Soil salinization of land poses a serious threat and harms the environment, agriculture, and the economy. Salinity stress in plants may cause changes at the molecular as well as the physiological level [1]. Some plants contain salt tolerance genes while many have a salt-sensitive genetic makeup. Various complex mechanisms may alter the genetic responses in plants under abiotic conditions. Modifications in the expression of salt-responsive genes make the plants more resistant to salinity stress. Ecophysiological traits of plants and their importance for biomass production in response to variable climate change are critical for sustainable agricultural productivity [2,3,4]. Plants can change their ecophysiological mechanism in five known constraints including growth, water dynamics, mineral nutrition, photosynthesis rate, and oxidative stability [5,6].

The adaptation of a plant to a stressful environment is a complex and sensitive phenomenon [7,8]. This acclimation is governed by multiple genes and regulatory pathways [9]. Once the plant detects a stress, it first senses and then transduces a stress signal. Plants utilize various components for signal transduction including transcription factors, ion transporters, kinases, calcium, and hormones [10]. During abiotic stress, many physical modifications occur such as alteration in protein and other metabolites along with changes in the cellular matrix and segregation of nucleic acid strands [11]. All these alterations may result in altered regulation of abiotic stress-responsive genes. It was observed by Tang [12] that superoxide dismutase is responsible for oxidative stress tolerance. Enhanced salt resistance in plants is due in part to the overexpression of chloroplast protein-increasing stress tolerance (CEST) [13]. The assimilation of methylglyoxal in a saline stressed potato plant was inhibited by glyoxalase activity [14]. Hasanuzzaman et al. [15] reported that selenium protects plants from damaging free radicals, improves the antioxidant defense system, and methylglyoxal detoxification. It was observed that the use of selenium nanoparticles with bitter melon induced alterations in the methylation of cytosine in DNA resulting in epigenetic modifications. The up-regulation of the WRKY1 transcription factor was induced by a high dose of selenium nanoparticles. The transcription of phenylalanine ammonia-lyase (PAL) and 4-CoA-ligase (4CL) genes have also been affected by selenium nanoparticles [10].

The application of nanoparticles to plants helps to mitigate salinity stress. Nanoparticles can be used to alter plant genetic makeup to become resistant to salt stress. Nanoparticles are identified as particles that have a size of less than 100 nm in diameter [16]. They are found naturally in various resources such as minerals or as a product of bacteria and clays. Nanoparticles have been used historically for coloring metals and other purposes, with new applications over the past several years [17]. Nanoparticles that are engineered have some significant specific properties. These nanoparticles have different sizes and shapes and their composition also varies, and they differ widely from naturally occurring nanoparticles [18]. Metal and metal oxide nanoparticles reveal various physiochemical properties such as high density and possess microscopic edges on their surface. The sizes of nanoparticles vary due to differences in composition, such as Cu⁺²O, Zn⁺²O⁻², Sn⁺⁴O⁻²₂, Al⁺³₂O⁻²₃, Mg⁺²O⁻², Ti⁺⁴O⁻²₂, and Ce⁺⁴O⁻²₂. Due to the changes in nanoparticles size, many properties including magnetic, electronic, and chemical properties are altered. Magnetic nanoparticles have achieved significant importance due to their variations in size and shape [19]. Surface, optical, thermal, and electrical properties can also be incorporated into these nanoparticles. The process of metal/metal oxide nanoparticle synthesis includes the reduction as well as oxidation of respective metal salts [20]. There are many different factors that contribute to nanoparticle reactivity with desired biomaterials. These factors are the size, dimension, and stability of the nanoparticles [21]. In the past few decades, synthesized nanoparticles have been used for various industrial and household purposes. There is continuing effort to synthesize new nanomaterials to enhance quality products. However, the environment can be contaminated due to the excess use of nanoparticles due to improper disposal of industrial wastes and other by-products [22].

Nanoparticles can be adapted for environmental conditions and their aggregation and oxidation state can be engineered [23]. The stability and behavior of nanoparticles can be affected by chemicals in the environment and by physical parameters. The properties of nanoparticles depend on their composition. The composition of nanoparticles also affects their rate of reaction, penetration ability, and translocation inside the plant. Hence, the same nanoparticles may show different responses in plants under different conditions. For instance, it was observed by Barrios et al. [24] that plant responses were influenced by citric acid-coated nanoparticles compared to bare nanoparticles. Plants constantly interact with the surrounding medium, such as water, air, and soil. The engineered nanoparticles can cause different effects caused by quantum dots, carbon-based and metal-based effects on plant growth variations, physiological and biochemical traits, food production, and quality of food. Thorough interaction studies between engineered nanoparticles and plants are needed to analyze the toxicity levels and the remediation scheme to build a sustainable environment for agriculture [25]. Plants play a significant part in the ecosystem and in the food chain. However, the effects of nanoparticles on plants are not well known. The study of nanoparticles is difficult due to a lack of detection methods in plants [26]. The most suitable technique for the identification of nanoparticles in plants is inductively coupled plasma mass spectroscopy (ICP-MS). Due to the size, shape, composition, and stability of nanoparticles, the plant may show positive or negative impacts due to nanoparticle application. Several reported studies showed that some nanoparticles have a negative impact on plants such as declines in plant growth, production rate, and pigments [27]. Conversely, some nanoparticles may be beneficial for plants. In order to maintain their stability in agricultural crop production, synthetic nanoparticles are mostly used. These nanoparticles are used as biofertilizers, growth stimulators, soil-improving agents, and are also used as sensors [28].

Engineered nanoparticles can interact chemically and mechanically with plants. These interactions are based on their properties such as size, surface area, and catalytic interactions. Few studies have been reported regarding the effect of nanoparticles at the molecular level [29,30,31]. Various plant species are highly affected by ZnO nanoparticles. Nanoparticles penetrate the plant leaf and accumulate in the edible parts while some assimilate into the soil in the surrounding area of the plant. Some metal and metal oxide nanoparticles are toxic to the environment, such as Ag⁺¹, Fe⁺³, Zn⁺², Al^+3, and Ti⁺⁴ [32]. It was observed that when Brassica juncea was treated with silver nanoparticles it resulted in increased levels of antioxidant enzymes, for instance, guaiacol peroxidase, catalase, and ascorbate peroxidase, which resulted in decreased levels of reactive oxygen species (ROS) activity [33]. The activity of enzymes such as super oxide dismutase, catalase, guaiacol peroxidase, ascorbate peroxidase, and glutathione reductase increased after the treatment of Brassica juncea with gold nanoparticles [34]. It was found that H₂O₂ and proline content increases in gold nanoparticle-treated plants. The activity of ascorbate peroxidase, glutathione reductase, and guaiacol peroxidase is stimulated in the presence of up to 400 ppm of gold nanoparticles, while on the other hand, the activity of guaiacol peroxidase increases with 200 ppm gold nanoparticles. Plant molecular responses to silver nanoparticle treatment were analyzed in Aradidopsis by reverse transcription-polymerase chain reaction [35]. A whole-genome cDNA expression microarray was also used for the transcriptional response analysis of Arabidopsis plants subjected to silver nanoparticles. This resulted in the identification of 286 upregulated genes, including those involved with metal and oxidative stress responses such as the vacuolar proton exchanger, SOD, cytochrome P450-dependent oxidase, and peroxidase. It also identified about 81 downregulated genes along with genes that help in the plant defense system. These included auxin-regulated genes, ethylene signaling pathway, and SAR against pathogens.

Iron (Fe) is considered to be essential for plant growth and development as it plays a significant role in enzymatic reactions, helps in photosynthesis, and aids to improve the performance of photosystems. In plants, Fe is present in the insoluble form, i.e., Fe³⁺. The increase in pH and aerobic conditions leads to a decreased concentration of Fe in the soil. The use of iron nanoparticles helps to improve plant resistance to different environmental abiotic stresses. The application of iron nanoparticles reacts at the molecular level of plants, which helps to enhance the nutrient uptake ability [38]. Toxicity in plants may be caused by an excess concentration of iron nanoparticles. A higher amount of free Fe ions such as Fe²⁺ and Fe³⁺ leads to the production of ROS in plants. It was reported by Rodríguez et al. [39] that in some plants, down-regulation of detoxifying proteins such as CAT2 (CATALASE 2; AT4G35090) protein and AP2 (PEROXIDASE 2; AT5G06720) protein has been observed. A deficiency of Fe in the roots of M. truncatula, P. dulcis, and P. persica was correlated with superoxide dismutase expression, i.e., ATMSD1 (ARABIDOPSIS-SIS-MANGANESE SUPEROXIDE DISMUTASE 1; AT3G10920) [39]. Fe deficiency may cause the production of non-enzymatic ROS. Under Fe deficiency in A. thaliana, two enzymes have been reported to be expressed: GST1 (ARABIDOPSIS GLUTATHIONE S-TRANSFERASE 1; AT1G02930) and MDAR1 (MONODEHYDROASCORBATE REDUCTASE 1; AT3G52880) [40]. The ROS-eliminating enzyme aids in the stimulation of the ascorbate-glutathione cycle from GPX3 (GLUTATHIONE PEROXIDASE 3; AT2G43350) [40]. Due to the magnetic properties of superparamagnetic iron oxides, Fe₂O₃ (maghemite) and Fe₃O₄ (magnetite) nanoparticles are widely used in various applications including the mitigation of salinity effects of plants. High Fe₃O₄ nanoparticle concentration has a high impact on seed germination and root elongation of cucumber [41]. In cucurbits, the Fe₃O₄ nanoparticle aggregation occurred in the stem and roots [42]. The toxicity of superparamagnetic iron oxide nanoparticles has been tested in Lemna gibba [43]. It has been observed that plant chlorophyll content decreased while the photosynthetic activity and growth were also highly affected. The size and stability of nanoparticles are responsible for their toxicity level. The effect of Fe₃O₄ nanoparticles has been investigated in Cucumis sativus, and it was observed that seed germination and root elongation were highly affected [41]. It has been shown that Fe₃O₄ nanoparticles are translocated towards the foliage, stem, and below-ground root. Aggregation of Fe₃O₄ nanoparticles in plants may decrease the root hydraulic movement and water transport. The growth parameters of S. lycopersicum were studied by the application of Fe₂O₃ nanoparticles. It has been observed that these nanoparticles were clogged in root hairs, root tips, and the nodal portion of plants. Increases in Fe₂O₃ nanoparticle concentration improved iron content in plants [44]. In Arachis hypogaea, root length and plant height increased due to the use of Fe₂O₃ nanoparticles in saline conditions [45].

While a number of genes with the potential for the engineering of salt tolerance have been identified and tested, additional genes and regulatory pathways need to be identified. Work in many labs is ongoing to develop genomic, transcriptomic, proteomic, and metabolomic resources.

The by-products of cellular regulatory mechanisms are metabolites. These metabolites are secreted in response to the external stimuli faced by the organism. More than 200,000 metabolites are secreted by plants and these metabolites are divided into two classes; these are primary and secondary metabolites for plant growth and development [46]. Primary metabolites are essential and include carbohydrates, fatty acids, vitamins, amino acids, and organic acids [47]. Polyketides, alkaloids, terpenoids, glucosinolates, and phenylpropanoids are secondary metabolites synthesized from primary metabolites and are required by plants for adaptation and defense responses [48]. Throughout the plant kingdom, primary metabolites are common in all plants and conserved in their structure, while on the other hand, plant secondary compounds may vary in their chemical composition and are species-specific. Figure 1 shows the metabolomics analysis of plants exposed to engineered nanomaterials. In xenobiotic plants, the modifications in plant physiology induced by engineered nanoparticles are monitored by molecular events. These molecular events also reflect the metabolites that participate in biological pathways, for instance, the citric acid cycle, glycolysis, gluconeogenesis, and amino acid and secondary metabolite biosynthesis, nitrogen, and fatty acid metabolism. In order to defend against or adapt to various abiotic stresses, plant roots excrete metabolites as signaling molecules. Plants also alter soil chemistry and biochemical pathways to enhance nutrient bioavailability [49].