ReviewRole of nanomaterials in plants under challenging environments☆
Introduction
Nanomaterials (NMs), once called by Paul Ehrlich as “Magic Bullets” (Kreuter, 2007), are one of the most studied materials of the century that gave birth to a new branch of science known as ‘nanotechnology’. The specific quality of NMs which make these tiny entities unique, is their size which ranges between 1 and 100 nm (1 nm = 10−9 m) (Ball, 2002). Although, NMs can be prepared from the bulk size materials but small size and shape of these particles make their chemical action entirely different from their parent material (Brunner et al., 2006). Smaller size of NMs helps them to penetrate specific cellular locations and their additional surface area facilitates more adsorption and targeted delivery of substances (Kashyap et al., 2015). The NMs exist in volcanic dust, mineral composites (natural NMs) as well as in anthropogenic waste materials like coal combustion, diesel exhaust, welding fumes etc. (incidental NMs) (Monica and Cremonini, 2009). Moreover, engineered NMs manufactured with nanoscale dimensions are generally grouped into four types viz. carbon based NMs, metal based NMs, metal oxides, dendrimers and composites (Yu-Nam and Lead, 2008).
Engineered NMs have revolutionized almost every field of science and of course, plant science could not remain unaffected. These NMs have been shown to affect plants at every stage of their life cycle (Caňas et al., 2008, Lahiani et al., 2013, Siddiqui and Al-Whaibi, 2014, Liu et al., 2016). Fertilizers are integral part of agriculture that assist growth and development of plants. However, recently employed nano-fertilizers have been proved more efficient alternatives to regular fertilizers. Smaller size of nanoparticles (NPs) provides additional surface area which enhances the availability and facilitates more absorption of fertilizers by the plants and thus reduces losses of fertilizers due to leaching, emissions, and long-term incorporation by soil microorganisms (Liu et al., 2006, DeRosa et al., 2010). Moreover, nano-fertilizers are released at slower rates which help in maintaining soil fertility by decreasing the toxic effects associated with over-application of traditional chemical fertilizers (Suman et al., 2010).
Being sessile organisms, plants have no choice to escape or hide from adverse environmental conditions such as drought, salinity, water logging, extreme temperature, UV-radiation, etc. These stresses create oxidative stress by inducing generation of reactive oxygen species (ROS) such as singlet oxygen (1O2), superoxide radical (O2−), hydroperoxy radical (HO2), hydrogen peroxide (H2O2) and hydroxyl radical (OH). Excessive accumulation of ROS damages membrane lipids, proteins and nucleic acids (Foyer and Noctor, 2000), triggers cytotoxicity, genotoxicity (Ghosh et al., 2015, Shen et al., 2010a, Shen et al., 2010b, Yadav et al., 2014) and suppresses growth (Begum et al., 2012). To counter oxidative stress, plants are fitted with a system of enzymatic antioxidants viz. superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), glutathione reductase (GR) and non-enzymatic antioxidants (glutathione, ascorbate) which continuously scavenge harmful ROS. Whereas, plants counter osmotic stress by enhancing the accumulation of organic osmolytes such as trehalose, polyols (glycerol, inositols, sorbitols etc.), amino acids (proline, glycine betaine and taurine) which maintain normal hydration level of cells. Under hypoxic conditions plants are deprived of proper supply of oxygen which causes energy depletion and settle the plants with low energy status, however, to maintain energy level plants alter their metabolism and switch over from carbohydrate metabolism to fermentation (Banti et al., 2013). To counteract metal stress plants synthesize metal-chelates, organic acids and polyphosphates that cause restriction and sequestration of toxic metals either in apoplasm or symplasm.
In addition to their role in plant growth and development, NPs play significant role in the protection of plants against various abiotic stresses (Table 1). The NPs mimic the activities of antioxidative enzymes and scavenge these ROS (Rico et al., 2013a, Rico et al., 2013b, Wei and Wang, 2013). Small size and large surface area of NPs provide access for toxic metals for binding and thus reduced availability and toxicity of metals (Worms et al., 2012). Under abiotic stresses, photosynthesis is highly susceptible cellular process, however NMs have been shown to protect photosynthetic system and improve photosynthesis by suppressing oxidative and osmotic stress (Haghighi and Pessarakli, 2013, Qi et al., 2013, Siddiqui et al., 2014). However, response of plants to NMs varies differently depending on plant species and NMs applied (Lin and Xing, 2007). Apart from their beneficial effects several NMs show toxicity symptoms (Slomberg and Schoenfisch, 2012, Begum and Fugetsu, 2012). Presence of NMs in the growth medium induces oxidative stress and causes reduction in germination rate, root and shoot length, and loss of photosynthesis, chlorophyll (Chl), biomass (Barhoumi et al., 2015, Da Costa and Sharma, 2016, Wang et al., 2016), and nutritive value of crop plants (Peralta-Videa et al., 2014). The NMs also alter gene expression involved in biotic and abiotic stress responses, cell biosynthesis, cell organization, electron transport, and energy pathways (Landa et al., 2012, Kaveh et al., 2013, Aken, 2015).
Therefore, role of NMs in growth and development and in the tolerance of plants to abiotic stresses is ambiguous and controversial. In the present article an attempt is made to shed light on the recent updates on the role of NMs under abiotic stress conditions.
Section snippets
Nanomaterials and plant growth
Plants growing under natural environmental conditions are constantly exposed to a combination of biotic and abiotic stresses. As far as abiotic stresses are concerned, drought salinity, water logging, heat, cold, metal, UV radiation etc. are some common stresses which plants face at some or the other stages of their life cycle. Several studies show that NMs play vital role in alleviating abiotic stresses and stress-induced alterations in plants (Table 1).
Seed germination is the first stage of
Nanomaterials and photosynthesis under abiotic stresses
Photosynthesis, the foundation for all the metabolic processes in plants, is considered as one of the most sensitive physiological processes to environmental stresses. Therefore, maintenance of optimal photosynthetic rate is vital for the endurance of plants under stressful conditions. Plants treated with NMs show protection against various abiotic stresses and exhibited improved rate of photosynthesis, stomatal conductance, transpiration rate, water use efficiency, and Chl and proline content
Nanomaterials and plants under abiotic stresses
Overproduction of ROS by various cell organelles is the signature effect of abiotic stress-induced oxidative stress. Apart from damaging effect, ROS are also known to trigger various defense systems through activating cell signaling cascade and inducing or suppressing the expression of many genes (Hancock et al., 2001). Nonetheless, plants are equipped with enzymatic and non-enzymatic system of antioxidants which continuously scavenge harmful ROS (Fig. 2). Regardless of such defense systems,
Phytotoxic effects of nanomaterials
As mentioned in the preceding pages, owing to their small size, shape and larger surface area to mass ratio, NMs enhance plant growth, productivity and provide protection against various abiotic stresses. On contrary, the same properties of NMs make them deliterious as they are known to induce oxidative stress, cytotoxic and genotoxic responses in plants (Tan and Fugetsu, 2007, Lin and Xing, 2007, Lin and Xing, 2008, Lin et al., 2009a, Lin et al., 2009b, Lin et al., 2010, Tan et al., 2009;
Mechanism of action of nanomaterials under abiotic stress
Although, plants are equipped with a network of defense units, but the precise perception and transduction of stress stimulus to the defense system and accurate temporal and spatial activation of these defense units in response to stress stimulus before the onset of stress-induced damage is crucial for the protection of plant’s cellular machinery. While going through the available information on NMs-plant interaction under abiotic stresses, it became evident that generation of ROS is a common
Conclusions
The available information reveals that NMs alleviate abiotic stress-induced damage through activating defense system of plants. Small size of NMs facilitates easy penetration and regulates water channels that assist seed germination and growth of plants; moreover, improved surface area facilitates more adsorption and targeted delivery of substances. On contrary, NMs have also been reported to trigger the generation of ROS and exhibit several toxic effects on plants. The enhanced ROS level by
Contributions
Khan M. N. planned and collected available literature and prepared first draft of the manuscript, Mobin M., Abbas, Z.K. and Siddiqui, Z.H. edited the MS and added their inputs on the section other stresses and AlMutairi, K.A. wrote the section phytotoxic effects of nanomaterials.
Acknowledgements
Financial support (Project no. S-0105-1436) by Deanship of Scientific Research (DSR), University of Tabuk is gratefully acknowledged. Authors are also thankful to the head of the Biology Department and Vice Dean of Higher Studies and Scientific Research, Faculty of Science, University of Tabuk.
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This article is part of a special issue entitled “Nanomaterials in Plant”, published in the journal Plant Physiology and Biochemistry 110, 2017.