Keywords

14.1 Introduction

Sustainable agriculture acts as a backbone for the development of the national economy as well as fulfills the aspiration of food demand in developing countries. To satisfy the demand for food supply for the upcoming future with the changing environmental conditions as well as rapidly increasing population, there is urgent need to increase crop yield and stability of plants in adverse conditions by exploiting the advance approaches like nanobiotechnology (Eckardt et al. 2009; Zhang 2007). Agricultural plants/production of crops together with their protection are reliant on the various parameters such as type as well as the quantity of applied fertilizers and pesticides. The growth and development of agricultural plants/crops entirely depend on ease of availability of optimal environmental as well as nutritional factors and any deviation from it leads to plant stress. Stress is a condition in which plants are not able to fully express their genetic potential for growth, development, and reproduction, thereby limits productivity owing to damage to biomass. Being sessile, plants cannot escape from adverse climatic conditions and, thus have to meet both the stresses, i.e., biotic stresses, for instance, interactions among organisms like microbial pathogens and so on and abiotic stresses that involve interactions among organisms with their physical environment. Abiotic (physical) stresses include temperature alteration (high or low), nutrient starvation, water deficit (drought), anoxia (during the flood), salinity and alkalinity of the soil, light intensity, submergence, mineral, and metal toxicity/deficiency (Cramer et al. 2011; Hirel et al. 2007; Wang et al. 2003). These stresses are unpredictable in nature in terms of their intensity, duration, and occurrence, so sustaining the development and survival of plants in an unfavorable environment turns out to be a difficult task. So, plants need to respond distinctly to protect themselves from physical stresses like cold, drought, heat, etc., that ultimately lead to the development of some adaptative mechanism in plants (Mittler 2002). Plants have the ability to sense abiotic stress and respond accordingly as per their past exposure so that in further repetitive stress can be adjusted (Ahmad et al. 2015; Hilker et al. 2015; Jiang et al. 2016). However, on the other hand, transgenic plants/crops are still not popular among the grower or farmers owing to their high level of safety concern. Therefore, in the current scenario, plant nanobiotechnology offers promising technological approaches for achieving food safety and security by increasing the efficiency of plants/crops, protecting them from different types of biotic as well as abiotic stresses via. modulating the mechanisms of different pathways, apart from those achieved through genetic and chemical production (Giraldo et al. 2019; Iqbal et al. 2020; Kah et al. 2019). Nanobiotechnology involves the cutting edge application-oriented research in the area of Nanoscience together with biotechnology. Nanomaterials (NMs) can be defined as materials depicting diameter in the range of 1–100 nm (Porwal and Sharma 2016; Pandey et al. 2018; Porwal et al. 2020; Rani et al. 2020; Singh and Porwal 2020; Singh, Pal, et al. 2018; Singh, Yadav, et al. 2018; Singh et al. 2020). The effect of various kinds of nanomaterials on plants under normal and/or abiotic stressed environment is presented in Table 14.1.

Table 14.1 Impact of nanomaterials on plants under normal and/or abiotic stressed condition
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14.2 ROS Scrounging Antioxidants of Plants

ROS (reactive oxygen species) are short-lived, unstable, and reactive (Halliwell 2006), which includes singlet oxygen (1O2), hydroxyl radical (OH·), superoxide radical (O2·−) as well as hydrogen peroxide (H2O2), etc. These are generated in different cellular compartments such as chloroplast, mitochondria, peroxisomes, plasma membrane (Apel and Hirt 2004) as a regular (unavoidable) by-product of aerobic metabolism such as photosynthesis and respiration in plants (Miller et al. 2010; You and Chan 2015) which is regulated by both enzymatic and non-enzymatic antioxidant defense system of the plant. The low or moderate level of ROS is responsible for plant growth (reproductive and senescence) and development including leaf shape, root hair elongation, trichome development (Gapper and Dolan 2006), stomatal closure, programmed cell death (Petrov et al. 2015), gravitropism (Wassim et al. 2013) as well as act as the second messenger in mediating different series of reactions in plant cells, and promotes the tolerance from biotic and abiotic stress conditions (Nath et al. 2017). However, excessive production of ROS due to both biotic and abiotic stresses (Bhattacharjee 2012; Khare et al. 2014; Kumar and Khare 2014) was not removed then results in damage to cell membranes (lipid peroxidation), proteins, nucleic acid (DNA as well as RNA), and several other cellular components of the plants, thereby affecting plant growth including development and ultimately yield (Demidchik 2015; Mittler 2002). Various abiotic stresses induced ROS generation and the role of nanomaterials enhancing stress tolerance in the plant is depicted in Fig. 14.1.

Fig. 14.1
figure 1

(Source Modified from Meena et al. 2017; Xie et al. 2019)

An overview of the abiotic stress-induced ROS generation in agricultural plants/crops and the role of nanomaterials in improving stress tolerance

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14.3 Stimulation of Antioxidant Mechanism in Response to Nanoparticle Exposure

Plants make use of enzymatic and non-enzymatic antioxidative systems and/or pathways to mitigate oxidative stress. The key enzymes involved in the ROS-scrounging include catalase (CAT), ascorbate peroxidase (APX), superoxide dismutase (SOD), peroxidase (POD), glutathione reductase (GR), glutathione peroxidase (GPX), glutathione S-transferase (GST), alternative oxidases (AOX), peroxiredoxin (PRX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), and many more (Catalá and Díaz 2016; Jaleel et al. 2009; Maxwell et al. 1999; Mittler et al. 2004). Non-enzymatic antioxidants comprised of low molecular weight metabolites such as flavonoids, polyphenols, glutathione (GSH), ascorbic acid (AsA), β-carotene, α-tocopherol, proline, glycine betaine, and many more (Gill and Tuteja 2010; Pandey et al. 2017). During stressed conditions plants protect themselves from ROS toxicity (leads to oxidative damage) by changing gene expressions as well as adapting ROS-scrounging antioxidant metabolic pathways such as ascorbate, aldarate, and shikimate phenylpropanoid biosynthesis routes (Zhang et al. 2018), using ROS as signaling molecules (Dietz 2015; Foyer and Noctor 2013; Ismail et al. 2014; Mignolet-Spruyt et al. 2016). Ascorbate-glutathione cycle (AsA-GSH) is a major ROS-scrounging pathway in plants (chloroplast, mitochondria, apoplast, and peroxisomes), which involves successive oxidation and reduction of ascorbate, glutathione, and NADPH catalyzed by APX, MDHAR, DHAR, and GR, thereby helps in combating oxidative damages triggered by abiotic stresses (Mittler 2002). Association of ROS in signaling reveals that there must be some regulation of network to maintain ROS at non-toxic level, needs a precise balance between ROS production (during cellular metabolism), ROS generating enzyme and ROS-scrounging pathways. Thus, stress tolerance of the plants/crop can be improved remarkably by manipulating the ROS levels. Numerous, research studies have demonstrated the role of nanomaterials (CeO2, C60 as well as Fe2O3) in scrounging the over-accumulation of ROS, generated during abiotic stress in plants, thereby improving abiotic stress tolerance in the plant and finally mitigating yield losses (Zhao et al. 2020).

14.4 Enzymatic Antioxidants

The agricultural plants/crops depict different types of antioxidants systems (Fig. 14.2) which are as follows:

Fig. 14.2
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Various types of antioxidant systems in plants

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  1. (a)

    Superoxide dismutases (SOD): SOD enzymes are present naturally in different living organisms like agricultural plants/crops and so on. They speed-up the dismutation of O •−2 to H2O2, so act as the first line of defense against ROS (Moustaka et al. 2015). Generally, due to the attachment of SODs to a metal ion such as Cu, Zn, Fe, Mn, and Ni, are distinguished based on their subcellular location and metal cofactor. In agricultural plants/crops, SODs encoding genes can be controlled and managed by development, tissue-specific, and abiotic stresses/signals (Scandalios 2005).

  2. (b)

    Catalases: These enzymes are mostly confined to peroxisomes, known for the exclusion of H2O2 by reducing it into 2H2O. The specific gene that encodes for CATs responds separately to each abiotic stress known to produce ROS (Scandalios 2005).

  3. (c)

    Glutathione peroxidases (GPX): These proteins are mostly confined to mitochondria, cytosol, and chloroplast. GPX is nonheme thiol peroxidases responsible for speed-up the reduction of organic H2O2 to H2O (Margis et al. 2008).

  4. (d)

    Ascorbate peroxidases (APX): These enzymes utilize ascorbate as an electron donor and are responsible for catalyzing the conversion of hydrogen peroxide into water. Different isomers of APX are present in the subcellular compartment of the plants like mitochondria, chloroplast, peroxisomes, and cytosol. The APX gene in plants is modulated by several environmental stresses (Caverzan et al. 2014) whereas the balance between APX, SOD, and CAT determines the intracellular level of O •−2 and H2O2. Any alteration in the balance of these three enzymes seems to induce defense-mechanism pathways (Scandalios 2005).

  5. (e)

    Peroxiredoxins: These antioxidant enzymes (thiol specific) are responsible for ROS detoxification in the chloroplast (Foyer and Shigeoka 2010), cell defense of plants by protecting them from oxidative damage, speed-up the reduction of peroxynitrite and various organic H2O2 to their corresponding alcohols (Wood et al. 2003).

  6. (f)

    Guaiacol peroxidases: These are heme-containing enzymes known to detoxify H2O2 and belong to class III or secreted plant peroxidases. Guaiacol peroxidases can also carry out hydroxylic reaction (second cyclic reaction), different from the peroxidative reaction. These class III peroxidases support many activities in plants such as auxin metabolism, germination to senescence, cell wall elongation, and protection from pathogens (Passardi et al. 2004).

  7. (g)

    Monodehydroascorbate reductase (MDAR): Different isomers of MDAR are found in the different subcellular compartments of plants such as mitochondria, peroxisomes, and cytosol. MDAR (flavin adenine dinucleotide enzyme) maintains the ascorbate pool in plants by catalyzing the regeneration of monodehydroascorbate radical utilizing NAD(P)H as an electron donor (Asada 1999; Leterrier et al. 2005).

  8. (h)

    Dehydroascorbate reductase (DHAR): It helps to maintain ascorbate (AsA) in its reduced form and speed up dehydroascorbate reduction into ascorbate by utilizing glutathione as reducing substrate (Gratão et al. 2005).

  9. (i)

    Glutathione reductase (GR): These enzymes are NAD(P)H dependent, speed-up the reduction of oxidized glutathione (GSSG) into reduced glutathione (GSH), and high GSH/GSSG ratio is required to protect the plant from oxidative damage (Foyer and Noctor 2005). GR plays a significant role in the ascorbate-glutathione cycle and maintains an appropriate level of reduced glutathione.

14.5 Impact of Nanoparticles on Plant Growth

Nanoparticles such as platinum (Pt), gold (Au), fullerene C60, Fe3O4, CeO2, Mn3O4 and many more are reported to improve in the functional activities of antioxidant enzymes like SOD, CAT, and POD, that results in more improved adaptation of plants to different abiotic stresses (Chen et al. 2018; Upadhyaya et al. 2018). The fabricated nanosheets of MoS2 resemble SODs, CATs, and PODs like activities. The nanoparticles of CeO2 at low concentration (5 µM) efficiently decrease ROS level and protect chloroplast (Boghossian et al. 2013), whereas CeO2 nanoparticles, when coated with polyacrylic acid, shows SOD and CAT activities, and successfully retained the photosynthetic capability of Arabidopsis plants under saline conditions (Wu et al. 2018). Foliar-sprayed CeO2 nanoparticles in sorghum under drought conditions mitigate the effect of oxidative damage (Djanaguiraman et al. 2018). γ-Fe2O3 nanoparticles in Brassica napus under drought conditions protect plants from oxidative stress by efficiently reducing H2O2 and malondialdehyde (Palmqvist et al. 2017). In the investigation conducted by Yao et al. (2018), suggested that Mn3O4 may be used to enhance plant stress resistance (as Mn is micronutrient for plants) due to their stronger ROS-scrounging ability over Ce nanoparticles. When Fe2O3 nanoparticles are applied on watermelon in different concentrations, the activities of SOD, CAT, POD, and seedling germination were found to significantly increase and, therefore help to mitigate abiotic stress (Li et al. 2013). Nanoparticles have shown a concentration-dependent impact on the growth and development of plants (Mishra et al. 2017). For instance, onion seedlings, when exposed to TiO2 nanoparticles, the SOD activity was increases with the increase in the concentration of TiO2 nanoparticles, whereas onion seed germination as well as seedling growth was enhanced at low concentration and suppressed at higher concentration of TiO2 nanoparticles (Dimkpa et al. 2017). Shallan et al. (2016) in their study, discovered that foliar spray of SiO2 (3200 mg L−1) or TiO2 (50 mg L−1) nanoparticles were found to enhance the drought tolerance of cotton plants. Siddiqui et al. (2014) reported that the application of SiO2 nanoparticles (1.5–7.5 g L−1) on squash (Cucurbita pepo L.) under saline condition upregulated the gene expression of SOD, CAT, POD, APX, and GR as well as increase the chlorophyll concentration, photosynthesis and biomass content of the plant. Under saline conditions, SOD and GPX gene expression are downregulated in tomato (Solanum lycopersicum), while on application of ZnO nanoparticles (15 and 30 mg L−1) showed positive growth response (Alharby et al. 2016). On similar lines, foliar spray of ZnO in finger millet (Eleusine coracana (L.) Gaertn) improved salinity stress tolerance (Sathiyanarayanan 2018). Dimkpa et al. (2019), reported positive effect on drought tolerance when ZnO nanoparticles (18 nm, 5 mg kg−1) are applied to soil-grown sorghum. However, several reports confirmed the negative impact of nanoparticles/engineered nanoparticles (Rico et al. 2015; Singh et al. 2016) on seed quality of plants like wheat (Rico et al. 2014) and common bean (Majumdar et al. 2015).

14.6 Effect of Nanoparticles on Plant Growth Under Salinity

Excessive accumulation of NaCl in the soil increases the salinity of soil and it affects the growth, development, and productivity of the plants in two ways: osmotic stress and ionic toxicity. Generally, osmotic pressure in the plant cell is more than the osmotic pressure in soil solution. Under high osmotic pressure, plant cell take-up water as well as other requisite minerals from soil solution into the root cells, but during saline conditions, this situation gets reversed and plant ability to take-up water and requisite minerals such as K+ and Ca2+ also disturbed, meanwhile Na+ and Cl− ions enter into cytosol that leads to low K+/Na+ ratio which is responsible for increased ROS production, electrolytes leakage, toxicity to cell membranes, and also affects metabolic activities in the cytosol (Khan et al. 2012; Kumar 2013; Kumar and Khare 2014). Overall, salinity has a negative effect on various biological and physiological processes of the plant. Some major negative effects of salinity stress on the plant include nutritional imbalance, increased ionic toxicity, ROS overproduction, reduced osmotic potential, the decline in photosystem II efficiency, and stomatal conductance (Negrão et al. 2017). Recently, nanoparticles have been reported to enhance the antioxidative defense mechanism of plants. This potential approach is being exploited to mitigate the salinity stress of the plants (Sabaghnia and Janmohammad 2015). Derosa et al. (2010) reported that SiO2 nanoparticles enunciate a layer inside the cell wall that facilitates them to conquer salinity stress and uphold yield. Silicon nanoparticles increase the rate of photosynthesis, proline accretion, seed germination, leaf water content, and antioxidant enzymes activities (Qados 2015). On the application of SiO2 nanoparticles, improvement in salinity stress was observed in Ocimum basilicum (Kalteh et al. 2014), Lens culinaris (Sabaghnia and Janmohammadi 2014) and Vicia faba (Qados 2015). Similarly, SiO2 nanoparticles were reported to enhance seed germination and antioxidant system in squash and tomato (Siddiqui et al. 2014). Further, mitigation in salinity stress was observed by the application of the foliar spray of Fe3O4 as well as ZnO (60 mg L−1) as nano-fertilizers on Moringa peregrina (Soliman et al. 2015). The efficiency of a chloroplast, as well as biomass, were increased in treating Brassica napus L. with CeO2 nanoparticles under both fresh and saline water irrigation (Rossi et al. 2016).

14.7 Impact of Nanoparticles on Plant Growth Under Drought Stress

Water is a prerequisite necessity for plant growth and survival, essentially needed for transporting nutrients, thus its crises result in drought stress. Drought stress affects the growth of plants, thereby ultimately influencing the agricultural plants/crops yield globally. During water crises situation, plants limit their various activities such as stomatal closure to prevent additional water loss, reduce CO2 fixation (photosynthesis), and NADP+ regeneration through the Calvin cycle (Gunjan et al. 2014). Drought stress tolerance of plant varies from species to species and depend to a larger extent on time and intensity they spend under stressful surroundings. Research studies confirm that during drought conditions plants overproduce ROS (H2O2, O •2 , 1O2, and OH•) which causes lipid peroxidation, denaturation of protein, DNA mutation, and eventually cell death (Molassiotis et al. 2016). However, plants protect themselves from negative effects of ROS by its several antioxidant enzymes like SOD, CAT, APX, and GR, while the degree of cellular oxidative damage depends on the capacity of their antioxidant defense system (enzymatic or non-enzymatic). Drought stress can be modulated by the application of different nanoparticles such as silica, silver, copper, ZnO, CeO2, and many more. On the application of silica nanoparticles improvement in drought tolerance was observed in two sorghum (Sorghum bicolor L. Moench) cultivars (Hattori et al. 2005), Crataegus sp., and hawthorns (Ashkavand et al. 2015). Similar results were observed in wheat on the application of 1.0 mM sodium silicate (Pei et al. 2010). Sedghi et al. (2013) reported an increased rate of germination on the application of ZnO nanoparticles in soybean under drought-stressed conditions. Foliar application of some micronutrients like iron and titanium nanoparticles were reported to improve drought stress in safflower cultivars and wheat, correspondingly (Davar et al. 2014). Further, Zn and Cu nanoparticles reported improving drought stress by enhancing SOD and CAT enzymes in wheat that results in limiting lipid peroxidation and increasing relative water content by enhancing photosynthesis (Taran et al. 2017). CeO2 nanoparticles when applied at 100 mg kg−1 reported enhancing photosynthesis and Rubisco carboxylase activity (Cao et al. 2017), while composite of CuO, ZnO, and B2O3 improve drought stress in Glycine max (Dimkpa et al. 2017). Encapsulated abscisic acid (ABA) was delivered successfully to Arabidopsis thaliana plant through glutathione-responsive mesoporous silica nanoparticles and their controlled release in plants increased the expression of ABA inducible marker gene (AtGALK2), ultimately improved drought resistance (Sun et al. 2018).

14.8 Impact of Nanoparticles on Plant Growth Under Metallic Stress

Excessive accumulation of metals in plants causes phytotoxicity, alters plant growth, and causes oxidative damage. Metal toxicity interferes with plant growth by suppressing activities of different plant enzymes, interrupting uptake of essential elements which leads to deficiency symptoms. Metals in growth medium are responsible for the overproduction of ROS, which leads to oxidative damage to biomolecules, cell structure, and cell membrane denaturation (Sharma et al. 2012). Biophysical barriers form the first line of defense against metallic stress. If metal passes through this barrier and enters cells, then plants resist metal uptake by its accumulated biomolecules such as organic acids, metal-chelates, and polyphosphates by activating cellular defense system which is responsible for ROS scrounging. However, timely and target-oriented stimulation of these antioxidant defense systems is essential to remove the effects of metallic stress. Nanoparticles (such as nano-selenium, nano-oxides of iron, manganese, and cerium) enters the contamination zone easily due to their smaller size and large surface area, possess a strong affinity towards metal/metalloids adsorption. Nanoparticles in plants retard metal-induced oxidative stress by regulating their energy metabolism, antioxidants, ROS production, and thereby mitigating abiotic stresses. Nanoparticles immobilize metal/metalloids in soil and improve the growth and development of plants during phytoremediation (Martínez-Fernández et al. 2017). Nano-TiO2 has been reported to limit cadmium (Cd) toxicity and enhance photosynthesis and plant growth rate (Singh and Lee 2016), nano-scale hydroxyapatite mitigates Cd toxicity in Brassica juncea (Li and Huang 2014), and ZnO nanoparticles attenuate uptake of Cd in plants (Venkatachalam et al. 2017). Tripathi et al. (2015) demonstrated that silicon nanoparticles hampers Cr accumulation in growth medium and prevents pea seedlings against Cr (VI) phytotoxicity by enhancing the antioxidant defense system. However, research studies reveal that nanoparticles may yield good or bad effects on plants at any level. Toxicological studies of nanomaterials done so far provide a great understanding of nanoparticle interaction with the plants and their potential risk hazards associated with the abiotic stress management and crop productivity improvement (Mustafa and Komatsu 2016; Venkatachalam et al. 2017).

14.9 Impact of Nanoparticles on Plant Growth Under Ultraviolet Radiation Stress

Sunlight together with the UV-B radiation (280-315 nm) is unavoidable abiotic stress for photosynthetic organisms due to the continuous depletion of the ozone layer in the stratosphere. On exposure to such non-ionizing radiation, structural changes occur in cellular components such as DNA, protein, chloroplast, and also induces accumulation of ROS, and free radical scrounging enzymes like SOD (Hideg et al. 2013). Moreover, plants also accumulate phenolic compounds which absorb detrimental UV-radiations (Shen et al. 2010). Nanoparticles are known to intensify the harmful effects of UV-B radiation on plants such as the application of CuO nanoparticles alone on Elodea nuttallii (waterweed species) shows no detrimental effects but in combination with the UV-B radiation, induces considerable negative effects on biochemical and physiological traits (Regier et al. 2015).

14.10 Effect of Nanoparticles on Plant Growth Under Flooding Stress

During flooding state, plants suffer from hypoxia conditions because the rate of diffusion of O2 is slower in water than in air. Flooding stress/hypoxia condition inhibits respiration, seed germination, root, vegetative and reproductive growth, hypocotyl pigmentation, and up-regulation of genes for ethylene synthesis (Komatsu et al. 2012). ATP formation is suppressed under hypoxic conditions, thus to sustain cellular energy level, flooded plants are required to shift their carbohydrate metabolism towards fermentation (Banti et al. 2013), and up-regulation of genes for alcohol dehydrogenase and pyruvate decarboxylase (Mustafa et al. 2015a). Nanoparticles mitigate flooding stress and improve plant growth by inhibiting ethylene biosynthesis (Syu et al. 2014). For instance, the silver nanoparticle treated plant shows less O2 distress under flooding stress. Besides, employing a gel-free proteomic technique by Mustafa et al. (2015b), reported that Al2O3 nanoparticles treated soybean plant under flooding stress has shown better growth performance as compared to plant treated with Ag and ZnO by regulating metabolic pathways and cell death.

14.11 Conclusion and Prospects

Globally, in the arena of agriculture, nanobiotechnology has been used to improve the productivity of crops with quality enhancement by improving cultivation methods. Plants being sessile encounter a variety of abiotic stresses such as salinity, drought, extreme low/high temperature, metal toxicity, UV-B radiation, flooding, and many more in their whole life-span. They accommodate themselves at the biochemical, physiological, and molecular levels by regulating their genes and enzymes responsible for the antioxidant defense system as well as maintaining homeostasis. Plenty of nanoparticles have been exploited for up-regulating various genes and enzymes to mitigate different abiotic stresses but still in its early stage. So far, very little work has been done on the phytotoxicity of nanoparticles on plants, and there exists a huge gap in our understanding of the eco-toxicity, tolerable limit, and uptake capability of various nanoparticles in plants. Therefore, to prevent negative effects of nanoparticles on the environment and living commodity (flora and fauna), and to harness best peculiar attributes of nanoparticles for improving plant growth, development and productivity in stressed conditions, further research is urgently needed to have a clear-cut understanding of the nanoparticle interaction with the plants and environments. Moreover, there is a need to develop a regulatory framework established on the various research evidence which will limit mankind’s exposure to undesirable bioengineered nanoparticles to a harmless level, although the application of nanoparticles had increase the productivity of crops. The remarkable applications of nanomaterials presents an optimistic prospect of nanobiotechnology with well understanding of their ecotoxicity and by including all the aspects like reutilizing, feasibility, manufacturing, and framework of policy to handle them securely and utilize them in an eco-friendly manner.