cherry tomatoes, whereas FeO NPs boost wheat growth, chlorophyll content, and antioxidant enzyme activities, reducing salt ion accumulation (Manzoor et al., 2021). Si NPs also improve lentils’ seed germination and growth under salt stress (Sabaghnia & Janmohammadi, 2015), and Mn NPs adjust molecular responses in salinity-stressed pepper plants (Ye et al., 2020). MWCNTs enhance rapeseed's salinity tolerance by lowering ROS production and improving the Na⁺/K⁺ ratio (Zhao et al., 2019). Silicon dioxide NPs increase cucumber's nutrient absorption and photosynthesis under water and salt stress (Alsaeedi et al., 2019).

Analcite NPs facilitate wheat germination and growth under high heat and dryness (Hossain et al., 2021). ZnO NPs increase soybean seed germination in dry environments. Cu and Zn NPs elevate wheat's antioxidant activities and moisture levels under drought, preserving photosynthetic pigments (Taran et al., 2017). SiO₂ NPs extend barley's shoot length and augment RWC while reducing superoxide production and membrane deterioration (Turgeon, 2010). TiO₂ NPs counteract yield decreases in drought-affected wheat (Jaberzadeh et al., 2013), and treating maize with Cu NPs enhances leaf water content, biomass, and pigment concentrations (Van Nguyen et al., 2022). SiO₂ NPs reduce photosynthesis and stomatal conductance in hawthorn under drought (Ashkavand et al., 2015), yet silicon NPs mitigate drought stress in bananas. Foliar application of Si NPs increases coriander's antioxidant activity and essential oil production under moderate drought.

SiO₂ and TiO₂ NPs via foliar application counteract drought's adverse effects on cotton (Shallan et al., 2016), and soil application of Si NPs improves chickpeas' moisture content under drought (Gunes et al., 2007). Si and Se NPs enhance growth and ion selectivity in roots, increasing rice yields under saline environments (Badawy et al., 2021). Despite drought stress exacerbating Cd's negative impacts in wheat, ZnO NPs effectively mitigate both stresses (Khan et al., 2021). In conclusion, NPs offer a promising avenue for enhancing plant resilience to abiotic stresses through a comprehensive approach that spans physiological, biochemical, and molecular interactions.

6.3 Regulation of signaling pathways (nano-regulators)

NPs act as nano-regulators, intricately modulating plant signaling pathways to enhance tolerance to abiotic stresses, including heavy metals, drought, salt, and heat stress. This modulation includes regulating gene expression related to antioxidant defenses and stress response, significantly boosting plant resilience. By activating specific pathways, NPs ensure efficient stress perception and response, leading to enhanced physiological and biochemical adaptation. They regulate a wide array of genes in antioxidant defense and stress response pathways, evidencing a broad regulatory impact (Zeeshan et al., 2023). For instance, ZnO NPs have been seen to alter cytosine methylation patterns, affecting gene expression linked to stress tolerance, which helps reduce genotoxic effects of salinity and enhance stress resilience in tomatoes (Choudhury et al., 2017).

In rice exposed to ZnO NPs, there is an increase in antioxidant gene expression, such as OsCu/ZnSOD1-3 and OsPRX11, and genes related to chilling responses like OsbZIP52 and OsMYB4, illustrating the depth of NP-induced genetic modulation (Song et al., 2021). Conversely, rapeseed treated with Zn NPs under salinity stress shows varied gene expression, highlighting NPs’ nuanced influence on plant genetics and stress mitigation strategies (Hezaveh et al., 2019). The use of Si NPs in wheat suppresses genes involved in Cd uptake and transport, like OsIRT1 and OsNRAMP5, mitigating heavy metal stress impacts (Biswas et al., 2023). NPs also affect the MAPK signaling cascade, critical for regulating stress-responsive gene expression, with components like MAPK2 playing key roles in modulating phytohormones and antioxidant defenses, emphasizing the complex interplay of NPs in stress signal transduction (Rahmani et al., 2016).

6.4 Enhancing phytoremediation potential of plants

NPs significantly boost phytoremediation, enhancing the uptake, translocation, and sequestration of pollutants and complementing the ability of hyperaccumulator plants to absorb ionic compounds from soils (Manoj et al., 2020; Nigam & Sinha, 2021). They form stable complexes with contaminants to reduce bioavailability and toxicity, leveraging their unique properties to increase plant cell membrane permeability and facilitate the efficient internalization of contaminants (Deng et al., 2017; Hong et al., 2021). Metal and metal oxide NPs act as catalysts for the degradation of organic pollutants and enhance heavy metal uptake (Tahir et al., 2020), whereas carbon-based NPs like CNTs improve pollutant degradation and translocation through stimulated plant enzymatic detoxification activities (Chang et al., 2020).

Iron-based NPs increase the solubilization and root uptake of heavy metals, demonstrating the quantitative benefits of NPs in phytoremediation (Latif et al., 2020). Studies have shown that aquatic plants like red clover, cucumber, and ryegrass can uptake TiO₂, Ag₂S, and ZnO NPs, highlighting their potential in water phytoremediation and offering eco-friendly solutions to mitigate NP pollution in aquatic ecosystems (Tripathi et al., 2017). Additionally, zero-valent iron NPs chelate metallic particles in soil, facilitating plant uptake and accelerating the remediation of soils contaminated with heavy metals like As, Pb, and Hg, thus enhancing plant growth and maintaining germination rates (Wang et al., 2016).

6.5 Nanoparticle-mediated modulation of plant immunity

NPs have revolutionized agricultural practices by modulating plant immunity against biotic stress factors such as pathogens and pests, enabling targeted delivery of agrochemicals, genes, or RNA molecules to enhance immune responses (Tortella et al., 2023). Ag NPs activate plants’ natural defense mechanisms, offering protection against bacterial and fungal pathogens, including P. syringae, which causes leaf spots and blights (Gogoi et al., 2020). Cu NPs have shown effectiveness against fungal diseases like downy mildew in grapes, caused by Plasmopara viticola (Kalia et al., 2021), by acting as carriers for defense elicitors recognized by specific plant cell receptors, thereby initiating defense responses including the fortification of cell walls and induction of pathogenesis-related proteins (Cai et al., 2020).

This process effectively enhances systemic acquired resistance, preparing plants to bolster their defenses against potential threats (Alghuthaymi et al., 2021). Ag NPs also enhance disease resistance in tomato plants against R. solanacearum by modulating defense-related enzymes and genes (Narasimhamurthy et al., 2022), and ZnO NPs have been effective in controlling powdery mildew in grapevines, demonstrating their utility in managing fungal diseases (Thounaojam et al., 2021). Moreover, NPs engineered from materials like Au NPs are utilized for transporting RNA molecules, such as siRNA or miRNA, directly into pathogens or pests. This targets and silences crucial genes, hindering their ability to infect or damage plants, as shown with Au NPs against B. cinerea (Ghosh et al., 2023; Holjencin & Jakymiw, 2022). This method offers a novel, highly specific approach to pest and disease management, aligning with sustainable agricultural practices by reducing reliance on traditional chemical pesticides.

8.1 Nano nutrient precision delivery systems

Slow-release nanofertilizers are an innovative approach that uses NPs to deliver essential nutrients to plants in a controlled and efficient manner as illustrated in Figure 4. These nanofertilizers can enhance nutrient use efficiency, reduce nutrient loss to the environment, and improve crop yields (Qureshi et al., 2018).

For instance, nano-hydroxyapatite has been used as a slow-release phosphorus fertilizer, which can gradually release phosphorus and improve its availability to plants (Weeks & Hettiarachchi, 2019). Similarly, nano-encapsulation of urea can increase nitrogen use efficiency and reduce the frequency of fertilizer application, thus minimizing its environmental impacts (Vega-Vásquez et al., 2020). Urea-hydroxyapatite nanohybrids have been reported to prolong nitrogen release, increase nitrogen use efficiency, and promote plant growth (Nkebiwe et al., 2016). Similarly, nano-sized hydroxyapatite has been shown to enhance phosphorus availability and reduce phosphorus loss in the soil, improving crop yield (Wang et al., 2016).

8.2 Nano-targeted agrochemical efficacy enhancers

Smart delivery systems utilize NPs to precisely deliver nutrients, agrochemicals, or other compounds to specific plant targets. These delivery systems can enhance the efficiency of agrochemicals, reduce off-target effects, and minimize the environmental impacts of conventional agriculture practices. One example is the use of MSNs as a smart delivery system for controlled release of plant growth regulators, such as auxins or gibberellins (Wang et al., 2022). MSNs can release these compounds in a pH-responsive manner, allowing for targeted and controlled delivery. Another example is the use of nano-capsules loaded with herbicides for selective delivery to weed species, reducing the risk of damage to nontarget plants (Singh & Kalia, 2019). MSNs have been used to deliver fungicides, herbicides, and insecticides, improve their efficacy, and reduce environmental risks (Ghormade et al., 2011; Vallet-Regí et al., 2001). Another example is the use of chitosan NPs as carriers for the controlled release of essential oils, demonstrating enhanced biopesticide activity against pests (Kashyap et al., 2015).

8.3 Nano-pesticides and patho-interrupters

NPs can play a vital role in protecting plants from pathogens and pests, thereby contributing to sustainable agriculture. Ag NPs are used as antimicrobial agents to control plant diseases caused by bacteria, fungi, and viruses (Kumar et al., 2019). Ag NPs can disrupt microbial cell walls and membranes, leading to cell death, and have demonstrated effectiveness against various plant pathogens. Additionally, NPs can be used for targeted delivery of biopesticides, enhancing their effectiveness against pests while minimizing off-target effects. For instance, chitosan NPs loaded with essential oils have shown promise in controlling insect pests such as the diamondback moth (Plutella xylostella) (de Oliveira et al., 2014). Ag NPs have demonstrated antifungal and antibacterial properties, protecting plants from various diseases (Paul & Roychoudhury, 2021). Use of nanocarriers for the delivery of biopesticides can improve their effectiveness, reduce the required dosage, and minimize nontarget effects (Kumar et al., 2019).

8.4 NPs and soil enhancement

NPs can aid in improving soil structure, thus enhancing water retention and root aeration (Rastogi et al., 2019). The addition of biochar NPs enhances soil fertility by retaining nutrients and reducing leaching (Alkharabsheh et al., 2021). A study found that Ag NPs, when applied to soil, resulted in decreased microbial biomass and altered substrate-use efficiency with increasing NP concentrations (Hänsch & Emmerling, 2010). Si NPs can significantly bolster the soil's water-holding capacity, which is beneficial during drought conditions. Certain NPs, like FeO, can remediate polluted soils by immobilizing contaminants, thus reducing their bioavailability. ZnO NPs have been noted to boost beneficial microbial activity, fostering a healthier soil ecosystem (Patra et al., 2016). Hence, the strategic application of NPs provides a multifaceted approach to soil improvement.

8.5 NPs and energy efficiency

Nanotechnology's role in the automotive industry is increasingly apparent in the modern era, with significant potential to enhance the production of recent vehicle models. It offers the opportunity to optimize safety performance while concurrently reducing environmental impacts, particularly in terms of pollutant emissions (Joost, 2012; Shafique & Luo, 2019). Nanomaterials can be effectively employed in automobile bodies, creating lightweight structures that maintain stiffness and crash resistance (Kulkarni et al., 2015). This results in reduced material usage and lower fuel consumption, ultimately leading to a decrease in greenhouse gas emissions. A linear regression analysis of curb weight versus CO₂ emissions (a measure of efficiency that is correlated with fuel consumption) for the model year 2008 vehicle fleet suggests that a 10% reduction in vehicle weight is associated with an 8% reduction of CO₂ emissions (Lutsey, 2010). A model that combines curb weight and fuel consumption data with a technique for normalizing vehicle performance indicates that a 10% reduction in vehicle weight yields a 5.6% reduction in fuel consumption for cars and a 6.3% reduction in fuel consumption for light truck (Baur & Silverman, 2007; Joost, 2012).

Nanotechnology has the potential to enhance the production of biofuels by improving the efficiency of breaking down cellulosic materials found in trees and plants. Wegner and Jones (2009) suggested that manipulating the nanoscale structures of cell walls, known as nanofibrils, can make it easier to disassemble these materials into their constituent parts for biofuel production, whether through fermentation, gasification, or catalysis. By employing nano-catalysis, it becomes possible to break down cellulose more effectively, which constitutes 15%–25% of the carbohydrate content in woody materials, thereby increasing the overall efficiency of biofuel generation. Additionally, nanotechnology can play a vital role in boosting biofuel efficiency by utilizing engineered nanoscale enzymes or enzyme systems such as glycol hydrolases and lignin-degrading enzymes to enhance the conversion of cellulose into sugars (Assad et al., 2022). This technology can be applied to tree biology, enabling the creation and storage of enzymes and enzyme systems within living trees until harvest. These enzymes can then be activated to facilitate the engineered disassembly of woody biomass. Furthermore, the possibilities for creating new symbiotic nanoscale biological systems that collaborate in the production of ethanol and other biofuels are opened by nanotechnology, as suggested by Ziolkowska (2018).