# Nanotechnology Potential in Seed Priming for Sustainable Agriculture

Anderson Do Espirito Santo Pereira, Halley Caixeta Oliveira, Leonardo

Fernandes Fraceto, Catherine Santaella

► To cite this version:

Anderson Do Espirito Santo Pereira, Halley Caixeta Oliveira, Leonardo Fernandes Fraceto, Catherine Santaella. Nanotechnology Potential in Seed Priming for Sustainable Agriculture. Nanomaterials, 2021, 11 (2), pp.267. 10.3390/nano11020267. [https://doi.org/10.3390/nano11020267](https://doi.org/10.3390/nano11020267)

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Review

# Nanotechnology potential in seed priming for sustainable agriculture

Anderson do Espirito Santo Pereira<fnref n="1" />,<fnref n="2" />, Halley Caixeta Oliveira<fnref n="3" />, Leonardo Fernandes Fraceto<fnref n="2" />*, Catherine Santaella<fnref n="1" />*

**Abstract:** Our agriculture is threatened by climate change and the depletion of resources znd bio-diversity. A new agriculture revolution is needed in order to increase the production of crops and ensure the quality and safety of food, in a sustainable way. Nanotechnology can contribute to the sustainability of agriculture. Seed nano-priming is an efficient process that can change seed metabolism and signaling pathways, affecting not only germination and seedling establishment but also the entire plant lifecycle. Studies have shown various benefits of using seed nano-priming, such as improved plant growth and development, increased productivity, and a better nutritional quality of food. Nano-priming modulates biochemical pathways and the balance between reactive oxygen species and plant growth hormones, resulting in the promotion of stress and diseases resistance outcoming in the reduction of pesticides and fertilizers. The present review provides an overview of advances in the field, showing the challenges and possibilities concerning the use of nanotechnology in seed nano-priming, as a contribution to sustainable agricultural practices.

**Keywords:** Seed nano-priming; seed coating; germination; plant resistance; nanoparticles; sustainability.

## 1. Introduction

Agriculture is currently facing many challenges, including production losses due to pests, the effects of global climate change, and the depletion of natural resources (Kah et al., 2019; Lowry et al., 2019). An additional difficulty is that traditional agricultural practices have been relying on continuous application of pesticides and fertilizers, resulting in environmental contamination(Lowry et al., 2019; Zhao et al., 2020).

The world population is expected to increase to 9-10 billion by 2050, implying that food production will need to rise by 25-70% compared to current levels (Scott et al., 2018). Therefore, new technologies need to be deployed in agriculture to ensure sustainability and increase productivity (Fraceto et al., 2016; Kah et al., 2019; Lowry et al., 2019; Panpatte et al., 2016).

Nanotechnology has the potential to contribute to a new technology-based agricultural revolution (Camara et al., 2019; Lowry et al., 2019; Shakiba et al., 2020). Many nanomaterials have been developed for agricultural applications, including new solutions for soil and water remediation as well as nanofertilizers and nanopesticides, designed to reduce the applied amounts of fertilizers and pesticides, while increasing food production and quality (Camara et al., 2019; Kah et al., 2019; Zhao et al., 2020).

Nanomaterials, especially nanoparticles, have a range of applications for crop protection (Fraceto et al., 2016; Kah et al., 2019; Lowry et al., 2019). This is a major field of re-

Citation: Pereira, A.d.E.S.; Oliveira, H.C.; Santaella, C. Nanotechnology potential in seed priming for sustainable agriculture. Nanomaterials 2020, volume number, x.

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<footnote n="1">¹ Aix Marseille Univ, CEA, CNRS, BIAM, Laboratory for Microbial Ecology of the Rhizosphere (LEMIRE), UMR7265 BIAM, Saint Paul-Lez-Durance, France F-13108. es</footnote>

<footnote n="2">² Department of Environmental Engineering, Institute of Science and Technology of Sorocaba, São Paulo State University (UNESP), Avenida Três de Março, 511, CEP 18087-180, Sorocaba, São Paulo, Brazil.</footnote>

<footnote n="3">³ Department of Animal and Plant Biology, Londrina State University (UEL), PR 445, Km 380, CEP 86057-970, Londrina, PR, Brazil.</footnote>

<footnote>* Correspondence: leonardo@sorocaba.unesp.br (L.F.F.); catherine.santaella@cea.fr (C.S.)</footnote>
search that has attracted the interest of companies in the agricultural sector, resulting in the inclusion of nanoparticles in formulations (Chau et al., 2019). The use of nanopesticides and nanofertilizers can contribute to pest control, plant nutrition, and environmentally friendly production methods (Acharya et al., 2020; Fraceto et al., 2016; Lowry et al., 2019).

Recent studies have shown that nanoparticles have effects on seeds and plants (Acharya et al., 2020; Lowry et al., 2019; Mahakham et al., 2017; Pérez-de-Luque, 2017). Some nanoparticles have side effects including the inhibition of germination or phytotoxicity in seedlings (Falco et al., 2020; Hayes et al., 2020; Pelegrino et al., 2020). However, others can act as stimulants, improving seed metabolism, seedling vigor, and plant growth by acting in cellular signaling pathways (Acharya et al., 2020; Mahakham et al., 2017). These effects depend on nanoparticle physical-chemical properties such as size, zeta potential, and concentration, which are factors that determine the biological responses (Acharya et al., 2019a; Pérez-de-Luque, 2017).

These characteristics have key roles in nanoparticle uptake and translocation in plants. As example, small-sized nanoparticles cross biological barriers more efficiently (Bombo et al., 2019; Hu et al., 2020; Palocci et al., 2017; Valletta et al., 2014). The surface charge of the nanoparticles is also decisive. Positively or negatively charged nanoparticles can be taken up by the leaves and translocated to the roots. However, only negatively charged nanos are taken up by the roots. Positive charges induce the production of mucilage, which prevents their uptake by plants (Avellan et al., 2019, 2017; Spielman-Sun et al., 2019).

Nano-priming can be applied to seeds in order to provide protection for seeds during storage, improve germination, germination synchronization, and plant growth as well as to increase the resistance of crops to abiotic or biotic stress conditions, which can help to reduce the required quantities of pesticides and fertilizers. The use of nanotechnology for seed priming is a new area of research, although studies have already shown promising results (Hussain et al., 2019; Kasote et al., 2019; Mahakham et al., 2017; Ye et al., 2020). Figure 1 shows the main benefits that can result from seed nano-priming.

<figure><img src="image_4.png" /><figcaption>**Figure 1.** Seed nano-priming and its potential benefits for sustainable agriculture. In addition to provide protection for seeds during storage, the use of seed nano-priming can result in improved establishment of plants in the soil with a reduced need for fertilizers. By growing faster, plants have an increased ability to compete with weeds for resources, consequently increasing productivity and food quality. The plants may also become more resistant to abiotic and biotic stresses, resulting in reduced use of pesticides.</figcaption></figure>
The aim of this review is to provide an update concerning the potential applications of seed nano-priming techniques in agriculture. Many different types of nanoparticles can be used, with effects on seed metabolism and on the subsequent plant growth and development.

## 2. Seed priming and nanoparticles: Definition and potential applications

### 2.1. Germination and principles of seed priming

Germination is one of the most important steps for the establishment of plants in agriculture and is fundamental for crop quality (Acharya et al., 2020; Savassa et al., 2018). The rapid development of seedlings ensures fast expansion of the leaves and elongation of the roots, which favor the uptake of nutrients, their translocation through the transpiration flow, and biomass production (Acharya et al., 2020; Chandrasekaran et al., 2020; Mahakham et al., 2016). Slow germination can expose the young seedling, which is one of the most vulnerable stages of plant life cycle, to many environmental stress conditions or pathogens, resulting in decreases in vigor and crop productivity, leading to economic losses for farmers (Acharya et al., 2019a).

The process of seed germination is usually divided into three phases, as summarized in Figure 2a (Nonogaki, 2014; Nonogaki et al., 2010). It begins with imbibition (phase I), when the fast water uptake triggers seed basal metabolism, as transcription, protein synthesis and mitochondrial activity. In phase II (activation or lag phase), water uptake is limited but the metabolism becomes hyperactive with the production of enzymes required for reserve mobilization and embryo development, including amylases, endoxylanase, and phytase. In phase III, the seeds exhibit again fast water uptake and embryo growth culminates in radicle protrusion (Ibrahim, 2016; Nonogaki et al., 2010).
<figure><img src="image_5.png" /><figcaption>**Figure 2.** Germination phases and reactive oxygen species (ROS) effects. a) The germination process is subdivided into three phases: phase I (water uptake), phase II (metabolic activity, with initiation of degradation of starch reserves and preparation for embryo development), and phase III (embryo development and emergence of the radicle), b) The ROS act by destabilizing cell wall, allowing water uptake and cell elongation, c) ROS level in seeds and hormones production.</figcaption></figure>

The seed germination process is finely regulated by signaling molecules that include reactive oxygens species (ROS) and phytohormones (Bailly, 2019; Choudhary et al., 2020; Oracz and Karpiński, 2016) (Figure 2b and c). Apoplastic ROS generation is directly related to cell wall loosening, which allows water uptake and cell extension (Bailly, 2019; Oracz and Karpiński, 2016). The abscisic acid and gibberellins work antagonistically to terminate seed germination or dormancy, and auxins could also act in the maintenance of seed dormancy (Wu et al., 2020). The ROS produced regulate gene expression and phytohormone signaling and the homeostasis of abscisic acid, gibberellins, auxins and ethylene, to control cellular events related to seed germination (Bailly, 2019; Choudhary et al., 2020; Shuai et al., 2016). However, when ROS levels are too high, extensive oxidative damages occur, hampering seed germination (Bailly, 2019). ROS content must be therefore spatio-temporally controlled to be enclosed in the so-called oxidative window, that ensures proper germination completion (Bailly, 2019).
Seed priming is a traditional technique used in agriculture to promote seed germination and plant establishment, based on a preliminary preparation of seeds prior to sowing (Acharya et al., 2019b; Bouroug et al., 2020; Ibrahim, 2016; Jisha et al., 2013). It is usually a water-based methodology whereby seeds are hydrated followed by drying, or some physical methods are applied as ultraviolet light (UV) priming (Thomas T.T. and Puthur, 2017). The absorption of water must be adequate to trigger the metabolic pathways for pre-germination (phases I and II), without resulting in radicle emergence (Figure 2).

This process affects the seed metabolism at molecular and cellular levels (e.g., transcriptomic reprogramming, enhanced capacity for reserve mobilization, cell wall loosening, increased potential for protein synthesis and post-translational modifications), inducing a particular physiological state, which speeds up or even improves the germination and vigor of the primed seeds on a novel imbibition (Acharya et al., 2019b; Bouroug et al., 2020; Ibrahim, 2016; Jisha et al., 2013). The moderate stress imposed by both soaking and subsequent drying may also induce stress-related responses (e.g., antioxidant mechanisms, heat-shock proteins), resulting in cross-resistance to other stress factors. Moreover, a faster germination shortens *per se* the exposition of the germinating seeds to adverse soil conditions. Thus, seed priming has been successfully used to accelerate and synchronize germination, improve seedling vigor, and make the plants more resistant to abiotic and biotic stresses, resulting in improvements of productivity and food quality (Acharya et al., 2019a; Carrillo-Reche et al., 2018; Mahakham et al., 2016).

Different types of seed priming can be used, such as hydro-priming or hydro-thermo-priming, where the seeds are hydrated using water treatment (usually limited to period of 7-14 hours), which allows the occurrence of germination phase II (Lemmens et al., 2019). The technique may be applied with temperature alternation (cold and hot) (Noorhosseini et al., 2018). In the case of osmo-priming, solutions of low water potential are used to control hydration (around 10-20%), altering seed metabolism through an additional abiotic stress factor (Lemmens et al., 2019). Other methodologies such as halopriming, hormo-priming, and bio-priming can be used as pre-sowing treatments, using solutions containing salts (Saddiq et al., 2019), plant growth regulators (Sytar et al., 2019), and microorganisms (Lemmens et al., 2019), respectively.

## 2.2. Nanoparticles for seed priming

Seed nano-priming is a new technology that uses nanomaterials, mainly nanoparticles, for seed priming (Kasote et al., 2019; Mahakham et al., 2017; Shukla et al., 2019). There is an important difference between seed priming and seed nano-priming, since conventional seed priming mainly employs water (hydropriming) or solutions containing substances (nutrients, hormones, or biopolymers) that adsorb on the seed and can result in seed coating (or dressing). In seed nano-priming, the media used are suspensions or nanoformulations, where the nanoparticles may or may not be taken up by the seeds (Acharya et al., 2019c). Even when nanoparticle uptake occurs, the greatest fraction is retained on the seed surface as coating (Duran et al., 2017; Falsini et al., 2019; Montanha et al., 2020; Savassa et al., 2018). Such seed coating can be used with fungicides or bactericides in order to protect against pathogens in the field or during storage (Gross et al., 2020).

One of the first studies showing the potential of nanomaterials to affect seed germination was reported by Khodakovskaya et al. (2009). Although seed priming was not used, these authors demonstrated that carbon nanotubes could be taken up by tomato seeds. The carbon nanotubes increased water uptake, resulting in tomato plants with a 2-fold higher number of flowers (Khodakovskaya et al., 2013). Other studies have similarly demonstrated that carbon nanotubes can modulate seed metabolism in plants such as tomato, barley, soybean, and maize, increasing the gene expression of several types of water channel proteins(Lahiani et al., 2013; Villagarcia et al., 2012).

Different nanomaterials, including metallic, biogenic metallic, and polymeric nanoparticles, have also shown potential for seed nano-priming (Acharya et al., 2019c; Falsini et al., 2019; Mahakham et al., 2017; Siddaiah et al., 2018), resulting in the stimulation of plant
growth and improvement in morphological and metabolic traits. This process can pro-
mote fast root and shoot development, with changes in the expression of genes that
modulate metabolic processes such as phytohormone production. Seed nano-priming
changes the activity of the defense system, increasing the antioxidant levels and enzyme
activities, so that the plants become more resistant to pests and other biotic and abiotic
stresses under field conditions (Abdel Latef et al., 2017; Acharya et al., 2020; Duran et al.,
2017; Itroutwar et al., 2019; Kasote et al., 2019; Li et al., 2019).

2.3. *Active nanoparticles and nanocarriers systems*

The potential applications of nanoparticles will be considered in two groups: (i) ac-
tive nanoparticles and (ii) sustained release nanocarrier systems (Figure 3). Table 1
shows systems that have been employed for seed priming/coating, together with their
potential effects as stimulants or against biotic and abiotic stress.

Active nanoparticles are nanoparticles that themselves can cause a biological effect, acting as a stimulant, an anti-pathogen, or both (Mahakham et al., 2017; Rizwan et al., 2019; Siddaiah et al., 2018). Nanocarriers with sustained release features are systems where the nanoparticle (being itself active or not) is loaded with an active ingredient (biological or synthetic) and provides extended release of this compound over time (Camara et al., 2019; Falsini et al., 2019; Kumar et al., 2020; Shakiba et al., 2020).

<figure><img src="image_6.png" /><figcaption>**Figure 3.** Seed nano-priming. The nanoparticles can be defined as (i) active nanoparticles, which are systems that can have a stimulatory effect on plant growth and development, or (ii) nanocarrier systems providing sustained release, where the nanoparticles can be active or inactive, and are loaded with an active ingredient.</figcaption></figure>

Metallic nanoparticles are an example of active nanoparticles that can be prepared employing either chemical or biological synthesis (Kasote et al., 2019; Mahakham et al., 2017; Maswada et al., 2018). Polymeric nanoparticles are other potentially active systems, since many of the polymers used for their preparation present biological activity (Divya et al., 2019; Li et al., 2019; Sathiyabama and Muthukumar, 2020). These systems can be used for sustained release after being loaded with substances including pesticides, fertilizers, biological compounds, or even other nanoparticles. The sustained release of these substances can result in higher biological activity and reduced toxic effects (Bravo Cadena et al., 2018; de Oliveira et al., 2019; Falsini et al., 2019). Many products that are used for seed priming can be loaded into nanocarrier systems, consequently improving their biological activities (Bravo Cadena et al., 2018; Falsini et al., 2019; Pereira et al., 2019).
### 2.3.1. Active nanoparticles

Metallic nanoparticles, including those produced using biogenic processes, are systems that are normally smaller than 100 nm. These systems have received special attention for seed nano-priming, since many of them are prepared with metals that play important roles in plant metabolism (Das et al., 2018a).

Some examples of these metals are iron, zinc, and manganese. Iron is a co-factor for enzymes such as cytochrome P450s and Fe(II)/2-oxoglutarate-dependent oxygenase, which makes it an essential nutrient in various metabolic pathways, including respiration and photosynthesis (Guha et al., 2018; Kasote et al., 2019). Zinc is another metal essential for plant metabolism. Around 30% of the global lands is deficient in this micronutrient, leading to problems in sugar production and the synthesis of cell membranes, hormones, and proteins, affecting seedling vigor, photosynthesis, and plant defense systems (Itroutwar et al., 2019; Savassa et al., 2018). Manganese acts as a co-factor for enzymes as superoxide dismutase (involved in antioxidant response) and the oxygen-evolving complex of photosystem II (Ye et al., 2020).

Seed nano-priming using nanoparticles based on these metals has shown considerable potential for agricultural applications. For example, Dileep Kumar et al. (2020) primed chilli seeds with metal oxide nanoparticles (zinc, titanium, and silver) and found that zinc oxide nanoparticles improve germination and seedling development (shoot and root length), while these effects are not observed for the other tested metal based nanoparticles (titanium and silver). According to Rizwan et al.⁵³, priming sorghum seeds with iron oxide nanoparticles improves the seed germination, water content, and photosynthesis (Rizwan et al., 2019).

Biogenic nanoparticles prepared using biological extracts from plants, fungi, or bacteria can have high contents of phytochemicals compounds, including phytomolecules rich in hydroxyl and carbonyl functional groups, such as phenolics, flavonoids, terpenoids, sugars, and proteins. These compounds are natural reducing agents for metals and can act as capping that stabilizes the nanoparticles in colloidal solutions (Acharya et al., 2019c; Mahakham et al., 2016).

As an alternative methodology for the synthesis of metallic nanoparticles, green synthesis is more economical compared to chemical methods, and has the advantages of increasing biocompatibility and avoiding the use of toxic chemicals (Acharya et al., 2019c). Many biogenic metallic nanoparticles have been shown to present improved biological effects as well as lower (or no) phytotoxicity compared to metallic nanoparticles synthesized using chemical methods (Itroutwar et al., 2019; Kannaujia et al., 2019).

Some metallic nanoparticles can become toxic to seeds or plants due to the re-oxidation process. For example, silver nanoparticles can be re-oxidized (from Ag⁰ to Ag⁺ ions) within seeds and plants, and the ionic form can inactivate proteins and enzymes, resulting in toxic effects (Kannaujia et al., 2019). However, bioactive compounds present as coatings on biogenic nanoparticles can act as stabilization agents, avoiding the re-oxidation reaction and consequently increasing the biocompatibility of metallic nanoparticles (Kannaujia et al., 2019; Spagnoletti et al., 2019). Different biogenic metallic nanoparticles have been shown to be able to enhance seed germination, improve plant growth and development, and increase the levels of chlorophyll and antioxidants (Acharya et al., 2020; Das et al., 2018b; Kasote et al., 2019; Mahakham et al., 2016).

Itroutwar et al., (2019) found that biogenic zinc nanoparticles produced using brown seaweed (*Turbinaria ornata*) extracts were able to increase the plant development and the level of antioxidant compounds. Mahakham et al., (2016) showed that aged maize seeds, primed with biogenic gold nanoparticles synthesized using galangal rhizome extracts, showed improved germination with a 3-fold increase in the seed emergence vigor index compared to the control, in addition to increased chlorophyll content. Mahakham et al., (2016) found that priming rice seeds with biogenic silver nanoparticles produced using kaffir lime leaf extract increased the α-amylase activity and water uptake of the seeds, improving both germination and plant biomass. Kasote et al., (2019) primed watermelon
seeds with biogenic iron nanoparticles produced using onion extracts, which resulted in metabolic improvements in the plants and enhancement of the innate immune system.

Biogenic metallic nanoparticles can act as delivery systems, since many secondary metabolites may adhere on the nanoparticle surface. These metabolites can have different biological actions that improve plant metabolism and growth (Itroutwar et al., 2019; Raja et al., 2019). Also, many biological extracts have fungicidal or bactericidal properties and can be used in the protection against pathogens (Fraceto et al., 2018; Maroufpour et al., 2020). Furthermore, metallic or biogenic metallic nanoparticles can be taken up by seeds more easily due their small sizes, and trigger secondary metabolites more efficiently, providing stronger action in development of the embryo (Kasote et al., 2019; Mahakham et al., 2017, 2016).

### 2.3.2. Sustained release nanocarrier systems

Biopolymeric nanoparticles are systems that can be made from polysaccharides, lipids, and proteins, resulting in nanoparticles that are biodegradable and biocompatible, besides being able to be designed to respond to different environmental stimuli (Camara et al., 2019; Fraceto et al., 2016; Shakiba et al., 2020). These systems can be loaded with various substances including fungicides (Gao et al., 2020; Maluin et al., 2020), essential oils (Campos et al., 2019; de Oliveira et al., 2019; Kavetsou et al., 2019), plant growth regulators (Falsini et al., 2019; Pereira et al., 2017; Yi et al., 2015) and fertilizers (Guo et al., 2018).

Polymeric nanoparticles used for seed coatings are normally larger than 100 nm (Falsini et al., 2019), providing slow release of active compounds used to modify plant metabolism or to combat pathogens (Falsini et al., 2019; Kumar et al., 2020; Muthukrishnan et al., 2019; Shakiba et al., 2020). Numerous nanocarrier systems based on biopolymers have the potential to be used for seed treatment, including alginate (Du et al., 2019; Ji et al., 2019), zein (Oliveira et al., 2019; Qi et al., 2020), cellulose (Coelho et al., 2018; Zhang et al., 2017), synthetic biopolymers (poly-epsilon-caprolactone, poly(lactic-co-glycolic acid), and poly(lactic acid)) (Shakiba et al., 2020), and lipid nanoparticles (Campos et al., 2015; da Silva Santos et al., 2019).

Chitosan is another biopolymer that has been used to prepare nanocarrier systems for agricultural applications. This polymer has fungicidal properties and also acts in plant metabolism, activating defense mechanisms (Chandra et al., 2015; Kashyap et al., 2015; Maluin et al., 2020). The concentration of this biopolymer is a key consideration, since high concentrations block seed germination (Sathiyabama and Muthukumar, 2020). This effect is because oligosaccharides and polysaccharides can elicit physiological responses in plants, high concentrations causing cell apoptosis, while low concentrations have stimulant effects (Li et al., 2019).

Biopolymeric nanoparticles can be used as carrier systems for metal micronutrients, allowing a slow release of these compounds. Saharan et al., (2016) showed that the priming of maize seeds with chitosan nanoparticles containing copper induced α-amylase and protease activities and promoted reserve mobilization, thus favoring seedling growth. In contrast, the treatment with bulk copper led to inhibitory effects. Similarly, the priming with zinc-loaded chitosan nanoparticles induced the activity of hydrolytic enzymes in maize seeds, favoring germination.

Nanocarrier systems for plant growth regulators have shown potential for seed priming, providing improvements from seed germination up to production (Falsini et al., 2019; Pereira et al., 2019). Falsini et al., (2019) used lignin nanoparticles containing gibberellic acid to coat arugula and tomato seeds. They observed an improved biological activity of the gibberellic acid in inducing seed germination and plant growth when the phytohormone was nanoencapsulated. The ability of active nanoparticles or nanocarrier systems to increase seed germination can be explained by the fact that at optimal concentrations, nanoparticles are able to penetrate the seed coat, increasing the number of holes in the coat and consequently increasing water uptake and oxygen transfer (Guha et al., 2018; Joshi et al., 2018; Khodakovskaya et al., 2013).
In another work with tomato seed priming, Pereira et al., (2019) reported that chitosan-based nanocarrier systems of gibberellic acid were able to improve not only initial seedling growth but also fruit production under field conditions. In this study, the seed nano-priming were able to increase the production in 225.5 % and 178,8% for chitosan/tripolyphosphate and alginate/chitosan nanoparticles containing gibberellic acid, respectively. The method used to prepare this nanoparticles do not involve organic solvents, being interesting method aiming a sustainable agriculture. (Jiménez-Arias et al., 2020; Pereira et al., 2017)

The improvement of the biological efficacy of active compounds by nanocarrier systems is associated to the enhancement of physical-chemical proprieties. Effects such as increased solubility, protection against degradation, and greater bioavailability can reduce the required concentration of the active compound and minimize toxic effects (Fraceto et al., 2016; Lombardo et al., 2019). Several studies have shown that seed-nano priming can be more efficient than conventional methods and can contribute to improved agricultural practices (Acharya et al., 2019a; Bravo Cadena et al., 2018; Mahakham et al., 2017). Many systems defined as nanopesticides or nanofertilizers, used in the control of pests or for plant development, have the potential for use in seed nano-priming (Camara et al., 2019; Fischer et al., 2019; Lowry et al., 2019).

## 2.4. *Seed nano-priming and effects on plant metabolism under abiotic and biotic stresses*

Abiotic and biotic stresses lead to reduced production and economic losses. Abiotic stresses are environmental factors such as drought, flood, heat or cold, salinity, or nutrient-deficient soils. Biotic stresses are caused by microbial pathogens (bacteria or fungi), insects, or weeds that compete for nutrients (Camara et al., 2019; Zhao et al., 2020).

The use of seed nano-priming in agriculture can improve the quality of seeds and increase resistance against stress conditions. Nanoparticles can act directly against pathogens, as well as alter the metabolism of seeds and plants, consequently enhancing the innate immune system, altering hormone production, and making the plants more resistant to diseases or abiotic stress (Panyuta et al., 2016).

The nanoparticles uptake under seed coating can promote the ROS production, acting in different metabolic pathways, increase the level of active gibberellins, and the mobilization of storage proteins (Chandrasekaran et al., 2020). Also, the effect of nanoparticles in increasing water uptake by the seeds can cause sufficient stress to activate germination, increasing the activities of enzymes in phases I and II of the process (Acharya et al., 2019c; Joshi et al., 2018) (Figure 4a). Seed nano-priming has been shown to increase germination, since these systems are able to keep ROS levels in the optimum range comprised by the oxidative window that promotes seed germination (Figure 4b). (Kasote et al., 2019; Guha et al., 2018)
## Nanoparticles and germination effects

<figure><img src="image_7.png" /><figcaption>b) Seeds</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>c)</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Figure 4.** Effects of seed nano-priming on seeds and plants under abiotic and biotic stresses. a) Effects of seed nano-priming during the germination, b) effects of seed nano-priming on ROS levels, and c) potential effects under biotic and abiotic stress.</figcaption></figure>

Under stress conditions, nanoparticles can act to reduce seed ROS levels, due to increased activity of enzymes such as superoxidase dismutase, catalases, and guaiacol-peroxidase, hence reducing seed cell damage (Guha et al., 2018).

The storage of seeds for long periods at low temperatures results in aging, which can heavily decrease the germination rate (Mahakham et al., 2016). After long periods, the seed cells initiate processes that increase ROS generation and reduce antioxidant levels, causing metabolic side effects that result in a reduction of the germination index (Guha et al., 2018).

The use of biogenic metallic nanoparticles has been shown to be able to control reactive oxygen species at optimum levels, resulting in improved germination of aged seeds. The biogenic nanoparticles can be coated with many compounds that are natural reducing agents and act to decrease ROS levels in seeds (Acharya et al., 2019c; Guha et al., 2018; Mahakham et al., 2017).

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### 2.4.1. Biotic stress

Seed protection is required at different times during agricultural practices. In the field, the seeds and seedlings need to be protected against insects, fungi, and bacteria that can damage the seed and reduce germination or seedling growth. During harvesting and storage, seeds carry microorganisms from the field. These may shorten the shelf-life of the product or lead to the accumulation of microbial metabolites that make the products dangerous for consumption (Schmitt et al., 2020).

In the field, the use of fungicides and neonicotinoids for seed coating can reduce the damage caused by these organisms during plant establishment (Gross et al., 2020). This method has shown excellent potential compared to foliar applications (Farias et al., 2019; Matyjaszczyk, 2017). Relative to foliar treatment, the use of pesticides for seed treatment can reduce the amounts of the chemicals applied in the field as well as decrease the number of runs of agricultural machines on the land (avoiding soil compaction); so, the technique can be considered economically and environmentally advantageous (Matyjaszczyk, 2017).

Seed coating techniques employ fungicides that may be released in the field during the sowing process, resulting in environmental contamination. For example, pesticides such as carbofuran and thiran are highly toxic and persistent in the environment (Chau et al., 2019), so the consumption of treated seeds by animals such as granivorous birds can lead to poisoning or death (Gross et al., 2020).

Because nanoparticles are reactive and physiochemically dynamic materials, they tend to heteroaggregate in environmental media (Lowry et al., 2012). This can result in an effective seed coating, limiting the release of these compounds in the environment (Lowry et al., 2012).

Nonetheless, the use of nanoparticles for seed priming and coating can provide eco-friendly solutions. Many of these systems are nontoxic, biodegradable, and can increase the bioefficacy of active ingredients used at low concentrations (Bravo Cadena et al., 2018; Mahakham et al., 2017; Sathiyabama and Muthukumar, 2020).

Several systems have been shown to be effective against pathogens. Ahuja et al., (2019) demonstrated that iron(II) sulfide nanoparticles were more effective than the fungicide carbendazim to control the fungus *Fusarium verticillioides* in rice seeds. Silver nanoparticles are active against plant pathogens, including *Aspergillus flavus*, *Aspergillus niger*, *Aspergillus fumigatus*, and *Colletotrichum capsica*, showing great potential for seed protection. Metallic nanoparticle systems can be used for both seed priming and seed protection, as demonstrated for nanoparticles composed of iron (Ahuja et al., 2019), copper (Abdel Latef et al., 2017), silver(Mahakham et al., 2017), and silica (Bravo Cadena et al., 2018). Sathiyabama and Muthukumar, (2020) primed rice seeds with chitosan/guar nanoparticles, which resulted in increased plant development and higher levels of chlorophyll and carotenoids, the nanoparticles showing anti-fungal activity (71%) against the rice pathogen *Pyricularia grisea*. For pearl millet, Nandhini et al., (2019) showed that seed nano-priming and foliar application of zinc nanoparticles reduced downy mildew caused by *Sclerospora graminicola*.

Siddaiah et al., (2018) showed that the treatment of millet seeds with chitosan nanoparticles resulted in alteration of the innate immune system of the plants and increased resistance against pathogens. The treatment improved the levels of antioxidant enzymes and increased expression of proteins involved in the salicylic acid pathway, which is related to plant resistance responses against biotrophic pathogens. The field studies of Choudhary et al., (2019) showed that the treatment of maize seeds with zinc-containing chitosan nanoparticles resulted in plants with higher activity of antioxidant enzymes and lignin concentrations, making the plants more resistant to pathogens. Bravo Cadena et al., (2018) showed that silica nanoparticles loaded with cinnamon essential oil and applied to pea seeds acted as seed primers and provided bactericidal activity against *Pseudomonas syringae* pv. pisi, even at low concentrations. The use of essential oils with bactericidal or bactericidal properties for seed priming is a promising eco-friendly alternative
to chemical fungicides or bactericides (Bravo Cadena et al., 2018; Campos et al., 2019; de Oliveira et al., 2019).

Techniques employed during the storage of cereals include the control of humidity and temperature to minimize the growth of microorganisms, as well as the use of chemical products, although the latter may be viewed negatively by consumers, who prefer natural products (Schmitt et al., 2020). For this purpose, the use of biopolymers or essential oils with activities against pests can provide a greener alternative to agrochemicals, avoiding problems associated with residual contamination.

### 2.4.2. Abiotic stress

Harvest yields can be drastically decreased by soil salinity and contamination (Maswada et al., 2018). Salinity represses plant growth due to water and nutritional deficits and direct ionic effects on plant metabolism (Abdel Latef et al., 2017). Contamination with heavy metals may be both anthropogenic (due to fertilizer applications, industrial processes, atmospheric deposition following waste incineration, and discharges of sewage and sludge) and geogenic (due to natural atmospheric deposition or regional geological processes) (Barbieri et al., 2018; Hussain et al., 2019; Qayyum et al., 2017).

Ye et al., (2020) showed that jalapeño seeds primed with manganese nanoparticles presented improved root growth under saline conditions, modulating the distribution of sodium between roots and shoots. The highest concentration was found in the shoots, which the authors attributed to a higher plants response mechanism to avoid root damage caused by high concentrations of salts changing the osmotic process and hormone regulation.

Maswada et al., (2018) reported that sorghum seeds treated with nano-iron(III) showed improved germination, with the plants showing increased chlorophyll contents and improved growth under saline conditions. These results indicate that this system could be used not only to improve seed germination, but also to avoid stress conditions. In another study, priming lupin seeds with zinc nanoparticles improved plant development under saline conditions, due to increased levels of photosynthetic pigments, phenols, organic molecules, and antioxidant enzymes (Abdel Latef et al., 2017).

In a case of heavy metal contamination, Rizwan et al., (2019) showed that priming of wheat seeds with iron and zinc nanoparticles reduced the absorption of cadmium, resulting in low cadmium concentrations in the grains. Hussain et al., (2019) also demonstrated that seed priming with silicon nanoparticles were able to reduce the cadmium concentration in wheat roots, shoots and grain.

Soil deficiency of nutrients such as iron and zinc can affect the production of phytohormones involved in plant defense responses, including jasmonic acid and salicylic acid (Choudhary et al., 2019; Kasote et al., 2019). Kasote et al., (2019) primed watermelon seeds with biogenic iron nanoparticles prepared using onion extract. The plants showed increased levels of jasmonic acid and of its precursor cis-(+)-12-oxophytodienoic acid during the early seedling stage, which could increase the resistance to stress.

Nguyen et al., (2020) showed that maize seed priming with cooper nanoparticles can enhance drought resistance in plants. The leaves kept high water content, increased the levels of anthocyanin, chlorophyll and carotenoids and reduced the oxidative stress.

These results showed the potential of seed-nano priming to relieve plant stress caused by saline conditions, drought, the presence of heavy metals, or nutrient deficiency, with the modulation of metabolism to resist stress conditions and improve plant growth (Figure 4 c). The use of seed nano-priming to trigger plant resistance to abiotic stress is a strong alternative to avoid side effects of global climatic crisis or anthropogenic and geogenic effects, in order to reduce losses in the field production.

### 2.5. Effects on microbiota

The effect of seed nano-priming on plant microbiota is a fairly new area requiring further studies. The interaction between plants and microorganisms is essential for agri-
culture, since these symbiotic relations are important for the absorption of nutrients, defense against pathogens, plant quality, and productivity (Shcherbakova et al., 2017).

The foliar application of nanoparticles can modulate the root microbial community. Raliya et al., (2016) showed that foliar application of zinc nanoparticles to mung beans led to an extension of the rhizosphere zone, with root volume increasing by 58.9%. They also reported increases in the activity of rhizosphere enzymes such as acid phosphatase (98.07%), alkaline phosphatase (93.02%), and phytase (108%). These factors are extremely important for the uptake of nitrogen and phosphorus by plants.

Dai et al., (2020) found that roots exposed to cerium oxide nanoparticles decreased the rhizosphere bacterial community, but enhanced the growth of microorganisms that promote plant growth.

Others studies have reported side effects in the soil microbiota following amendments with nanoparticles. Zhang et al., (2020) found that silver nanoparticles (at 100 mg/kg) altered the soil pH and negatively affected microorganisms involved in nitrogen, carbon, and phosphorus cycles. In another study, Li et al., (2020) showed that the amendment of soils with silver nanoparticles (10.4 mg kg⁻¹) reduced plant development and resulted in silver bioaccumulation.

Other metallic nanoparticles, such as titanium and zinc nanoparticles, have been found to affect the soil microbiota, depending on the concentration and the duration of exposure (Chavan and Nadanathangam, 2020; Zhai et al., 2019). Apart from the intrinsic properties (chemical nature, size, coating) and concentration, the impact of nanoparticles on plant root and rhizosphere microbiota are largely dependent on soil texture, pH, organic matter content, and fertilization (Hamidat et al., 2016; Santaella and Plancot, 2020; Simonin et al., 2018).

Few studies have investigated how seed nano-priming might affect the interactions of microorganisms with roots. Rahman et al., (2020) showed that seed priming using platinum nanoparticles stabilized with poly(vinylpyrrolidone), resulted in side effects on the root microbiota of pea plants, with a decrease in mycorrhizal fungi and rhizobial colonization, while these effects were not observed for the same system with gold or silver nanoparticles.

More studies in this field must be encouraged in order to elucidate the mechanisms by which seed-nano priming can affect seed metabolism and alter the microbiome selected by the roots during development of the plant. Many nanoparticle systems have shown great potential for use in agriculture, but could be toxic towards soil microbiota. The treatment of seeds may offer a safer way to improve the establishment of plants and avoid side effects in the soil, since the treatment is performed on the seeds, avoiding applications in soils.

## 2.6. Improving crop quality and production

Seed nano-priming can increase the productivity of different crop species, due to the positive effects on plant metabolism and development. Fast root development increases the potential of the plant to access nutrients and water, accompanied by faster expansion of the leaf area, consequently increasing the use of light energy for plant growth.

The effects on the innate immune system can improve the resistance against pathogens, so that smaller quantities of pesticides need to be applied. Consequently, the concentrations of residual agrochemicals in food are lower, making the products safer for consumption. These effects have been demonstrated in studies evaluating the effects of seed priming, up to harvest, in species with agronomical value.

Yasmeen et al., (2017) showed that in wheat plants, seed priming with iron and copper nanoparticles led to improvements of spike length, number of grains per spike, and grain weight. Rahman et al., (2020) reported that priming of pea seeds with platinum nanoparticles stabilized with poly(vinylpyrrolidone) increased production by 163.5%, although the seed weight was reduced by 66.7%.
Acharya et al., (2020) demonstrated the potential benefits of priming watermelon seeds with biogenic silver nanoparticles produced using onion extracts. The results revealed a burst in plant development and increased metabolic activity throughout the life of the plant, resulting in production increases between 31.6% and 35.6%.

Pereira et al., (2019) reported that the priming of tomato seeds with alginate/chitosan nanoparticles containing gibberellic acid greatly improved fruit production, increasing productivity by almost 4-fold.

Another issue concerns biofortification. For example, the treatment of seeds with iron, zinc, and manganese nanoparticles resulted in plants and grains with higher contents of these minerals (Sundaria et al., 2019). Other studies have also reported that increased concentrations of these elements in roots, shoots, leaves, or grains can result in vegetables richer in essential minerals (Rizwan et al., 2019; Sundaria et al., 2019).

Seed nano-priming can be an effective way to reduce the amounts of fertilizers applied to crops. A major problem in agriculture is that only 30-50% of the nitrogen and 45% of the phosphorus applied in the field are absorbed by the crops (Das et al., 2018b). These losses can be harmful to the environment, since they can lead to the eutrophication of aquatic systems and contamination of native terrestrial ecosystems. The use of seed nano-priming has been shown to improve root development and increase the production of enzymes required for nutrient uptake, which would enable reductions of the amounts of fertilizer used in the field (Guha et al., 2018).

### 3. Concerns

Although nanoparticles have potential for use in seed priming or seed coating, caution must be exercised in the application of nanomaterials. The safe use of these systems demands the development of appropriate regulations based on sound research, not only in agriculture, but also in many other industrial sectors (Kah et al., 2018; Scott et al., 2018). Legal frameworks are required for industrial production of nanomaterials, treatment of industrial waste, and agricultural applications, together with evaluation of the fates of these nanoparticles in the environment, considering their possible ecotoxicity.

Agricultural activities are connected with many ecosystems that may be directly impacted by nanomaterials (Lowry et al., 2019). Therefore, it is crucial to understand the mechanisms of action of these materials and to develop nanoparticles that are safe in both the field and the wider environment.

Before seed treatment, it is necessary to evaluate the conditions employed for priming, considering the nanoparticle size and concentration, and the duration of exposure, since these factors can cause side effects such as germination inhibition, reduced plant development, detrimental alterations of metabolism and cell structure, and modification of root-microbiota interactions (Duran et al., 2017; Rahman et al., 2020; Rizwan et al., 2019).

It is essential to understand how the physical-chemical properties of nanoparticles affect seeds and other associated organisms, in order to be able to design nanoparticles that are both effective and present minimal toxic effects (Camara et al., 2019).

The different kinds of nanoparticles cited in this article (metallic, biogenic metallic and polymeric nanoparticles) not only differ in their physical chemical characteristics, but also in their biological activity. The design of these nanoparticles for seed priming can be used for different strategies, as example seed protection, biofortification, plant resistance against pests and abiotic stresses or even the mix of this effects.

However, the use of nanoparticles for seed priming can provide great advantages. The treatments in seeds reduce the exposition of nanomaterials compared with foliar and soil applications. Another positive point is the low concentrations of nanoparticles used for seed priming, that can be a through a controlled form by factories, avoiding high release of this materials in the environment. Probably, the residual of nanoparticles in plants will be very small or even none, but studies are necessary to elucidate how different nanoparticles as metallics, metallics biogenic and polymerics interact with the plant development.
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**4. Conclusions**

Nanotechnology is a promising area for exploitation in agriculture, and seed-nano priming is one of the tools that can be employed to promote sustainability. These systems can promote the establishment of plants, as well as provide protection against biotic and abiotic stresses, resulting in improvements of productivity and food quality.

There are many issues that need to be addressed concerning the industrial production of these technological systems and their application in the field, including scale-up, seed priming conditions, and toxic effects in plants and other organisms. However, it is clear that the adoption of nanoparticle systems can alter crop management, with reduction of the applied quantities of pesticides and contamination risks, resulting in agricultural practices that are safer for farmers, consumers, and the environment.

**Declaration:** The authors declare that they have no competing interests.

**Author's contribution:** Anderson do Espirito Santo Pereira: Conceptualization, Writing - original draft - review & editing, Halley Caixeta Oliveira: Conceptualization, Writing - original draft - review & editing, Leonardo Fernandes Fraceto: Conceptualization, Writing - original draft - review & editing and Catherine Santaella: Conceptualization, Writing - original draft - review & editing

**Acknowledgments:** The authors are grateful for financial support provided by CAPES-COFECUB (project #88881.191767/2018-01) and the São Paulo State Research Foundation (FAPESP, #2017/21004-5). A.E.S.P is grateful for a post-doctoral grant provided by CAPES-COFECUB (#88887.363975/2010-00).
<table><caption>**Table 1.** Nanoparticles employed for seed priming and coating, showing the nanoparticle systems, their characteristics, and the main effects on the species evaluated. Each article is classified according its application: A1 (seed priming), A2 (effects in the field), A3 (effects against pathogens), and A4 (relieving abiotic stresses).</caption><thead><tr><th>Nanoparticle system</th><th>Characteristics</th><th>Main effects</th><th>Applications</th><th>Reference</th></tr></thead><tbody><tr><td>Biogenic silver nanoparticles produced using kaffir lime leaf extracts</td><td>Spherical nanoparticles with particle size of 6-26 nm.</td><td><strong>Concentrations:</strong> 10 and 20 mg mL<sup>-1</sup>. <strong>Species:</strong> Rice seeds (Oryza sativa L. cv. KDML 105). <strong>Effects:</strong> Increases of water uptake and aquaporin gene expression, enhanced α-amylase activity (1.2-1.3-fold higher than control group), faster germination, high vigor index of seedlings, and increases of root, shoot, and seedling biomass. The seeds showed greater uptake of silver ions compared to the silver nanoparticles.</td><td></td><td>Mahakham et al., 2017</td></tr><tr><td>Iron oxide nanoparticles</td><td>Particle size &lt;50 nm, with surface area of 180 m<sup>2</sup>/g<sup>2</sup>.</td><td><strong>Concentrations:</strong> 10, 50, 100, and 500 mg L<sup>-1</sup>. <strong>Species:</strong> Sorghum (Sorghum bicolor (L.) Moench) KDML 105. <strong>Effects:</strong> The treatments increased seed germination and plant growth. Treatment at 500 mg mL<sup>-1</sup> increased photosynthetic pigments and, biomass. Treatments at 100 and 500 mg mL<sup>-1</sup> increased the relative water content in leaves.</td><td></td><td>Maswada et al., 2018</td></tr></tbody></table>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>
<table><tr><td>Biogenic iron nanoparticles produced using onion extracts</td><td>Particle size of 19-30 nm, with low-crystalline or amorphous Fe2O3.</td><td><b>Concentrations:</b> 20, 40, 80, and 160 mg L-1 <b>Species:</b> Watermelon (Citrullus lanatus (Thunb.) Matsum and Nakay varieties). <b>Effects:</b> Treatment with the nanoparticles increased germination and growth of shoots and roots. The nanoparticles did not cause adverse antioxidant and chlorophyll effects, compared to the effects of the bulk counterparts (FeCl3 and Fe2O3). The nanoparticles were taken up and translocated into the seed endosperm. Increased non-enzymatic antioxidant levels and induction of jasmonates-linked defense responses in the seeds.</td></tr><tr><td>Zinc oxide and iron oxide nanoparticles</td><td>Zinc oxide nanoparticles with sizes of 20-30 cm; iron oxide nanoparticles (Fe3O4) with sizes of 50-100 nm.</td><td><b>Concentrations:</b> Zinc nanoparticles at 25, 50, 75, and 100 mg L-1; iron nanoparticles at 5, 10, 15, and 20 mg L-1. <b>Species:</b> Wheat (Triticum aestivum L.). <b>Effects:</b> The seed priming (high concentrations) improved plant development, increasing the spike length, plant biomass, and photosynthetic pigments, while reducing cadmium uptake. The zinc nanoparticles reduced cadmium levels in shoots, roots, and grains by 38%, 55%, and 83%, respectively. The iron nanoparticles reduced cadmium levels in the shoots, roots, and grains by 54%, 56%, and 84%, respectively. The plants presented high concentrations of Zn and Fe after seed priming.</td></tr><tr><td>Silicon nanoparticles</td><td>Spherical nanoparticles with size around 90 nm.</td><td><b>Concentrations:</b> The nanoparticles were evaluated at concentrations of 300, 600, 900, and 1200 mg L-1. <b>Species:</b> Wheat (Triticum aestivum L.). <b>Effects:</b> The seeds treated with nanoparticles and germinated in soil contaminated with cadmium showed increases of plant biomass, photosynthetic rate, and levels of carotenoids and chlorophylls a and b, while there was decreased formation of reactive oxygen species and antioxidant enzymes</td></tr></table>

<figure><img src="image_12.png" /></figure>

Kasote et al., 2019

<figure><img src="image_13.png" /></figure>

Rizwan et al., 2019

<figure><img src="image_14.png" /></figure>

Hussain et al., 2019
Biogenic zinc nanoparticles produced using brown seaweed (Turbinaria ornata) extracts

Spherical and hexagonal nanoparticles with average size of 15-52 nm.

activity. Cadmium uptake by the plants was reduced, with the greatest effect for the concentration of 1200 mg L⁻¹.

**Concentrations:** Nanoparticles at concentrations of 5, 10, 25, 50, 100, and 200 mg L⁻¹. **Species:** Rice (Oryza sativa L.). **Effects:** The treatment at 10 mg L⁻¹ had the greatest effect, increasing seed germination, vigor index, and seedling development. The plants showed high contents of antioxidant enzymes. There was a dose-response accumulation of zinc in the seedlings, according to the concentration used for seed priming.

<figure><img src="image_15.png" /></figure>

Itroutwar et al., 2019

Nanoparticles of zinc, titanium, and silver

Zinc oxide nanoparticles (35-40 nm), titanium oxide nanoparticles (100 nm), and silver nanoparticles (85 nm), with spherical, cylindrical, and needle-like morphologies, respectively.

**Concentrations:** Nanoparticles at concentrations of 750, 1000, and 1250 mg kg⁻¹. **Species:** Chilli (Capsicum annuum L.). **Effects:** The zinc nanoparticles increased germination, as well as shoot and root growth. These effects were not observed for the titanium and silver nanoparticles. Evaluation against the pathogens *Aspergillus flavus*, *Aspergillus niger*, *Aspergillus fumigatus*, and *Colletrotrichum capsica* showed that silver nanoparticles at 1000 and 1250 mg kg⁻¹ could reduce pathogen infection.

<figure><img src="image_16.png" /></figure>

Dileep Kumar et al., 2020

Chitosan/tripolyphosphate nanoparticles

Nanoparticles with size of 259.4 ± 4.7 nm, PDI (polydispersity index) of 0.28 ± 0.0016, and zeta potential of 40.0 ± 2.9 mV.

**Concentrations:** Nanoparticles at concentrations of 1-100 µg mL⁻¹. **Species:** Wheat (Triticum aestivum L.). **Effects:** The nanoparticles promoted plant development, increasing the chlorophyll content at low concentration (5 µg mL⁻¹), and showed metabolic effects, with enhancement of auxin content in roots and shoots.

<figure><img src="image_17.png" /></figure>

Li et al., 2019
Biogenic gold nanoparticles
synthesized using galangal
rhizome extracts

Spherical nanoparticles
with size of 10-30 nm.

**Concentrations:** Nanoparticles at concentrations of 5, 10, and 15 ppm.
**Species:** Maize (Zea mays L.). **Effects:** The treatment at 10 ppm improved the emergence from aged seeds by 83% and seed vigor index by 3-fold, compared to the control. The plants showed increased chlorophyll content. The nanoparticles were detected in the seeds, roots, and shoots.

<figure><img src="image_18.png" /></figure>

Mahakham et
al., 2016

Manganese(III) oxide nano-
particles

Spherical nanoparticles
with size of 50 nm and
zeta potential of 29.2 ± 1.8
mV.

**Concentrations:** 0.1, 0.5, and 1 mg mL⁻¹. **Species:** Jalapeño (Capsicum annuum L.). **Effects:** Under salt stress conditions, the seedlings showed greater root length (around 55% greater than for the control). Molecular evaluation (expression of manganese superoxide dismutase) showed that the seed priming could control oxidative stress. Seed priming avoided changes of proteins, lignin, or distribution of nutrients, which are effects caused by salt stress in plants. The nanoparticles were taken up by the seeds and formed nanoparticle-corona complexes.

<figure><img src="image_19.png" /></figure>

Ye et al., 2020

Iron(II) sulfide aqua nano-
particles

Spherical nanoparticles
with size around 6-20 nm

**Concentration:** 30 µg mL⁻¹. **Species:** Rice (Oryza sativa L.). **Effects:** The treatment of seeds with nanoparticles resulted in increased germination, vigor index, and root length. Evaluation against the fungus Fusarium verticillioides showed that the nanoparticles presented anti-fungal activity, with 10-fold greater efficiency than the fungicide carbendazim for control of the fungus and reduction of rice rot.

<figure><img src="image_20.png" /></figure>

Ahuja et al.,
2019
<table><tr><td>Copper nanoparticles</td><td>Nanoparticles with sizes of 25, 40, and 80 nm, and zeta potentials of 15-25 mV.</td><td>Concentrations: 1, 10, 100, and 1000 mg Cu L<sup>-1</sup>. Species: Common bean (Phaseolus vulgaris L.). Effects: The effects depended on nanoparticle size and concentration, with low concentrations and larger nanoparticles increasing plant weight. High concentrations inhibited seed germination, independent of nanoparticle size.</td></tr><tr><td>Chitosan nanoparticles of and carbon nanotubes</td><td>Chitosan nanoparticles with size of 95 ± 2 nm and zeta potential of +123.5 mV; carbon nanotubes with size of 40 ± 0.4 nm and zeta potential of -8.5 mV</td><td>Concentrations: 10% of both nanomaterials were used, with a concentration of 20 µg L<sup>-1</sup>. Species: Common bean (Phaseolus vulgaris L.). Effects: Seed priming with the nanomaterials led to side effects. The plants presented reduced growth parameters, stress effects (increases of electrolyte leakage and hydrogen peroxide), and reduced concentrations of carbohydrates, proteins, and vitamin C. The authors evaluated the same systems for foliar treatment, with the results showing improvement of plant quality. The carbon nanomaterial led to a reduction of 30 days for harvesting the crop.</td></tr><tr><td>Zinc nanoparticles</td><td>Zinc nanoparticles with size of 20 nm and spherical shape, and sizes of 40 and 60 nm with elongated shapes.</td><td>Concentrations: 1, 10, 100, 1000, and 5000 mg L<sup>-1</sup>. Species: Common bean (Phaseolus vulgaris L.). Effects: None of the treatments affected germination. After 5 days, higher fresh weight was obtained for the treatment with 40 nm nanoparticles. Few nanoparticles were detected in the cotyledons, with the greatest part being retained in the seed coat.</td></tr></table>

<figure><img src="image_21.png" /></figure>

Duran et al., 2017

<figure><img src="image_22.png" /></figure>

Abdel-Aziz et al., 2019

<figure><img src="image_23.png" /></figure>

Savassa et al., 2018
<table><tr><td>Chitosan nanoparticles</td><td>Chitosan nanoparticles with sizes of 20-170 nm.</td><td>Concentrations: 0.5, 1, 1.5, 10, 15, and 20 mg mL<sup>-1</sup>. Species: Rice (Oryza sativa L.). Effects: The nanoparticles increased seed germination, seedling growth, and plant weight.</td><td>Divya et al., 2019</td></tr><tr><td>Zinc nanoparticles</td><td>Mean size of 21.3 nm</td><td>Concentrations: 20, 40, and 60 mg L<sup>-1</sup>. Species: Lupin (Lupinus termis L.). Effects: A saline condition was imposed by the use of 150 mM NaCl. The treatment at 60 mg L<sup>-1</sup> mitigated the stress condition, avoiding reductions of growth parameters (root and shoot lengths; fresh and dry weights) and photosynthetic pigments. Soluble sugar and protein contents were increased, lipid peroxidation was reduced.</td><td>Abdel Latef et al., 2017</td></tr><tr><td>Biogenic silver and gold nanoparticles produced using onion extract</td><td>Silver nanoparticles with size of 11.6 ± 2.40 nm and zeta potential of -2.20 ± 0.29 mV, with spherical and ellipsoidal morphology; gold nanoparticles with size of 93.68 ± 2.06 nm and zeta potential of -8.51 ± 1.26 mV, with anisotropic morphology.</td><td>Concentrations: Silver nanoparticles at 31.3 µg mL<sup>-1</sup> and gold nanoparticles at 31.3 µg mL<sup>-1</sup>. Species: Onion (Allium cepa L.) Effects: The studies were performed for 2 years; the treatments resulted in increases of seedling emergence, number of leaves, plant weight, and productivity, compared to unprimed and hydro-primed seeds. The results also showed metabolic effects, with increases of chlorophylls a and b. The gold nanoparticles provided better results than the silver nanoparticles. The uptakes of nanoparticles by the onion seeds were 29.75% for silver nanoparticles and 4.86% for gold nanoparticles.</td><td>Acharya et al., 2019a</td></tr></table>

<figure><img src="image_24.png" /></figure>

<figure><img src="image_25.png" /></figure>

<figure><img src="image_26.png" /></figure>
Platinum nanoparticles stabilized with poly(vinylpyrrolidone)

Nanoparticles with size of 3.2 ± 0.8 nm and spherical morphology.

**Concentrations:** Concentrated solution at 1.0 mM. **Species:** Pea (*Pisum sativum* L.). **Effects:** Seed priming for 3 hours resulted in side effects, with decreases of germination and root length. The treatment reduced colonization by microorganisms (arbuscular mycorrhizal fungi and rhizobia).

<figure><img src="image_27.png" /></figure>

Rahman et al.,
2020

Biogenic silver nanoparticles produced using Phyllanthus emblica

Nanoparticles with size of 10-35 nm, irregular shape, and zeta potential of -23.8 mV.

**Concentrations:** 0, 5, 10, 25, and 50 mg L⁻¹. **Species:** Wheat seeds (*Triticum aestivum* L.). **Effects:** The biogenic silver nanoparticles reduced ROS production by root systems compared to non-biogenic silver nanoparticles, although the effects differed according to the wheat variety. The non-biogenic silver nanoparticles reduced the germination index, while the biogenic nanoparticles at a concentration of 10 mg L⁻¹ increased root growth. Coating with plant extract reduced nanoparticle uptake by wheat seeds.

<figure><img src="image_28.png" /></figure>

Kannaujia et
al., 2019

Chitosan guar nanoparticles

Nanoparticles with size of 122 nm, PDI of 0.358, and zeta potential of -30 mV.

**Concentrations:** 0.05, 0.1, and 0.2%. **Species:** Rice (*Oryza sativa* L.). **Effects:** At concentrations of 0.05% and 0.1%, the nanoparticles promoted seed germination, while the use of 0.2% reduced seed germination. The plants showed increases of root and shoot lengths as well as of fresh and dry weights. The levels of chlorophyll and carotenoids were increased. The formulation showed anti-fungal activity (71%) against the rice blast pathogen *Pyricularia grisea*.

Sathiyabama
and
Muthukumar,
2020
<table><tr><td>Iron nanoparticles</td><td>Nanoparticles with size of ~80 nm and zeta potential of -44 mV.</td><td><b>Concentrations:</b> 25, 50, 100, 200, 300, 400, 500, and 1000 µg mL<sup>-1</sup>. <b>Species:</b> Wheat (<i>Triticum aestivum</i> L.), types WL711 (low-iron genotype) and IITR26 (high-iron genotype). <b>Effects:</b> Increased germination index at 300 and 400 µg mL<sup>-1</sup>. The best treatment for WL711 was 200 µg mL<sup>-1</sup>, which increased shoot and root lengths by 64% and 46%, respectively. High concentrations caused inhibition of plant growth. At 25 µg mL<sup>-1</sup>, the concentrations of iron at harvest were 45.7% (IITR26) and 26.8% (WL711).</td></tr><tr><td>Zero-valent iron nanoparticles (priming of aged seeds)</td><td>Nanoparticles with size of 33.8 ± 3.59 nm and zeta potential of -39 mV.</td><td><b>Concentrations:</b> 10, 20, 40, 80, and 160 mg L<sup>-1</sup>. <b>Species:</b> Rice (<i>Oryza sativa</i> L.). <b>Effects:</b> At 20 mg L<sup>-1</sup>, there was increased water uptake, while above this concentration, the water uptake decreased by 25-50%. Increases were observed for root and shoot lengths, chlorophyll and carotenoid levels, and amylase and protease activities. Control of ROS levels and membrane integrity was the greatest at 20 mg L<sup>-1</sup>. The highest concentration (160 mg L<sup>-1</sup>) had negative effects on seedling growth. Iron content in roots and shoots increased in line with the nanoparticle concentration.</td></tr><tr><td>Biogenic silver nanoparticles and turmeric nanoemulsions</td><td>Turmeric nanoemulsion with particle size of 171.3 ± 0.52 nm, PDI of 0.25, and zeta potential of -1.23 ± 0.16 mV; biogenic silver nanoparticles with size of 141.3 ± 0.78 nm, PDI of 0.18 ± 0.03, and zeta potential of -0.03 ± 0.01.</td><td><b>Concentrations:</b> 10-fold diluted turmeric nanoemulsion and biogenic silver nanoparticles at 31.3 µg mL<sup>-1</sup>. <b>Species:</b> Watermelon (<i>Citrullus lanatus</i> (Thunb.) Matsum. & Nakai). <b>Effects:</b> The biogenic nanoparticles reduced the germination time. There were increases of the catabolism of disaccharides (sucrose) to monosaccharides (fructose and glucose), shoot length, stem diameter, and chlorophyll content. There was no difference in the concentration of phenolic compounds, compared to the control. Productivity increased by between 31.6% and 35.6%. The formulations were taken</td></tr></table>

<figure><img src="image_29.png" /></figure>

Sundaria et al., 2019

<figure><img src="image_30.png" /></figure>

Guha et al., 2018

<figure><img src="image_31.png" /></figure>

Acharya et al., 2020
<table><tr><td>Multi-walled carbon nanotubes</td><td>Nanotubes with diameter of 13-14 nm.</td><td>up by the seeds.<br>Concentrations: 70, 80, and 90 µg mL<sup>-1</sup>. Species: Wheat (Triticum aestivum L.). Effects: Increases of seed germination, root length, number of root hairs, shoot length, plant weight, number of stomata, and size of average length of vascular systems (xylem and phloem). At 90 µg mL<sup>-1</sup>, there were increases of 21% and 27% for spike length and weight, respectively, 20% for the number of spikelets, and 32% for grain production.</td></tr><tr><td>Nano-pyrite (FeS<sub>2</sub>)</td><td>Nanoparticles with sizes in the range 25-100 nm, with spherical morphology</td><td>Concentrations: 50 µg mL<sup>-1</sup>. Species: Rice (Oryza sativa L.). Effects: Increased α-amylase activity and improved germination. Field studies showed that seeds treated with nano-pyrite (FeS<sub>2</sub>), with seedlings grown in the field without fertilizers, had the same productivity found for untreated seeds, with seedlings grown in the presence of fertilizers.</td></tr><tr><td>Mesoporous silica nanoparticles containing cinnamon essential oil</td><td>Nanoparticles with size of ~100 nm and pores of around 2-2.8 nm.</td><td>Concentration: 2 mg mL<sup>-1</sup>. Species: Pea seeds (Pisum sativum L.). Effects: The seeds showed fast germination and in biological evaluations against Pseudomonas syringae pv. pisi, the cinnamon essential oil provided a 90,000-fold increase in bactericidal activity in comparison the oil non-encapsulated.</td></tr><tr><td>Copper and iron nanoparticles</td><td>Copper nanoparticles with sizes of around 15-30 nm; iron nanoparticles with sizes of around 20-30 nm.</td><td>Concentrations: 20, 25, 30, 35, and 40 ppm Species: Wheat (Triticum aestivum L.) seeds of varieties galaxy-13, Pakistan-13, and NARC-11. Effects: Different effects were observed, depending on the wheat variety and the nanoparticle priming. The nanoparticles increased starch degradation, glycolysis, and the tricarboxylic acid cycle. Both types of nanoparticle increased the sugar content and superoxide dismutase activity, while the</td></tr></table>

<figure><img src="image_32.png" /></figure>

Joshi et al.,
2018

<figure><img src="image_33.png" /></figure>

Das et al.,
2018b

<figure><img src="image_34.png" /></figure>

Bravo Cadena
et al., 2018

<figure><img src="image_35.png" /></figure>

Yasmeen et al.,
2017
<table><tr><td>Cobalt and molybdenum oxide nanoparticles</td><td>Both types of nanoparticle had sizes of 60-80 nm and spherical morphology.</td><td>copper nanoparticles had greater potential to improve stress tolerance. Treatment with the iron and copper nanoparticles at 25 µg mL<sup>-1</sup> increased the spike length, the number of grains per spike, and the grain weight. However, high concentrations decreased all these parameters, depending on the wheat variety.<br/><b>Concentrations:</b> 1 L of stock commercial solution for 40 kg of seeds. <b>Species:</b> Soybean seeds (Glycine max (L). Merr.). <b>Effects:</b> The nanoparticles increased seed germination and, when associated with plant growth regulators (auxin and gibberellic acid), increased the shoot and root length and dry biomass. The molybdenum oxide nanoparticles increased the activities of amylases and lipase.</td></tr><tr><td>Biochar nanoparticles produced from rice and wood sawdust</td><td>Nanoparticles with sizes of ~22-56 nm and spherical morphology.</td><td><b>Concentrations:</b> 0.5, 5, and 50 mg L<sup>-1</sup>. <b>Species:</b> Rice (Oryza sativa L.) and tomato (Lycopersicon esculentum Mill.). <b>Effects:</b> For rice, the treatments with rice biochar nanoparticles increased seed germination, with the exception of the highest concentration (50 mg L<sup>-1</sup>). The treatments increased root growth by 50%, compared to the control, and also increased the plant biomass. The biochar nanoparticles showed no stimulant effects on tomato seeds. For the other species, the nanoparticles presented side effects.</td></tr><tr><td>Chitosan nanoparticles loaded with thiamine</td><td>Nanoparticles with size of 560 nm, polydispersity &lt;1, and zeta potential of 37.7 mV.</td><td><b>Concentration:</b> 0.1%. <b>Species:</b> Chickpea (Cicer arietinum L.) <b>Effects:</b> The nanoparticles increased seed germination, induced a 10-fold increase of the auxin level in seedlings, and increased the quantity of secondary roots.</td></tr></table>

<figure><img src="image_36.png" /></figure>

Chau et al., 2019

<figure><img src="image_37.png" /></figure>

K. Zhang et al., 2020

<figure><img src="image_38.png" /></figure>

Muthukrishnan et al., 2019
<table><tr><td>Lignin nanoparticles loaded with gibberellic acid</td><td>Nanoparticles with sizes around 200-250 nm, polydispersity of 0.17-0.38, and spherical morphology.</td><td><b>Concentrations:</b> 0.5, 1, and 1.5 mg mL<sup>-1</sup>. <b>Species:</b> Arugula (Eruca visicaria (L.) Cav. subsp. sativa), tomato (Solanum lycopersicum L. cv. Ciliegino), and chickpea (Cicer arietinum L.). <b>Effects:</b> The effects varied according to the time of sowing after the treatment. For the arugula seeds, the formulations improved germination and increased the stem and root lengths and the plant biomass. Similar effects were observed for tomato seeds, varying according to the parameter and the concentration used. The system could be taken up by the seeds and provided a seed coating.</td></tr><tr><td>Chitosan nanoparticles loaded with gibberellic acid</td><td>Alginate/chitosan nanoparticles with 450 ± 10 nm, PDI of 0.3 and zeta potential of -29 mV. Chitosan/tripolyphosphate nanoparticles with 195 ± 1 nm, PDI of 0.3 and zeta potential of 27 mV. Spherical morphology</td><td><b>Concentrations:</b> 0.05, 0.005, and 0.0005 mg mL<sup>-1</sup>. <b>Species:</b> tomato (Solanum lycopersicum var. cerasiforme). <b>Effects:</b> The chitosan nanoparticles had a biological response according the concentration. The chitosan nanoparticles improve the shoot and root dry weight with 30 days. After 90 days the seeds treated with chitosan nanoparticles containing gibberellic acid, improve the shoot plant length and weight. Also improve the production of fruits in 225% for chitosan/tripolyphosphate and 178% for alginate/chitosan nanoparticles. The plants treated with alginate/chitosan nanoparticles loaded with gibberellic acid increase the fruit weight in 30%</td></tr><tr><td>Biogenic silver nanoparticles produced using the fungus Macrophomina phaseolina</td><td>Nanoparticles with size of 5-30 nm.</td><td><b>Concentrations:</b> 10, 20, and 50 µg mL<sup>-1</sup>. <b>Species:</b> Soybean (Glycine max (L.) Merr.). <b>Effects:</b> The nanoparticles showed bactericidal effects against gram-positive and gram-negative bacteria. There were no observed effects on germination or seed development.</td></tr></table>

<figure><img src="image_39.png" /></figure>

Falsini et al., 2019

<figure><img src="image_40.png" /></figure>

Pereira et al., 2019

<figure><img src="image_41.png" /></figure>

Spagnoletti et al., 2019
<table><tr><td>Chitosan nanoparticles</td><td>Nanoparticles with size of 400 nm and spherical morphology.</td><td><b>Concentration:</b> 250 mg kg<sup>-1</sup>. <b>Species:</b> Pearl millet (Pennisetum glaucum (L.) R.Br.). <b>Effects:</b> The chitosan nanoparticles increased seed germination. The treated seeds showed increased resistance against downy mildew disease caused by the biotrophic oomycete Sclerospora graminicola. The seed priming improved the innate immune system. The plants showed increased the expression of genes of phenylalanine ammonia lyase, peroxidase, and polyphenoloxidase. The plants overexpressed PR1 and PR5 proteins, markers of salicylic acid pathways.</td></tr><tr><td>Biogenic zinc oxide nanoparticles produced using Eclipta alba extracts</td><td>Nanoparticles with size of 32 nm.</td><td><b>Concentrations:</b> 50, 100, 150, 200, 250, and 500 ppm. <b>Species:</b> Pearl millet (Pennisetum glaucum (L.) R.Br.). <b>Effects:</b> In vitro, at 100-250 ppm, the nanoparticles increased seed germination and vigor. In the field, at 200 and 250 ppm, there were increases of plant growth (fresh weight and surface leaves). The combination of seed-nano priming and the foliar application of nanoparticles inhibited sporulation and growth of Sclerospora graminicola.</td></tr><tr><td>Chitosan nanoparticles containing copper</td><td>Nanoparticles with size of 374.3 ± 8.2 nm and zeta potential of 22.6 mV</td><td><b>Concentrations:</b> 0.01, 0.04, 0.08, 0.12, and 0.16% w/v. <b>Species:</b> Maize seeds (Zea mays L.). <b>Effects:</b> Inhibitory effects on plant development were induced only by the highest concentration. The other concentrations improved seed germination and seedling growth by inducing α-amylase and protease activities and promoting reserve mobilization.</td></tr></table>

<figure><img src="image_42.png" /></figure>

Siddaiah et al.,
2018

<figure><img src="image_43.png" /></figure>

Nandhini et al.,
2019

<figure><img src="image_44.png" /></figure>

Saharan et al.,
2016
<table><tr><td>Chitosan nanoparticles containing zinc</td><td>Nanoparticles with size of 387.7 ± 4 nm, spherical morphology, polydispersity of 0.22, and zeta potential of +34 mV.</td><td>Concentrations: 0.01, 0.04, 0.08, 0.12, and 0.16% w/v. Species: Maize seeds (Zea mays L.). Effects: The nanoparticles showed anti-fungal activity against Curvularia lunata. The seeds showed increased germination, due to high activity of α-amylases and protease enzymes. The combination of seed-nano priming and application of the nanoparticles to leaves increased the activities of antioxidant enzymes and of phenylalanine ammonia lyase and polyphenol oxidase. Lignin accumulation was also increased. In field experiments, the seeds/plants treated with nanoparticles were less susceptible to diseases, compared to the control. The grain yield increased by 39.8% and the seeds showed higher zinc contents.</td></tr><tr><td>Molybdenum nanoparticles combined with the bacteria Mesorhizobium ciceri ST282 and Bacillus subtilis Ch13</td><td>Nanoparticles with size of 35-50 nm.</td><td>Concentrations: 10 mg L<sup>-1</sup> of nanoparticles, 10<sup>8</sup> CFU mL<sup>-1</sup> of M. cicero ST282, and 10<sup>7</sup> CFU mL<sup>-1</sup> of B. subtilis Ch13. Species: Chickpea (Cicer arietinum L.). Effects: Priming of the seeds with nanoparticles plus microorganisms resulted in increases of germination, plant growth, harvest yield, and antioxidant activity.</td></tr></table>

<figure><img src="image_45.png" /></figure>

Choudhary et al., 2019

Shcherbakova et al., 2017
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