Research into the human genome using omics techniques found that zinc binds to around 10% of all human proteins [15, 38, 39]. This remarkable discovery has dramatically uncovered the importance of Zn in molecules that are involved in cellular processes. Several enzymes, transcription factors, and other proteins rely on Zn for proper functioning [40]. Zn-finger domains, LIM domains, and zinc finger domains are examples of regions in these proteins that might facilitate zinc interaction. The skeletal muscles and bones are the primary Zn reservoirs in the body (Fig. 1), but unable to hold more minerals than the organism requires. For the proper functioning of cells, it is necessary to consume Zn daily [41]. Even though zinc is generally safe and has low toxicity, its lack or overabundance can cause serious health problems [15]. Too much zinc could be toxic, resulting in symptoms like nausea, vomiting, fever, and headaches, while too little zinc can cause eye and skin lesions, hair loss, immune dysfunction, taste abnormalities, and growth retardation [42]. Supplementing with zinc alleviates zinc deficiency symptoms, demonstrating the importance of zinc as a trace mineral and the significance of zinc homeostasis as a physiological process [43]. Zinc is necessary for healthy development and differentiation, immune system function, neurological processes, and protein synthesis. There are reports of health benefits from supplementing with zinc and zinc complexes containing other compounds [44].

Fig. 1.

Human body zinc reserves and distribution. Zinc absorption from food occurs in the small intestine and then travels throughout the body. The skeletal system, specifically the bones, and muscles, serves as a major Zn reservoir tissue.

More and more physiological roles for Zn in cellular responses have been uncovered recently. Zinc serves multiple purposes within the cell, including as a neuromodulator during synaptic transmissions and as an intracellular signal transducer. Zinc homeostasis is partly maintained by various zinc transporters, which are also essential for normal cellular processes [45, 46, 47].

An estimated 9% of the eukaryotic and 6% of the prokaryotic proteomes are composed of zinc-binding proteins [48]. Enzymes such as carbonate dehydratase, aldehyde dehydrogenase, Zn/Cu superoxide dismutase (SOD), and Zn-finger DNA-binding proteins contain zinc in plant species [49, 50]. Zn-Cu-SOD protects membrane proteins and lipids from oxidation by quenching the radicals produced by the regular metabolism of the body. This vitamin is crucial for seed development because it increases cytochrome production [51, 52]. Multiple physiological processes in plants require zinc, including the production of tryptophan, a precursor of indole acetic acid, as well as the transmission of inputs via mitogen-activated protein kinases [53]. The control of auxin, the rejuvenation of photosystem II, the stabilization of CO${}_2$ quantity in the mesophyll, and other physiological responses are plant-specific functions that depend on zinc availability [54, 55]. Zinc ions mitigate several detrimental effects on plant cells by interacting with functional groups or displacing other cations from binding sites [33, 56, 57]. For plants to thrive, Zn must be absorbed, transported, and distributed throughout the plant tissues, cells, and intracellular locations [33]. Despite its significance, zinc deficiency causes stunted growth and leaf discoloration, which can reduce nutrient absorption and eventually lead to zinc deficiency in the human diet [33, 48].

Top-down and bottom-up approaches are two of the most common classical methods for producing NPs with desirable shapes and sizes (Fig. 2A) [99]. Despite using distinct synthesis principles, these approaches yield NPs with the desired properties [99, 100]. Top-down methods involve breaking up large materials into smaller pieces, reducing them to eventually form NPs [99, 100]. These NPs were created via various methods, such as photolithographic processes, grinding, sputtering, and milling [101]. There are benefits and drawbacks to using each method to maximize the output efficiency of NPs. The top-down approach is feasible and can potentially produce a sizable number of NPs. The disadvantages of top-down methods include the imperfection on the surfaces of NPs and the risk of damage [102]. As the optical, physical, and chemical functions of NPs are dependent upon the surface architecture of NPs, top-down synthesis of NPs is only applicable in a few cases [103]. Another prominent technique for NPs production at the moment is the bottom-up method, which entails the blending or assembly of individual atoms, molecules, or clusters to create a wide variety of NPs [104]. NPs can be produced through self-assembling monomer and polymer molecules, nano structural precipitation, laser pyrolysis, flame spraying synthesis, sol-gel processing, and bio-assisted synthesis techniques [105]. Therefore, bottom-up is a versatile technique for creating nanoclusters that can be applied to various settings.

NPs are synthesized in a variety of physical methods including plasma arcing, pyrolysis with sprays, ball milling, electron irradiation, sputter accumulation, lithography methods, layer-by-layer growth, thermal evaporation, pulsed laser desorption, or the diffusion flame method (Fig. 2B) [106]. These methods employ a tube furnace and atmospheric pressure to create metal NPs via evaporation and condensation. Nanomaterials can be manufactured using various physical techniques, such as deposition, sputtering, ball milling, and plasma-based processes [105]. The production rate of metal NPs using most of these methods is extremely low. For example, 50% of nanomaterials are produced by ball milling [107]. The particle size distribution is large because 6–8% of the sputtered material is smaller than 100 nanometers. Furthermore, high amounts of power are required for laser ablation and plasma techniques. Most physical processes are too costly to be practical owing to the low throughput, uneven size distribution, and excessive energy requirements [107].

ZnO-NPs can be physically synthesized through high-energy ball milling, melt mixing, physical vapor deposition, laser ablation, sputter deposition, electric arc deposition, and ion implantation [108]. ZnO-NPs are produced at excessively high rates during most physical/mechanical processes and are typically used in manufacturing. Since its inception in 1961, the high-energy ball milling method has become the most popular and widely used process for the synthesis of various type of NPs [108]. Ball mills are used to grind materials into extremely fine powder by subjecting them to high-velocity collisions between the mill’s steel balls [109]. Burmeister and colleagues reported high-energy ball milling processes as efficient, cost-effective, and simple techniques for preparing ZnO nanostructures. ZnO nanopowder particles were identified to have a crystallite size of 15 nm, a particle size of approximately 60 nm, and a lattice strain of 0.67% using X-Ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) [110]. Additionally, Salah et al. [109] employed a similar approach with ZnO for antimicrobial activity. Particles can be removed from the surface of a solid or a liquid using a laser ablation technique. Ismail and coworkers found that the average diameter of ZnO spheres is 35 nm. In double-distilled water, they used pulsed laser ablation [111]. The energy absorbed by the laser causes the material to heat up and evaporate at low reflux, but may turn into plasma at high reflux. Vapor-solid-liquid (VLS), physical-vapor-deposition (PVD), and chemical-vapor-deposition (CVD) are other common and well-studied techniques [112]. Metals are deposited onto the surfaces using physical vapor deposition (PVD) techniques. PVD utilizes two different types of processing methods: evaporation and sputtering. The term “sputtering” describes the process by which particles are ejected from a surface after being struck by higher-energy particles. The sputtering process utilizes ions provided by plasma [113].

The chemical synthesis of NPs is performed using a bottom-up strategy (Fig. 2B) [114]. One such process is nucleation which reduces metal cations to metal monomers using water-soluble cations as a precursor [115]. Where a cluster of reduced metal atoms has assembled, the growth of particles is automatically halted by the reducing agent [116]. Consequently, the particles grew to a thermally stable size [117]. Atoms and smaller molecules interact to create nanomaterials. Sol-gel synthesis, chemical vapor deposition (CVD), pyrolysis, microemulsion, hydrothermal, polyol synthesis, and plasma-enhanced chemical vapor deposition are examples of different chemical synthesis methods [115]. Metal NPs (MNPs) are synthesized in a chemical solution during chemical preparation; different chemical reactions and chemical compositions are used for this purpose. For instance, the most widely used method for the preparation of MNPs is the chemical reduction of metal ions inside reversed micelles in a nonpolar solvent [118]. The reversed micelles provide a safe environment for the chemical reduction of a metal slat that has been dissolved in water, and the ratio of water to the volume of reversed micelles influences the particle size.

Zinc salts are typically precipitated and coprecipitated upon reaction with a reducing agent (typically inorganic alkalis). To obtain the desired morphology and characteristics of the NPs, the resulting soluble or insoluble precipitate is washed and calcined at various temperatures. By adjusting the reaction conditions, the shape and size of the ZnO can be controlled [119]. Colloidal techniques rely on basic chemistry. In the literature, however, Faraday’s synthesis of gold nanoparticle colloids via the reduction of HAuC${}_{\text{l4}}$ with phosphorus was first reported in 1857 [108]. Colloidal chemistry is essential to sol-gel processes. Colloidal solutions, or “sols”, are solid particles with a diameter of a few hundred nanometers that are suspended in a liquid phase. Polycondensation and polyesterification are the most common methods for creating gels, and they are typically followed by aging to induce phase transformations and Ostwald ripening [108]. To obtain metal oxide NPs, the gels must be dehydrated at temperatures as high as 8000 °C and then densified at temperatures higher than 8000 °C. Sol-gel techniques have the potential benefit of guaranteeing structurally uniform and highly pure ZnO [120]. Khan and colleagues reported the formation of thorn-like ZnO via the sol-gel method. Moreover, mechanical stirring added a slight twist to the process [120].

The spray pyrolysis method, in which the zinc salt precursor is heated in a flame to produce aerosol droplets, is widely used. Dehydration causes the droplets to become smaller and disperse throughout the gas. The final steps are the decomposition and sintering of the necessary material [121]. Another technique is inert gas condensation, which can be divided into physical vapor deposition (without catalytic interaction) and chemical vapor deposition (with catalytic interaction). These techniques primarily involve resistive heat evaporating a zinc source inside a chamber. Various heat sources like electron, laser, or radio frequency beams are also used. The vapors are compelled to move from the heating chamber into the cooler, inert-gas chamber, where they are gathered for further consolidation [108]. The aggregation and coalescence of ZnO NPs is the main drawback of this method. The one-step levitational gas condensation method for ZnO that Uhm and colleagues reported is another significant technique. The apparatus is comprised of a levitation and evaporation chamber, a 2.5 kW induction generator, and an oxygen concentration control unit. The obtained NPs, which had a diameter of 30 nm, displayed excellent morphology [122].

Biological methods of NP synthesis are becoming increasingly popular in the field of nanotechnology [123]. Eco-friendly materials for the biological synthesis of NPs include plant extracts [124], bacteria [125], and fungus [126] (Fig. 2B). Since no toxic chemicals are used in this synthesis of Ag-NPs, it is therefore environmentally friendly and suitable for use in the pharmaceutical and biological industries. Green synthesis is superior to traditional methods because it is cost-effective, safer for the environment, and can be easily scaled up to meet high production needs [127].

NPs can be described by their size, shape, surface area, and distance from each other [158]. The consistency of these values is crucial in many contexts. Standard methods for determining the nature of NPs include Fourier transform infrared spectroscopy, powder X-ray diffraction, energy-dispersive spectroscopy, atomic force microscopy, UV-visible spectrophotometry, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and dynamic light scattering (Fig. 2C) [159]. NPs of ZnO-NPs are typically characterized using wavelengths between 350–380 nm [160]. Dynamic light scattering (DLS) showed that it could be used to characterize the surface charge and size distribution of particles suspended in a liquid solution [161]. Standard techniques for morphological characterization on the nanometer to micrometer scale include SEM and TEM [127, 162]. However, the resolution of the TEM is one thousand times that of the SEM. Fourier-transform infrared spectroscopy (FTIR) spectroscopy is an effective method for elucidating the chemical composition of surfaces [130, 131, 163, 164]. Surface chemical residues and organic functional groups of plant biomolecules (such as hydroxyls and carbonyls) can be identified by FTIR. Nanoparticle crystal structure, characterization, and phase identification can be established with XRD [165]. This method involves penetrating the nanomaterial with X-rays and comparing the resulting diffraction pattern to reference patterns to determine its internal structure. The elemental composition of the metal nanoparticle is determined using energy-dispersive spectroscopy [165].

The effectiveness of Zn fertilizer applied to the soil depends on the pH of the soil; Zn is more accessible in acidic soils. Specifically, each increment of pH has a one-hundred-fold negative effect on soil solubility [166]. There is evidence that liming acidic soils can decrease Zn availability [167]. The formation of insoluble calcium zincate, a pH increase caused by CaCO${}_3$, and the direct sorption of Zn to the precipitated CaCO${}_3$, are responsible for the Zn deficiency in calcareous soil [168]. Approximately 30% of the global farmland is covered by alkaline soil, which has low Zn availability for plants [169, 170]. Complexation with organic matter, occlusion in minerals, diffusion into micropores and interparticle space, solid-phase diffusion, and co-precipitation with other metals are additional factors besides pH that affect Zn fixation. Application of 40, 80, and 120 ppm ZnO-NPs in sandy loam soil during pot culture experiment shows that 80 ppm of ZnO-NPs is the more effective concentration for enhanced Zn content (19.94 g) along with increases in plant height, seeds per spike, 100 seed weight, yield, and biomass [62]. Under drought conditions, the application of ZnO-NPs at 1, 3, and 5 mg Zn/kg in the soil can enhance the Zn content in grains of wheat up to 94% [63]. These concentrations of ZnO-NPs also accelerate plant growth and development, promoting fortified edible grains with critically essential nutrients and higher production under drought stress (Fig. 3). In soil, ZnO-NPs at concentrations of 40 to 400 mg/kg can increase the Zn content in the soil as well as in grains of soybean, consequently increases seed yield, antioxidant enzymes activities. This indicates the potential use of ZnO-NPs in addressing malnutrition globally as the NPs improve Zn-deficient in soils, thus boosting the yield of Zn-enriched soybeans [171]. In soil application, ZnO-NPs can enhance the Zn content with the production parameters of different crops (Table 1).

Fig. 3.

Application methods of ZnO-NPs for fortification of crops.

Seed priming is a pre-planting process that causes a physical change in the seed that increases its germination rate [172]. NPs-based pre-soaking treatments of seeds, known as seed nanopriming, induce nanopore formation in the shoot, which promotes the uptake of water (Fig. 3). Additionally, it activates antioxidant mechanisms against reactive oxygen species (ROS) in the seeds, which form hydroxyl radicals that loosen the cell walls and act as catalysts for the rapid hydrolysis of starch and uptake of nutrients [173, 174]. Seeds with a high zinc concentration have been shown to boost seedling health and crop stand (Fig. 3) [175]. Plants grown in nutrient-poor soil produce nutrient-deficient seeds, reducing the overall vigor, growth, and yield of subsequent generations of plants [175]. Since plant roots absorb Zn via diffusion, a lack of moisture negatively impacts Zn availability [167]. Therefore, the effectiveness of Zn applied to the soil may vary depending on the soil irrigation. The outcome of Zn nutrition can be highly variable under rainfed conditions, where soil moisture content entirely depends on rainfall. Application of pre-soaked ZnO-NPs from Zn-rich seeds increases plant growth and yield under Zn-deficient conditions, particularly in rainfed conditions (Table 1) [175].

Seed priming with 4% ZnO-NPs can improve the red bean’s morphological and phenological traits, yield and yield components, third trifoliate leaf, maturity, grain filling period, plant height, and number of seeds per pod [8]. Nanopriming moringa (Moringa oleifera L.) with 10 mg/L ZnO-NPs can increase the percentage of germination, length, dry weight of the plumule and radicle, concentration of photosynthetic pigments, phenolic compounds, flavonoids, vitamin C, glutathione (GSH), Zn content and antioxidant activity, including ascorbate peroxidase (APX), catalase (CAT), and glutathione (PAL) [64]. Pre-sowing treatment with 25 ppm of ZnO-NPs can enhance the agronomic profile of rice by increasing yield traits and grain Zn nutritive value [65]. The priming of wheat seeds with ZnO-NPs at a concentration of 10 mg/L significantly and positively influences seed germination performance, vigor index, fresh biomass, water uptake, and photosynthetic efficiency [66]. Pretreating direct-seeded rice with 10 mol ZnO-NPs increases seed vigor, promotes early seedling emergence, and boosts plant growth, grain nutrients, and productivity in Direct Seeded Rice (DSR) [67]. However, under poor resources and stressed conditions, nanopriming seeds may play an essential role in overcoming the Zn deficiency problem.

The effectiveness of foliar Zn application is highly dependent on the type of fertilizer used, crop characteristics (particularly leaf characteristics), and the genetic potential of the crop (Table 1) [176]. The foliar application method has many benefits, including a reduced need for fertilizer, eliminating Zn fixation, and preventing antagonist nutrient interference with Zn uptake (Fig. 3, Table 1) [167]. When applied to the leaves, Zn travels through the phloem to reach the developing grains [169]. Foliar application of Zn has been proven to be more effective than soil application at raising grain Zn concentration in crops like wheat (Fig. 3). [177]. When applied, foliar applications exert a greater or lesser effect on increasing grain Zn concentration [178]. Foliar applications of 10 ppm biogenic ZnO-NPs significantly contribute to nutrient recovery, increased yield, and productivity compared to nutrient inputs with higher Zn contents in amaranth [68]. Applying 100 µM ZnO-NPs via foliar spraying can potentially improve zinc biofortification in cauliflower [69]. Tomatoes treated with a foliar spray of ZnO-NPs with a concentration of 100 ppm show improvements in growth parameters, physiological traits, yield attributes, yield, and quality traits [70]. Research has revealed that a concentration of 20 ppm of ZnO NP in mung bean seedlings produces the best results concerning growth, photosynthetic pigments, protein content, activity, and the amount of both enzymatic and non-enzymatic antioxidants present [71].

Bio-transformations in the soil determine the bioavailability and toxicity of NPs. In response to root contact, NPs translocate to the aerial parts of the plants, where they accumulate in cells and organelles (Fig. 4). Plant roots are responsible for the initial stage of bioaccumulation, which is the uptake of NPs from the soil [179]. Numerous studies have analyzed various NPs and proposed that NPs accumulate in plants through crystal phase dissolution, biotransformation, and bioaccumulation, which play a role in the spread of absorbed compounds from the roots to the rest of the plant. Likewise, size plays a role in the accumulation of NPs, toxicity, and the rate at which they are transported into plant cells [180]. This is because the size of NPs depends on how they enter into cells (i.e., plasmodesmata) or plant cell organelles. Regarding the reactivity with cells or plant structures, the surface area, shape, and agglomeration of NPs are interconnected [181]. The area and morphology of the NP surface are crucial parameters in localizing the point of contact between NPs and plant cells. Due to the negative charge of the cell wall, the NPs can only cling to the plant cells via their electrical charges. Hydrophobicity on the plant surface was the third most important factor in uptake and translocation after charge and particle size [182]. NP absorption, translocation, and aggregation in plants can be better understood by observing the fundamental structures of the nanomaterials [183]. Standardized laboratory experiments assessing NPs in plant tissues at various levels are required to pinpoint the precise effect of NPs supported by their physical-chemical properties. Therefore, a thorough investigation is necessary to fully understand and explain the processes of absorption, translocation, and accumulation, and knowledge of the composition of NPs [184]. The distribution and localization of NPs within plants and their cellular organelles require careful tracking and monitoring. Osmotic pressure, capillary forces, and translocation through root epidermal cells have been reported as pathways for the tiny NPs (diameters ranging from 3 to 5 nm) to access plant roots [185]. Due to the small pore size, root epidermal cells are only partially permeable, preventing large NPs to enter the root (Fig. 4).

Fig. 4.

Mode of actions of ZnO-NPs based nanofertilizers depicted diagrammatically (A) plant system; (B) from soil to root and (C) transport through xylem and phloem in different parts of plants via symplastic and apoplastic pathway.

The uptake of some NPs is aided by the formation of new pores in the epidermal cell wall [186]. The xylem can grow vertically after NPs travel apoplastically to the central vascular cylinder through cell walls and extracellular spaces. However, NPs must symplastically cross the casparian strip barrier to reach the central vascular cylinder (Fig. 4). This is accomplished by binding to carrier proteins in the membrane of endodermal cells, which triggers endocytosis, pore formation, and transport. When NPs are taken up into the cytoplasm, they can be transferred to neighboring cells via plasmodesmata [187]. The casparian strip is a collection point for NPs that cannot internalize, however, NPs that reach the xylem are transported to the shoots, where they are returned to the roots via the phloem (Fig. 4) [102]. Additionally, NPs have been detected in plant nuclei, cytoplasm, and epidermal cell walls [188]. Nutrient absorption can be altered by NPs that are not absorbed in the root surface of the soil aggregate [189]. Seeds can absorb NPs directly through the parenchyma and into the intercellular spaces, where they are diffused to the cotyledons [190]. When leaves are applied with NPs, the particles are absorbed through the stomata or the cuticles. The cuticle is the first line of defense in protecting a leaf from foreign particles, the cuticle is the first line of defense, keeping NPs smaller than 5 nm out. NPs gain access to the plant vascular system via stomata and are subsequently transported to the organs of photosynthesis and transpiration via apoplastic and symplastic pathways, respectively [189]. NPs (10–50 nm) are transported to new locations via the cytoplasm of neighboring cells (symplastic route). Henceforth, bigger NPs (in the range of 50 to 200 nm) are shuttled around in the intercellular spaces of the body (apoplastic route).

Internalized NPs travel through the phloem sieve tubes with the sugar solution. An effective sap sink is found in the roots, young leaves, stems, fruits, and grains of plants, thus NPs travel unrestrictedly via phloem transport and accumulate to varying degrees [183]. As a nonselective path with least resistance, the apoplastic pathway is well known. The apoplastic route is commonly accepted as the most efficient route for translocating many water nutrients and non-essential metal complexes [189]. The capacity of NPs for adsorption following foliar application was found to be primarily influenced by application method, particle size, concentration, and environmental conditions [181]. Several factors that affect the trapping of NPs on the leaf surface include morphology, chemical makeup, trichrome, waxes, and exudates on the leaf [191]. ZnO-NPss were examined using synchrotron X-ray absorption spectroscopy to determine their transformation after contacting different plants [192]. The various forms of ZnO-NPs, such as Zn-phosphate, accumulate most heavily in the roots and shoots of Zea mays when exposed to hydroponic conditions. It might be a consequence of increased rhizosphere ionic Zn dissolution, plant uptake, or translocation [193]. Soil-grown wheat crops exhibited similar Zn accumulation and speciation [194]. The translocation of ZnO NPs from the soil into glycine max was observed [195].

Micronutrient transporters in plants are responsible for the uptake of Zn from the soil. ZRT-IRT-like proteins directly absorb the Zn${}^{2+}$ after applying ZnO-NPs (ZIPs) (Fig. 5). ZIP transposases are found in various organisms, from bacteria and fungi to mammals and plants. ZIPs proteins are predicted to have eight transmembrane helices, and in most cases, there is a loop region located between transmembrane domains 3 and 4 that contains a histidine-rich sequence that may bind metals, suggesting a role in zinc transport or regulation (Fig. 5).

Fig. 5.

Predicted diagrammatic structure depicting the protein structure of membranes of ZIP family of micronutrient transporter that aids in transporting zinc at the molecular level in plants. C: C terminus; N: N terminus.

Gene expression studies in yeast and experiments on roots show that the proteins encoded by ZIP1, ZIP3, and ZIP4 possess a high affinity for zinc and may also aid in transporting Cd and Cu into the plant. While ZIP1 and ZIP3 are expressed solely in the roots during Zn deficiency, ZIP4 is expressed both in the roots and the shoots. Plants typically utilize various members of multigene families to facilitate the ZnO-NPs metal transport along with ZIPs, e.g., YSL1, FER1, FER2, and PHT4 (Fig. 4) [196]. ZIPs transporters are involved in the transport of external ZnO-NPs, continuously supplying Zn to different parts of plants during Zn deficiency [197].

Overall Zn transport from the roots to the shoot is affected by the transport of Zn into the vacuole [198], and metal chelators like nicotianamine (NA) are important for this movement (Fig. 6) [199]. In ZnO-NPs, the zinc ions are transported in the xylem either as a free cation or as a complex with organic acids like citrate [200]. ZIP family proteins are involved in the remobilization of sugars from the leaves during grain filling, as well as the transfer of sugars from the root or basal part of the shoot to the phloem (Fig. 6). Phloem is the only route for nutrient transport in some cereals due to the discontinuous nature of the xylem [201]. Zn is carried through the phloem in complexes with NA or small proteins. Several members of the ZIP and metal tolerance protein (MTP) families have been proposed as transporters from maternal tissue into the endosperm cavity, aleurone, and embryo (Fig. 6) [202].

Fig. 6.

Diagrammatic representation depicting the molecular basis of translocation of ZnO-NPs into Zn supplement in wheat plants. Translocation of Zn from roots to grain via shoots (stalk) assisted by ZIP (ZRT-IRT like protein), YSL (Yellow stripe like transporter), MFS (Major facilitator super family transporter), MTP (Metal tolerance protein), HMA (Heavy metal ATPase), FER (Ferritins transporter), PHT 1 (Phosphate transporter 1), NA (Nicotianamine), PHY (Phytate), and PSV (Protein storage vacuoles).

The COVID-19-led severe acute respiratory syndrome is a global pandemic [214]. As a precaution, freedom of movement has been restricted in most parts of the globe. The Centers for Disease Control and Prevention (CDC), National Institutes of Health (NIH), and World Health Organization (WHO) have developed therapeutic guidelines for infected people [215]. However, no universally accessible preventative measure has yet been established besides handwashing and vigilance surrounding commonly encountered oronasal symptoms and fever. The availability of a biologically active substance for use as a preventative measure during a pandemic is crucial. The ideal solution would be affordable, accessible, and low-risk for the majority of the population. The mineral zinc has been shown to inhibit viral replication and stimulate the immune system. Clinical studies are warranted to determine whether zinc consumption (within the recommended upper safety limits) can be used as a prophylactic measure to protect against the development of COVID-19. A low zinc status can affect vaccination responses even after vaccination [10]. Zinc is a vital mineral that facilitates the functions of over a hundred enzymes, and is required for proper immune system operation, synthesis of DNA and RNA, and proper cell integration. Because the body is unable to synthesize or store zinc, it is an important nutrient. Zinc needs to be consumed at a rate of 10 to 15 mg per day by an adult Indian to meet the RDI [216]. A higher incidence of COVID-19 has been linked to immune suppression (Fig. 8). Among these factors is eating a healthy diet, which contributes to a healthy immune system. The antioxidant nutrient zinc has an extensive impact on the immune system. Zinc plays an important role in the maturation and operation of neutrophils and natural killer cells, which mediate innate immunity. Zinc deficiency also has an effect on macrophages, the cells responsible for phagocytosing invading pathogens [216]. There is a correlation between zinc deficiency and the development and activity of T and B lymphocytes. In addition to its anti-inflammatory properties, zinc may also act as an antioxidant. COVID-19, like SARS-COV-2, is caused by an RNA virus. RNA viruses store their genetic information in ribonucleic acid (RNA). The central enzymes of the RNA-synthesizing machinery of RNA viruses are RNA-dependent RNA polymerase (RdRp) enzymes, therefore, RdRps are highly prioritized for antiviral drug development [18, 217].

Fig. 8.

Diagrammatic representation of COVID-19 SARS-CoV-2 virus infection mode, spreading process, and impact of nano zinc enabled agriculture food to boost the immune system.

A recent study shows that intracellular Zn${}^{2+}$ concentrations effectively inhibit the replication of RNA viruses like COVID and arteriviruses by inhibiting RdRp activity [18]. High concentrations of Zn${}^{2+}$ and the addition of ionophores (compounds that stimulate cellular import of Zn${}^{2+}$) such as hinokitiol (HK), pyrrolidine dithiocarbamate (PDTC), and pyrithione (PT) have been shown to inhibit the replication of various RNA viruses in cell culture studies. Through nano-fortification, Zn content in cereal and horticulture crops can be enhanced, fulfilling humans’ need for daily routine zinc. The availability of Zn can prevent major diseases like COVID-19. Nano-fortified crops fulfill the need for Zn to support a healthy life for human beings (Fig. 8).

Zinc deficiency endangers crop production and food safety due to its decreasing accessibility in arable soils worldwide [218]. Low zinc contents, high pH, lower reduction potential, sustained flooding, rhizosphere microbial communities, high organic matter, high iron/manganese oxide contents, high calcium carbonate and bicarbonate contents, high sodium and phosphorus availability, high exchangeable magnesium/calcium ratio, and numerous other factors all negatively impact the available zinc in soils for plant growth [219]. Zn deficiency disrupts protein metabolism because it leads to the breakdown of ribosomes [220]. Low zinc levels inhibit RNA and protein synthesis because zinc is an essential component of several enzymes required for protein synthesis, along with glutamate ribonuclease, ATPase, and dehydrogenase [221]. Zn deficiency reduces the production of protein via reducing the protein synthesis process which varies among plant species. Yellowing immature leaves, reduced leaf size, and wilted stems are the most noticeable symptoms of zinc deficiency in plants under severe Zn deficiency stress; nevertheless, older leaves frequently exhibit leaf bronzing, restricted growth, rolling, and wilting [222]. Phyto-siderophore and organic acid ion generation, arbuscular mycorrhiza symbiosis, and improved resistance to oxidative stress caused by zinc shortage are a few of the defenses plants have evolved to survive [223]. These plant modifications influence the transcription of Zn-required processes and boost zinc uptake and transport mediated by zinc transporters [224, 225]. The complete acceptance and propensity of the agricultural sector to implement nanotechnologies are limited by rising concerns about bioavailability, the toxicity of NPs, and an improper regulatory framework [226]. It is important to note that NPs are a double-edged sword, and improper application of them can have detrimental effects on plant growth by inhibiting the production of proteins and pigments in plants [227]. Plants react differently to high concentrations of NPs at the morphological, physiochemical, and molecular levels [228]. Soil bio/geo transformations of NPs are a key factor in determining their bioavailability and toxicity [226]. In-depth analyses of the variables that influence the phytotoxicity of NPs have been conducted previously [229]. Studies have shown that various factors, such as the type of nano-material, test plants used, NP concentration, NP size distribution, NP morphology, and NP surface charge, play a role in determining the phytotoxicity of NPs. Furthermore, these studies have reported that the mediated effect of NPs on plant growth and development is concentration dependent, with the use of high concentrations of these NPs leading to problematic aftermath for plants, animals, and humans [230].