Effects of nanomaterials on abiotic stresses.
The response of plants to abiotic stress is complex and involves changes in their morphology, physiology and metabolism. A number of strategies are being followed to enhance the tolerance of abiotic stress conditions, including the development of genetically-engineered varieties containing various gene constructs believed to enhance the performance under stress conditions. Nanotechnology is a versatile field and has found application in almost all the existing fields of science. The application of nanoparticles increased germination and seedling growth, physiological activities including photosynthesis and nitrogen metabolism, leaf activities of CAT, POX and APX, chlorophyll contents, protein, carbohydrate contents and yield, and also positive changes in gene expression indicating their potential use in crop improvement. Nanoparticles enhances the water stress tolerance via enhancing root hydraulic conductance and water uptake in plants and showing differential abundance of proteins involved in oxidation-reduction, ROS detoxification, stress signaling, and hormonal pathways. The mobility of the nanoparticles is very high, which leads to rapid transport of the nutrient to all parts of the plant. In particular, the most actual is to find ways to increase the adaptation potential of cultivated plants with the use of nanopreparations in stressful conditions.
- abiotic stress
- crop plants
World population is increasing day by day and by 2050 it is expected to reach 9.1 billion, but agricultural production is not rising at a parallel pace. Raising productivity is a challenge as the area under cultivation is likely to remain constant or even decrease due to increasing pressure on land for nonagricultural uses. While increased investments and technological breakthrough can improve availability, these may not necessarily translate into increased accessibility and absorption of food. With climate change on the trail, abiotic stresses are considered to be a major constraint for sustaining crop productivity. As per one of the estimates approximately 70% of yield reduction of crops is directly or indirectly influenced by abiotic stresses . Abiotic stress leads to a series of morphological, physiological, biochemical and molecular changes that adversely affect plant growth and productivity. Drought, salinity and extreme temperatures are the most prevalent abiotic stresses, threatening the global food security. Development of stress tolerant plants can be a worthwhile strategy to win over the problem of decreasing global food production. Conventional breeding methods have met with limited success in improving the stress tolerance of crop plants involving inter-specific or inter-generic hybridization. The conventional breeding approaches are limited by the complexity of stress tolerance traits, as well as the low genetic variability of yield components under stress condition and lack of efficient selection criteria. It is important, therefore, to look for alternative strategies to develop stress tolerant crops. All traditional breeding methods including selection, hybridization, polyploidy and mutation have utilized for genetic improvement of crop plants. Albeit supplementary success in history of crop improvement in agricultural crops, their yield at present reached a plateau and there exists food insecurity and poverty in many developing countries. For this purpose exploration of novel strategies and their exploitation in complement to existing traditional and advanced breeding tools is the need of the hour. Now-a-days the global demand is to increase food production with limited available resources and minimum but efficient use of fertilizer and pesticides that can check pollution in the environment which ushered in new agricultural technologies to reshape modern agriculture. Among the latest technology, nanotechnology is most promising one in the era of agriculture and plant biotechnology . The application of nanoparticles or nanodevices affect various developmental stages both positive and negative impact on plant growth. Nanotechnology comprises novel properties of nanomaterial that make easy for agricultural research in crop improvement program as well as alleviation to stresses . Nanotechnology has been provisionally defined as relating to materials, systems and processes which operate at a scale of 100 nanometers (nm) or less. ‘Nano’ usually refers to a size scale between 1 and 100 nm. Nano materials are composed of components with very small size, and these components have impacts on the properties of materials at the macro level. Nanomaterials have a relatively larger surface area when compared to the same mass of material produced in a larger form. Nano particles can make materials more chemically reactive and affect their strength or electrical properties. The particles have high surface to volume ratio that increases their reactivity and possible biochemical activity .
2. Types of nanomaterials used
Nanomaterials have applications in the field of nanotechnology, and displays different physical chemical characteristics from normal chemicals (i.e. silver nano, carbon nanotube, fullerene, photocatalyst, carbon nano, silica). Common types of nanomaterials include nanotubes (Single walled carbon nanotube, multi walled carbon nanotube), dendrimers, quantum dots, fullerenes, metal (Ag, Si, Au, etc.) and metal oxide (TiO2, SiO2, ZnO, CuO, etc.) based. Nano-scale materials can occur in nature; both natural and manmade processes (such as volcanic activity or diesel combustion) can also give rise to nanometer-sized materials unintentionally. There are two processes for nanomaterial creation including “bottom-up” processes (such as self-assembly) that create nanoscale materials from atoms and molecules, and “top-down” processes (such as milling) that create nanoscale materials from their macro-scale counterparts. Nanomaterials can be nanoscale in one dimension (e.g. surface films), two dimensions (e.g. strands or fibers), or three dimensions (e.g. particles). They can exist in single, fused, aggregated or agglomerated forms with spherical, tubular, and irregular shapes.
3. Uptake, translocation, and accumulation of nanoparticles (NPs) into the plants
The uptake of CB and MB NMs by plants is a very recent field of study. Among CB NMs, the most studied materials are the fullerene C70, the fullerol (C60(OH)20) and CNTs; while the most studied MB NMs are TiO2, Au, Ag, Cu, CeO2, FeO, and ZnO NPs. Uptake, translocation, and accumulation of NPs depend on the species of plant and the size, type, chemical composition, functionalization, and stability of the NPs. Usually, NPs enter the plant root system through the lateral root junctions and reach the xylem through the cortex and the pericycle . The mechanism behind interaction of nanoparticles with plant system is primarily based on chemical processes which create reactive oxygen species, ion cell membrane transport activity, oxidative damage and lipid peroxidation. Once enter in the plant cells NPs react with sulfhydryl, carboxyl groups and ultimately alter the protein activity. The NPs may form complexes with membrane transporters or root exudates and subsequently be transported into the plants [6, 7]. Nanomaterials move from leaves to roots, stem, and developing grain, and from one root to another. One of the main passages of uptake and transportations to the shoot and leaves of plant is the Xylem [8, 9]. The nanomaterials are capable of penetrating through the leaf cuticle and into the cell cytoplasm . In the cytoplasm, the NMs may bind with different cytoplasmic organelles and interfere with the metabolic processes at that site . One of the pathways also reported particle size of 20 nm Ag nanoparticles may be transported inside the cells through plasmodesmata [12, 13]. A study on generational transmission of C70-NOM in rice plants and find the presence of black aggregates of C70 in the leaves of the second generation of the plants treated with fullerenes only in their first generation .
4. Plant response to abiotic stress
Among the abiotic stresses, drought, salinity, alkalinity, submergence and mineral toxicity/deficiencies are considered as major factors that contribute to decrease crop growth and productivity . Plants face various environmental stresses throughout their life cycle, therefore they develop their defense against environmental stresses at various levels by modulating molecular, biochemical and physiological pathways. In order to cope these stresses, plants adopt molecular routes by appropriate alteration of gene expressions. There are several studies which indicated that nanoparticles mediated effect on plants growth and development is concentration dependent. Nanoparticles are involved in upregulating the activities of antioxidant enzymes like, SOD, CAT and POD .
4.1 Effect of nanoparticles on drought stresses
Water is a vital component for plant survival and essentially required for transport of nutrients, therefore water deficiency leads to drought stress, which resulted into weakened vitality of plants . Nanotechnology promises the significant effort to mitigate the drought stresses. Several recent studies (Table 1) have evaluated nanoparticle-mediated in different stresses [18, 19].
|Abiotic stresses||Nanomaterial||Concentration||Plant species||Stress responses||Refs.|
|Drought stress||Nano TiO2||0.01, 0.02, and 0.03%||Wheat (||Increasing growth, yield, gluten and starch content of wheat|||
|Nano TiO2||0, 10, 100, and 500 mg L−1||Flax (||Enhancing chlorophyll and carotenoids content, improving flax growth and yield attributes, decreasing H2O2 and malondialdehyde (MDA) content|||
|Nano TiO2||0%, 0.01% and 0.03%.||Basil (||Improving the negative effects of drought stress on basil plants|||
|Nano Zero valent Fe||Activation of plasma membrane H+-ATPase, stomatal opening, increasing Chl content and plant biomass, maintaining normal drought sensitivity, increasing CO2 assimilation in thale cress plants|||
|Nano SiO2||0, 10, 50 and 100 mg L−1||A positive significant effect on photosynthetic rate, stomatal conductance and plant biomass, non-significant effect on chlorophyll and carotenoid content|||
|Nano ZnO||0.5, 1 g L−1||Soybean (||Increasing germination percentage and germination rate, decreasing in seed residual fresh and dry 8 weight of soybean|||
|SiO2||0, 10, 50 and 100 mg L−1||Hawthorns (||SNPs increased plant biomass, xylem water potential and MDA content, especially under drought conditions, RWC and ELI were not affected by the SNP pre-treatments.|||
|Silicon||Sorghum (||Increase in leaf area index (LAI), specific leaf weight (SLW), chlorophyll content (SPAD), leaf dry weight (LDW), shoot dry weight (SDW), root dry weight (RDW), total dry weight (TDW)|||
|TiO2 and SiO2||25, 50, 100 and 200 ppm) or nano-SiO2 (400, 800, 1600 and 3200 ppm)||Cotton||Increased total phenolics, total soluble proteins, total free amino acids, proline content, total reducing power, total antioxidant capacity, catalase activity peroxidase activity and superoxide dismutase activity in comparison with control|||
|Salinity stress||Nano SiO2||25 mM||Tomato (||Lower levels of nano-SiO2 enhanced seed germination potential, root length and dry weight. Higher levels suppressed seed germination characteristics|||
|Nano SiO2||Basil (||Increasing fresh and dry weight, chlorophyll content and proline content|||
|Nano SiO2||Squash (||Improving seed germination and growth characteristics, reduced levels of MDA, H2O2 and electrolyte leakage, reducing chlorophyll degradation and oxidative damage, enhancing photosynthetic parameters antioxidant enzymes|||
|Nano SiO2||Tomato (||Up-regulating the expression profile of four salt stress genes and six genes were down-regulated, suppressing the effect of salinity on seed germination rate, root length and fresh weight|||
|Nano ZnO and Fe3O4||30,60,90 mg L−1||Moringa peregrina||Reduction in Na+ and Cl− contents, increasing N, P, K+, Ca2+, Mg2+, Fe, Zn, total chlorophyll, carotenoids, proline, carbohydrates, crude protein and enzymatic and non-enzymatic antioxidants|||
|Nano ZnO||2 g L−1.||Sunflower (||Increasing growth, net CO2 assimilation rate, sub-stomatal CO2 content, chlorophyll content, Fv/Fm and Zn content and decreasing Na+ content in leaves|||
|ZnO||2 g L−1.||Sunflower (||Increase growth, proline content, and some antioxidant enzyme activities|||
|Flooding stress||Nano Ag||40, 80 or 120 ppm||Blocking of ethylene signaling, promotion of root growth|||
|Nano Al2O3||Soybean (||Regulation of energy metabolism and cell death, improved growth|||
|Nano Ag||Soybean (||Reducing generation of cytotoxic byproducts of glycolysis, increasing the abundance of stress-related proteins, enhancing seedling growth|||
The application of different concentrations of silica nanoparticles improves the plant tolerance toward drought stress in Hawthorns (
Treatment with Rutile (TiO2) has led to increase germination, germination indices, vigor indices, plant dry weight, chlorophyll formation, activities of ribulose bisphosphonates carboxylase and oxygenase, rate of evaluation of oxygen in the chloroplast leading to promoted photosynthesis [39, 40]. TiO2 NPs augmented wheat plant growth and yield with its components under water deficit stress condition  and also regulates enzymes activity involved in nitrogen metabolism such as nitrate reductase, glutamate dehydrogenase, glutamine synthase, and glutamic-pyruvic transaminase that helps the plants to absorb nitrate. The effects of nano-TiO2 improved germination, light absorbance, photosynthetic activity and activate Rubisco  also promoted antioxidant stress by decreasing the accumulation of superoxide radicals, hydrogen peroxide, malonyldialdehyde content and enhance the activities of superoxide dismutase, catalase, ascorbate peroxidase, guaiacol peroxidase in spinach . Nano- TiO2 also was observed to promote the growth of spinach through an increase in photosynthetic rate and nitrogen metabolism in spinach . Nano-TiO2 can enhance plant water and nitrogen use and stimulate some antioxidant enzyme activities, such as SOD, POD and CAT such as in canola . The application of nano zinc oxide has potential to increase seed germination percentage and germination rate in soybean as compared to those were subjected to water stress. It was further suggested that nano zinc oxide application under drought stress decrease seed residual fresh and dry weight, which shows that zinc nanoparticles were effective for using of seed reservoirs to seedling growth and enhance drought tolerance . A study revealed the significant effect of iron nanoparticles under drought stress in plants on traits like number of boll per branch, number of seeds per boll, the 1000 seed weight and yield at probability level of 1%. Foliar application of iron nanoparticles exhibited drought stress mitigating effects on yield components and oil percentage of Goldasht spring safflower cultivars. Application of Fe nanoparticles also enhance yield and yield components at two stages of flowering and granulation, although it was better at flowering stage than seed formation in contrast to drought stress conditions without Fe nanoparticles application . Advances of silver nanoparticles (AgNPs) application of silver nanoparticles (AgNPs) could be attributed toward mitigating water stress mediating loss of plant growth and yield .
4.2 Effect of nanoparticles in salinity stress
Salinity is the major concern of scientific community to attain sustainable crop production, it is estimated that more than 20% of cultivated land worldwide is experiencing salinity stress and the amount is increasing day by day. Salinity stress causes the negative impact on various biochemical and physiological processes which are associated with plant growth and yield. Lowering of soil osmotic potential, creation of nutritional imbalance, enhancing specific ionic toxicity (salt stress) or one or more combination of these factors, are some of the common implications of salinity stress experienced by plants . The Application of nanofertilizers could be a potential approach to address such issues of soil toxicity and other associated stress problems. It is reported that silicon nanoparticles and silicon fertilizer exhibited promising effects on physiological and morphological traits on vegetative features of basil under salinity stress. It was evident from results which indicated significant increase in growth and development indices, chlorophyll content and proline level in basil (
5. Effect of nanoparticles on antioxidant and molecular aspect of plants
Nanoparticles can interact with biological systems such as plants chemically or mechanically; and these specific interactions originate mainly from their small size, large surface area, and intrinsic catalytic reactivity. There are only few studies describing nanoparticles impact on antioxidant and molecular level. The treatment of silver nanoparticles in
6. Conclusion and perspectives
Application of nanotechnology in agriculture, even at its global level, is at its nascent stage. Nanoscience is leading to the development of a range of inexpensive nanotech applications for enhanced plant growth, biotic and abiotic stress responses. Nanoparticles enhances the stress tolerance via enhancing root hydraulic conductance and water uptake in plants and showing differential abundance of proteins involved in oxidation–reduction, ROS detoxification, stress signaling and hormonal pathways. Nanoparticles interaction with plant cell results in modification of plant gene expression and biological pathways which ultimately affect plant growth and development. Research on nanotechnology in agriculture a vast study on fabrication, characterization, standardization, biodegradability, ecofriendly nature and also possible uptake and translocation of nanoparticles by plants is needed.
The authors are grateful to Prof. Chittaranjan Kole, former Vice-Chancellor of BCKV for guiding and providing necessary information.
Conflicts of interest
The authors declare no conflict of interest.