Keywords

4.1 Introduction

4.1.1 Cascade of Signaling Behind Plant-NPs Interaction and Stress Tolerance

Plants have a complex system for defense against environmental stresses, yet the mechanism of perception of stimulus to the transduction and activation of responsive units against stimulant before stress prompted destruction is decisive to cellular machinery. As per available information about plant-NPs interaction against abiotic stresses, it is obvious that synthesis of ROS is vital phenomenon in plant cell against environmental stimuli (Khan et al. 2017). Although the mechanism of action of plants by the application of NPs is not well understood, however the omic approaches help to conceive the signaling pathways in plant cells.

Signaling molecules predominantly activate a defense mechanism which triggers molecular network to encounter a particular challenge. Calcium ions (Ca2+) being secondary messenger molecules act as vital components in signal transduction. Upon receiving a stress signal Ca2+ start transport from stores to cytosol via Ca2+ gated channels and ultimately increase the cytoplasmic level of secondary messenger, which is identified my calcium binding proteins (CaBPs) that start a cascade of reaction downstream to alter the gene expression of plant against that particular stress (Khan et al. 2014a, b; Tuteja and Mahajan 2007). It has been evident that nitric oxide (NO) promotes the cytosolic Ca2+ in cell upon the onset of environmental stress and pathogen attack (Khan et al. 2012a, b; Lamotte et al. 2006) and inevitably Ca2+ promote the NO synthesis (Del Rio et al. 2004; Corpas et al. 2004).

Proteomic studies on Oryza sativa roots have declared the role of Ag NPs-responsive proteins in oxidative stress pathway, second messenger (Ca2+) regulation, signal transduction and expression and post transcription changes, cellular growth and programmed cell death (Mirzajani et al. 2014). The study is a prove of previous work by Goyer (1995) who anticipated the Ag-NPs role in cellular metabolism by binding Ca2+ gated channels and Ca/Na pumps (Goyer 1995). It has been further observed that C60 nano-crystals prompted the functional regulation of Ca2+/calmodulin- dependent protein kinase II (CaMKII) (Miao et al. 2014). Further studies on Arabidopsis thaliana investigated the overexpressed Ca2+ dependent protein (CML45) and CaMPK23 caused by the activity of cadmium sulfide QDs (Marmiroli et al. 2015). These overexpressed proteins play a pivotal role in stress resistance in various plants (Delk et al. 2005; Xu et al. 2011; Boudsocq and Sheen 2013) however Ca2+ in the proteins can be switched with NPs (Cheung 1980).

NPs have been reported to involve in increased nitrate reductase activity that is key enzyme in nitric oxide (NO) biosynthesis to regulate plant immune system (Carpenter et al. 2012; Shahrokh et al. 2014; Chandra et al. 2015). On the other hand NO interacts NP-induced toxicity mechanism and antioxidant genes to increase their transcript level and decrease ROS and lipid degradation (Chen et al. 2015). NPs are involved in upregulated expression of genes related to stress responsive and cell growth and development (Almutairi 2016; Khodakovskaya et al. 2011a, b, 2012).

The studies on NPs-plant interaction have exhibited the increased generation of ROS that play not only signaling molecules in plant but also increase phytotoxicity (Oukarroum et al. 2012; Qi et al. 2013; Ma et al. 2010; Van Hoecke et al. 2008; Simon et al. 2013; Wei and Wang 2013).

The dual action of NPs needs a comprehensive study to evaluate how NPs protect plants against ROS while acting as inducers of oxidative stress at the same time. Genomics and proteomics approaches aid to existing knowledge by investigating the role of NPs in plants under environmental stress. A proteomic study using Ag NPs and AgNO3 in Eruca sativa demonstrated the altered level of proteins involved in redox regulation and metabolism of sulfur as a result of physiochemical nature of NPs (Vannini et al. 2013). A differential expression of genes realted to abiotic stress has been observed in Arabidopsis thaliana by the application of Ag NPs and Ag+ covered with polyvinylpyrrolidone (PVP) (Kaveh et al. 2013). The study concluded that the Ag NPs induced stress is partially due to Ag+ toxicity and partially it is the consequence of nanoparticle-specific effects.

Expression of miRNA in response to NPs paves another way to understand the mechanism of action of NPs against environment in plants. Increased expression of miRNA in Tobacco plant on application of optimum concentration of TiO2 and Al2O3 was demonstrated against metal stress however increased level of NPs negatively affected the growth and development of plant (Frazier et al. 2014; Burklew et al. 2012). AHA2 (gene involved in stomatal opening) in Arabidopsis is upregulated by the treatment of zero valent NP , which cause tolerance to drought stress (Kim et al. 2015). Moreover studies conducted on the application of TiO2 and MWCNTs on Arabidopsis showed the downregulated gene expression involved in development and phosphate deficiency (García-Sanchez et al. 2015).

4.2 Nanoparticles and Abiotic Stress Resistance

4.2.1 Salinity Stress

Salinity , due to the deposition of anions (chloride and sulfate) or cations (primarily sodium but occasionally of calcium and magnesium) in arid to coastal areas soil, is one of the major abiotic stresses limiting food production. Over 20% of agricultural land is affected by salinity and the limit is ever increasing. Salinity is detrimental as it causes reduction in growth and development by influencing physiological, biochemical and molecular pathways in plants. It not only shifts the osmotic stress but also ionic imbalance in plants due to high accumulation of salts. Due to Osmotic stress plants’ nutrients and water uptake is decreased while ionic stress engenders decreased proportion of K+/Na+ (Khan et al. 2012) and over production of ROS that affects molecular mechanisms in plants leading to electrolytes splits and damages metabolic pathways such as protein and lipid metabolism, and photosynthesis in cytol (Sharma et al. 2012; Ismail et al. 2014; Khan et al. 2010).

Current advancement in nanotechnology highlighted that the NP of Silica (SiO2) the second most abundant natural element and titanium dioxide (TiO2) contributed much to enhance vegetative growth and overall crop production under salinity. The tolerance is attributed to silicon NP that might better absorbed by maize roots than its micro or bulk counterparts Suriyaprabha et al. (2012) and form a thin layer in cell wall to enhance resistance against stress to maintain yield (Latef et al. 2018; Derosa et al. 2010).

The stressed tomato and squash plants exhibited better seed germination, the anti-oxidative enzyme activities, photosynthetic rate and leaf water absorption rate when treated with Si NP (Haghighi and Pourkhaloee 2013).

Na+ ion toxicity led a reduced yield in maize; however SiO2NPs alleviated the plant response under salinity stress by reducing Na+ ion concentration in cell wall through lower absorption of the ions (Gao et al. 2006a, b). Similar studies were conducted in tomato plants that lead to better plant growth (Savvasd et al. 2009). A remarkable elevation in germination and seedling growth was observed in Lens culinaris under salinity stress by application of Si NP (Sabaghnia and Janmohammadi 2014). A promising effect of Si nano fertilizer was highlighted by Kalteh et al. (2014) in Ocimum basilicum, where increased chlorophyll content, proline level and other physiological traits were recorded under salinity stress. Under high concentration of salt Squash (Cucurbita pepo) showed a lethal reduction in roots and shoots growth, vigor length and yield of plant (Siddiqui et al. 2014). Use of SiO2 NP ameliorated the traits by decreasing the electrolyte leakage and level of hydrogen peroxide (H2O2), malondialdehyde (MDA) and chlorophyll degradation.

Fe2 NPs proved to be auspicious addition in Nano biotechnology. The application revealed that it positively affected foliar fresh and dry weights and mineral contents of peppermint (Mentha piperita) however it didn’t show any effect on sodium content. Maximum activities of anti- oxidant enzymes were recorded under salinity stress but masked by the application of Fe2O3 NP (Askary et al. 2017). Torabian et al. (2016) have described high level of chlorophyll content, photosynthesis rate, CO2 concentration, osmotic regulation and reduced Na content in Helianthus annuus by the use of nanoZnO under salt stress.

Contemporary studies on the significance of chitosan NPs (maize and tomato), multi-walled carbon nanotubes (broccoli) and silver NPs (wheat) have demonstrated their mitigating effect on salinity (Bruna et al. 2016; Hernandez-Hernandez et al. 2018; Martinez-Ballesta et al. 2016; Abou-Zeid and Ismail 2018; Mohamed et al. 2017).

4.2.2 Drought Stress

Water is the vital component for life on the planet and its deficiency leads to severe conditions (Drought stress ) in living organisms including plants. Drought is most commonly occurring environmental stress which affects almost 45% of global agriculture area (Dos Reis et al. 2016). Water scarcity in plants leads to decrease in water potential and turgor of cell, which increase the level of molecules in the cytol and extracellular surfaces. Later, decreased cell size elicits the retarded growth and reproduction failure in plants. Ultimately cell starts to accumulate abscisic acid (ABA) and proline (Osmotic regulator), which leads to excessive production of ROS, glutathione and ascorbate (radical scavengers) which exasperates the severity (Hussain et al. 2019; Ahmad et al. 2017). Drought not only affects cell water potential but it also influences the stomatal closure, gaseous and ionic exchange, photosynthesis and transpiration rate (Schulze et al. 2019).

During the past decade tremendous efforts have been made to counter the water induced stress in plants using NP . The nanoparticles of TiO (Rutile) exhibited intense effect by exogenous application in spinach plant. The morphological, biochemical and physiological changes occurred in plant resulted in high rubisco activase activity, chlorophyll synthesis and promoted photosynthesis which leads to increase in dry weight of plant (Gao et al. 2008). Foliar application of TiO2 NP might cause an increase photosynthesis rate which augmented the overall seed yield in cow pea (Vigna unguiculata L.) (Owolade et al. 2008). As the effect of TiO2 NP varies among the species and with different applied environments, 0.02% of foliar spray of TiO2 NP enhanced the vigor of wheat plants by improving yield traits such as plant height, ear number and weight, 1000-kernal weight and seeds/plant, harvest index, and starch and gluten content of plant under water scarcity (Jaberzadeh et al. 2013). Dragonhead (Dracocephalum moldavica) plants treated with TiO2 NP (10 ppm) exhibited more proline level with less H2O2 and MAD content as compared to control plant under water deficit state (Mohammadi et al. 2014a, b). It was established that drought-prompted mutilations in plants like membrane damage and oxidative stress can be mitigated by optimal concentrations of TiO2 NPs. Silica NPs protects cell wall during water deficit conditions by reducing cell wall permeability of leaves resulting low lipid peroxidation (Zhu et al. 2004). SiO2 has proved to increase proline content with addition of escalated CAT and POD activities in plants under stress vs. controlled plants of tomato (Siddiqui and Al-Whaibi 2014), faba bean (Qados and Moftah 2015; Qados 2015) and alfalfa (Cakmak et al. 1996). Reative water content (RWC), water use proficiency and turgor pressure in leaf cells are the physio-chemical processes effected by Silica NPs that directly influence xylum transport plant. At different level of water deficiencies the response of Silica NPs varies in Hawthorns (Crataegus sp.). Enhanced tolerance was observed in plant against drought at different concentrations of NPs by positively effecting physio-biochemical processes (chlorophyll, carotenoid, carbohydrateand proline contents, and increased photosynthesis rate, MDA, (RWC) and membrane electrolyte leakage (ELI)) within the cell (Ashkavand et al. 2015).Silicon NP posed a positive effect on two sorghum cultivars with relatively different drought vulnerability irrespective of level of stress by maintaining photosynthesis rate and improved root growth (Hattori et al. 2005).

ZnO and CuO NPs act as fertilizers as these are source of Zn and Cu to plants. At different doses the NPs react on different parts of roots as Zn NPs causes increased lateral roots whereas Cu NPs induce proliferated and elongated root hair close to root tip in Triticum aestivum seedlings under drought (Yang et al. 2017). The short root length may reduce access to water. CuO possibly change the water supply thus shrink the cell wall in Arabidopsis and mustard thus increase lignification. The altered water transport may also be the reason of Cu-pectin association in cell wall (Nair and Chung 2017). The continuous drought stress elevates proline and anthrocynin in cell. The high level of ROS during the stress which leads to increased ABA may cause differential gene expression for drought tolerance (Dimkpa et al. 2012). Silver nanoparticles (AgNPs) are one of the most abundant NPs in use to mitigate abiotic as well as biotic stresses. AgNPs provided an inhibitory role against microorganisms (Beyene et al. 2017). In water deficit lentil plants, application AgNPs resulted in high germination rate and high growth and production parameters (Hojjat and Ganjali 2016).

Sodium nitroprusside (SN) along with Multi walled carbon nanotubes (MWCNTs) provide tolerance in Hordeum vulgare against water and salt stress by not only improving water absorption capacity of seed as well as seedling water concentration (Karami and Sepehri 2017). Increased antioxidants and high germination rate was recorded in Hordeum vulgare, Glycine max and Zea mays using MWCNTs (Lahiani et al. 2013; Liu et al. 2016). High root and shoot growth in Triticum aestivum suggested the drought tolerance though MWCNTs (Srivastava and Rao 2014).cerium oxide (CeO2) provided another source of NP to enhance crop production under water defict condition in Glycine max (Cao et al. 2018). In addition the in vitro use of iron (FeO2) NPs alongside salicylic acid manifested to be an effective tool against water deficiency in strawberry at pre transplantation to soil (Mozafari et al. 2018).

A comprehensive knowledge of metabolic and molecular mechanisms of plant through NPs to ameliorate abiotic stress will pave a way to develop stress resistant crops (Singh et al. 2017). Syntheses of dehydrins in susceptible plants, by application of NPs cause mitigation of drought stress (Lopez et al. 2003). Production of compatible molecules like proline, betaine, etc., is initiated by dehydrins which in turn maintains cell integrity and water deficiency (Paleg et al. 1984). Once it is known that at which stage of metabolic pathways NPs counter abiotic stresses, the massive increase in their use will be possible.

4.2.3 Temperature Stress

Temperature is vital factor which determines plant growth, development and crop yield. It is characterized by the ideal point beyond that plant growth is effected badly, though the optimum temperature varies between species and genera. Temperature stress includes high temperature stress (above the threshold temperature) and low temperature or chilling stress (very low than ideal) for a certain time span to cause a permanent damage to plant (Wahid 2007).

4.2.3.1 Heat Stress

Thermal stress implies the increase in temperature beyond the optimum level for a longer time span that causes permanent loss to development and vegetative growth (Wahid 2007). The stress negatively affects growth and yield of crop globally. High temperature elevates the synthesis of ROS and cause oxidative imbalance which leads to breakdown of organic molecules (proteins), degradation of lipids and escape of ions in cell membrane (Karuppanapandian et al. 2011; Moller et al. 2007; Savicka and Skute 2010;) that may affect chlorophyll content ultimately photosynthesis (Prasad et al. 2011).

Selenium (Se) nanoparticles provide an alleviated response of high temperature when used in low concentration by modulation of hydration potential and chlorophyll content (Haghighi et al. 2014). High level of Se is associated with oxidative stress while the low concentration is considered to be responsible for antioxidative response (Hasanuzzaman et al. 2014; Hartikainen et al. 2000). Heat shock proteins (molecular chaperones) are produced by plant during high temperature stress which along with other proteins cause stress tolerance by retaining their stability under challenging condition (Wahid et al. 2007). It has been reported that MWCNTs are involved in up regulation of gene expression associated with stress tolerance including HSP 90 (Khodakovskaya et al. 2011a, b). Furthermore a study in susceptible corn plant confirmed the role of heat shock proteins by application of cerium oxide (CeO2) NPs that lead to the higher synthesis of H2O2 and high expression of HSP70 (Zhao et al. 2012). TiO2 NPs also play role in heat stress by enhanced photosynthesis by regulation of stomata opening in plant leaves (Qi et al. 2013).

4.2.3.2 Cold Stress

When plants are exposed to the temperature very lower than their optimum temperature, the cell and tissues are damaged due to physiological and morphological changes, the phenomenon is known as Cold stress (Hasanuzzaman et al. 2013). Electrolytes imbalance and degradation in cell membrane are the adversities related to cold stress which eventually leads to decreased germination, reduced growth and crop production (Welti et al. 2002; Suzuki et al. 2008). Nevertheless, sensitivity to the stress may differs inter species and inter genera with tolerant plants showing least membrane damage than the susceptible (Maali Amiri et al. 2010; Heidarvand et al. 2011). Despite plants vulnerability to stress, NP like TiO2NPs possesses the potential to mitigate the chilling effect by reducing membrane degeneration and maintain electrolyte imbalance (Mohammadi et al. 2013). However its accumulation ratio is more in sensitive (thinner membrane layer and wide stomata) to tolerant genotype (Giacomo et al. 2010). Photosynthesis is essential process of plant that is affected by the chilling stress. Plants subjected to cold result in photosystem damage by decreasing in chlorophyll content, transpiration rate, deterioration in photosystem enzymes (Liu et al. 2012; Yordanova and Popova 2007).

Ameliorations of NPs on photosystem have been inferred by elevated synthesis of Rubisco (photosystem enzymes) (Gao et al. 2006a, b), chlorophyll capacity to absorb light, (Ze et al. 2011), rate of electron movement and and suppressed ROS synthesis in chloroplast (Giraldo et al. 2014). TiO2 NPs manifested the increased expression level in genes associated with Rubisco and chlorophyll binding proteins (Hasanpour et al. 2015), improved activities of antioxidant enzyme such as CAT, APX and SOD (Mohammadi et al. 2014a, b), finally increase resistance against chilling stress.

When plants are subjected to cold stress the transcript level of antioxidant genes like MeAPX2 and MeCu/ ZnSOD is up regulated, dehydroascorbate reductase, monodehydroascorbate reductase and glutathione reductase activities are elevated. As a result ROS scavenging which leads to repressed oxidative stress factors (lipid peroxidation, pigment degradation and H2O2 production) ultimately develop tolerance (Xu et al. 2014). While the application of NPs on Chiling stress have been reported with enhanced growth and physiochemical processes in plants under cold (Azimi et al. 2014; Hawrylak-Nowak et al. 2010; Kohan-Baghkheirati and Geisler-Lee 2015; Haghighi et al. 2014).

4.2.4 Heavy Metals Stress

Metal with high molecular weight and toxic at very low concentrations are termed as heavy metals . Metal stress has become one of the alarming environmental issues plants are facing worldwide that cause toxicity and serious crop loss (Chibuike and Obiora 2014; Rahimi et al. 2012). Heavy metal causes reduction in plant growth by disruption of important plant activities like reduced up take of vital elements, repressed enzyme activities which results into deprivation of important element (Capuana 2011). Plant Growth medium augmented with metals accelerates ROS synthesis, resulting in to oxidative stress in plant cell by disruption of cell structure including organic molecules and plasma membrane (Sharma et al. 2012; Rascio and Navari-Izzo 2011).

To combat heavy metal stress plant evolve a special defense mechanism by producing metal chelate, polyphosphates and organic acids which restrain the uptake of metal ions regulation of anti-oxidative pathways (CAT, POD and SOD)and ultimately ROS scavenging. Although plant defense mechanism is pivotal to counter heavy metals, artificially induced NPs play key role in reducing heavy metal phyto-toxicity (Gunjan et al. 2014; Tripathi et al. 2015).

ZnO NPs along with other micronutrients (Zn, Cu, Mn) are reported to play crucial response to reduce efflux of cadmium (Cd) in plants (Baybordi 2005; Venkatachalam et al. 2017). River tamarind (Leucaena leucocephala) possesses Cd and Pb phytotoxicity, which can be ameliorated by the foliar spray of ZnO NPs. The NPs are responsible for elevated soluble proteins, chlorophyll and carotenoid content in leaves, and decrease in oxidative damaged to lipids membrane (Venkatachalam et al. 2017). The induced level of antioxidative enzymes (CAT, SOD, POD) in leaves of L. leucocephala and lipid perodixadation was confirmed at seedling stage. Similar effects were recorded by Si NPs application to reduce Cr toxicity through activation of anti-oxidative pathways in pea plant (Tripathi et al. 2015). Enormous studies have been conducted using TiO2 against abiotic stresses in plants. Besides mitigating effects on environmental stress, TiO2 NPs proved to limit Cadmium (Cd) phytotoxicity by augmented growth and increased energy driven pathways (Singh and Lee 2016). In addition TiO2 multiple NPs evinced positive role against heavy metal challenge in plants (Table 4.1). Li and Huang (2014) exhibited that nano- hydroxyapatite (Ca5(PO4)3) may regulate toxicity of Cd in Brassica chinensis.

Table 4.1 physiochemical effects of nanoparticles in plants against various Abiotic stresses

4.3 Conclusion and Future Perspectives

Nanoparticles minimize the damage caused by environmental stresses by activating the defense system of plants. The activated defense system is the result of high ROS activities, which exhibited the toxic effects. Taking advantage of their size NPs become permeable and modulate ion channels to promote growth and germination of plants. The large surface area helps in absorption and delivery of molecules. The exact mechanism of action of MPs is still not very well studied however the omic studies revealed that NPs mimic secondary messengers and related proteins are activated. A cascade of reactions starts that leads to altered gene expression responsible for abiotic stress tolerance and plant growth. In conclusion it is merely important to further study the exact role of NPs at molecular level to confirm whether these molecules are involved in stress tolerance or stress induction.