Agro-Nanotechnology as an Emerging Field: A Novel Sustainable Approach for Improving Plant Growth by Reducing Biotic Stress

: In the present era, the global need for food is increasing rapidly; nanomaterials are a useful tool for improving crop production and yield. The application of nanomaterials can improve plant growth parameters. Biotic stress is induced by many microbes in crops and causes disease and high yield loss. Every year, approximately 20–40% of crop yield is lost due to plant diseases caused by various pests and pathogens. Current plant disease or biotic stress management mainly relies on toxic fungicides and pesticides that are potentially harmful to the environment. Nanotechnology emerged as an alternative for the sustainable and eco-friendly management of biotic stress induced by pests and pathogens on crops. In this review article, we assess the role and impact of different nanoparticles in plant disease management, and this review explores the direction in which nanoparticles can be utilized for improving plant growth and crop yield.


Introduction
Crop cultivators suffer from high yield loss caused by various diseases. Biotic stress induced by microbes on crop plants reduces the crop yield and decreases the quality. Biotic stress causes disease in crops, which leads to the suffering of the plant. Diseases of the plant need to be controlled to maintain the abundance of food produced by farmers around the world. The management of crop diseases is very necessary to fulfill the food demand. Potato blight disease caused by plant pathogenic fungus Phytopthora caused more than one million deaths in Ireland [1]. Around 20-40% of agricultural crop yield losses occur globally due to various diseases caused by phytopathogenic bacteria, phytopathogenic fungi, pests, and weeds [2].
It is estimated that in 2050 the world's human population will reach around 10 billion, and around 800 million people in the world will be hungry and around 653 million people in the world will be undernourished in 2030, thus fulfilling the food demand will remain a huge challenge. The current research progress and disease management strategies are not enough to fulfill the food demand by 2050 [3]. The first green revolution made a huge difference in yield and food production, but in the last few years' crop production has been stagnant and food demand is increasing sharply, so now we need a second green revolution to fulfill the food demand of the population. Different approaches are used by farmers to mitigate the impact of plant diseases. The agriculture system mainly relies on chemicals to manage crop diseases and inhibit the growth of phytopathogens, which cause diseases before and after crop harvesting. The excessive use of chemical pesticides, herbicides, and fungicides that are mainly used to control plant diseases causes harmful environmental and human health consequences. Tilman et al. [4] observed that the high use of chemical pesticides increases resistance in pathogens and pests, reduces nitrogen fixation, and the bioaccumulation of toxic pesticides occurs.
An example is the synthetic chemical pesticide DDT, dichlorodiphenyltrichloroethane, which was extensively used in agriculture for controlling plant pathogens and was found to be genotoxic in humans, causing endocrine disorders [5]. Water and soil pollution is also caused by the excessive use and misuse of these chemicals. There is an increasing demand day by day to reduce the use of synthetic chemicals. Consequently, the harmful effects of chemicals on wildlife, the environment, and human health have increased the need for alternative measures in the control of plant pathogens, so that some phytopathologists have focused their research on developing a new alternative that should replace the use of chemicals in controlling plant diseases.
Nanotechnology has revolutionized agriculture and can control plant diseases, although the field of nanotechnology is still in the nascent stage and needs more research analysis [6].The use of nanomaterials in agriculture will reduce the excessive use of toxic chemicals used for plant disease management (Figures 1 and 2).
"Nano" denotes one-billionth part, thus nanotechnology deals with small things. The word nano is used for materials with a size range of 0.1 to 100 nanometers [7,8]. The first time the term nanotechnology was used was by Taniguchi in 1974 to the science that largely deals with particles of nano size (1.0 × 10 −9 m). When a bulk material is reduced to nano size, it has a high surface-to-volume ratio that may increase its reactivity and express some new properties [7,9]. The control of plant diseases and improving plant growth by the use of nanomaterials are some of the possible key applications in the area of plant pathology. Approximately 260,000-309,000 metric tons of nanoparticles were produced in 2010 globally, and the worldwide consumption of nanomaterials was approximately from 225,060 metric tons to 585,000 metric tons in 2014 to 2019 [10,11].
In this review article, recent research progress and the application of various nanoparticles for the sustainable management of the biotic stress of crop systems and impact on plant growth have been discussed. We try to cover the various problems associated with crop cultivation and plant diseases and the use of different nanomaterials to control phytopathogens and improve plant growth.

Nanomaterials in Improving Plant Growth and Yield
Currently, around 1300 nanomaterials, with widespread potential applications, are available [13,14]. Nanoparticles can penetrate the cell wall because the cell wall is porous to 3.5-20 nm macromolecules. Nanoparticles can enter through stomatal openings. When stomata are present at the lower surface of leaves, the entry of nanoparticles (NPs) becomes difficult [15]. It is reported that nanoparticles of size ≤43 nm can penetrate and enter into stomata [16,17].
The effect of nanoparticles on crop plants is concentration-based. Many plant processes such as seed germination and plant growth are affected by NP concentration [18]. Many NPs have been reported to be beneficial for plant growth. Mahmoud et al. [19] used Zn, B, Si, zeolite NPs on a potato plant and found that these nanoparticles have a

Nanomaterials in Improving Plant Growth and Yield
Currently, around 1300 nanomaterials, with widespread potential applications, are available [13,14]. Nanoparticles can penetrate the cell wall because the cell wall is porous to 3.5-20 nm macromolecules. Nanoparticles can enter through stomatal openings. When stomata are present at the lower surface of leaves, the entry of nanoparticles (NPs) becomes difficult [15]. It is reported that nanoparticles of size ≤43 nm can penetrate and enter into stomata [16,17].
The effect of nanoparticles on crop plants is concentration-based. Many plant processes such as seed germination and plant growth are affected by NP concentration [18]. Many NPs have been reported to be beneficial for plant growth. Mahmoud et al. [19] used Zn, B, Si, zeolite NPs on a potato plant and found that these nanoparticles have a

Nanomaterials in Improving Plant Growth and Yield
Currently, around 1300 nanomaterials, with widespread potential applications, are available [13,14]. Nanoparticles can penetrate the cell wall because the cell wall is porous to 3.5-20 nm macromolecules. Nanoparticles can enter through stomatal openings. When stomata are present at the lower surface of leaves, the entry of nanoparticles (NPs) becomes difficult [15]. It is reported that nanoparticles of size ≤43 nm can penetrate and enter into stomata [16,17].
The effect of nanoparticles on crop plants is concentration-based. Many plant processes such as seed germination and plant growth are affected by NP concentration [18]. Many NPs have been reported to be beneficial for plant growth. Mahmoud et al. [19] used Zn, B, Si, zeolite NPs on a potato plant and found that these nanoparticles have a positive effect on potato plants and they improve the plant growth. Khan and Siddiqui [20] treated eggplant with ZnONPs and found a foliar spray of ZnONPs causes the highest improvement in eggplant growth. Awasthi et al. [21] reported that ZnONPs have a positive effect on seed germination in the Triticum aestivum plant. Zinc oxide nanoparticles (ZnONPs) can enhance plant biomass and agriculture production [22]. Sabir et al. [23] also showed that nanocalcite (CaCO 3 ) application with Fe 2 O 3 , nano SiO 2 , and MgO improved the uptake of Mg, Ca, and Fe, and also notably enhanced the intake of P with micronutrients Zn and Mn. Venkatachalam et al. [24] found that ZnONPs increase in photosynthetic pigment in the Leucaena leucocephala plant. Narendhran et al. [25] reported high chlorophyll-a', chlorophyll-'b' and total chlorophyll content in the Sesamum indicum plant when treated with ZnO NPs. Taheri et al. [26] observed that treatment of ZnONPs increases the increase in shoot dry matter in Zea mays. Tarafdar et al. [27] found that ZnONPs enhanced shoot and grain yield in the Pennisetum glaucum plant.
The application of titanium dioxide (TiO 2 ) on crops promotes plant growth parameters and can enhance the photosynthetic rate. Siddiqui et al. [28] usedTiO 2 and ZnONPs on beet root plants. They found that both NPs increased chlorophyll and carotenoid content, improved plant growth, and also improved super oxide dismutase (SOD), catalase (CAT), H 2 O 2 , and proline content in plants. ZnONPs were found to be better than TiO 2 NPs on beetroot plants. Raliya et al. [29] reported that TiO 2 NPs treatment improved shoots in the Vigna radiate plant. Lawre and Raskar [30] observed that TiO 2 NPs at a lower concentration enhanced seed germination and seedling growth in onion plants. Rafique et al. [31] found a positive effect of TiO 2 NPs on the Triticum aestivum plant. Mahmoodzadeh et al. [32] found a positive effect of TiO 2 NPs on the seed germination of the Brassica napus plant. Qi et al. [33] reported that treatment of TiO 2 NPs promotes photosynthetic rate in tomato plants.
Silicon is an important element that plays a key role in several metabolic and physiological activities in plants [34]. SiO 2 nanoparticles have the potential to enhance the germination and seedling growth of Agropyron elongatum [35]. Nano-SiO 2 can be used to produce effective fertilizers for crops and to minimize the loss of fertilizer through slow and controlled release, allowing for regulated, responsive, and timely delivery [36]. Siddiqui et al. [37] found improved seed germination in the Cucurbita pepo plant after treatment with Nano SiO 2 . Haghighi and Pessarakli [38] reported that Nano Si treatment on the tomato plant improves photosynthetic rate in treated plants.
Copper is an essential element for plant growth and development. Copper plays a key role in the activity of many plant enzymes. Copper nanoparticles (Cu NP) are used as antimicrobial agents, gas sensors, catalysts, electronics, etc. [39]. Wang et al. [40] found that CuO NPs improved photosynthesis in the Spinacia oleracea plant. Zhao et al. [41] reported that Cu(OH) 2 NPs improved the antioxidant system of the Lactuca sativa plant. Shinde et al. [42] found that Mg(OH) 2 NP treatment promotes seed germination and seedling growth in the Zea mays plant. Hussain et al. [43] reported that MgO NPs improve the antioxidant system in Raphanus sativus plants. Cai et al. [44] observed that MgO NPs can promote the plant growth of the Tobacco plant. Imada et al. [45] found that MgO NPs can induce resistance in the tomato plant.
Iqbal et al. [46] reported that AgNP treatment improved plant growth and tolerance to heat stress in the Triticum aestivum plant. Mehta et al. [47] found that AgNPs' foliar application enhanced growth and biomass in the Vigna sinensis plant. Pilon et al. [48] observed that chitosan NPs protect apple plants after post-harvest. Van et al. [49] found that chitosan NPs improve plant growth in Robusta coffee.
Reduce the salt stress effect [37] Nano Si Tomato Enhancement of germination rate and dry weight [38] CuO NPs Spinacia oleracea Improved photosynthesis in treated plants [40] MgO NPs Tobacco Promote plant growth [44] MgO NPs Tomato Induce resistance in tomato plant [45] AgNPs Wheat Regulate antioxidative defence system [46] AgNPs soil bacterial diversity Regulate soil bacterial diversity [47] Chitosan NPs Apples They reduce microbial growth [48] Chitosan NPs Robusta cofee Improved growth parameters [

Nanoparticles Plant Effect on Plants in a Dose-Dependent Manner Reference
SilicaNPs Zea mays. L Improve silica content in plants [58] SiO 2 NPs Maize Improved growth parameters and increased seed stability [59] SiO 2 and TiO 2 NPs Soybean Enhance germination of seeds [60] Cu(OH) 2 Lactuca sativa Improve antioxidant system [61] Cu(OH) 2 Spinach Improve the antioxidant system [62] ZnO NPs Glycine max
Shahryari et al. [79] reported that AgNPs and a silver-chitoson composite show antibacterial activity against Pseudomonas syringae pv. syringae bacteria. Divya et al. [80] reported that chitoson NPs have antifungal activity against Macrophomia phaseolina and Alternaria alterneta fungi. Xing et al. [81] reported that chitoson NPs have antifungal activity against Fusarium solani and Aspergillus niger fungi. Dang et al. [82] reported that AuNPs have antibacterial activity against E. coli bacteria. Attar and Yapaoz [83] observed that ZnO and AuNPs have antibacterial activity against E. coli bacteria. The gold nanoparticles showed toxic effect on bacteria, Salmonella typhimurium, in which the macro gold did not exhibit. Jayaseelana et al. [84] synthesized gold nanoparticles from Abelmoschus esculentus and reported their antifungal activity. The antifungal activity of AuNPs was tested against Puccinia graministritci, Aspergillus niger, Aspergillus flavus and Candida albicans using the standard well diffusion method. The maximum zone of inhibition was observed in the Au NPs against P. graminis and C. albicans.

Toxic Effect of Nanoparticles
Nanomaterials' effect on organisms is largely dependent on the dose, size, and shape, the types of NPs, concentration, and the duration of exposure to NPs and the plant/animal species [117,118]. Nanoparticles at optimum concentration augment the plant's growth, but high concentrations of nanoparticles could be toxic for plants. The inhibitory action of nanoparticles on fungi and bacteria includes disruption by pore formation in the cell membrane, disturbance in membrane potential, cell wall damage, direct attachment to the cell surface, DNA damage, cell cycle arrest, the inhibition of enzyme activity and reactive oxygen species (ROS) generation, and this finally leads to death. Nanoparticles generate the ROS, which causes damage to the cellular structures. The different components of reactive oxygen species include free radicals, such as hydrogen peroxide (H 2 O 2 ), superoxide (O 2 − ), singlet oxygen ( 1 O 2 ), carbon dioxide radical (CO 2 − ), hydroxyl (HO · ), hydroperoxyl (HO 2 ), carbonate (CO 3 − ), peroxyl (RO 2 ), and alkoxyl (RO), and nonradicals, such as ozone (O 3 ), nitric oxide (NO), hypobromous acid (HOBr), hypochlorous acid (HOCl), hypochlorite (OCl − ), peroxy nitrite (ONOO − ), organic peroxides (ROOH), peroxo monocarbonate (HOOCO 2 − ), peroxy nitrous acid (ONOOH) and peroxy nitrate (O 2 NOO−), and these nanoparticles accumulate in the membrane of bacteria or fungi, which leads to change in the permeability of the cell membrane and disturbs the proton motive force (PMF).Oxidative stress due to the higher concentration leads to single-and double-strand breaks and nitrogen base and pentose sugar lesions [103,104].

Toxic Effect of Nanoparticles
Nanomaterials' effect on organisms is largely dependent on the dose, size, and shape, the types of NPs, concentration, and the duration of exposure to NPs and the plant/animal species [117,118]. Nanoparticles at optimum concentration augment the plant's growth, but high concentrations of nanoparticles could be toxic for plants. Kushwah and Patel [119] observed that the optimum concentration of nano TiO 2 in the Vicia faba plant ranged from 5-50 mg/L. Other studies proved that TiO 2 NPs may induce stress in plants such as tomato, cucumber and spinach at high concentration [120]. Silver nanoparticles cause chromosomal aberrations in Vicia faba [121]. Lopez-Moreno et al. [122] reported that CeO 2 nanoparticles can induce DNA damage in soybean.

Conclusions
In summary, the literature shows that food demands will increase with time, and to fulfill the demand of people, the present agricultural practices are not sufficient and chemicals used in agriculture as pesticides have a severe toxic effect on the environment. Thus, we need to develop an alternative approach that has a less toxic effect on the environment and that could help in fulfilling food demands. According to estimates, around 192.8 Mt chemical fertilizers were used in 2016-2017 in the whole world. The use of toxic chemicals and pesticides causes environmental pollution, which affects fauna and flora. Pathogens and pests induce resistance against fungicides and pesticides. Hence, optimizing of the use of toxic chemical pesticides and fungicides is needed. Nanotechnology is flamboyant and has provided nanostructure materials as pesticide and fertilizer carriers. Nanomaterials can develop smart fertilizers as they can enhance nutrient availability and reduce environmental pollution [123]. Novel nanotechnology can be an alternative that can reduce crop diseases and enhance crop yield. Previous studies reported a significant positive effect of nanomaterials on crop plants. This novel technology can reduce the use of toxic chemicals and pesticides that contaminate soil, the environment, and groundwater. Further research is needed to develop this technology on a large scale (Figure 4). trient availability and reduce environmental pollution [123]. Novel nanotechnology can be an alternative that can reduce crop diseases and enhance crop yield. Previous studies reported a significant positive effect of nanomaterials on crop plants. This novel technology can reduce the use of toxic chemicals and pesticides that contaminate soil, the environment, and groundwater. Further research is needed to develop this technology on a large scale (Figure 4).

Informed Consent Statement: Not Applicable
Data Availability Statement: The raw data used for this proposed work have been cited in the manuscript. Moreover, the derived data supporting the findings of this study have been graphically depicted and are available with the corresponding author on request.