1. Introduction

Over the last few decades, crop production has decreased even as food demands have risen due to an increasing human population [1]. According to the reports, 2 billion people have micronutrient deficiencies, 800 million people experience chronic hunger, and 653 million people will still be undernourished in 2030 [2]. In 2011, the Food and Agriculture organization estimated a loss of 1.3 billion tons of food per year around the globe. As a result, food security will remain a major concern as the world’s population is predicted to reach 10 billion in 2050 [3]. Climate change is considered a major limiting factor, having a detrimental effect on plant growth, development, crop yield, and productivity. Abiotic stresses such as drought, salinity, soil erosion, flooding, and nutrient depletion are the byproducts of climate change and are one of the key factors for stunted plant growth, resulting in reduced yield and quality of crop plants [4]. The development of stress-tolerant plants could be a promising and practical solution to this challenge. However, conventional breeding methods have not yielded the desired results in improving crop plant tolerance to stress through inter-generic or inter-specific hybridization [5]. Thus, novel and promising agricultural strategies capable of reforming modern agriculture are required to increase food productivity while limiting pesticide and fertilizer use to reduce abiotic stress. Nanotechnology is currently an emerging and rapidly developing field of study that has gained tremendous attention recently due to its broad-spectrum applications in various sectors, particularly in plant biotechnology and agriculture. Nanotechnology, in particular, can help provide effective solutions to agricultural problems and achieve a sustainable and secure future for agriculture [6]. Nanotechnology uses nanoparticles as a tool to mitigate abiotic stress factors and to improve crop yield and food quality.

Nanoparticles (NPs) are small materials ranging in size from 1 to 100 nm. In contrast to their larger-sized counterparts, NPs have distinct and diverse physicochemical properties including high adsorption efficacy, large surface area-to-volume ratio, and effective linking and work efficiency. These qualities make them a suitable candidate for disease management strategies as bactericides/fungicides/pesticides to boost plant health. Moreover, NPs can also help to alleviate nutrient deficiency symptoms and supplement essential elements when used as macro and micro-nano fertilizers in plants [7]. Titanium dioxide (TiO₂), iron oxide (Fe₃O₄), zinc oxide (ZnO), silicon oxide (SiO₂), copper (Cu-NPs), and selenium (Se-NPs) have recently caught significant attention due to their non-threatening use in agriculture [8, 9, 10]. Different biological, physical, and chemical techniques can be used for NPs synthesis. The impact of NPs on plants varies and primarily depends upon the size, origin, time of application, and concentration [11]. Recent studies reported that the application of NPs resulted in improved plant tolerance against abiotic stresses by increasing the antioxidant activities, decreasing hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) accumulation, and maintaining the efficiency of the photosynthetic apparatus. Moreover, they also have the ability to reach the photo system-II (PS-II) reaction center and increase the transmission of electrons and light absorption by penetrating chloroplast, resulting in improved photosynthetic efficiency and plant growth [12, 13].

Despite their extensive and commercial use in different products, concerns about NP’s toxicological and environmental impacts are circulating in the research community [14]. It has been reported that the excessive use of NPs can result in physiological abnormalities and oxidative stress in plants, leading to reduced antioxidant activities and gas exchange characteristics. Moreover, disruption of cell division, mitotic index, and root growth has also been reported [15, 16]. Most of the published literature emphasizes the positive effects of NPs on growth and crop productivity. However, they lack an in-depth understanding of NP-plant interaction at the biochemical, physiological, and molecular levels. Therefore, this chapter will focus on a detailed understanding of interaction of NPs with plants at the molecular level and help to fill in research gaps that will enable potential future use of NPs. This comprehensive review will be a valuable source of information to conduct more studies and develop new eco-friendly NPs of different metals to counter the devastating impact of abiotic stresses on plants.

2. Effects of abiotic stress on crop plant

Climate change has led to an increase in abiotic stress conditions, which are gradually affecting crop production in various respects. Different physiological and biochemical changes occur in plants during stress, resulting in disruptions in plant growth, development, and productivity. Nanobiotechnology could be the most efficient adaptation strategy to cope with climate and abiotic stress factors, thereby achieving sustainable production [17, 18]. These stressors encompass heat, drought, high soil salinity, and heavy metal (HM) toxicity—cadmium (Cd), lead (Pb), arsenic (As), nickel (Ni), chromium (Cr), cobalt (Co), and zinc (Zn) [17]. Drought and heat stress are chief contributors to reduced crop yield around the globe. Prolonged drought and heat increase reactive oxygen species (ROS) production, affecting oxidative processes and causing reductions in stomatal opening, leaf size, water potential, and root growth, resulting in a general decrease in crop yield (Figure 1) [18, 19].

Figure 1.
Abiotic stresses and their response in crop plant. (The figure is created using BioRender https://biorender.com/).

Salinity and heavy metal stress are also considered among the environmental factors that limit crop yield in many countries. Soil salinity causes low osmotic potential around the seeds, preventing water uptake and thus resulting in inhibition of seed germination. Oxidative stress in legumes induced by sodium chloride (NaCl) decreases seed nutrient quality, lowers nodulation, and inhibits plant growth. Ascorbate and carotenoids are important non-enzymatic salinity defense mechanisms, whereas proline (Pro) is a known osmoregulatory stress-related compound [20, 21].

Various factors contribute to heavy metal accumulation in agricultural soils, such as chemical fertilizers, atmospheric deposition, sewage sludge, and rapid industrial growth [22]. Heavy metals significantly impede plant growth by disrupting essential functions like food intake, antioxidant enzymes, and photosynthetic machinery. The increased generation of reactive oxygen species (ROS) during heavy metal stress worsens the decline in agricultural productivity [20, 21]. The annual economic cost of heavy metal pollution exceeds $10 billion, primarily attributed to anthropogenic and industrial waste deposition. Mitigating heavy metal-induced stress requires a comprehensive approach, including transgenic crop engineering, marker-assisted breeding, and traditional breeding. Sustainable agricultural practices are vital for enhancing plant resilience in the face of heavy metal contamination [23].

Efforts to mitigate the impact of abiotic stressors on crop productivity require integrated strategies that address drought, salinity, cold, heavy metal contamination, and other environmental challenges. Sustainable agricultural practices, advanced breeding techniques, and a focus on resilience are essential components of a comprehensive approach to ensure food security in the face of a changing climate [24, 25].

3. Mechanisms of nanoparticle-plant interaction in crop plant

Plants and nanoparticles interact in a complicated and multidimensional way that is controlled by molecular processes, physiological reactions, barrier crossing, uptake, and defense [26]. Therefore, comprehending these dynamics is crucial in assessing the possible advantages and drawbacks of employing nanoparticles in agricultural and environmental settings [27]. In addition to that, optimizing the characteristics of nanoparticles, reducing their negative impacts, and maximizing their potential for environmental remediation and sustainable agriculture techniques are also essential [28]. At specified concentrations, distinct NPs have been shown to boost the growth and development of plants through standard mechanisms [29]. A detailed mechanism of action for nanoparticles within plants is given below (Figure 2).

Figure 2.
Detailed pathway of NPs uptake, movement, and translocation inside plant. a. Application of different types of NPs via different routes, i.e., ground and foliar application. b(i). The small NPs (diameters range from 3 to 5 nm) are reported to penetrate plant roots along with osmotic pressure and capillary forces or pass directly through the root epidermal cells. The epidermal cells of the root cell wall are semipermeable, containing small pores that restrict the large NPs. Some NPs induced new pores in the epidermal cell wall, which facilitated its entrance. b(ii). The NPs applied by the leaves can enter the leaves through the stomata or cuticles. The cuticle acts as a primary leaf barrier, restricting the entry of NPs to a size of <5 nm. The NPs > 10 mm enter through stomata, and their cellular transport occurs through apoplastic and symplastic routes into the vascular system of the plant. c(i). Foliar application of NPs (between 10 and 50 nm) is favored through the adjacent cell’s cytoplasm (symplastic route). Thus, larger NPs (between 50 and 200 nm) are translocated between the cells (apoplastic route). c(ii). After crossing the cell walls, NPs are apoplastically transported through extracellular spaces until they reach the central vascular cylinder. NPs, however, need to symplastically cross the Casparian strip barrier to enter the central vascular cylinder. This happens by binding to the endodermal cell membrane’s carrier proteins through endocytosis, pore formation, and transport. NPs travel from one cell to another through the plasmodesmata as internalized in the cytoplasm. d. Internalized NPs entered via roots reach the central vascular cylinder, allowing the xylem to move unidirectionally upward. Meanwhile, NPs entered through leaves are transported along with the sugar flow through the phloem sieve tubes. As a result of vascular transport by phloem, NPs can travel bidirectionally and accumulate in roots, stems, fruits, grains, and young leaves to varying degrees because these organs serve as potent sinks for the sap. (The figure is created using BioRender https://biorender.com/).

3.1 Uptake mechanisms

Adsorbed nanoparticles (NPs) from the soil can adhere to the surface of the roots. Subsequently, they can enter the root system by direct absorption, root junctions, or wounds. Additionally, these particles may enter plants through their above-ground sections, primarily through cuticles (a non-polar channel) and stomata or trichomes (a polar pathway) [30, 31]. Artificial exposure might happen when they are sprayed on or directly injected into the leaf [31].

3.2 Absorption and intracellular movement

The first line of defense against NPs is the cell wall of plants, wherein the cellulose permits the penetration of tiny NPs (5–20 nm) while obstructing the bigger ones [32]. After navigating the cell, there are two ways for them to navigate through tissues: the apoplast and the symplast. Apoplastic transport occurs outside the plasma membrane through the extracellular spaces, cell walls of adjacent cells, and xylem vessels while symplastic transport uses plasmodesmata and sieve plates to move water and substances between the cytoplasm of adjacent cells [33, 34]. The apoplastic pathway is essential for radial movement within plant tissues and helps nanomaterials reach the root central cylinder and the vascular tissues. When reached inside the central cylinder, nanoparticles make their way to aerial parts of the plant by using a transpiration stream [35]. However, because Caspary strips function as a resistive barrier, nanoparticles can form aggregates and concentrate in the endodermis. The nanoparticles must then enter the symplast for effective translocation to shoots [36].

Using plasmodesmata and retina structures, symplastic transport is the transfer of chemicals and water between the cytoplasm of adjacent cells [37]. Since the transfer of nanomaterials to the symplast allows for their efficient translocation to the shoots, this mechanism is thought to be more significant for the transportation of nanoparticles. The symplastic transit of nanoparticles across the membrane and their accumulation influences the charge inside the cell, which modifies the electrochemical potential of the membrane. These changes can have an impact on the movement of other materials over the membrane, so to sustain turgor pressure, water and nutrient intake, and plant development, the membrane’s electrochemical potential must stay in equilibrium [38].

In the case of foliar applications, nanomaterials must cross the barrier the cuticle presents, following the lipophilic or the hydrophilic pathway [39]. Numerous other pathways also allow nanoparticles to penetrate the intracellular area [40]. They could get through by attaching to transport proteins, ion channels, aquaporins, endocytosis, or by starting the creation of new pores, some of which might be bigger than the ones that already exist [41]. The processes of nanoparticle adsorption, translocation, and accumulation are heavily influenced not only by the type of plant but also by the physical and chemical properties of the nanoparticles themselves [42].

The way NPs move inside the plant is a key indicator of their accumulation and storage in different parts of the plant. For example, if an NP took the apoplastic pathway and used the xylem for translocation, then it would most probably move from roots to shoots and aerial parts of plants but would not move downwards toward roots. Thus, application methods for these types of NPs should be carried out at the root level, ensuring a good distribution in the plant. On the other hand, if an NP translocates via phloem and takes the symplastic pathway, then it will accumulate in plant organs acting as sinks, such as fruits and grains. Thus, appropriate measures should be taken while applying NPs to plants to avoid further human or animal ingestion of nanomaterials. Moreover, translocation is not essentially limited to a specific cell type, and lateral movement of nanomaterials between xylem and phloem is also possible [43].

The crystallinity of the nanoparticles utilized is one of the most critical elements influencing accumulation [42], the hydrodynamic dimension of the NPs, the nature of the surrounding soil, such as whether it is “organic” or inorganic, and the influence of zeta potentials [44]. The plant’s large concentration of nanoparticles results in a high amount of phytotoxicity. The phytotoxic impact of NPs is determined by their size, shape, exposure period, and concentration [45].

3.3 Nanoparticle-plant interaction

Plants’ regular cellular activities can be disrupted by NPs, resulting in oxidative stress. This disruption is frequently caused by NPs interfering with the electron transport chain in mitochondria and chloroplasts, where these particles cause plants to produce ROS, resulting in an oxidative burst [46]. This increased ROS level can induce cellular damage such as lipid peroxidation, DNA damage, and protein alterations [47]. But despite its destructive nature, it is essential for various biological activities like stress tolerance. However, fortunately, plants are gifted with antioxidant defense systems to counterbalance the negative effects of high ROS levels. Enzymatic (e.g., superoxide dismutase, catalase, and guaiacol peroxidase) and non-enzymatic (e.g., ascorbate, glutathione, and carotenoids) molecules are produced in response to scavenge excess ROS [48]. NP-plant interactions can disturb hormonal homeostasis, influencing plant metabolism and stress responses. In response to NPs, many hormonal pathways may be activated or downregulated, regulating plant growth, development, and stress tolerance [49]. A study conducted on Capsicum annuum indicated increased cytokinin levels in response to AgNP stress while another study reported on Gossypium sp. showed decreased levels of ABA and auxin in response to CuO-NPs, suggesting the alteration of plant hormonal balance and metabolism upon plant-NP interaction [50].

Nanoparticles cannot work successfully unless they interfere with the molecular processes and gene expression of plants. The diverse gene expressions of micro-RNA were studied in nanoparticle-treated cells. Nanoparticles have an effect on miR-398 and miR-408, which are responsible for seed germination, root length, seedling development, and free radical scavengers [51]. The random drop in miR-164 expression, which is involved in auxin hormone signaling, is, nevertheless, linked to the nanoparticle-mediated rise in roots [52].

4. Synthesis of nanoparticles and their role in alleviating abiotic stress

As their name implies, nanoparticles have distinct physicochemical features and range in size from 1 to 100 nm. Usually, both top-down and bottom-up synthesis methods are used to synthesize nanoparticles [53]. Under the top-down method, bigger molecules break down to generate nanoparticles. Examples of top-down methods include electro-explosion, mechanical milling, chemical etching, laser ablation, and sputtering. Metal-oxide nanoparticles are mostly synthesized by [54]. In contrast, simpler materials mix and react to generate nanoparticles during bottom-up synthesis. Examples of bottom-up techniques include pyrolysis, condensation, the sol-gel process, and spinning. The bottom-up approach includes the green production of nanoparticles, typically from biological material [55]. Besides, NPs can also be synthesized by using the Green synthesis approach which involves reacting processed extracts of plants, fungi, and microorganisms with metal salt solutions, followed by reduction and capping to create metal nanoparticles coated with target biomolecules. Green-synthesized nanoparticles help improve results and facilitate efficient transportation to the target spot due to their small size [56].

The success of nanoparticle production depends on the following characteristics:

Characterizations of morphology by microscopy (scanning and transmission electron microscopy).
Characterization of structures by Zeta-size analyzers, Raman, XRD, and XPS.
UV-visible and photoluminescence optical characterizations [57].

4.1 Commonly used nanoparticles for abiotic stress

Several metals including silver (Ag), copper (Cu), gold (Au), iron (Fe), titanium (Ti), and zinc (Zn) and their oxides have been used for the green synthesis of NPs using plants and their extracts, microorganisms and membranes, and DNA of viruses including diatoms.

Recently, it has been reported that the cerium oxide (CeO) NPs application maintained the quantum yield of photosystem (PS) II and CO₂ assimilation through ROS scavenging, particularly hydrogen peroxide, induced by abiotic stress. The application of titania (TiO₂) NPs improved the activity of catalase (CAT), glutathione peroxidase (GPOX), and superoxide dismutase (SOD) and reduced oxidative stress in Duckweed (Lemna minor) plants [58, 59]. Several beneficial and stress-countering effects of NPs application in various crops have been reported, such as improved growth in Solanum lycopersicum L. [60] and Allium cepa L. [61]; S. lycopersicum L. [62], Oryza sativa [63], Capsicum annuum [64]. Therefore, it can be concluded that the stress ameliorative potential of NPs can be exploited to combat various negative effects caused by abiotic stresses in crop plants. Different types of NPs that are found to be effective in mitigating stress response are listed in Table 1.

Nanoparticles	Dosage (mg/L)	Duration (days)	Plant species	Abiotic stress	Application	References
Selenium dioxide	20–40 mg	30	Phaseolus vulgaris	Salinity	Enhanced the plant growth and yield	[65]
Calcium oxide	25 mg	30	Hordeum vulgare	Heavy metal	Increased plant growth, photosynthesis efficiency, and antioxidant enzymes	[66]
Silicon	2–50 mg	45	Citrus x sinensis	Salinity	Improved oxidative stress tolerance	[67]
Gold	10 mg	07	Triticum aestivum	Salinity	Improved the plant defense systems	[68]
Cerium oxide	50–100 mg	21	Oryza sativa	Salinity	Improved the crop yield by modulating the plant’s physiological and biochemical mechanisms	[69]
Cerium oxide	50–100 mg	30	Gossypium	Salinity	Improved the plant growth by maintaining cytosolic K+/Na + ratio	[70]
Zinc oxide	0.5–1 mg	1–7	Carthamus tinctorius	Salinity	Enhanced plant germination and salinity tolerance by improving the activities of antioxidant enzymes	[71]
Iron oxide	10 mg	07	Oryza sativa	Cadmium and drought stress	Increased the tissue dry weight and reduced the Cd accumulation	[63]
Iron oxide	10 mg	05	Triticum aestivum	Salinity and heavy metal	Facilitates photosynthetic pigments and restricts cadmium uptake	[72]
Silver	50–75 mg	10	Satureja hortensis	Salinity	Improved plant growth and germination	[73]
Iron	5–20 mg	15	Triticum aestivum	Heavy metal and drought stress	Improved photosynthesis and alleviated oxidative stress	[12]
Zinc oxide	0.5–1 g	07	Triticum aestivum	Heavy metal and drought stress	Increased plant growth and reduction in chromium bioavailability	[74]
Selenium	100 mg	03	Triticum aestivum	Drought	Enhanced plant growth and development	[75]
Titanium oxide	2–5 ppm	01	Dracocephalum moldavica	Salinity	Promote plant growth and ameliorate salinity stress	[76]
Copper	50, 100, 200 mg	30	Triticum aestivum	Heavy metal	Increased biomass, antioxidant enzyme contents, and photosynthesis efficiency	[77]
Silica	2–50 mg	21	Cucumis sativus	Drought and salinity	Improved the growth and productivity of cucumber plants by balancing nutrient uptake	[78]
Titanium oxide	1.5–10 ppm	07	Vicia faba	Salinity	Improved growth and enhanced tolerance against salinity	[79]
Cerium oxide	50–100 mg	14	Glycine max	Salinity	Enhanced plant growth by regulating photosynthesis and water use efficiency	[80]
Chitosan	100 mg	01	Zea mays	Salinity	Mitigates the deleterious effects of salinity	[81]
Titanium oxide	2–5 ppm	03	Cicer arietinum	Cold	Increased plant growth and antioxidant activity	[82]
Titanium oxide	50–100 mg	30	Solanum lycopersicum	Heat	Enhanced plant growth and photosynthesis efficiency	[83]

Table 1.

List of the NPs applications in plant abiotic stress.

4.2 Delivery methods of nanoparticles to crop plants

The delivery of nanoparticles to crop plants can be achieved through various methods to enhance plant growth, increase nutrient uptake, and mitigate environmental stresses. Different application methods are described in detail below (Figure 3).

Figure 3.
Diagrammatic representation of different delivery methods used for nanoparticle application in crop plant. (The figure is created using BioRender https://biorender.com/).

4.2.1 Seed coating

Nanoparticles are applied as coatings on the surface of seeds. Seeds are immersed or coated with a solution containing nanoparticles before planting. This method ensures direct contact between nanoparticles and emerging roots, promoting early interaction with the plant [84].

4.2.2 Soil drenching

Nanoparticles are applied as a solution directly to the soil around the plant root zone. Nanoparticle solutions are poured or sprayed onto the soil surface, allowing the roots to absorb the particles. Effective for delivering nanoparticles to the root system, promoting nutrient absorption and root development [85].

4.2.3 Foliar spray

Nanoparticles are applied as a spray directly to the leaves and stems of plants. In this method, a nanoparticle solution is sprayed onto the plant’s vegetative parts. Plants absorb foliar NPs usually through stomata, ion channels, protein carriers, cracks or water pores, endocytosis, stigma, wound and trichomes. This approach allows rapid uptake, controlled release of NPs at targeted locations and improved absorption and assimilation of foliar fertilizer [86].

4.2.4 Hydroponic systems

Nanoparticles are added to the nutrient solution in hydroponic systems. Nanoparticles are dissolved in the nutrient solution, and plants absorb them through their roots in a soilless growing medium, which provides precise control over nutrient delivery and uptake [87].

4.2.5 Root injection

Nanoparticles are injected directly into the plant root system. Nanoparticle solutions are injected into the soil around the root zone or directly into the roots, which offers a targeted and efficient delivery method [88].

4.2.6 Biological vector-mediated delivery

Nanoparticles are delivered into plants using biological vectors, such as bacteria or fungi. Engineered microorganisms carrying nanoparticles are introduced to the plant, facilitating uptake, which utilizes natural biological processes for nanoparticle delivery [89].

4.2.7 Nanoparticle-embedded fertilizers

Nanoparticles are incorporated into conventional fertilizers. Nanoparticles are blended with fertilizers, allowing for simultaneous delivery of nutrients and nanoparticles during standard fertilization practices, which integrates nanoparticle application with routine agricultural practices [90].

NPs were taken up by roots and/or leaves, and they moved through two different channels: apoplastic and symplastic [91]. After passing through the first barrier, the cell walls, NPs were distributed between the plasma membrane and the walls of the cells; osmotic pressure and capillary forces may have an impact on the NPs’ subsequent migration [92]. In addition to ion channels and transport or carrier proteins like aquaporin, NPs can also enter cells by cell membrane barrier breaking or endocytosis [93]. The precise location and function of NPs in plant organelles are not exploited by any mechanistic method [94]. The translocation and uptake kinetics of NPs should be investigated further, as well as the biochemical, molecular, and physiological patterns of NPs uptake and transport kinetics, as these are crucial to comprehending the accumulation behavior of NPs in plants.

5. Molecular responses of plants to nanoparticles under abiotic stress

The complex and diverse array of molecular responses triggered in plants under abiotic stresses leads to the activation of stress phytohormones, accumulation of reactive oxygen species (ROS), kinase cascades, and stimulation of ion channels, contributing to electro-physiology and signaling [95, 96, 97]. Subsequently, a sophisticated species and stress-specific regulation of gene networks occur, targeting defense and repair mechanisms against stress-induced injuries [98]. Stress-induced proteins play roles in ROS neutralization, mitogen-activated protein (MAP) kinase, and salt overly sensitive (SOS) kinase signaling cascades [96, 99]. Recent studies reveal functional outcomes extending to ion and water transport, uptake, and transcription regulation for secondary response elements (Figure 4) [100, 101].

Figure 4.
Schematic pathway of genetic regulation in plants against different abiotic stresses upon application of different types of nanoparticles. (The figure is created using BioRender https://biorender.com/).

Nanoparticles (NPs) have proven to be useful in triggering these molecular responses when they are applied to plants facing abiotic stress conditions. Noteworthy NPs include aluminum oxide (Al₂O₃) NPs, silicon-NPs, zinc oxide (ZnO) NPs, multi-walled carbon nanotubes, and titanium dioxide (TiO₂) NPs. Research by Zhao et al. underscores the potential of nanomaterials like silver and copper oxide, inducing stress responses and defense mechanisms and enhancing crop stress resilience [102]. Proteomic and transcriptomic approaches have helped to investigate the molecular effects of NPs on various plant species, with observed morphological and physiological effects influenced by NP dosage, type, size, and shape.

5.1 NPs mitigate salinity stress in plants

Nanoparticles (NPs) play a crucial role in mitigating salinity stress in plants by influencing cellular mechanisms and gene expression [103]. Studies on micro-RNA expression in NP-treated cells revealed the impact of NPs on miR398 and miR408, crucial for regulating seed germination, root and seedling growth, antioxidants, and free radical scavengers [104, 105]. Foliar application of Zn-NPs on rapeseed plants under salinity stress altered gene expression, reducing some genes (e.g., SKRD2, MYC, and MPK4) and increasing others (e.g., ARP and MPK) related to physiological, hormonal, and developmental responses [106].

Carbon-based NPs, specifically multi-walled carbon nanotubes [107], affected genes involved in antioxidant and salt sensitivity systems in rapeseed plants under salinity stress [108]. According to Zhao et al., under salinity stress, multi-walled carbon nanotubes can change gene expression, affecting the antioxidant and salt overly sensitive 1 (SOS1) system in rapeseed plants [109]. Cerium oxide NPs decreased ROS levels and increased calcium content in plants, influencing ROS and Ca²⁺-mediated signaling genes [110]. Silicon-NPs demonstrated positive effects on Cannabis sativa L., improving growth and inducing molecular changes [111]. Proteomic studies in tomato plants under salinity stress revealed that silicon affected cytochrome b6f (Cytb6f) light-harvesting complexes and ATP-synthase complex genes [112]. Copper-NPs enhanced tomato growth under salt stress by upregulating SOD and jasmonic acid genes, contributing to salinity tolerance [113]. The role of Cu-NPs in reducing the harmful impacts of salinity on plants was explored, showing potential in imparting salinity stress tolerance [114]. While AgNPs hold promise for crop improvement, their environmental and health impacts, particularly the release of toxic silver ions (Ag+), require careful consideration and further research [108, 115]. Overall, NPs offer a promising avenue for enhancing plant resilience to salinity stress, but comprehensive understanding and caution are essential for their safe and effective use in agriculture (Table 2).

Abiotic stress	Nanoparticles	Plant species	Genes	Gene regulation and plant response	References
Drought	Fe, Cu, Co, and ZnO	Glycine L. max	WRKY27	Increased expression, regulates ABA biosynthesis and signaling pathways involving ABA hormones	[116]
	Fe, Cu, Co, and ZnO	Glycine L. max	MYB118	improve expression in leaves, Fe-NP exhibits improved expression in both roots and leaves, ↑ drought tolerance by upregulating drought-related genes and, subsequently, ↓ ROS levels, regulating osmolytes, and increasing flavonoid biosynthesis.	[116]
	Fe, Cu, Co, and ZnO	Glycine L. max	NAC11	Increased expression in roots with all NPs, regulate gene expression by activating transcription factors like DREB2 or ERD1.	[116]
	Fe, Cu, Co, and ZnO	Glycine L. max	RD20A	↑ Expression in roots with Fe- or Co-NP, part of the ABA-dependent pathway, high expression in roots results in drought tolerance.	[116]
	Fe, Cu, Co, and ZnO	Glycine L. max	ERD1	↑ Expression in both root and leaves, functions in the ABA-independent pathway, ↑ Expression in the cascade of genes acting directly in response to abiotic stress.	[116]
	Fe, Cu, Co, and ZnO	Glycine L. max, Cucumis melo L.	DREB2, DREB3	Increased expression in roots with Fe, Cu, and ZnO-NPs, regulate ABA-independent pathways & improve drought tolerance.	[116, 117]
	ZnO	Triticum aestivum	CAT1	Upregulation of CAT1 in leaves, ↑ CAT activity, ↑ stress tolerance.	[118]
	ZnO	Triticum aestivum	P5CS	↑ Expression of proline biosynthesis in leaves, improves drought stress.	[118]
	ZnO	Triticum aestivum	Wdhn13	↑ Expression in leaves encodes LEA D-11 DHN (dehydrin), part of the ABA-dependent drought tolerance pathway.	[118, 119]
Drought	Silica	Triticum aestivum	ABC1	↑ Expression in leaves resulting in ↑ expression of plasma membrane-localized transporters → ↑ movement of substances like abscisic acid → increased tolerance.	[119]
	Silica	Triticum aestivum	CHP zinc finger protein	↑Regulation in leaves plays a role in mitigating the effects of drought tolerance.	[119]
	ZnO	Cucumis melo L.	SOD, POD, CAT, APX	↑ Expression of antioxidant genes→ inhibit H₂O₂, resulting in improved drought tolerance.	[117]
	Chitosan-GSNO	Glycine L. max	DEFENSIN	↑ Expression in drought stress, a multi-member family of cysteine-rich proteins having strong antimicrobial activity and play a key role in protection against drought.	[120]
	Chitosan-GSNO	Glycine L. max	GOLS	Increased expression, early response gene of drought stress	[120]
Salinity	Fe₂O₃	Dracocephalum moldavica	PAL, TAT genes	Over expression of PAL and TAT genes, ↑production of RA that showed a positive correlation with MDA and scavenging of DPPH radicals.	[121]
	Chitosan	Catharanthus roseus	MAPK3, GS, and ORCA3	High expression, improve the plant growth and development under salinity by increasing the alkaloid contents in the leaf and roots.	[122]
	BioSeNPs	Brassica napus	BnPIP1-1 and BnPIP2-1	Over expression of BnPIP1-1 and BnPIP2-1, ↑ROS scavenging attributes (enzymatic and non-enzymatic antioxidants) and enhanced water potential.	[123]
	PEC	Gossypium hirsutum	DEGs	Upregulation of DEGs, 46% decreased ROS accumulation in roots, improved plant germination and morphological traits	[110]
	Fe₃O₄ and ZnO	Solanum lycopersicum	APX, SOD, and GR	↑expression of APX and SOD genes. ↓ Regulation of the GR gene which activated the antioxidants against salinity stress.	[124]
	Se	Melissa officinalis	PAL and RA	Overexpression of PAL and RA genes, increased activity of SOD, CAT, and POD.	[125]
Salinity	PNC	Oryza sativa	nia1	↑Expression of nia 1, increase the production of nitric oxide up to 34%, ion of nia1 gene.	[126]
	ZnO	Brassica napus	MYC, MPK4, SKRD2, and ARP	Downregulation of MYC, MPK4, and SKRD2 genes. 44% upregulation of ARP mRNA expression, which resulted in improved plant physiology and morphology.	[106]

Table 2.

Genetic regulation in plant upon application of nanoparticles in drought and salinity stress.

5.2 NPs alleviate the impact of drought stress in plants

Sessile organisms such as plants are constantly exposed to a range of abiotic elements such as drought, resulting in a substantial decrease in yield. Drought stress impairs the photosynthesis, nutrient uptake, osmotic, and antioxidant activities of plants. Photorespiration can lead to overproduction of reactive oxygen species (ROS) in drought-stressed plants, resulting in denaturation of proteins, DNA damage, and lipid peroxidation, which hinders cell growth and elongation, resulting in poor plant growth and productivity [127].

Recent studies have reported the role of metal-based and carbon-based NPs in mitigating drought stress by inducing tolerance. Metal-based NPs, such as ZnO, TiO₂, Fe, and Cu-NPs, and carbon-based NMs such as graphene, fullerene, fullerol, and carbon NTs, have been extensively used to control drought stress by increasing water and nutrient uptake through stress tolerance and upregulation of genes involved in cell growth [128, 129].

The expression of the P5CS gene enhances a plant’s ability to withstand diverse environmental stress, including both biotic and abiotic challenges, due to its role in encoding proline biosynthesis [130]. In response to various stress conditions, MAPK2, a member of the MAP kinase gene family, plays a pivotal role in regulating phytohormones and antioxidant defense mechanisms, working in tandem with Ca21 and ROS [131]. Transcriptional regulators AREB/ABF are crucial for overseeing the AREB gene, which controls abscisic acid and is vital for fostering resilience against demanding environments like drought and salt stress [131, 132]. The reduction in ZFHD gene expression mitigates the adverse impacts of salt and drought stress, governed by the abscisic acid biosynthesis pathway. Conversely, lowering the expression of the TAS14 gene alleviates osmotic pressure and enhances solute aggregation, including K1 and sugars, thereby strengthening a plant’s resistance to drought and salt stress [131]. Applying AgNPs (5 and 10 mg/L) to rapeseed plants brings about changes in the metabolic pathways of glucosinolate and phenolic-related genes, linked to both biotic and abiotic stresses, while suppressing carotenoid genes [133]. The use of Ag and Ag1-NPs on Arabidopsis plants induces the overexpression of genes related to oxidative stress and metal response, alongside the downregulation of ethylene and auxin-related genes [131]. Notably, three of the genes overexpressed by AgNPs are involved in thalianol biosynthesis, contributing to the plant’s antioxidant defense mechanism (Table 2).

5.3 NPs mitigate extreme temperature effects in plants

The tolerance of plants to heat and cold stress involves the regulation of numerous genes, transcription factors, and proteins. The expression levels of these components, whether upregulated or downregulated, directly impact the plant’s ability to survive under challenging conditions. For instance, in rice, the foliar application of ZnO-NPs induced the expression of genes related to the antioxidative system and chilling-response transcription factors in chilling-treated seedlings [134]. In soybeans, ZnO-NPs led to the upregulation of various genes, including EREB, R2R3MYB, HSF-34, WRKY1, MAPK1, HDA3, and CAT, resulting in increased photosynthetic pigments, proline concentration, antioxidant enzyme activity, and plant yields [135]. Cu-based NPs of 50 nm size were found to modulate genes associated with oxidative stress, brassinosteroid biosynthesis, and root formation [136]. Additionally, 40 nm-sized Cu nanoparticles in wheat were observed to accumulate secondary metabolites involved in cell signaling and defense responses [137]. The application of silicon in wheat induced the overexpression of aquaporin genes under heat stress, leading to increased relative water content [138]. In Arabidopsis thaliana, ZnO-NPs enhanced the alleviation of heat stress-induced genes, while nano-anatase in maize increased the concentration and activity of Rubisco activase, improving photosynthetic carbon production rates [139]. In maize seedlings, the application of lanthanum oxide (La₂O₃) affected the expression of aquaporin genes at root tips [140]. The varied effects of different NPs involve the regulation of numerous cold and heat stress-inducing/regulatory transcription factors and genes, ultimately enhancing plant stress tolerance through physiological, molecular, and biochemical modifications (Table 3) [130].

Stress	NPs	Plant	Genes	Major regulated genes	References
Cold Stress	TiO₂	Cicer arietinum L	Rubisco	Upregulation of LRubisco; SRubisco; chlorophyll a/b-binding protein; PEPC.	[82]
Cold Stress	TiO₂	Cicer arietinum L	APX, CHN, RLK	Upregulation of genes, Differential expression of genes related to cellular defense, cell signaling transcriptional regulation, and chromatin modification.	[141]
Heat Stress	CeO₂	Zea mays	HSP70	Upregulation of Heat Shock Proteins (HSP70) and excessive production of hydrogen peroxide H₂O₂.	[142]
Heat Stress	MWCNTs	Solanum lycopersicum	HSP90	Upregulation of HSP90	[143]
Flooding	AL₂O₃	Glycine L. max	Nmr A-Like, PABP, FQRI.	Differential expression of proteins related to protein synthesis/degradation; glycolysis; lipid metabolism. Upregulation of Nmr A-Like, PABP2. Downregulation of FQRI.	[144]
Flooding	Ag	Glycine L. max	ADH1, PDC	Differential expression of root proteins for cell signaling and cell metabolism.	[145]
Flooding	Ag	Glycine L. max	BKR1	Upregulation of proteins related to protein metabolism; cell division/organization; amino acid metabolism; BKR1	[146]
Cd Stress	Si	Oryza sativa	LCTI, NRAMP, HA3 and LSI1	Downregulation of LCTI and NRAMP5 Upregulation of HA3 and LSI1	[147]
Fluoride stress	Ag	Cajanus cajan	P5CS1	Downregulation of NADPH oxidase and P5CSI	[148]

Table 3.

Genetic regulation in plant upon application of nanoparticles in different abiotic stresses.

5.4 NP intervention in alleviating the adverse effects of heavy metals in plants

Cong et al. outlined the impact of silicon nanoparticles (Si-NPs) in diminishing the absorption and toxicity of cadmium (Cd) in rice [149]. Si-NPs inhibit genes linked to the transportation and uptake of Cd from the root to the shoot, specifically targeting low-affinity cation transporter (LCT1) and natural resistance-associated macrophage protein 5 (NRAMP5). Upregulation is observed in genes related to the transport of Cd into vacuoles, such as heavy metal ATPase 3 (HMA3) and the silicon uptake gene, low silicon rice 1 (LSI1). By using Si-NPs, silicon is enhanced from the roots, concurrently suppressing Cd uptake. In the study by Ahmed et al., the Cd transporter genes, including OsHMA2, OsHMA3, and OsLCT1, were identified [13]. The application of iron oxide nanoparticles (FeO-NPs) and hydrogel NPs substantially decreased the expression of these genes in rice. The discussion also delves into the natural resistance-associated macrophage protein (NRAMP) gene family, which is responsible for heavy metal transport in various plant species like rice, potato, pepper, tomato, Arabidopsis, and soybean [150]. Si-NPs treatment leads to the downregulation of Cd uptake and transport genes, fostering wheat growth and mitigating heavy metal stress. Additionally, nanoscale zero-valent iron (nZVI) has been recognized for its ability to alleviate the accumulation of heavy metals in plants such as rice seedlings and facilitate growth by downregulating genes (IRT1, IRT2, YSL2, and YSL15) responsible for iron and cadmium uptake [151]. Venkatachalam et al. observed notable genomic alterations in plants treated with zinc oxide nanoparticles (ZnONPs), which manifested as the emergence of new DNA bands and/or the absence of standard bands in the random amplified polymorphic DNA (RAPD) pattern under cadmium (Cd) and lead (Pb) stress [152]. Detailed information about gene activation is listed in (Table 3).

5.5 Flood stress mitigated by NP in crop plants

Flooding induces severe oxygen depletion in plant roots, leading to significant physiological changes [153]. In a study using Glycine max L. (soybeans) exposed to Ag-NPs and Al₂O₃-NPs during flooding stress, alterations in the proteome and transcriptome were observed. The analysis revealed changes in 172 proteins, with 107 being root proteins. NPs influenced the expression of genes related to stress response, including downregulation of FQR1, ADH, and PDC, and upregulation of NmrA-Like [144]. Al₂O₃-NPs showed better alleviation of flood stress compared to Al-NPs. NPs affected energy metabolism, glycolysis, protein, and lipid metabolism during flooding. Notably, glyoxalase II downregulation and reduced PDC expression suggested a potential role of NPs in reducing cytotoxicity and alleviating anaerobic stress. NPs also influenced lipid metabolism, with BKR1 expression increased during flooding and decreased during the recovery period [146]. The upregulation of translation initiation by PAB2 and the antioxidant activity of NmrA-like family proteins contributed to stress alleviation. These effects were observed specifically with NPs of 15 nm size, highlighting the size-dependent impact on differentially expressed protein classes (Table 3) [146].

6. Nanotechnology for sustainable agriculture in crop plants

The integration of nanotechnology in agriculture offers a sustainable approach, providing a cornerstone for resilient and resource-efficient crop production in the face of evolving environmental challenges. Nano-enabled technologies, ranging from nanofertilizers to genetic engineering, present a paradigm shift in promoting plant growth and resilience [154]. The superior attributes of nanoparticles (NPs), including a high surface-area-to-volume ratio and efficient nutrient supply, distinguish them from conventional fertilizers [155]. Studies demonstrate the potential of NPs, such as iron oxide (Fe₃O₄) NPs and nano-silicon (Si), in enhancing crop growth under diverse stress conditions, encompassing heavy metal-contaminated soil and drought stress [156, 157]. Notably, green-synthesized silver nanoparticles (AgNPs) enhance antimicrobial activity in Tephrosia apollinea under drought stress [158], mitigating membrane damage and elevating hydrogen peroxide (H₂O₂) content in plant roots, thereby exemplifying the multifaceted benefits of NPs in sustainable agriculture [159].

Moreover, examples such as cerium oxide (CeO) NPs preserving photosystem yield [58] and titanium dioxide (TiO₂) NPs boosting antioxidant enzyme activity underscore the diverse avenues through which NPs positively influence plant health [160, 161]. Leveraging the stress ameliorative potential of NPs, particularly through green synthesis methods, emerges as a promising tool to counteract negative effects induced by abiotic stresses in various crops [162].

6.1 Applications of nanotechnology for crop protection

In the context of plant abiotic stress management, nanotechnology can play a role in enhancing plant tolerance to environmental stressors. Here are some applications of this emerging technology that have been studied for their potential to mitigate abiotic stress in plants:

6.1.1 Nanofertilizers

Nanofertilizers are nutrients encapsulated or coated within nanomaterial to enable controlled release, and its subsequent slow diffusion into the soil. These fertilizers help to restore degraded soils, manage soil health, improve seed germination, enhance plant growth, and increase crop yield like nitrogen nanoparticles designed to improve the efficiency of nitrogen delivery to plants. Phosphorus nanoparticles enhance the availability and uptake of phosphorus by plants. Potassium nanoparticles improve the delivery of potassium to plants for better growth and stress resistance [155, 163].

6.1.2 Nanomediators

They act as efficient carriers for targeted delivery of genetic material, regulating gene expression to improve stress response in plants. These nanoparticles can also aid in nutrient management by facilitating the controlled release and mitigation of persistent organic pollutants, Pesticides, and HM. Facilitate the targeted delivery of genetic material for gene therapy [164].

6.1.3 Nanopesticides

Nanopesticides revolutionize agricultural practices by offering targeted and controlled release of pesticides. This reduces environmental impact by minimizing pesticide usage while effectively managing both target and non-target plant pests. Additionally, nanopesticides contribute to plant disease management, enhancing crop health and ultimately improving overall yields. As a result, these innovative formulations showcase potential advancements in sustainable and efficient pest control strategies for modern agriculture [165, 166].

6.1.4 Nanosensors

Nanosensors, incorporating nanoparticles, play a pivotal role in detecting biological molecules and assessing environmental conditions. In agriculture, they help monitor heavy metal levels in soil, ensuring optimal plant growth and preventing contamination. Nanosensors also enhance the sensitivity and selectivity of gas sensors, allowing for precise detection of gases related to environmental health. Their versatility extends to identifying pollutants and contaminants in the environment, offering a promising avenue for real-time, high-precision environmental monitoring and management [167].

6.1.5 Nanomaterials

Nanomaterials such as carbon nanotubes (CNTs), graphene nanoparticles, and metal nanoparticles are employed to enhance plant resilience to various environmental stresses. In the context of salinity stress, these nanomaterials have shown promise in increasing plant tolerance by regulating ion balance and minimizing salt-induced damage. Additionally, they play a crucial role in reducing the bioaccumulation of heavy metals in plants and mitigating environmental contamination [168, 169].

7. Challenges and risks associated with nanobiotechnology

Although nanobiotechnology harbors immense potential for future agricultural technology revolutions, particularly considering climate change and growing populations, it also presents numerous challenges and associated risks. This requires more study and research on nanomaterials and their properties, as they behave very differently than in bulk form. Some materials are suspected of being toxic at the nanoscale due to significant, yet unknown, hazardous properties related to their unique physiochemical features. This can put manufacturers, formulators, handlers, applicators, and consumers at greater risk. Thus, nanoparticles can cause heterogeneous effects as they can leave toxic effects on non-target organisms upon contact, i.e., nanoparticles can come into direct contact with humans and cause toxic effects on them. The hazardous effects of nanoparticles on non-target organisms include dermal absorption of nanoparticles and translocation to go deep into the lungs and brain through inhalation and crossing the brain barriers, respectively.

Nanoparticles can enter various human body parts and cause harm by disrupting cellular pathways, enzymatic actions, and organ functions. Nanomaterial disposal could result in the formation of a new class of non-biodegradable pollutants in the environment [170, 171]. The potential risks associated with nanomaterials lead to the challenges of basic and applied nanobiotechnology in different fields including crop production. The major possible challenges are the following [170, 171]:

High-quality mass production of nano-based products at an economical cost.
The development of a personalized nanomaterial production system to fulfill local needs.
Environmental and human health safety and protection during the use and disposal of nanomaterials.
The challenge is to overcome the gap between basic and applied nano-based research.
The production cost, severe risks, and practical knowledge gaps are also considered as chief concerns/challenges in nanobiotechnology applications.

Hence, a collaborative effort, based on comprehensive scientific research and technological advancements, between regulatory bodies and workforce research should be made to cater to the above-mentioned challenges and associated risks. This would help to provide the necessary information to devise appropriate guidelines for comprehensive risk management and applications of nanobiotechnology for sustainable crop production.

8. Conclusion and future prospects

The impact of climatic variations in recent years has contributed significantly to reduce crop productivity and gave birth to various environmental stress factors. Abiotic stresses are the key player responsible for reduced crop yield, quality and stunted plant growth around the globe. Efforts to cater to challenges associated with abiotic stress led to the development of nanobiotechnology, which encompasses small-size nanoparticles that aid in enhancing the farming sector and managing global food demand. Several NPs, such as TiO₂, SiO₂, and AgNPs, can reduce the negative effects of abiotic stress by activating the genetic makeup of plant defense mechanisms via the induction of ROS production and phytotoxicity. Their small size allows them to penetrate easily into plant tissues, after which they positively influence plant morphological, physiological, and biochemical processes, promote plant development, and improve crop productivity in plants under various abiotic stresses. Furthermore, NPs have a large surface area that improves the absorption and delivery of various targeted nutrients. Despite the positive effects of nanoparticles, there are some risks and challenges associated with them that need to be addressed. These risks will aid in exploring their dimensions and help to improve crop productivity by understanding the underlying interaction pathways at the molecular level.

It is important to conduct future research exploring different uses of nanoparticles to enhance crop plant productivity. Nanoenzymes including Mn₃O₄-NPs gain much attention due to their potential role in mitigating stress conditions in plants. Mechanisms responsible for nanopriming-induced seed germination, breaking seed dormancy, and their interactions with seeds need to be investigated. A better understanding of plant–NP interaction will enable researchers to design tailor-made nanomaterials targeting agricultural challenges. Moreover, targeted delivery approaches using nanomaterials will prove to be another emerging way to enhance plant productivity. Recently, a study conducted with this approach used nanomaterials to convert chloroplasts into a “chloroplast factory,” resulting in improved plant photosynthesis under low light conditions [172]. In addition to that, the use of nanomaterial CRISPR-Cas genome editing in cargo delivery will also improve the efficiency of genetic engineering to enhance plant stress tolerance [173]. However, the management of biosafety hazards associated with the use of nanoparticles demands the development of strict policies and procedures so that we can utilize and benefit from their potential role in enhancing crop tolerance to abiotic stresses.

Conflict of interest

There are no conflicts of interest among the authors.

Authors contribution

All authors have contributed equally to writing and drafting this article. Hafiza Fizzah Zulfiqar also contributed to designing the images in this article.

Abbreviations

HM	heavy metal
Cd	cadmium
Pb	lead
As	arsenic
Ni	nickel
Cr	chromium
Co	cobalt
Zn	zinc
ROS	reactive oxygen species
NPs	nanoparticles
CuO	copper oxide
DNA	deoxyribose nucleic acid
RNA	ribose nucleic acid
UV	ultraviolet
PS	photosystem
TiO2	titania
CAT	catalase
GPOX	glutathione peroxidase
SOD	superoxide dismutase
MAP	mitogen-activated protein
SOS	salt overly sensitive
ZnO	zinc oxide
MAPK	mitogen-activated protein kinase
Myc	myelocytomatosis oncogene
ARP	auxin-repressed protein
EREB	ethylene responsive element binding protein
HSF	heat shock transcription factor
LCT1	low-affinity cation transporter
NRAMP5	natural resistance-associated macrophage protein 5
nZVI	nanoscale zero-valent iron
RAPD	random amplified polymorphic DNA
CeO	cerium oxide

Abd Jalil Nordin

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