1 Introduction

The agricultural industry is currently facing the dual challenge of satisfying the increasing global food demand at the same time adopting sustainable farming methods [1]. Plants exist in conditions that are continually fluctuating and frequently unsuitable or daunting to their growth, life span, and consistent development. These unfavourable environmental factors include biotic stressors such as microbial contamination and herbivore attacks, as well as abiotic stressors like drought, heat, cold, nutritional shortage, excessive salt, or harmful elements like aluminium, arsenate, and cadmium in the land [2]. Amidst all those adverse conditions, salt stress is one of the most serious universal agrarian issues, threatening crop growth and production, impeding the equitable advancement of contemporary agriculture, and jeopardising the availability of food. According to various investigations, salinity afflicts over one-third of the globe's fertile farmland [3]. Salinity stress in crops is caused by a number of circumstances, including elevated transpiration rates, limited rainfall, inadequate drainage, excessive fertiliser use, and soil geological properties [4].

The two main contributors to soil salt toxicity amid all of these variables are deteriorating sewage and drainage systems and increasing levels of groundwater with elevated levels of salt [5, 6]. Elevated salt content in the ground or irrigation water raises the osmotic pressure, which causes osmotic stress in crops. It hinders the roots' ability to absorb water and throws off the crop's internal water balance. Plants thus endure a lack of water, constrained cell growth, and compromised metabolic functions. Intake and transit of vital minerals, including potassium (K+), magnesium (Mg2+), and calcium (Ca2+), may be hampered by the increased absorption of Na+. Several metabolic functions, such as enzyme activity, protein synthesis, and photosynthesis, can be impacted by this imbalance, which can result in nutritional deficits [4]. Additionally, Sodium Chloride (NaCl)-induced salt stress manifests fatal oxidative gradients in plant cells through the abundant production of reactive oxygen species (ROS) and reactive nitrogen species (RNS)−+. By oxidising macromolecules including proteins, lipids, and DNA, ROS, such as superoxide radicals (O2⋅−), hydrogen peroxide (H2O2), and hydroxyl radicals (OH⋅), injure cells. This oxidative damage interferes with cellular function and hinders the normal growth curve and developmental phases of plants [7]. Consistent exposition to elevated levels of NaCl impairs crop advancement and growth, leading to constricted roots and shoots. This decreased development results in shorter plants, slower biomass buildup, and eventually poorer agricultural yields. Another effect of salt stress on the reproductive system is a decrease in flower emergence, pollen viability, and seed output [4]. Therefore, increasing crops' ability to withstand the effects of salt stress is crucial for maintaining world agricultural sustainability [8, 9]. Various methods were adopted from time to time to alleviate the impacts of salt stress in plants. These methods include, conventional breeding methods and modern methods such as gene silencing, gene mutation, gene knock out and knock in etc. Conventional breeding methods have played a significant role in development of stress tolerance in crops. But all these conventional breeding techniques are tedious and have limitations like requirements of large man power, energy, etc. [8, 10, 11].

Nanotechnology is a novel approach towards the improvement in the agricultural sector as it puts forth new ways to impart tolerance against various stresses and enhances the productivity [12,13,14,15,16,17]. Nanoparticles (NPs) refer to the particles of size from 1 to100 nm, with distinct chemical and physical properties such as different shapes (spheres, rods, tubes, fibres, discs, worms, squares, urchins, ellipsoids), large surface area by volume ratio, crystal structure, pore size, geometrical orientation, having strong activity at the cellular and molecular level in living organisms and enhanced conductivity at the surface have led to extraordinary efficiency, reduced environmental contaminants, and limited components in precise agriculture, making them a competitive alternative to conventional crop-protection methods [10]. Plant nanotechnology is predicted to be a successful approach to increasing agricultural output in areas like the treatment of seeds and the germination process, the progression and development of plants, pathogen evaluation, genetic modification, plant stress tolerance, nutritional management, and the identification of harmful agricultural chemicals [18,19,20,21,22]. In the current situation, NPs have the potentiality to boost plant morphogenesis, used as herbicides, nano-pesticides, and nano-fertilizers, etc., that can proficiently release their content in required amounts to target cellular organelles in plants [23]. It has also been clear that employing NPs to increase plants' ability to withstand stress is anticipated to be a profitable, cost-effective, and sustainable technique of agricultural production.

Plants are sessile so they have to face extreme abiotic stress conditions, such as salinity, drought, high and low temperatures, heavy metals, flooding, high and low light intensities, ultraviolet (UV), and others [23]. Different types of NPs are developed such as those containing inorganic non-metallic NPs, carbon-based NPs, metallic NPs, and organic polymeric materials based on the application and usage [24]. Researchers have revealed that NPs help plants to overcome abiotic stresses by their concentration-dependent impact on plant growth and development [25]. For example, application of silica (SiO2) NPs in hawthorns improved seedling growth and physiological parameters under drought stress [26]. Moreover, the application of selenium (SeO2) NPs in sorghum improved their antioxidant machinery to scavenge ROS produced as a result of heat, thus alleviating heat stress [27]. Application of silver (Ag) NPs augmented the stress tolerance potential of soybean seedling by downregulation of protein mis-folding induced by flooding stress [28]. NPs such as titanium oxide (TiO2) NPs play a significant role in mitigating light stress by catalysing the redox reaction, which leads to the generation of superoxide and hydroxide radicals [23]. Moreover, external application of NPs have exhibited significant promise in enhancing plant salt tolerance [29]. Some of the NPs presently being used to boost crop resistance to salt include SiO2 NPs, cerium oxide NPs (CeO2 NPs), put-carbon quantum dots (put-CQD) NPs, TiO2 NPs, carbon nanotubes (CNTs), and nano-zinc. For instance, multi-walled carbon nanotubes (MWCNTs), SeO2 NPs, and zinc oxide (ZnO) NPs may all significantly lessen the negative impact of salt stress on the growth of rapeseed seedlings. Seed priming with CeO2 NPs, Se NPs, and ZnO NPs significantly improves the germination indices of rapeseed seeds under salt stress [14, 30,31,32,33,34].

NPs increase plant tolerance to salinity by preserving photosynthesis, promoting ROS clearance, and lowering osmotic and ionic stress [35,36,37,38,39]. By regulating the functioning of various genetic material, transcription elements, proteins, and metabolites, NPs have become a viable technique to improve the crop's resistance to salt. Studying the plant's evolutionary reactions to salinity and increasing agricultural yield requires the integration of omics methods such as genomic, transcriptomic, proteomic, and metabolomic investigations [40,41,42]. Research on the morphological, anatomical, and biochemical alterations observed in crops when subjected to salt, as well as the function of phytohormones in the face of salt stress, has received a lot of attention in recent publications. Additionally, this review considers significant new research that explores the utility of NPs to reduce salt stress and explains their impact on plant molecular responses (Fig. 1). We summarised the molecular principles underpinning plant resistance to salt in the following assessment and emphasised the pragmatic relevance of nanotechnology in enhancing plant salt resistance [29].

Fig. 1
figure 1

Diagrammatic representation of mode of action of salinity stress responses. Salinity stress in plants causes the ionic stress (reduces the uptake K+ ions and accumulation of Na+ ions from soil) and osmotic stress in plants which leads to the generation of ROS in plants, which affects the photosynthetic activity and reduces photosynthetic pigments and damages the chloroplast membrane which ultimately decreases the plant growth and agricultural yield

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2 Salt and NPs transport in plants

2.1 Uptake and transport of salt into plants under salinity stress

Considerably, the most agitating danger to plants is salt toxicity, which causes osmotic and water stress. Plants absorb salt through apoplastic and symplastic mechanisms (Fig. 2). For salt transport, monocots primarily use the apoplastic pathway, while dicots prefer the symplastic pathway. Transporters and channels are crucial for the symplastic NaCl transport pathway.

Fig. 2
figure 2

Uptake of salt from soil to leaves via stem with the help of different membrane transporters

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The influx of Na+ requires the action of the Na+/H+ antiporter (NHA) or SOS1 [43,44,45]. Ca2+ serves as an inhibitor of NSCCs, whose molecular expression is modulated by the family of genes encoding glutamate receptor-like channels (GLRs) and cyclic nucleotide-gated channels (CNGCs) [46, 47]. Multiple isoforms of high-affinity K+ transporters (HKTs) and cation/H+ exchangers (CHX) have to actively engage in the long-distance ion transport of Na+ across the xylem and phloem [44, 48]. By stimulating Na+ absorption, HKTs significantly elevate the amount of Na+ that accumulates in the plant. Aquaporin and distinct forms of the plasma membrane intrinsic protein (PIP) are contributors to the facilitation of Na+ absorption in plants. Low-affinity cation transporters (LCT1) and the kinase AKT1 indirectly mediate Na+ entry. Therefore, Na+ uptake is influenced by HKTs, NSCCs, NHA, and CHX, whereas NSCCs, NHA, PIP2, LCT1, and AKT1 are connected to the long-distance distribution and movement of potentially hazardous Na+ [49]. Cl influx into root cells is governed by H+/Cl− symporters, Cl−/H+ co-transporters, and nitrate transporters (NRT) [44]. Different anion channels facilitate the passive transport of Cl− during salinity. Cl− influx is facilitated by cation chloride cotransporters (CCCs), which are also harboured in the plant root and shoot tissues. Cl transport in plants necessitates the presence of the chloride channel (CLC) and the aluminium-activated malate transporter (ALMT), both of which are found on the tonoplast. Long-distance Cl− transport is also interconnected with anion channel-associated homolog 1 (SLASH) and nitrate transporter/peptide transporter (NPF) [44]. Consequently, it makes practical sense to assume that the synergistic actions of all the transporters involved in the transportation of Na+ and Cl− result in salinity and ion toxicity [44, 50].

2.2 Interaction and uptake method of NPs

2.2.1 Interaction of NPs with plants

NPs must be internalized by plant cells and go through the plasma membrane to reach the symplastic pathway. Several mechanisms facilitate this process, although their understanding is more extensive in animal cells compared to plants [51, 52]. One method for NPs to get fixed in cells is by endocytosis, which involves the penetration of the plasma membrane and the creation of a vesicle that may move to certain cell compartments [53,54,55]. Another mechanism involves the disruption of the plasma membrane by certain NPs, which induces the formation of pores for direct entry into the cell without encapsulation in organelles [56]. NPs can also bind to surrounding proteins, including cell membrane proteins, which function as potent carriers for their absorption and internalization into the cell [57]. Aquaporins, for instance, have been suggested as transporters for NPs [58]. However, due to their small pore size (ranging between 2.8 and 3.4 Å), it is unlikely for them to act as channels for NPs penetration unless modifications are made to increase their pore size [59]. Plasmodesmata, specialized structures for intercellular transport, also play a role in NP entry into plant cells but require the NPs to already be present in the symplast. This mechanism is particularly significant for NP translocation through the phloem [60]. Although ion channels have been proposed as possible pathways for NP entry, their typical size of around 1 nm makes it highly improbable for NPs to cross them effectively without substantial modifications [55, 61,62,63]. It must be noted that the mechanistic actions associated with NP uptake in plant cells continue to be under vigorous investigation, and further studies are a prerequisite to thoroughly comprehending these bioprocesses.

2.2.2 Uptake of NPs from soil

The main and most important entry points for NPs into the soil are the pores in the soil and the roots [64]. The lateral root system and emerging root hairs in the rhizosphere are the most effective locations for NP absorption [64]. The initial interface between NPs and plant roots is through root-mediated adsorption of the NPs from the soil and onto their surface. Positively charged NPs are more likely to accrue and get deposited there more quickly due to the root surface's negative charge brought on by the secretion of phytochemicals from root hair openings [65, 66]. Organic acids and mucus, to name some of the aforementioned chemicals, NPs can freely move through the tissues of plants once they have entered them using apoplastic and symplastic movement [67,68,69,70]. Sattelmacher et al. [71] asserts that the apoplastic pathway facilitates the radial transport of NPs into vascular tissues. The symplast and the cytoplasm of the adjacent cell exchange water and other materials [55]. The origin of lateral roots could create a new adsorption interface that will allow NPs to penetrate the root column [72, 73]. The root epidermis resembles the leaf surface both morphologically and functionally. The root tip epidermis and root hair surfaces of primary and secondary plant roots, however, are not yet fully formed. When NPs are exposed here, they directly penetrate and come into contact with the root epidermis [74, 75]. Root epidermal cells possess a porous cell wall, which permits some water and nutrients to transverse through. Root cells contain densely packed cell walls to filter out undesirable smaller and bigger particles [61]. Small molecules like steroids can transverse through the physical barrier that the root cell's plasma membrane creates between the root and the soil, but hydrophilic and macromolecules cannot. Through the use of channels and carriers like ATPase and the GLUT1-4 transporter/carrier, these molecules are moved across the membrane [76, 77]. The transport of NPs from the soil to the root cells may also involve these membrane channels and transporters [66, 78]. NPs can enter the centrally located vascular column, or xylem, of the root when it lacks an exodermis [79]. According to other studies, some NPs have been shown to cause damage to the plasma membrane and to create new pores in the epidermal cell wall that permit the uncomplicated yet efficient passage of larger NPs [65]. Numerous entry routes exist for the cells to absorb NPs when they are introduced to plant tissue [72, 73]. The ion pathway, endocytosis, protein binding to cell membranes, and physical harm are a few important mechanisms.

The accreted NPs in plants are distinct from the applied form of NPs [80, 81]. Due to their biochemical and geometrical stability, NPs like TiO2 and SiO2 are present in plants in their native form, while other NPs, including but not limited to ZnO, NiO, CuO, Yb2O3, CeO2, and La2O3, undergo transformation. For instance, following application, Zn was taken up and translocated as Zn2+ in maize since ZnO NPs undergo transformation in the rhizosphere [65]. However, most concentrations of Zn detected in maize underground roots and aerial shoots were in the form of ZnPO4. Wheat crops treated with ZnO NPs and CuO NPs accumulated ZnPO4 and Cu(I)-sulfur complexes, as indicated by Dimkpa et al. [82]. Both rice and maize plants displayed evidence of Cu (II) being reduced to Cu (I). The translocation of CuO NPs from root zones to shoots of rice results in Cu reduction (II). In addition to being reduced to Cu2O, Cu (II) confers the propensity to complex with cysteine and citrate [72, 73]. The transformed end-products of CeO2, Yb2O3, and La2O3 NPs were found in the rhizosphere and other intercellular plant regions by Zhang et al. [83]. When hydroponic cucumber culture was treated with NPs, 7 nm CeO2 NPs were found to be reduced to Ce (III) and accumulate as CePO4 in the root's intercellular regions. The root exudates, rich in organic acids, aided in the dissolution of CeO2 NPs. Ma et al. further confirmed the transformation sites of CeO2 NPs in cucumber plants [30]. The significance of the rhizosphere in the degradation of CeO2 NPs was elucidated at the time of the discovery of an accumulation of Ce (IV) and Ce (III) as a response to root exposure to NPs, but not in the case of foliar spray. Research indicated that in cucumbers, Ce migrates from the roots to the shoots via the xylem as Ce (IV) and Ce (III); however, it is transported from the shoots to the roots as CeO2.

2.2.3 Foliar application of NPs and uptake

The leaf surface is daubed externally with NPs, which settle there and are absorbed by the plant via specialised appendages including trichomes, hydathodes, or stomatal pores. Wax, cutin, and pectin collectively constitute the majority of the waxy cuticle covering the leaf epidermis. It operates as a vital inherent barricade against NPs from entering while preventing water loss in developing leaves [84]. The waxy stratum corneum, on the flip side, has two distinct channels, one of which is lipophilic and the other hydrophilic. The diameters of hydrophilic and lipophilic channels range from 0.6 nm to 4.8 nm [85]. Hydrophilic NPs designed to have a geometric diameter of less than 4.8 nm can diffuse through the hydrophilic channels [86]. Diffusion and infiltration through the lipophilic channels in the cuticle allow lipophilic NPs to be absorbed by leaves [87]. Recently, Hu et al. [62] used high-resolution confocal fluorescence microscopy to show that carbon dots as small as 2 nm can pass through the cuticular pathway and enter cotton leaves. However, the assimilation of NPs through the epidermis is constrained by the tiny size of the pore channels within the cuticle. When NPs are sprayed onto a leaf, they tend to accumulate in the veins and skin. In the meantime, numerous studies have established that NPs can be moved to various plant tissues. The research group postulated that NPs might be absorbed by plants through the stomatal pathway [55]. NPs are administered as a spray that lands on the outermost layers of leaves and is then absorbed by the plant by trichomes, hydathodes, or stomata. The major bulk of the waxy cuticle of the leaf epidermis is comprised of wax, cutin, and pectin. It serves as a key natural barrier to prevent NPs from entering and water from leaving [84]. There are two separate channels, one hydrophilic and one lipophilic, in the waxy stratum corneum. The width of either hydrophilic or lipophilic channels can range from 0.6 to 4.8 nm [85]. Hydrophilic NPs smaller than 4.8 nm can diffuse through the hydrophilic channels [86]. The lipophilic routes in the cuticle facilitate the infiltration and diffusion of lipophilic NPs into the leaf [87]. However, the miniscule openings of pore channels serve as a regulatory element in the cuticle, which limits the optimum absorption of NPs through the epidermis. When NPs are applied to a leaf's surface, studies show that they accumulate in the leaf's vascular tissue and outer epidermis. Meanwhile, numerous studies have established that NPs can be moved to various plant tissues. The study's authors speculated that NPs might be absorbed by plants through their stomata.

The absorption, migration, and accretion of foliar NPs are influenced in part by a number of factors, one of which is the plant species. The unequal distribution of stomatal pores in dicotyledons and monocotyledons may account for the difference in uptake behaviour. Plants with fewer veins, shorter petioles, and more leaf surface area tend to accumulate more NPs. Foliar supplementation of NPs to leaves largely depends on their developmental stage and life cycle. Foliar NP penetration was thwarted by physio-chemical attributes such as NPs size, leaf cross-sectional surface area, epidermal structure, and growth stage, among other factors [88]. While lipophilic epidermal wax aids the entry of hydrophobic NPs, hydrophilic nanosuspensions can be absorbed through cell walls. Additionally, phyllosphere microorganisms generate organic and inorganic molecules that might acidify, decrease, and chelate NPs, as well as have an impact on NP entry into leaves. Humic acid, for instance, is implicated in the chelation of ZnO NPs [89], while ethylene diamine-N and N-bis(2-hydroxyphenylacetic acid) chelate Fe NPs [90]. Metal- and carbon-derived NPs are less able to be absorbed and transferred in plants because they bind to the thiol group of diverse chemical compounds found in vacuoles after entering the leaves. The endodermal casparian strip acts as a final line of defence against NP translocation, preventing it from entering the bloodstream [91]. Numerous abiotic environmental elements, including temperature, light, and humidity, may detrimentally affect the uptake of NPs via foliar spray. For instance, elevated temperatures can cause the epidermis of leaves to shrink, while shade and low humidity can cause stomata to close. High relative humidity lowers the leaf's osmotic potential, which stimulates NP uptake. Abiotic factors have an impact on the intake of NP through foliar spray and also hinder epidermal development [92]. The stomatal apparatus closes, and aerial uptake of NPs declines when the epidermis thins and weakens in response to abiotic stressors (shade, high temperature, and low humidity). As a direct consequence of its impact on the osmotic potential of the leaf surface, high relative humidity promotes the uptake of NPs [93]. The foliar uptake of NPs is affected by abiotic factors like light and temperature, which in turn affect photosynthetic efficiency and the development of the leaf epidermis [94]. The sprayed NPs are washed away by the rain, reducing their absorption, as reported by Alshaal and El-Ramady [95].

3 Effect of NPs on plants under salinity stress

3.1 Seed germination

The development, growth, and productivity of plants all begin with their seeds germinating. The results of using a natural method to germinate seeds are slow and modest. Yet, from the perspective of treated seeds, improved germination indices have been achieved, rendering nanotechnology a promising approach for augmenting both germination and productivity yield. There have been a lot of studies conducted to see if NPs can improve germination. There is still much mystery surrounding how NPs treatments improve seed germination rates. Increased germination may be attributable to the enhanced seed absorption and water retention brought about by the NPs treatments [96].

Salinity is a leading extrinsic stress factor that threatens to slow plant growth and, in turn, product yield [20]. The osmotic and oxidative stress brought on by salinity in seeds is linked to a more gradual and extended germination period [97, 98]. Priming seeds with Mn NPs ameliorates their ability to withstand salt, increases their root length, and changes the way macro- and micronutrients like Mn, Na, and Ca are redistributed [37]. Despite the oncoming salt stress, TiO2 improved the germination of maize seeds, the morphological profiles of their shoots and roots, the biomass indices of their seedlings, their relative water content, the total phenol content, and the functionality of their antioxidant enzymes [99]. Priming rapeseed with cerium oxide improved salt tolerance and lowered ROS accumulation while also increasing germination, water absorption, SOD, POD, amylase activities, total soluble sugar content, and the Na+/K+ ratio [100,101,102,103,104]. The NPs of chitosan in milk thistle and Fe nano-chelates in lentil seeds [105] and cucumber seeds [106] reduced salt stress by increasing physiological salt stress. Under circumstances of salt stress, priming lupine seed with ZnO NPs optimised photosynthetic pigments (chlorophylls, anthocyanins, etc.), total phenolic content, Zn and APX, POD, SOD, and CAT activities while alleviating malondialdehyde levels and Na+ ions [107]. Similarly, nano priming with ZnO enhanced wheat salt resilience by increasing electron transport efficiency in respective photosystems and sugar metabolism in distressed crop plants [108]. This was accomplished by initiating the activation of the antioxidant enzyme machinery. The use of carbon NPs (CNPs) that dissolve in water improved germination and chlorophyll content in lettuce seeds subjected to salt stress [109].

3.2 Plant growth, development, and yield

The physiological activity of seeds is altered by salinity stress, and the result is a decrease in germination. It inhibits germination in many plants because of its effect on water potential, protein content, and stored food reserves within seeds [110]. Reduced phosphatase activity caused by salt stress inhibited germination in Arabidopsis thaliana seeds [111]. The germination rate of plants was decreased by salt because it elevated the content of soluble sugars, starches, and ABA while decreasing the amount of gibberellic acid (GAs). Additionally, abnormal salinity had a major effect on the plant productivity index and its constituents. All of these plant metrics were reduced due to the stress inflicted by surplus salt in the rhizosphere of plants [112]. Cotton yield, cotton ball weight, lint output percentage, and overall crop quality are all highly correlated with soil salinity [113].

NPs have been shown to exhibit numerous positive effects in several avenues associated with sustainable crop improvement. These include enhanced seed germination, longer shoots and roots, more fruit, better metabolite content, and a marked increase in seedling and plant vegetative biomass in many crops (Table 1) [35, 37, 114, 115]. NPs have been indicated to affect a variety of biochemical attributes relevant to the holistic development of plants, including nutrient utilization efficiency, antioxidant defence system performance, and osmotic stress tolerance. Many studies have shown that Zn NPs have positive effects on plant growth [116]. Under normal conditions [117] as well as stress conditions, including salinity stress [14, 117,118,119,120], it was validated that treatment with Zn metal-derived NPs directly impacted plant morphological and physiological attributes in a positive manner (Table 2) [118, 121]. Applying bulk ZnO and ZnO NPs to okra (Abelmoschus esculentus L.) plants found that the latter strengthened light harvesting machinery and its pigments and CAT and SOD activities while deteriorating proline and total soluble sugar content [118]. According to Farouk and Al-Amri [122] when Zn NPs were applied to salt-stressed canola (Brassica napus L.) plants, the negative effects of salt were mitigated through increased antioxidant activity, osmolyte biosynthesis, and ionic regulation. Mahmoud et al. [123] found that applying zeolite, zinc, silicon, and boron NPs to soil increased potato growth, yield, and physiology under salinity stress. Supplementing plants with zeolite, Zn, Si, and boron-based NPs in the face of salt toxicity, the advantageous alterations were reflected as enhanced water and nutrient retention, nutrient uptake and metabolic turnover, photosynthesis, and enzymatic antioxidant activities [123]. Therefore, it can be certainly concluded that Ag NPs enhance a wide assortment of plant growth characteristics, from germination rate to growth indices, by monitoring and altering accordingly a wide range of physio-biochemical traits [124, 125]. It has further been established that Ag NPs can boost crop growth and beneficial parameters in extreme environments (Table 2). Dose-dependent exposure of Pennisetum glaucum seeds to Ag NPs led to increased levels of antioxidant enzyme activity in the resulting plants, which in turn reduced oxidative damage under saline conditions [126]. Ag NPs also reduced the Na+/K+ ratio in the leaves while increasing the flavonoid and phenolic content [127]. Seed pre-germinative treatment with Ag NPs (nano-priming) increased the growth, proline content, soluble sugars, and POD functionality of salt-stressed wheat seedlings, as shown by Mohamed et al. [128]. Plants under salt stress may benefit from Si fertilisation by retaining more water [129]. Water loss through the leaf cuticle is reduced due to silicon storage in the epidermal cell walls, and transpiration is increased [129]. Si application to salt-stressed plants has been shown in various reports to increase photosynthesis, vegetative growth, and dry matter production while simultaneously decreasing shoot Na+ and Cl− deposition and increasing K+ accumulation until an equilibrium is maintained [81, 129]. The micronutrient Cu has considerable effects on a plant's stability index and seed output. Additionally, when Fe NPs were administered as a foliar supplement to Helianthus annuus flourishing in a saline environment, the catalytic efficiency of polyphenol oxidase, CAT, and POD were improved [130]. Therefore, it can be deduced that Fe-derived NPs have considerable capability to alleviate NaCl toxicity, though an adequate information pool on the precise, underlying metabolic mechanisms they regulate is lacking in contemporary literature. Several studies have elucidated that Fe NPs play a crucial mediating role in the development of salt tolerance in plants (Table 2). Under salt stress, the results of the application of nano-sized Fe2O3 were evaluated in a study conducted by Moradbeygi et al. [131]. The authors observed that plants exposed to salt stress grew better and had higher enzyme activity profiles upon treatment with 60 mg L−1 of nano Fe2O3.

Table 1 Effect of salinity stress on plants at different morphological, physiological, and biochemical traits
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Table 2 Application of NPs in salinity stress to management of antioxidant responses for ROS homeostasis
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Abdoli et al. [148] revealed that the combination of Fe2O3 NPs and SA mitigated salt stress through altering the K+/Na+ ratio, the Fe content, the activities of the antioxidant machinery (SOD, CAT, POD, and polyphenol oxidase), endogenous SA, and some important osmolytes. They examined the effects of Fe2O3 (3 mM) on Trachyspermum ammi under salt stress. The changes improved the plant's rooting and shooting patterns and growth, leaf pigmentation, membrane structural stability, and seed output. Additionally, spray supplementation of Fe NPs in Helianthus annuus supported by salt-stressed soil resulted in improved activities of polyphenol oxidase, CAT, and POD [130].

3.3 Effect on the photosynthesis process

Photosynthesis is a vital physicochemical process, but it is extremely vulnerable to saltwater. Salinity has been observed to restrict photosynthesis-dependant autotrophic modes of nutrition in salt marsh plant species by increasing the sensitivity of the stomatal apparatus in the leaves [149]. The amount of chlorophyll and other associated phytopigments in wheat declines. In addition, the oxygen-evolving complex (OEC) and quinine acceptors are deactivated by high salinity, which further reduces PSII activity. Therefore, PSII is more receptive and primarily susceptible to salt than PSI [149]. Reduced stomatal conductance and impaired transpiration due to salinity result in inefficient gas exchange and a slower photosynthetic rate. Halophyte grasses have less photosynthetic pigment because of salinity [150]. Manganese (Mn+2) contributes to an array of cellular processes (occurring in active organelles such as mitochondria and chloroplasts) and is an integral cofactor contributing to the structural stability of various enzymes, etc. that optimise photosynthetic electron transport rate and oxygen evolution [151]. Mn NPs were also found to aid in keeping photosynthesis at a healthy level even when plants were subjected to abiotic tenacity [152]. A pilot study published that adding Mn to Vigna radiata plants afflicted with salt stress enhanced their nitrate reductase activity, chlorophyll content, and membrane stability index [153]. In line with preceding investigations, integrating copper into maize plants could reduce the detrimental effects of salinity on water relations and photosynthesis [154]. The induction of photosynthesis by Mn and Cu NPs, however, has been demonstrated to boost plant tolerance to NaCl stress. To ascertain the specific and new roles that Mn and Cu NPs play in influencing salt stress tolerance, additional analysis in this area is warranted. By fixing carbon dioxide, the RuBisCO enzyme plays a crucial role in the Kelvin cycle. Through genetic research of the smaller RuBisCO subunit, Xuming et al. [155] discovered that foliar spray application of TiO2 NPs greatly boosted the synthesis of this enzyme and plant photosynthetic quanta. Ullah et al. [88] recently evaluated and published the effects of TiO2 NPs on wheat expansion under salinity stress. The researchers concluded that the conspicuous improvement in plant growth disregarding saline stress was facilitated by the NPs' capacity to capture light and consequently improve photosynthesis and amplify water uptake by the wheat plant. As reported by three studies focusing on how TiO2 NPs affected spinach plants, the NPs' photocatalytic qualities shielded the chloroplasts from oxidative damage and increased their life span. Recent research by Hezaveh et al. [156] on the impact of exogenously applied ZnO NPs on rapeseed grown under salinity stress indicated that the NPs minimized ion leakage and enhanced Hill reaction, which in turn affected the expression of stress response genes like ARP while downregulating the expression of SKRD2, MYC, and MPK4. Avestan et al. [157] revealed that the accumulation of osmolytes in strawberry plants treated with nano-silicon dioxide was lower than in plants treated with salt, despite maintaining the structure of the epicuticular wax and enhancing photosynthetic pigments. Cu application to plants has been proposed to curb the adversities of abnormally high salt concentrations on water relations, photosynthesis, and nutrient profile in maize plants by boosting the antioxidant defence and elevating osmo-protectants and amino acid levels [154]. In an imperative evaluation, chlorophyll content and photosystem II efficiency were found to be significantly higher in foliar-sprayed NP-treated Arabidopsis plants compared to control plants after exposure to salinity stress (100 mM NaCl). Higher rates of photosynthesis and carbon assimilation were also observed in foliar-sprayed plants [46]. The effects of cerium NPs and salt stress on Brassica napus were studied by Rossi et al. [30]. The implications were that plants treated with cerium NPs had increased photosynthetic efficiency and biomass compared to controls. Researchers delineated that diffusing MWCNTs into the soil in saline circumstances could improve photosynthetic turnover and water uptake of broccoli [158]. Low concentrations of MWCNTs have been found to have a salt-relieving effect on Ocimum basilicum plants by increasing the content and efficiency of photosynthetic pigments, as reported by Gohari et al. [159]. Salt-stressed lettuce varieties were used in an experiment with carbon NPs by Baz et al. [160]. Under high temperatures and salinity stress (150 mM), they discovered that pretreatment with carbon NPs enhanced seed germination. Seedling root development and photosynthetic pigment accumulation were also aided by the application of carbon NPs.

4 NPs mediated tolerances aspects towards salinity stress

4.1 NP-mediated regulation of ionic homeostasis and osmotic adjustment

4.1.1 Regulation of ionic homeostasis

A multitude of ecological concerns, such as soil salinity, lead to the soil suffering from an ionic imbalance, which impairs mineral uptake and results in acute or chronic mineral deficiencies, which further hinder plant maturation, root architecture, and physiological profile [20]. High levels of salt inhibit the chelation followed by the absorption of essential metals like Fe, Ca, K, Zn, B, and Mg. Salinity also causes an imbalance in the mineral content of the plant. Salinity reduces N, K, and Zn contents in leaves, whereas P, K, Ca, and Mg contents are increased in roots [161]. The revelations of Sheldon et al. [162] indicate that salinity alters the osmotic potential of the soil, making it increasingly trickier for plant roots to absorb mineral nutrients. High salinity reduced bioavailability and inhibited the utilization of macronutrients like P, K, and Mg as well as micronutrients like Fe, Zn, and Mn, promoting ion toxicity and resulting in a disproportionate nutrition supply in many plant organs. High salinity results in mineral shortages and unwarranted modifications in the interplay between the soil and plants' microbiomes [163]. It is known that NPs can alter the ionic ratio, which improves the osmotic potential of the plant and helps it grow better when it is stressed by salt. One report published that by increasing leaf K+ concentration, Nano SiO2 promoted the growth of soybean seedlings exposed to salt stress [164]. Yet another evaluation found that increasing the growth performance and Na+/K+ osmotic ratio of tomato plants through foliar application of Cu-NPs reduced the negative impact of salt stress [165]. Abdoli et al. [148] also investigated the role of Fe2O3 NPs on salinity stress, finding that they reduced the latter by increasing the plant's K+/Na+ ratio and Fe content. Ye et al. [152] tested Mn-NPs for their effect on salinity-stressed pepper plants. A report described that plants that were subjected to both salt and Mn-NPs illustrated substantially decreased root growth promotion and less fixation of Ca, Mn, Na, and K contents within the root and air-borne sections. Zn-NPs were administered to salt-stressed canola plants to trigger defensive counteractions against the harmful effects of the salt through ionic and osmotic management, according to Farouk and Al-Amri [122]. As stated by Liu et al., a prime approach for enhancing plant health via targeting the K+-to-cytoplasmic Na+ ratio in cotton plants is the administration of Ce NPs in the face of salt stress.

An essential adaptation for surviving in salty settings is the plant's capacity to control the entry of ions and their subsequent compartmentalization. The excess salt is either transported and filled into vacuoles or stored in the old plant tissue since plants are unable to cope with large amounts of salt in the cytosol. A series of pumps (Na+/H+ antiporter) assists Na+, which penetrates into roots from surrounding soil and travels through the cytoplasm and into the vacuole (Fig. 2) [166]. The two most prevalent forms of H+-pumps found on tonoplasts are H+-ATPase and V-ATPase. The dominant ATPase in a plant, H+-ATPase (also known as V-ATPase), is responsible for solute transport, balance, and growth [167]. Na+ sequestration in vacuoles is mediated by NHXs like NHX1 and NHX2, and the tonoplast Na+/H+ exchanger plays a role [168]. To regulate ionic equilibrium and lessen salt toxicity, the protein Salt Overly Sensitive (SOS) is vital. A paper by Gupta and Huang (2014) elucidated that the Na+/H+ antiporter (encoded by SOS1) controls cytosolic Na+ efflux and makes it more facile for detrimental ions to travel from the roots to the leaves [166]. SOS2 and SOS3 are involved in Ca2+ processes that govern Na+ efflux. The HKT family of proteins also serves as Na+/K+ symporters. Na+-selective transporters are used to prevent Na+ from entering plant cells, resulting in Na+/K+ equilibrium. Additionally, many studies have shown that preserving the stability of the K+/Na+ ratio is a vital factor in imparting a state of salinity tolerance. Therefore, a high concentration of K+ keeps Na+ out of the cell [169]. In response to elevated salinity, plants prohibit Na+ from entering their shoots. Na+ efflux out of the cell is passively regulated by specific weak voltage-dependent or voltage-independent non-selective cationic channels (NSCCs). The three categories of NSCCs correspond to those that are voltage-insensitive (VI-NSCCs), triggered by hyperpolarization (HA-NSCCs), and activated by depolarization (DA-NSCCs). Although VI-NSCCs enable the roots to absorb Na+, they also allow diffusion of K+, which helps the plant build up K+. Ca2+ can travel through NSCCs and has a potent blocking impact on the NSCCs that control Na+ current and influx in plant roots when it travels from the shoot. As an outcome, the phenomenon helps to deter the implications of salt stress. For effective photosynthesis, the chloroplasts of salt-tolerant plants have the proper ion balance (Na+, Cl−, and K+) [170]. As mentioned by Liu et al., one of the primary strategies for enhancing plant growth by raising the K+ to systolic Na+ ratio in cotton plants is the administration of cerium NPs during salinity stress. A possible approach for making plants more stress-tolerant is to increase the K+ level in the cell, as these NPs did in this experimental study [171]. Contrary to popular perception, it has been established that Ce NPs have more proficient long-term OH scavenging abilities than other materials, due to the enhanced transportation efficiency of K+-selective (GORK) channels embedded within the plasma membrane [172]. The studies provide evidence that, in addition to constrained NPs, they may minimise plant stress in salty conditions by modifying the uptake of other nutrients (through nutritional homeostasis). This is in addition to preventing Na+ absorption. However, while growing plants treated with NPs under salt stress, nutritional balance in plants is equally crucial to consider. It has been proposed that Zn NPs can enhance nutrient uptake; however, as Zn uptake increases, other nutrients, like P, become more challenging to transport and absorb, knocking the Zn-to-P ratio out of sync [173]. More research must be undertaken to gain insight into how NPs might increase plant nutrition while lowering salinity stress.

4.1.2 Regulation of osmotic homeostasis

Plant water content and absorptive load shrink under extreme alkalinity. High salt accumulation in the soil strata increases osmotic stress, which interferes with plant water uptake and alters leaf water content, stomatal conductance, leaf growth (accelerating leaf senescence and leaf death), and photosynthesis (decreasing chlorophyll content and impairing light harvesting machinery) [174]. As a result, plant growth is greatly compromised. Plants can maintain their absorptive capacity by increasing the concentration of solutes within their cells, a process known as the osmotic adjustment. Osmotic stress triggers two distinct mechanisms for osmotic adjustment. However, the solute can also serve as an osmolyte. On the other hand, solutes protect biological macromolecules by preventing their structure from becoming unstable. Hydrophobic regions of protein surfaces have poor binding affinity for water molecules, especially in hypertonic conditions, which may explain the mechanism. In a hypertonic medium, this fraction of water is lost from cells first. Proteins can improve their adhesion to water by forming hydrogen bonds, which increase their surface polarity and allow more osmotic regulators to dissolve in the binding water. There are two main types of osmotic regulators: organic compounds and inorganic ions. Different cells will show varying degrees of upregulation of these substances against persistent osmotic stress.

There exists documented verification that NPs can improve water status and water utilization proficiency in a plethora of plant species, which can minimise salt-driven osmotic stress [175]. Likewise, numerous studies have demonstrated that NP-treated plants sustain a higher rate of stomatal conductance and transpiration rate, as well as higher percentages of leaf water content, root hydraulic conductance, and whole-plant hydraulic conductance [76]. The fundamental intrinsic protein superfamily contains a type of channel protein identified as aquaporins, which are essential for plants’ ability to restore water relations. Aquaporins are primarily responsible for transporting water and miscellaneous small neutral molecules across plant cell membranes [57]. According to some other investigations show that NPs’ heightened production of the plasma-membrane intrinsic protein aquaporins influences root hydraulic conductance, which might facilitate water absorption while reducing oxidative stress and membrane damage [58, 76, 176]. Considering that they elevate water absorption and retention, NP treatments are widely adopted to boost seed germination [177]. In substrate containing CNTs, tomato seeds were sown, and after two days, their moisture content had risen by 19% in comparison to unsown seeds. The results therefore suggested that the NPs help to absorb and store water. Uncertainty encircles the associated mechanism; however, as explained by Sanborn et al. [178] the NPs may form microspores that enable water molecules to penetrate the seed coat while escaping miscellaneous membrane regulatory barriers are hypothesised to regulate the aquaporins found in otherwise impermeable seed coats. In addition, according to research by Martínez-Ballesta et al. [179] the presence of MWCNTs in broccoli enhances aquaporin transduction, strengthens water absorption capacity, and confers salinity tolerance. Under salt overload, one of silicon’s primary utilitarian functions is to improve aquaporin efficiency [180]. It will take more research to determine whether other mechanisms are at play in the relationship between NPs and aquaporin expression and activity. Root hydraulic conductance is known to be negatively impacted by oxidative stress and salt-induced stress through its effects on plasma membrane injury and aquaporin activity (transmembrane aquaporin channels are also impacted by negative mechanistic regulation) [181]. Prospective studies need to be performed to validate the idea that applying NPs can improve plant water balance when exposed to salinity stress by maximising the functioning of critical antioxidant biocatalysts.

4.2 NP-mediated antioxidant responses for ROS homeostasis

Unwarranted damage causing ROS and non-radical ROS are liberated during metabolism. Examples of non-radicals are H2O2 and singlet oxygen (1O2), while free radicals include alkoxyl radicals (RO⋅), OH⋅, and O2⋅. ROS are generated as byproducts of several aerobic metabolic processes.

Under typical circumstances, the antioxidant protective system in plants strikes a steady balance between ROS generation and scavenging. Moreover, ROS are signals that control a wide variety of fundamental cellular and organismal activities. They are useful as injury signalling molecules to bring about a plant’s reaction to stress. However, amid stress, cells excessively generate ROS, distorting the ratio of scavenging to the buildup of ROS, which ultimately causes oxidative damage. To manage ROS surplus as a response to a wide range of abiotic challenges, plants have developed a sophisticated network of antioxidant defences. The antioxidant defence system is comprised of both enzyme-based antioxidants (like SOD, CAT, POD, ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), and dehydroascorbate reductase) and non-enzyme-based antioxidants (like ascorbic acid, GSH, and glutathione S-transferase (GST).

4.2.1 NP-mediated enzymatic antioxidant responses for ROS homeostasis

The primary frontier of resistance against oxidative stress in plants is SOD. It releases H2O2 from O2⋅−). There are different types of SOD, such as CuZn-SODs, Mn-SODs, and Fe-SODs, which are in various cellular compartments. Peroxidase (POD) plays a role in oxidizing phenolic compounds, generating phenoxyl radicals. H2O2 acts as an electron acceptor in this reaction, eventually being converted to water (2H2O). POD is widely distributed in plant tissues and influences growth, development, metabolism, and adaptation to the environment. CAT rapidly breaks down H2O2 into water and oxygen. Higher CAT activity in plants corresponds to a smaller increase in H2O2 levels, indicating its ability to mitigate oxidative damage. CAT is found in peroxisomes and glyoxysomes and comprises three isoenzymes: CAT1, CAT2, and CAT3. APX drives the conversion of ascorbic acid (AsA) to MDHA using H2O2 as an oxidant. Plants possess different types of APX, including cytosolic APX (cAPX) and chloroplast APX (chlAPX), all involved in H2O2 detoxification and AsA regeneration. Other enzymes of the AsA-GSH cycle participate in the sequestering of ascorbic acid (AsA) and GSH. MDHAR regenerates AsA from MDHA, while dehydroascorbate reductase (DHAR) catalyses AsA regeneration from dehydroascorbate (DHA). Glutathione reductase (GR) reduces oxidized glutathione (GSSG) to reduced GSH. Glutathione peroxidase (GPX) and GST, along with GSH, collaborate to reduce H2O2 and safeguard cells against oxidative damage. Evidence from multiple research analyses shows that NPs can boost antioxidant enzyme levels. The investigators of the study assert that some NPs have attributes similar to particular antioxidant enzymes, permitting them to support plants in their defence against oxidative stress [148, 189]. Cs, Mn, Cu, and Fe NPs, on the contrary, operate more like peroxidase (POD) enzymes than cobalt, iron, and Cs NPs [190]. In Arabidopsis plants administered with CeO2 NPs, researchers discovered that promoting ROS-NSCC activity improved ROS scavenging [46]. Khan et al. (2020) evaluated the effects of seed priming with Ag-NPs (10, 20, and 30 mM) on pearl millet (Pennisetum glaucum L.) exposed to salinity stress (0, 120, and 150 mM NaCl) [191]. The perceived improvement in growth aspects can be credited to increased catalytic turnover capacity of antioxidant enzymes such as SOD, CAT, and GPX, as well as a decrease in the Na+/K+ ratio. A report from a study by Sami et al. (2020) reveals the most notable mechanistic improvement by Ag NPs in low doses for plant growth is improved antioxidant enzyme activity [192]. In a study that examined the impact of TiO2 NPs on Dracocephalum moldavica at 0, 50, 100, and 200 mg L−1 under salinity stress (0, 50, and 100 mM NaCl), TiO2 NPs increased antioxidant content and decreased H2O2, especially at a dosage of 100 mg L−1 [193]. MWCNTs were enriched with carboxylic acid by Gohari et al., who subsequently researched how they benefited basil seedlings (Ocimum basilicum L.) exposed to salt toxicity at concentrations of 0, 25, 50, and 100 mg L1. According to the study, 50 mg L−1 was determined to be the optimum dose for this plant, and using it during alkalinity stress optimized the production of photosynthetic phytopigments and antioxidant enzymes and compounds [159]. The empirical study on the effects of bulk, nano, and ionic Cu on the maize (Zea mays L.) plant [194]. One of the interesting findings was that Cu, regardless of the source, rendered the polyphenol oxidase (PPO) enzyme used in this experiment non-functional. Cu also had no effect on the CAT enzyme gene's activity. At low concentrations, Cu complemented the expression of the APX enzyme gene, while at high concentrations, it sharply decreased the activity of the gene. Cu also influenced the expression of metallothionein genes. Ortho-hydroxyphenols are converted to orthophenols by an enzyme that also contains copper, and orthophenols are oxidised to orthoquinones by the same enzyme. In addition, Ce NPs enhanced the antioxidant enzyme activity in cotton plants, which facilitated the internal removal of ROS. The NPs also accelerated plant development in highly salinized environments [195]. By increasing photosynthetic pigments and inducing antioxidant defence system enzymes, which include APX, CAT, and GP, as well as non-enzymatic components like phenolics, Gohari et al. [193] and Gohari et al. [159] elucidated that low concentrations of MWCNTs had a salt stress relieving effect on Ocimum basilicum plants. According to observations, Fe promotes the development of essential antioxidative enzymes, which in ultimately foster salt tolerance [196]. Findings on the impact of these NPs on plants' enzymatic antioxidant reactions to stressors like salt stress have been conducted. They are indispensable in preserving redox equilibrium and shielding plants from the oxidative harm imposed by ROS.

4.2.2 NP mediated nonenzymatic antioxidant responses for ROS homeostasis

Some of the most dangerous ROS (1O2 and ⋅OH) are resistant to enzyme-based antioxidant defences [197, 198]. Therefore, plants must rely solely on non-enzymatic methods of ROS elimination. Most cases of lipid peroxidation and MDA elevation can be traced back to ROS [20]. In response to stress, the body produces ROS, which can be neutralised by a combination of nonenzymatic compounds such as GSH, compatible solutes (CS), phenolics (phenols), alpha-tocopherol (vitamin E), carotenoids (CA), and flavonoids (FLA) [199]. Using GSH, a ubiquitous thiol tripeptide, GPX catalyses the degradation of H2O2 (Szalai et al., 2009). It has been identified to participate in the formation of a conjugate through a GPX- and GST-catalysed process in the elimination of H2O2 and/or the degradation of H2O2 and lipid peroxides [199, 200]. Ascorbic acid, often known as vitamin C, operates as an electron donor for APX and an essential co-factor of POXs in the AsA-GSH pathway to ensure its functional integrity [201, 202]. AsA regenerates tocopherols and xanthophylls, which help quench this excitation energy [203]. Cell signalling pathways and other biochemical mechanisms are at the forefront of efficiently attenuating oxidative damage afflicted by abiotic stress. Both enzymatic and nonenzymatic antioxidants are essential tools for stringent regulation of ROS within plant cells. Studies have demonstrated that the complementary effect of NPs and non-antioxidants makes it possible to dampen cellular oxidative damage under salt stress (Table 2) [76]. A scientific investigation confirmed that augmenting more non-enzymatic antioxidants such as phenol, flavonoid, and anthocyanin to the Dracocephalum moldavica L. plant through foliar application of Fe NPs (30, 60, and 90 mg L1) mitigated the detrimental effects of salt stress (50–100 mM) [131]. Under salinity stress, Ocimum basilicum L. plants treated with C NPs (25, 50, and 100 mg L−1) exhibited elevated levels of non-catalytic antioxidants like phenol [193]. Improved relative water content (RWC), K+ concentration, seedling vigour index, morphological traits of shoot and root, fresh and dry biomass of the seedling, seed dormancy, energy consumption during radical emergence, and germination rate were observed after seed priming with TiO2 NPs (40, 60, and 80 mg L−1) in the presence of salinity stress [204]. Carotene, GSH, yellow carotenoids, flavonoids, and phenols were all increased in the Capsicum annuum L. plant upon exposure to salinity stress at concentrations of 10 and 50 mg L−1 of Se NPs, 200 and 1000 mg L−1 of Si NPs, and 100 and 500 mg L−1 of Cu NPs [205]. On the grounds of extensive experimentation concluded that the non-enzymatic system is more favoured by the plant metabolism than the enzymatic system in mitigating the negative effects of stress on this plant [131]. The plant's enzymatic defence system promptly participates to help neutralise and scavenge the free radicals that caused the damage once the plant's non-enzymatic defence system has been overwhelmed. These findings demonstrate that various NPs have the propensity to attenuate oxidative damage in plants by modulating antioxidant defences to manage lethal levels of ROS and RNS (through both enzymatic constituents and non-enzymatic constituents). Despite these discoveries, much remains unclear about the interactions between NPs and the antioxidant enzyme system in plants, and more diligent and systematic omics studies must be undertaken to elucidate the intricate mechanisms underpinning the NPs capacity to regulate salt stress responses [206]. More study is needed to determine how NPs trigger these reactions. Additional field trials are required because many of these conclusions are derived from controlled or hydroponic studies [206].

4.3 NPs mediated regulation of phytohormone responses

4.3.1 Auxin

Auxins are important for normal and saline plant growth, physiological development, cell elongation, and vascular system stability and integrity (Fig. 3). The plant hormone indole-3-acetic acid (IAA) regulates germination and maintains ionic homeostasis. Auxin dictates gene expression by up- and down-regulating it, thereby controlling root development in response to salt stress. A total of 72 genes in the auxin signalling pathway had their expressions tweaked. In plants under salt stress, the expression of the genes PIN2, auxin influx carrier, GH3, small auxin-up RNA (SAUR), and auxin response factor was downregulated. Vegetative cell elongation is facilitated by the transcriptional regulation of genes including gp1 and indole-3-acetic acid-amido synthetase, and IAA can be critical for stress tolerance since it produces a variety of DEGs. To combat the negative impacts of salinity, a set of genes known as auxin-responsive genes express themselves more vigorously. These specific genes are members of the auxin/indoleacetic acid (Aux/IAA), SAUR, or GH3 families [207]. The main auxin, IAA (indole-3-acetic acid), is biosynthesized from tryptophan via two chemical reactions involving the YUC (YUCCA) family of flavin monooxygenases and the TAA1 (Tryptophan Aminotransferase of Arabidopsis-1) family. However, auxin transport is controlled by the PINFORMED (PIN) family of efflux carriers and the AUX1 (AUXIN TRANSPORTER CARRIER1/AUXIN TRANS-PORTER-LIKE PROTEINS) influx carriers. New evidence illustrates that plants encountering NPs exhibit regulatory responses in the transport and production of auxin in plants, which in turn affect plant development. Numerous prior investigations indicate that auxin content spikes when exposed to metal-based NPs [42]. Following a study, foliar applications of Se-NPs (10 and 20 mg L−1) on strawberry plants can improve the accumulation of IAA that aids in growth and yield parameters of plants grown on non-saline and diversely salinized soils (0, 25, 50, and 75 mM NaCl). This was attributed to their propensity to preserve photosynthetic pigments, as well as increased key osmolytes, including total soluble carbohydrates and free proline, in comparison to untreated blank samples [32]. A separate assessment points out that adding 1 mg L−1 of Ag NPs to wheat under salt stress increased the accumulation of indole-3-butyric acid (IBA), 1-naphthalene acetic acid (NAA), and 6-benzylaminopuri, resulting in improved GP, growth profile, phytopigment contents, and chlorophyll stability index (CSI) [168].

Fig. 3
figure 3

NPs based regulation of membrane transporters, signalling pathways, synthesis of osmolytes like proline, glycine betaine and antioxidant enzymatic based ionic and osmotic homeostasis

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4.3.2 ABA

ABA caused stomata to close, delayed ageing, regulated metabolism, and catabolism, and disrupted transpirational exchange of gases. ABA is an endogenous signalling molecule that maintains equilibrium between a plant's leaves and soil. At the molecular level, the genes responsible for stomat218a closure and Osmo protectants synthesis are upregulated by ABA. ABA improved several proteins, including dehydrin, which mitigates the disadvantageous effects of salt stress on plants. Plants optimise their genes and the transcription factor ABI5 (ABSCISIC ACID INSENSITIVE 1) in response to salt acclimation. In saline environments, ABA also prevents the development of lateral roots (SALT AND DROUGHT-INDUCED RING FINGER 1). ABA phytohormone plays a critical role in modulating a plethora of physiological and biochemical processes through differential expression of the ABA receptor gene (Fig. 3), SnRK2 (serine/threonine-protein kinase), ABA-responsive element-binding genes, and other related proteins.

To foster a reliable salt-tolerant state, genes involved in ABA production, such as 9-cis-epoxycarotenoid dioxygenase (NCED), zeaxanthin epoxide (ZEP), and ABA 8′-hydroxylase (ABA 8′-OH), were upregulated. To counteract salt toxicity-induced anomalies in plants, DEGs such as Snf1-related kinase 2 (SnRK2), also known as SAPKs, are expressed; these proteins phosphorylate TRAB1 to enhance the transcriptional activation of the ABA response. LeNCED1, which encodes the key enzyme in ABA biosynthesis (9-cis-epoxycarotenoid dioxygenase) and Yang et al. [208] was found to be abundantly expressed in tomatoes grown in a saline regime. Some studies have found that the NPs ability to alter the ABA balance is responsible for the enhanced stress tolerance of plants [126, 127, 168, 209, 210]. The findings of Abou-Zeid and Ismail [168] suggested that the large-scale application of Ag NPs (1 mg L−1) as priming agent can enhance the ABA content and improve seed germination parameters of wheat plants under salinity stress. El-Badri et al. [211] have recently elucidated the advantageous effect of 150 µmol L−1 Se NP, and 100 mg L−1 ZnO NPs on ABA activity that improved the seed germination, promoted the root fresh weight, leaf soluble sugar content, and leaf and root total protein contents in Brassica napus under salt stress. Additionally, ABA was administered to Arabidopsis using mesoporous SiO2 NPs to bolster resilience against salt toxicity [207].

4.3.3 GAs

The plant hormone known as GAs aids in reducing a variety of environmental stresses (Fig. 3). In plants exposed to salinity, the amount of GA increases endogenously. To counter salinity, it modifies several physiological and metabolic processes and underlying molecular cues in the plant. The plant's GA also boosts the production of osmolytes, sugar signalling, and antioxidant machinery, which all aid in quenching ROS and preserving cell turgor under salinity. The passage of products of photosynthesis from source to sink and the preservation of the plant's fluid relationships are all rendered plausible by GAs, which reduce the detrimental effects of NaCl contamination and raise the plant's tolerance. Scientists and investigators affirm that small quantities of GA mitigate the consequences of salt stress by decomposing DELLA and complementing SA synthesis. Particularly, GA4, the active form of GA, helps to lessen the harmful effects of salt.

The GID1-GA-DELLA complex is created when GA binds to gibberellin insensitive dwarf 1 (GID1), sequentially leading to the degradation of DELLA. Genes that code for the DELLA protein and favour the production of ABA positively control plant responses against ever-fluctuating salt concentrations in crop plants [215, 216]. Rigorous experimentation suggested that application of NPs can enhance the activities of gas under salinity stress (Table 3) [42, 128, 217,218,219].

Table 3 Application of NPs and their mode of action to regulating the phytohormone response under salinity stress
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4.3.4 JA

JA provides sufficient protection for plants against the negative effects of microbial and ecological stress. JA plays a pivotal role in various developmental processes, including germination, flowering, callus development, and both normal and stressed bulb and tuber growth (Fig. 3). Given that JA upregulates the expression of a multitude of genes encoding for biocatalysts involved in metabolic pathways, including invertase, Rubisco, and arginine decarboxylase, the harmful effects of salinity are alleviated. JA reduced the toxicity of ions by regulating protein synthesis and CO2 fixing. Gene expression associated with DELLA formation is upregulated by JA, while GA-GID-binding genes are downregulated by JA [220, 221]. To counteract the toxicity of salt, plants upregulate MYC2 and TGA genes while downregulating JAZ protein-making genes. Fatty acid metabolism involves both JA and methyl jasmonate (MeJA). Sb05g002750 and Sb01g048200 genes, which manufacture phospholipase A1 (PLA1) and acyl-coenzyme A oxidase (ACXs) proteins, respectively, are a pair of genes that are transcribed in Sorghum under salt overload (Fig. 3) [220, 222, 223]. The genes are involved in fatty acid and JA precursor metabolism, signal transduction, and biosynthesis. The TIFY gene family is a transcription factor (TF) family that aids plants in getting accustomed to salt exposure by having an amino acid domain that is conserved [215, 224]. Application of 10 and 100 mg/L TiO2 NPs increases root biomass and the number of lateral roots of wheat [225]. Chitosan NPs upregulate many antioxidant enzymes, such as SOD, CAT, and peroxidase, that act as the ROS scavenging system, via enhanced activity of JA under salinity stress [226]. The finding of Hernández-Hernández et al. [227] suggested that the application the Chitosan-PVA and Cu NPs activates the antioxidant defence mechanisms of a plant and the octadecanoid pathway by enhancing the activity of jasmonates. Similar to another research finding by Hernández-Hernández et al. [228], chitosan–PVA and Cu NPs can enhance JA activity that upregulate the growth and antioxidant enzymatic activity of tomatoes under saline stress. Si NPs may additionally encourage or activate JA under anomalous alkaline conditions, therefore bolstering rice’s antioxidant defensive systems and causing the generation of osmolytes [124].

5 Nano-phytotoxicity

Increasing concerns regarding bioavailability, toxicity of NPs, and impropriety of the regulatory framework restrict the complete acceptance and inclination of the agricultural sector to implement nanotechnologies [229]. It should be noted that the use of NPs is like a double-edged sword, if not used properly, their application can have negative effects on plant growth and reduce the formation of proteins and pigments in the plants [230]. In the soil, NPs undergo a series of bio/geo-transformations, which determine the bioavailability and toxicity of NPs [229]. Previous studies have reviewed the factors that affect NPs phytotoxicity (Table 4). According to these studies, the type of NPs and test plants, the concentration of NPs used, the size distribution of NPs, the morphology of NPs, chemical composition, chemical structure, and the surface charge of NPs are some factors influence on NPs phytotoxicity. In addition, these studies reported that NPs-mediated effect on plants growth and development is concentration dependent and using high concentrations of these NPs can lead to a series of issues to the plants, animals, and finally to humans. The presence of NPs on the root surface can change the surface chemistry of the roots and consequently affect the uptake of nutrients into the plant root [231, 232]. In addition, NPs release toxic substances to exposed plant, such as metal ions into plants, attributing to the phytotoxicity of NPs [233]. The toxic effect of heavy metals and the adsorption of pollutants by carbon NPs and plastic NPs might also be the reasons for the negative impact on plant growth [223, 234, 235]. The exposure of NPs to plants produces cytotoxicity, increase the content of ROS, and result in oxidative stress, leading to the increase of antioxidant enzyme activity and antioxidant content [236]. Meanwhile, ROS also acts on cell membranes and mitochondria, causing damage to cell membranes and mitochondria [237]. Oxidative stress may indirectly lead to genotoxicity, such as chromosome aberration and micronucleus formation that change the expression of genes and the level of biological components in plants [238]. Although the impacts of NPs on terrestrial plants have gradually attracted the attention of researchers, the understanding about the mechanism of phytotoxicity induced by NPs is still limited. Most of the studies are carried out in the early development stage of plants rather than the whole life cycle. Therefore, it is of great significance to conduct long-term exposure experiments of NPs, deeply explore their mechanism of phytotoxicity and investigate the impact of NPs on the environment and human health [239].

Table 4 Phytotoxicity  effect of NPs on different crop plants
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6 Conclusion and future prospects

Salinity stress remains a pressing concern in contemporary agriculture, posing a significant threat to global food security. As discussed in this review, NPs have emerged as promising candidates for mitigating the adverse impacts of salinity stress on crop productivity and quality. The diverse array of NPs, including metallic, metal oxide, and carbon-based NPs has demonstrated the potential to enhance salt tolerance in various crop species. Because of their small size, NPs may readily penetrate plant tissues and positively impact the morphology, physiology, and biochemistry of plants. This review article shows that NPs act through multiple mechanisms, such as the regulation of ion homeostasis, osmotic adjustments, and reinforcement of antioxidant defence systems, which collectively contribute to encourage plant development and boosts agricultural productivity, particularly under salinity stress situations. It has also been shown that the deliberate use of NPs protects the plant's photosynthetic system from damage caused by salinity stress and improves membrane integrity and nutrient absorption. It appears that using NPs properly (i.e., at the optimum concentrations) may promote plant health and be important for low-input sustainable agriculture of both food and non-food crops. However, the successful incorporation of NPs in agriculture always comes with its challenges and concerns. For instance, unknown effects of NPs on seed germination emphasise the necessity to learn more about their involvement in germination mechanisms, which include water intake, radical protrusion, and the activation of food-mobilizing enzymes. Furthermore, since abscisic acid and gibberellins are necessary for seed germination, it is imperative to look at how NPs impact their metabolic processes. NPs also boost the production of genes that respond to stress and antioxidants, which fortifies the defence system against salt stress. At the genomic, metabolomic, and proteomic levels, further research is still needed to determine the precise mechanism(s) by which they alter plant physiology by enhancing host defence systems. The impact of different NPs on plant biochemistry and microbial gene expression must be investigated further. Additionally, applied research is required to determine the ideal concentration of NPs, as well as the best times and techniques for applying NPs to agricultural plants, particularly when the plants are stressed by salt. The research above shown that NPs were administered under different salinity stress conditions, but in nature, plants are subjected to many stressors at once. Therefore, it is recommended to examine NPs' impact on enhancing resistance to salt and drought stressors concurrently because most environmental stresses, particularly salinity and drought, are present simultaneously in agricultural areas. Most of the NPs-related subcellular research has focused on the cell wall. To further understand the biological functions of NPs in improving plant tolerance to salt stress, it would be fascinating to investigate how NPs are distributed in organelles and the nucleus of cells. However, combined application of NPs and salt-tolerant microbes is required to mitigate salinity stressors, but their respective and combined impacts on plants must also be compared. Because the fundamental processes by which NPs controlled the relationships between plant-salinity-tolerant microbes and plants have not been discovered. Moreover, Environmental considerations, including NP toxicity, mobility, and long-term effects on soil ecosystems, necessitate a cautious and responsible approach to their application. Continued research and innovation in this field are crucial to refine NP applications, address environmental concerns, and optimize their benefits for the agricultural sector. Sustainable agricultural practices and strict regulatory guidelines are essential to ensure the safe and effective use of NPs in the field. As we move forward, it is imperative that the scientific community, policymakers, and stakeholders collaborate to strike a balance between harnessing the potential of NPs and safeguarding the environment. By doing so, we can cultivate a more sustainable and resilient agricultural system capable of meeting the challenges of a changing world and ensuring the availability of nutritious food for all.