Titanium and Zinc Based Nanomaterials in Agriculture: A Promising Approach to Deal with (A)biotic Stresses?

Abiotic stresses, such as those induced by climatic factors or contaminants, and biotic stresses prompted by phytopathogens and pests inflict tremendous losses in agriculture and are major threats to worldwide food security. In addition, climate changes will exacerbate these factors as well as their negative impact on crops. Drought, salinity, heavy metals, pesticides, and drugs are major environmental problems that need deep attention, and effective and sustainable strategies to mitigate their effects on the environment need to be developed. Besides, sustainable solutions for agrocontrol must be developed as alternatives to conventional agrochemicals. In this sense, nanotechnology offers promising solutions to mitigate environmental stress effects on plants, increasing plant tolerance to the stressor, for the remediation of environmental contaminants, and to protect plants against pathogens. In this review, nano-sized TiO2 (nTiO2) and ZnO (nZnO) are scrutinized, and their potential to ameliorate drought, salinity, and xenobiotics effects in plants are emphasized, in addition to their antimicrobial potential for plant disease management. Understanding the level of stress alleviation in plants by these nanomaterials (NM) and relating them with the application conditions/methods is imperative to define the most sustainable and effective approaches to be adopted. Although broad-spectrum reviews exist, this article provides focused information on nTiO2 and nZnO for improving our understanding of the ameliorative potential that these NM show, addressing the gaps in the literature.


Introduction
Nanotechnology is gaining prominence in the agro-food system as consequence of the positive reports released in the last half decade highlighting the promising applications that nanomaterials may have in plant fortification, enhancing crop tolerance to abiotic stresses, and improving plant defence against pathogens [1][2][3][4][5][6][7][8][9][10][11][12]. Besides, nanotechnology offers a route to make agriculture more sustainable and precise, as it can contribute to the reduction of the amount of agrochemicals used in farming and their consequent accumulation in the environment, decrease the production cost of conventional fertilizers, smartly deliver active molecules to enhance crop performance and improve plant disease prevention and control, and mitigate the effects of environmental pollutants, pesticide degradation, micronutrients for efficient use, etc. [13]. Thanks to the reduced size of nanomaterials (NM), they show an increased surface-to-volume ratio, reactivity, and frequently distinct properties from their bulk or ionic counterparts [14]. Their particular physicochemical properties enable them to increase the efficiency of agrochemicals at the same time as decreasing the number of active compounds and raw materials needed to produce them and to be used in agriculture. These features will decrease the environmental impact of agrochemicals and contribute to the development of new strategies to overcome the challenges of modern agriculture, plants [42]. In turn, it was also demonstrated that nTiO 2 induces the activation of the biosynthesis of antioxidants, such as vitamin E [43].
The beneficial effects of nZnO in several plant processes, such as photosynthesis and the antioxidant system, are related to the putative increase of Zn availability and/or to the molecular effects of these NM [44,45]. Zn is an essential element that acts as a cofactor of a large number of key enzymes (e.g., SOD, carbonic anhydrase, and glutathione dehydrogenase), and it is involved in the metabolism of carbohydrates and proteins [46]. nZnO acts at the pigment synthesis level, promoting carotene and chlorophyll biosynthesis [47]. Moreover, these NPs can strengthen the plant vascular system, particularly the metaxylem tissues, improving the nutritional status [44]. Additionally, nZnO is described to modulate many genes and transcription factors (e.g., ARP, MPK4, MKK2, SKRD2, MYC, bHLH, EREB, HsfA1a, R2R3MYB, and WRKY1) associated with physiological, hormonal, and developmental responses, and abiotic stress tolerance [44,48]. Moreover, a recent study highlights that nZnO can induce epigenetic modifications, downregulating the histone deacetylases gene (HDA3) [44]. Furthermore, nZnO also modulates the transcription of genes of the antioxidant system, leading to a protective response by increasing the activity of several antioxidant enzymes [49].

The Potential of nTiO 2 and nZnO in Increasing Abiotic Stress Tolerance to Plants
Abiotic stresses are environmental factors that can limit plant growth, development, and productivity, and global climate change scenarios have contributed substantially to the intensification of these factors [69]. Therefore, strategies that reduce their adverse impact on plants need to be implemented to increase the resilience of plants to stress conditions. Among the several strategies adopted to mitigate the negative effects of abiotic stresses in plants, nanotechnology-particularly the use of NM, mostly NP-is one of the most promising [11].
Several metal-based NM, such as nTiO 2 and nZnO, have been extensively studied in the last years due to their environmentally favorable use in agriculture, particularly in the promotion of plant growth and their protective role under stress conditions [39]. These studies have been mostly conducted in plants exposed to drought and salinity, since these abiotic stresses are the most common and produce a stronger impact on plant productivity [70][71][72]. The beneficial effects of the application of nTiO 2 and nZnO in plants exposed to other abiotic stresses, such as high and low temperature [11,73], were less studied. For instance, under cold stress, nTiO 2 foliar application (5 mg L −1 ) in Cicer arietinum increases the antioxidant enzymes activity, RuBisCO and phosphoenolpyruvate carboxylase, and the levels of pigments [74,75]. Moreover, these NPs reduce H 2 O 2 content and membrane damage. The same NM applied in Lycopersicon esculentum (nano-anatase with 16 nm; 0.05, 0.1 and 0.2 g L −1 ) exposed to heat stress enhanced photosynthesis and promoted stomatal opening [76]. In wheat plants under heat conditions, foliar application of nZnO (10 ppm, size 80 nm) increased the antioxidant enzymes activity (e.g., SOD, CAT, GST, and peroxidase) and reduced the levels of lipid peroxidation [77].

Drought and Salinity
Climate changes have contributed to the increase of global drought, changing the precipitation patterns and increasing the periods without or with low precipitation [69]. Drought is therefore considered one of the most natural hazards, with important consequences in the agriculture sector and food security [70]. For instance, the occurrence of drought events in the European Union (particularly in southern and western parts) resulted in annual agriculture economic losses of around 10% [71]. Furthermore, global soil salinization is increasing due to climate change [72]. Intensive farming together with low-quality irrigation water and poor drainage have strongly contributed to soil salinization [78]. Currently, around 62 million hectares of the world's irrigated area suffer from salinity, and this situation will be aggravated, particularly in the arid and semi-arid regions [79]. Therefore, drought and salinity are major global concerns and key factors that decline plant performance, yield, and productivity [78,80].
A common feature of these abiotic stresses is that they affect one of the most important key processes in plants-photosynthesis-reducing photosynthetic reactions and pigments levels but increasing the production of reactive oxygen species (ROS) leading to oxidative stress [72]. To control the levels of ROS and oxidative damages, plants can activate the antioxidant system, composed of enzymatic (e.g., superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), and ascorbate peroxidase (APX)), and non-enzymatic (e.g., glutathione, ascorbate (AsA), and flavonoids) antioxidants [51]. In addition, plants can develop stress tolerance mechanisms to avoid negative effects of stress; nevertheless, they vary between species and depend on the intensity and duration of the stress event [72].
The use of stress mitigation agents to alleviate the negative impact of these abiotic stresses can also be an affordable strategy to maintain plant growth and productivity. Therefore, the search for new stress mitigation strategies that ensure food and nutritional security under the rising global population has increased. Several metal-based NM have been investigated and their potential to mitigate drought and salt stress adverse effects demonstrated in several works in different species (e.g., [28,39]). NP application in drought and salt-stressed plants has been reported to increase the availability of water in leaves and to promote growth (above and below-ground), biomass production, nutrient uptake, carbohydrate accumulation, and photosynthesis (Tables 1 and 2). NM can also modulate phytohormone and osmolyte levels under drought and salinity conditions and contribute to the reduction of oxidative stress by the upregulation of the antioxidant battery (Tables 1 and 2). Additionally, under salt stress conditions, NP can help to regulate ion balance, reducing Na + toxicity and increasing the uptake of K + in plants [28].
3.1.1. Ameliorative Effects of nTiO 2 in Plants Grown under Drought or Salinity Stress nTiO 2 is one of the most studied NPs, with applications in several areas, such as pharmaceutical, medicinal, industrial, and agricultural fields [39]. Most of the benefits of the application of these NM in plants, particularly at the photosynthesis level under both optimal and abiotic stress conditions, are related to the photocatalytic properties of nTiO 2 [81]. Within the three crystalline structures of nTiO 2 (anatase, rutile, and brookite), anatase exhibits the highest catalytic activity. The several advantages of nTiO 2 application in plant species exposed to drought and salt stress conditions are summarized in Tables 1 and 2. Concerning drought, several studies have been conducted with wheat plants treated with nTiO 2 . Faraji and Sepehri [82,83] reported several positive effects in a controlled experiment using different nTiO 2 concentrations for seed priming or soil amendment, respectively, before water deficit treatments (Table 1). These authors described that seed priming with nTiO 2 promoted wheat shoot and root length and fresh weight, and soil amendment increased leaf water availability (relative water content-RWC) despite the higher stomatal conductance and transpiration and photosynthetic pigment levels (chlorophylls and carotenoids). Furthermore, in the same species and adding nTiO 2 in pot soil, Mustafa et al. [81] demonstrated that they modulated the levels of hormones (increase of IAA and GA), proline, and carbohydrates under drought stress conditions. The wheat root length and nutrient uptake (K and P) were also improved by this NP under stress [81]. In maize plants, Karvar et al. [84] reported that nTiO 2 foliar application increased the leaf RWC, F v /F m , carotenoids, chlorophylls, proline, soluble protein, and grain yield when plants were under drought. Besides photosynthesis, the production of secondary metabolites is also improved by the application of nTiO 2 under drought conditions. Moreover, the enzymatic antioxidant system is also activated by nTiO 2 in response to drought [39]. The activity of CAT, APX, and POD increased in basil, wheat, and maize plants treated with nTiO 2 [81,82,84,85], reducing the levels of oxidative stress by decreasing the production of H 2 O 2 and lipid peroxidation [82].
In the case of salt stress, the benefits of nTiO 2 application are very similar to drought. In a study conducted by Sheikhalipour et al. [86], the nTiO 2 foliar treatment of stevia plants exposed to different levels of salinity (50 mM and 100 mM NaCl) improved leaf availability (RWC), chlorophylls and carotenoids levels, and photosynthesis (net CO 2 assimilation rate and F v /F m ), leading to higher plant height and weight. At increased levels of salinity (180 mM), faba bean plants also respond to foliar nTiO 2 treatment, increasing the levels of photosynthetic pigments, sugars, and proline, resulting in an improvement of growth [42]. nTiO 2 application also ameliorated oxidative stress, activating several antioxidant enzymes (SOD, CAT, APX, and CAT) and reducing the levels of lipid peroxidation and H 2 O 2 production [42,86]. In barley plants exposed to high levels of salinity (100 and 200 mM), nTiO 2 application in soil improved photosynthesis (net CO 2 assimilation rate, stomatal conductance, and transpiration rate) and chlorophyll and proline levels [87]. Moreover, barley leaf relative water content (RWC) and root length increased, as well as the activities of CAT and SOD, which contributed to the decrease of lipid peroxidation under salinity [87]. nZnO has been widely used in several areas (e.g., cosmetic and medicine), but its higher popularity arises from its use in fertilizers and pesticides manufacture [89]. Zn is an important cofactor of several essential enzymes, and the benefits of nZnO application in plants are in part based on the increased availability of this nutrient to the plant, which leads to improvements on several metabolic pathways [45]. Several advantages of nZnO application in plant species have been reported under both drought and salt stress conditions (Tables 3 and 4). Concerning drought stress, positive effects of nZnO treatments (seed priming, soil amendment, or foliar application) on wheat, cucumber, and aubergine plants were reported, increasing water availability, both stages of photosynthesis, light-dependent and independent reactions, and photosynthetic pigments [80,[90][91][92]. nZnO stimulated carbohydrate (e.g., leaf sugar levels) and amino acid (e.g., proline, glycine betaine, and free amino acids) metabolism and increased shoot and root growth (length, fresh and dry weight) in maize and cucumber plants [80,93]. Furthermore, nZnO seems to induce a strong boost of the antioxidant system, upregulating the expression and activity of several antioxidant enzymes (SOD, POD, APX, CAT, glutathione reductase (GR), dehydroascorbate reductase (DHAR), monodehydroascorbate reductase (MDHAR), and phenylalanine ammonia lyase (PAL) activity) and non-enzymatic antioxidants (ascorbate, glutathione, total phenols, and flavonoids) [80,91,93] leading to lower oxidative stress due to less lipid peroxidation, membrane leakage, and O 2 −• and H 2 O 2 accumulation under drought stress conditions [80,90,91,93]. A study conducted with sorghum plants also showed that nZnO application in soil improves grain yield [37] and in wheat grains enhances nutrient levels under drought conditions [92].
Under salinity conditions (30-150 mM), different nZnO applications (soil amendment or foliar spray) induced positive effects in potato, tomato, and flax plants (Table 4), with improvements at the shoot and root growth attributes (length, fresh and dry weight), leaf area, photosynthetic parameters (including both light-dependent and independent reactions), chlorophyll, protein, and proline [94][95][96]. In these species, nZnO treatment increased leaf nutrient uptake and stimulated the antioxidant system (antioxidant enzymes), leading to lower oxidative stress (reduction of lipid peroxidation, O 2 −• and H 2 O 2 production). In addition, Mahmoud et al. [96] verified an increase of the RWC and gibberellic acid (GA) levels in salt-stressed potato plants. Furthermore, nZnO foliar application stimulated the carbohydrate and amino acid metabolism in mango plants watered with salinized drainage water [97]. These authors also reported an enhancement of nutrient levels and antioxidant capacity (antioxidant enzymes SOD, POX, and CAT).

Foliar application 2 times
Increased leaf RWC and F v /F m . Improved membrane stability [90] nZnO foliar application improved the levels of carotenoids, proline, sugars, and antioxidants (enzymes, ascorbate, and total phenolic compounds) in salt-stressed rapeseed plants [98]. These authors reported that nZnO decreased the levels of oxidative stress (H 2 O 2 production, lipid peroxidation, and membrane leakage) in this species caused by salinity (108 mM). In Lupinus exposed to 150 mM NaCl, seed priming with nZnO increased protein, sugars, free amino acids, including proline, and pigments levels, as well as antioxidant enzyme activities (SOD, POD, APX and CAT) and non-enzymatic antioxidants (total phenols and ascorbate) resulting in lower levels of lipid peroxidation [89]. Furthermore, rapeseed seeds primed with nZnO lead to improved levels of sugars, soluble protein, proline, and increased activity of SOD under 150 mM NaCl [99]. These authors also verified that these NPs upregulated the gene BnPER (peroxiredoxin antioxidant family gene) in priming seeds, contributing to oxidative damage reduction under salinity.

Environmental Contaminants
Climatic stresses combined or separated with xenobiotics induce major damage in sensitive crops, making them major threats to food security and the financial safety of farmers [100]. Soil contamination by metals, in particular heavy metals, is an environmental problem worldwide, with a negative impact on agriculture as a consequence of their phytotoxicity, low mobility, non-biodegradable nature, and high persistence [101,102]. Moreover, the contamination of soils by metals is a health problem due to the trophic transfer in the food chain and the bioaccumulation and biomagnification of metals [103].
The consumption of crops contaminated with metals causes risks to human health and animals, as some of these elements are not essential to humans (e.g., Pb, Cd, As, Hb, Al), and others, despite having functional roles in humans (e.g., Zn, Fe, Mn, Mg, Cr, Ni, Cu, Mo, Se), can have adverse health outcomes at high doses. Some of the putative effects may include alterations in reproductive health [104], impacts on the nervous system, the induction of carcinogenesis, oxidative stress, and loss of cellular functions [105,106].
In crops, multiple effects have been described to be induced by metal exposure, frequently dependent on the dose, exposure period, crop species and/or genotype, and soil physicochemical characteristics (e.g., pH) [107]. Impairments in plant morphology, physiology, biochemistry, and adjustments in plant metabolism are transversal to a high number of metal contaminants, as well as their cytotoxicity and genotoxicity [107,108].
Both metallic and metal oxide nanomaterials, including SiO 2 , TiO 2 , Fe 2 O 3 , Fe 2 O 4 , ZnO, Mn 3 O 4 , and CeO 2 , have been described to mitigate, at some level, the toxicity of metals in plants. Nevertheless, the benefits of these materials are frequently linked with low doses, whereas higher concentrations are more prone to induce toxicity [100,151], highlighting the need to optimize the dose when it is intended to use this kind of materials in agriculture.
3.2.1. Ameliorative Effects of nTiO 2 in Plants Exposed to Environmental Contaminants nTiO 2 has been described, during the last half decade, to be able to ameliorate the toxic effects of several environmental contaminants, including Cd, Cu, Pb, Al, Sb, As, 2,4-dichlorophenoxyacetic acid, and tetracycline, although most works focus on Cd toxicity (Tables 5 and 6). In the majority of works, it is not stated whether the crystalline phase of nTiO 2 is considered, making it impossible to establish a relationship between the crystalline phase and the effects reported.
Concerning the nTiO 2 application method, two main routes are used: (1) via roots in a solid matrix, by mixing nTiO 2 powder [152] with the soil/substrate or by spiking with nTiO 2 suspensions [153][154][155][156], or in hydroponic systems using NP suspensions [157][158][159][160][161]; (2) via leaves by foliar spray with nTiO 2 suspensions [157,162,163]. Besides, seed priming was analyzed by Sardar et al. [164], whereas Dai et al. [165] applied nTiO 2 during the seedling stage in petri-dishes with moistened paper, and Katiyar et al. [166] treated the seeds and seedlings with nTiO 2 suspensions, both simultaneously with the contaminant.   (3 days) followed by exposure to 2,4-D in a hydroponic system Modulated K, N, P accumulation; increased biomass; increased the activity of the enzyme invertase; promoted the nitrogen metabolism Increased shoot and root biomass; decreased tetracycline content in shoots and roots; modulated nutrient accumulation; decreased antioxidant enzymes activity; showed antagonist effect with tetracycline [160] Besides the application methods, the NP concentrations and size used, as well as the treatment period, are also very diverse, with concentrations ranging from 50 to 500 mg kg −1 in soil experiments, 10 to 2000 mg L −1 in hydroponic systems, and 10 to 200 mg L −1 in foliar application, whereas the treatment period may range from 24 h to 90 days (Tables 5 and 6). Nevertheless, despite these differences, it is evident that nTiO 2 application may alleviate toxic symptoms induced by several contaminants.
Under Cd, soil amendment with nTiO 2 improved several physiological attributes in white clover [153], rice [152], and soybean [154]. In white clover, nTiO 2 (500 mg kg −1 ) stimulated plant growth with the increase of plant length and biomass [153]. Similarly, nTiO 2 (500 mg kg −1 ) application in rice improved plant growth and also decreased MDA content simultaneously with the decrease of CAT, SOD, and POD activity [152]. In soybean, nTiO 2 (100-300 mg kg −1 ) increased the protein and chlorophyll b contents and decreased the proline content [154]. Furthermore, in rice, but using a hydroponic system, nTiO 2 treatments (10-1000 mg L −1 ) showed some potential to stimulate plant growth, reducing the MDA content, modulating the antioxidant response, altering the levels of some phytohormones (indole-3-acetic acid, isopentenyl adenosine, methyl jasmonate, and zeatin riboside), and stimulating photosynthesis by increasing the net photosynthetic rate and the chlorophyll content [158]. In addition, and in contrast to what was described by Zhang et al. [152], in this case, the Cd uptake decreased. Root application of nTiO 2 using a hydroponic system in maize also decreased Cd content in roots (250 mg L −1 ), likewise increasing the membrane integrity and upregulating the carbohydrate metabolic pathways [157].
The foliar spray with nTiO 2 also shows potential in mitigating Cd phytotoxicity. In cowpea [162] and rice [157,163], leaf application of nTiO 2 decreased Cd uptake and increased membrane integrity. In both species, the increment of pigment levels (Chl b in cowpea and Chl a, b and carotenoids in maize) was also reported, as well as the stimulation of the antioxidant response by increasing the activity of antioxidant enzymes [162,163]. Besides the pigment levels, nTiO 2 also stimulated the photosynthesis in maize by improving the gas exchange parameters, including the net photosynthetic rate, transpiration rate, and stomatal conductance, which may have contributed to the observed stimulation of plant growth [163]. Interestingly, the mitigating effects observed by Rizwan et al. [163] were induced despite using 10 times lower concentrations of nTiO 2 than Ogunkunle et al. [162] and Lian et al. [157].
At the seedling stage, sodium dodecyl benzene sulfonate-coated and uncoated nTiO 2 (1000 mg L −1 ) increased the root length of wheat exposed to 8.5 mg L −1 of Cd [165]. Finally, the priming of coriander seeds [164] with nTiO 2 (40, 80 and 160 mg L −1 ) also showed promising results and a potential strategy to mitigate Cd phytotoxicity (50 mg kg −1 ) once: it enhanced the germination rate, plant growth, and biomass; increased the pigment levels (80 mg L −1 ); stimulated the non-photochemical phase of photosynthesis by increasing the intercellular CO 2 content, stomatal conductance, transpiration rate, and net photosynthetic rate; stimulated an antioxidant response by increasing the activity of the enzymes CAT, SOD and APX, and the proline content; improved membrane integrity; improved the seed yield; and decreased Cd uptake.
Under Cu and using a hydroponic system, nTiO 2 (10 mg L −1 ) decreased the translocation factor of Cu in soybean [159]. In lettuce grown in soil, Mariz-Ponte et al. [155] reported ameliorative effects induced by nTiO 2 (5 mg kg −1 ) amendment in the presence of Pb or Al. In both cases, nTiO 2 decreased the relative membrane permeability and promoted the intercellular CO 2 content, stomatal conductance, transpiration rate, and net photosynthetic rate. Furthermore, under Pb, nTiO 2 also increased the pigment content (chlorophyll a, b and carotenoids), whereas in the presence of Al, it enhanced the effective efficiency of photosystem II [155]. nTiO 2 (2500 mg L −1 ) also showed positive effects to mitigate As effects in mung bean by upregulating the expression of antioxidant enzymes (SOD and CAT), which may have conferred greater protection against oxidative damages, as supported by the decrease of MDA content and ROS levels (H 2 O 2 and O 2 −• ) [166]. Besides metal contaminants, nTiO 2 was able to mitigate some effects of 2,4dichlorophenoxyacetic acid (2,4-D), a systemic herbicidal, and tetracycline, an antibiotic widely used in agriculture and livestock industries. In the case of 2,4-D, nTiO 2 increased the biomass, promoted the accumulation of N and K, despite reducing P content, upregulated the activity of soluble invertase to values closer to the control (despite decreasing the amount of cell wall bounded invertase), and promoted the nitrogen metabolism by increasing nitrate reductase and glutamine 2-oxoglutarate amino transferase activities [167]. Concerning the tetracycline, nTiO 2 application alleviated the negative effects on plant/pod biomass in Arabidopsis [161] and rice [160] grown in hydroponics. Furthermore, in both species, nTiO 2 modulated the antioxidant response, altering the activity of antioxidant enzymes, and in Arabidopsis, it upregulated the expression of adenyltransferase (APT), adenosine-5 -phosphosulfate reductase (APR), and sulfite reductase (SiR) in the roots. It is worth mentioning that in rice, nTiO 2 significantly decreased the levels of tetracycline in shoots and roots and altered the nutrient content, showing a trend to increase P, S, and Zn [160].
Taking into consideration all the works presented in Tables 5 and 6, it seems that nTiO 2 has the capability to decrease metals and non-metals contaminants uptake and translocation to shoots when the NPs are applied as suspensions to the leaves/seeds or are added to a nutritive solution together with the contaminant (hydroponic system). When nTiO 2 is applied to the soil, it looks to promote metal uptake, nevertheless without increasing its toxicity. In fact, an overall mitigation of toxic symptoms was observed. In both cases, an antagonistic effect is reported, as was particularly stated by Mariz-Ponte et al. [155] and Ma et al. [160], reducing metal phytotoxicity and enhancing plant performance. The mechanism underlying these results may be related to a reduction in contaminant bioavailability due to their immobilization by nTiO 2 [10,156,160].

Ameliorative Effects of nZnO in Plants Exposed to Environmental Contaminants
The studies available using nZnO as a strategy to cope with the adverse effects of environmental contaminants in plants, despite being limited and fractionated, unveil their potential to improve plant physiology under stress conditions. Similar to nTiO 2 , nZnO was mostly explored to ameliorate Cd toxic effects, despite a couple of studies existing with As, Pb, Cu, and Co (Tables 7 and 8). Most of the works focus on plant growth, ROS production, oxidative stress, and the antioxidant response, and show the capability of nZnO to modulate those physiological processes under metal stress.
Cd phytotoxicity was alleviated in rice [168,169], maize [170], and wheat [73,171] plants sprayed with nZnO suspensions (25-100 mg L −1 ). In all these species, plant growth improvements were described, such as plant length and biomass; promotion of gas exchange, including the net CO 2 assimilation rate; an increase of pigment contents; and a decrease of Cd uptake and/or translocation. In addition, the stimulation of the antioxidant response with the upregulation of antioxidant enzymes (CAT, SOD, G-POX) [73,168,170,171] and proline [168] was reported, together with the reduction of ROS and MDA content and decrease of electrolyte leakage. Similar responses were obtained in wheat after seed priming with nZnO (25-100 mg L −1 ) [172] and when nZnO (25 mg L −1 ) was supplemented in a hydroponics system in Leucaena seedlings [61].
In Leucaena, nZnO induced the enhancement of soluble protein and genomic alterations (presence of new DNA bands and/or absence of normal bands in the RAPD pattern of the exposed plants); nevertheless, in contrast to the previous works, in this case, it augmented the Cd accumulation [61]. Besides Cd, Venkatachalam et al. [61] also conducted the same experiment under Pb stress, with similar ameliorative results as those obtained for Cd. Under combined exposure of low Cd concentration (1 mg L −1 ) and high Pb (100 mg L −1 ), the amendment with polyvinylpyrrolidone-coated nZnO (100 mg L −1 ) decreased Cd content in shoots of cilantro, parsley, and spinach, whereas it increased the Pb levels in cilantro and did not affect the Pb content in parsley and spinach [173]. In roots, Sharifan et al. [173] described that Cd content decreased in parsley and spinach and that Pb was significantly reduced in all three species. The authors attributed the Cd and Pb mitigation to the adsorption of metals onto the nZnO surface, despite its overall significance possibly being affected by the nZnO surface charge plus the presence of roots exudates [173]. Additionally, nZnO altered the dynamic translocation and uptake of essential minerals such as Cu, Fe, and Zn: Fe increased in shoots of parsley and spinach; Zn increased in all species and both organs; and Cu decreased in cilantro shoots [173]. Furthermore, under combined stress, sunflower plants irrigated with heavy-metal-contaminated wastewater (mainly Cr, Cd, and Pb) showed improved performance when foliar sprayed with nZnO (60 mg L −1 ), as well as a decrease in metal content [174].
Under As, nZnO amendment (10-200 mg L −1 , with higher effects when were used 10-50 mg L −1 nZnO) promoted the rice seedlings' growth, increasing root and shoot biomass, enhanced the chlorophyll levels, and upregulated the activity of CAT and SOD [146]. Furthermore, nZnO reduced the MDA content, as well the levels of As in both roots and shoots, whereas it increased the Zn concentration. In soybean plants, nZnO (2 mg L −1 ) amendment of the nutritive solution containing As (V) reduced several cellular toxicants such as ROS (H 2 O 2 and O 2 −• ), MDA, and oxidized glutathione [175]. On the other hand, in both roots and shoots, several antioxidant pathways were activated, which included the upregulation of the expression of detoxification-encoding genes (GmSOD, GmG-POX, GmAPX, GmCAT, GmGR, GmGST) and the activity of antioxidant enzymes (SOD, G-POX, CAT, APX, GR); the increase of compatible organic solutes (proline and glycine betaine) levels and GmP5CS expression; and the stimulation of the ascorbate-glutathione cycle [175].   Germination and seedling growth in petri dishes The lowest doses enhanced seedling length and the pigment content, and stimulated the antioxidant response by increasing the activity of SOD and CAT; decreased MDA content and As accumulation and translocation [146] As(V); Na 2 HAsO 4 25 µM 25 µM (2.0345 mg L −1 ); PS 20 nm Glycine max L. Hydroponic system (10 days) Increased root and shoot dry weight; decreased As content in roots and shoots; decreased ROS (H 2 O 2 and O2 −• ) and MDA in roots and shoots; enhanced the activity of antioxidant enzymes (SOD, G-POX, CAT, APX, GR); upregulated the expression of defenseand detoxification-encoding genes; increased GSH/GSSH, and AsA, proline and glycine betaine contents [175]  The single work under cobalt (Co) stress revealed several beneficial effects in maize induced by seed priming with nZnO (500 mg L −1 ) [176]. The pre-treatment with nZnO enhanced maize growth (length and biomass), promoted Zn uptake while reducing Co levels in shoots and roots, and increased chlorophyll contents, which may have contributed to the detected improvement of Fv/Fm. Besides, seed priming reduced the damage induced by Co in guard cells and restored, at some level, the stomatal aperture, as well the chloroplast and thylakoid ultrastructure. These changes may be the reflection of oxidative stress mitigation, as proven by MDA reduction and the superior activity of antioxidant enzymes, and together be responsible for the restoration of gas exchange, including the net CO 2 assimilation rate [176].
Finally, Cu phytotoxicity was reduced in tomato plants when foliar sprayed with nZnO (50 mg L −1 ) [177]. Treated plants showed lower Cu content, superior length and biomass, higher chlorophyll index, and superior fluorescence of chlorophyll a, with the increase of Fv/Fm. However, concerning photosynthesis, nZnO improved the net photosynthetic rate, internal CO 2 content, stomatal conductance, and transpiration rate, and promoted carbonic anhydrase activity. Seed priming also promoted an antioxidant response with the upregulation of the activities of several enzymes (CAT, APX, and SOD) and the increase of proline, which may have contributed to control ROS production/scavenge (reflect of O 2 −• and H 2 O 2 reduction) and reduce the oxidative stress (MDA decrease) [177].

Protective Effects of ZnO and TiO 2 against Biotic Stress
Plants are affected by numerous pathogens that are able to induce diseases and diminish plant performance and yield. Crop production is globally affected by pests and phytopathogens such as viruses, bacteria, and fungi, with losses reaching up to 40% of crop local or global production [178][179][180], and thus affecting global food security. NM have been explored as a sustainable alternative to the conventional synthetic agrochemicals, which lack selectivity and sensitivity and are a threat to the environment and human health. This nano-based approach shows desirable properties for agro-application, such as slow and controlled release of active compounds, low cost, efficient drug delivery, multi-site mode of action, ameliorative effects, antimicrobial and/or fungicidal activity, among others [32]. Hence, NM are promising strategies for both plant health monitoring and disease management in smart agriculture. When NM became to be explored for agricultural proposes, these materials were mostly synthesized by conventional methods. Nevertheless, as their potential was revealed, emerged bio-based synthetic methods, where NM were prepared from plants and microbes, as an environmentally-friendly alternative to chemical synthesis with promising results in agricultural fields, such as in crop diseases management [181].
NP induce the generation of ROS, such as hydroxyl, hydroperoxyl, peroxyl, alkoxyl and carbon dioxide radicals, superoxide anions, hydrogen peroxide, and carbonate, and nonradicals, such as ozone, nitric oxide, peroxy nitrite, hypobromous acid, hypochlorite, and organic peroxides [25,182], increasing the level of oxidative stress. Moreover, oxidative stress induces single and double-strand breaks and lesions on nitrogen base and pentose sugar [182], cell damage, injury of cell membrane with leakage of cytoplasmic material, proteins and nuclei acids [183,184]. The accumulation of NP in the membrane of bacteria or fungi induce alterations in cell membrane permeability, leading to disturbances in the proton motive force [182]. Several metal and oxide-NM show direct action against bacteria, fungi, and viruses and even nematodes. Among them are silver (nAg), gold (nAu), cupper (nCu), and nickel (nNi) NP, as well as nZnO (Table 9), nTiO 2 (Table 10), copper oxide (nCuO), aluminum oxide (Al 2 O 3 ), iron oxide (nFe 2 O 3 ), and magnesium oxide (nMgO) NP (for review see [32]).
In particular, nZnO doped with Fe and Mn showed antibacterial activity against Pantoea ananatis, and in the pathosystem P. anantis-corn, nZnO doped with Fe, Mn, Cu, or Ni reduced the diseases progression when the NM were foliar sprayed to plants before and after plant inoculation with the bacteria [189]. In the bacterial blight diseases complex of pea, caused by M. incognita and Pseudomonas syringae pv. pisi, nZnO was able to reduce the index diseases and galling population and improve plant growth and pigment content [188]. In tomato, soil amendment with nZnO reduced the diseases incidence induced by Ralstonia solanacearum and stimulated plant growth and the antioxidant response (with the decrease of MDA content) and improved soil microbial community [185] (Table 9).
nZnO synthesized from a flower extract presented antibacterial activity against R. solanacearum and decreased the bacterial wilt disease in tomato [190], whereas nZnO synthesized from Citrus medica peel extracts showed antimicrobial activity against Streptomyces sannanesis, Bacillus subtilis, Pseudomonas aeruginosa, Salmonella enterica, Candida albicans, and Aspergillus niger [191] (Table 9). Biogenic nZnO NPs synthesized from Trichoderma harzianum, Trichoderma reesei, and co-culture [192], or from Paenibacillus polymyxa strain Sx3 [183], showed antibacterial activity against Xanthomonas oryzae pv. oryzae, responsible for the bacterial leaf blight diseases in rice. Besides, Ogunyemi et al. [183] reported a decrease of bacterial leaf blight diseases in plants foliar sprayed with biogenic nZnO, together with the improvement of plant growth.
CuZn@DEG and ZnO@PEG nanoflowers showed antifungal activity against Botrytis cinerea and Sclerotinia sclerotiorum, and in lettuce plants inoculated with S. sclerotiorum, both NM reduced the disease index and improved the net photosynthesis, photosynthetic quantum yield, and photosynthetic efficiency [193]. The antifungal activity of nZnO was also evaluated against Fusarium oxysporum in tomato plants, decreasing the diseases incidence and severity and improving plant growth [194].
Biogenically synthesized nZnO (Table 9) using lemon peel extract showed antifungal activity against Alternaria citri, responsible for citrus black rot disease [195], whereas using leaf extract of Cinnamomum camphora (L.) Presl, nZnO presented antifungal activity against Alternaria alternate, responsible for early blight disease in Solanum lycopersicum [184]. In A. alternate, nZnO induced an excessive accumulation of MDA and caused the damage of the cell membrane, leading to the leakage of protein and nucleic acid [184]. In addition, ZnO bio-synthesized using Penicillium chrysogenum showed antifungal activity against Fusarium solani, Fusarium oxysporum, Sclerotium sclerotia, and Aspergillus terreus [196]. An innovative approach in plant defense is the photoactivation of nZnO, and using this approach, it was possible to inactivate Escherichia coli B. and F. oxysporum in contaminated seeds [197], whereas in strawberry, photoactivated nZnO reduced B. cinerea incidences, promoted crop production, and increased fruit shelf-life [198].

Foliar spray
Induced systemic acquired resistance (SAR) and reduction of viral accumulation levels and of disease severity; increased plant growth; up-regulated the transcriptional levels of PAL, PR-1, CHS, and POD genes. [199] Concerning plant diseases induced by viruses, Abdelkhalek et al. [199] reported the decrease of Tobacco mosaic virus diseases incidence in tomato plants after being sprayed with green-synthesized nZnO. These particles improved plant growth and upregulated tomato-innate defense genes (PAL, PR-1, CHS, and POD) [199].

nTiO 2 Potential for Crop Diseases Control
The photocatalytic activity of nTiO 2 contributes to its antifungal and antibacterial activity [1]. For instance, Sar et al. [200] highlighted the antifungal activity of nTiO 2 (anatase; 3-12 nm; 50, 100, 150, and 200 ppm) against F. oxysporum f. sp. radices lycopersici and F. oxysporum f. sp. lycopersici. Application of nTiO 2 (10-100 nm; 20, 40, 60 and 80 mg L −1 ) in wheat plants reduced the severity of the diseases caused by the fungus Bipolaris sorokiniana [201]. Similarly, Hamza et al. [202] demonstrated that these NPs can control Cercospora beticola infection in sugar beet (Table 10). Boxi et al. [203] demonstrated that nTiO 2 at 0.75 and 0.43 mg/plate induces a growth inhibitory effect in two potent phytopathogens: F. solani, which causes Fusarium wilt diseases in potato and tomato plants, and Venturia inaequalis, which is responsible for apple scab disease. nTiO 2 foliar application in cucumber (1.6%) and poinsettia and geranium (25 and 75 mM) showed antibacterial action against the pathogenic P. syringae pv. lachrymans and Pseudomonas cubensis and Xanthomonas hortorum pv. pelargonii, Xanthomonas axonopodis pv. poinsettiicola [204,205] ( Table 10). The antibacterial activity of TiO 2 (0.5 mol L −1 ) against the bacteria Dickeya dadantii, which causes the stem and root rot diseases in sweet potato, was reported by Hossain et al. [206]. Similarly, nTiO 2 has a strong antimicrobial activity against nematodes and viruses [32]. Ardakani [207] found nematocidal activity of nTiO 2 against the root-knot nematode M. incognita in tomato plants. nTiO 2 can also control the pathogenic activities of the virus Turnip mosaic in tobacco plants by limiting the replication of DNA. In faba bean plants, the foliar treatment with nTiO 2 helped to control the spread of the broad bean stain virus [208]. An insecticide effect of nTiO 2 was also observed in tomato plants infected with Bactericera cockerelli Sulc [209]. The nTiO 2 treatment induced a high insecticidal effect after 24 h, with a mortality around 93% for the concentrations above 100 ppm.

Conclusions
In the last years, nanotechnology has gained much attention in the agro-food system, mostly due to the potential to increase plant performance, enhancing tolerance to biotic and abiotic stresses. In this review, we highlighted the most recent studies on the application of NPs, particularly nZnO and nTiO 2 , in several species exposed to the most common climatic stresses, such as drought and salinity, as well as environmental contaminants, such as heavy metals, and phytopathogens and pests (Figure 1). The beneficial effects of nZnO and nTiO 2 on plants exposed to these stressors at the molecular, metabolic, and physiological levels are well demonstrated in several works performed under controlled and field conditions. These effects can already depend on several factors such as the type of NP used, method of application, concentration, and the type and extent of stress exposure. In general, these NPs show the potential to improve plant performance and may represent a sustainable strategy to alleviate the negative impacts of (a)biotic stresses in agricultural species (Figure 1).  Data Availability Statement: This is a review article; however, data used and/or analyzed during the review are available on request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.