**5. Effect of nanoparticles on antioxidant and molecular mechanism of plants**

Nanoparticles have an impact on plants' antioxidant system at the molecular level as they increase the capability of plants to tolerate oxidative stress. When *Brassica* 


#### **Table 3.**

*Effect of nanoparticles on plants for ROS and antioxidant system.*

*juncea* plant was treated with silver NPs, antioxidant enzyme activities (CAT, ascorbate peroxidase APX and guaiacol peroxidase) were increased which reduced the ROS [78]. The antioxidant system of *Spirodela polyrhiza* plants was activated when copper NPs were used to induce the activity of CAT, POD, and SOD. Moreover, ROS level also increased remarkably because of malondialdehyde and glutathione [85]. When seedlings of *Brassica juncea* were treated with gold NPs, the activity of antioxidant enzymes (guaiacol peroxidase, CAT, glutathione reductase, and APX) was significantly increased, in addition to the accumulation of the higher amount of proline and hydrogen peroxide. The activity of glutathione reductase was maximum at 200 ppm and the activity of other antioxidant enzymes, such as APX and guaiacol peroxidase, were also increased at 400 ppm of gold NPs treated plants [86]. When roots of kidney beans were exposed to CeO2 NPs for a longer time, then antioxidant enzymes' activities were reduced and soluble protein was increased. While leaves treated with CeO2 NPs showed increased activity of guaiacol peroxidase [87]. Plants exposed to ZnO NPs increase the Zn and SOD antioxidant enzyme minimizing the effect of oxidative stress [88].

The molecular mechanism of plants can be studied by using the model plant species. *Arabidopsis thaliana* treated with AgNPs gene expression analysis done by RT-PCR and cDNA microarray analyzed for transcriptome behavior [89] showed 281 upregulated genes associated with metal and oxidative stress and 80 downregulated genes associated with hormonal stimuli and plant defense system. The effect of AgNPs on rice has also been studied and some responsive proteins were associated with transcription, oxidative stress, protein degradation, cell division, calcium signaling and regulation, and apoptosis [90]. Hence, the effect of different nanoparticles on different plant species for the functioning of ROS and antioxidant enzymes has been briefed in **Table 3**.

*Improvement of Abiotic Stress Tolerance in Plants with the Application of Nanoparticles DOI: http://dx.doi.org/10.5772/intechopen.110201*

## **6. Mechanism of nanoparticle (NPs) absorption in plants**

Absorption and translocation of NPs in plants are one of the latest disciplines of study. The most commonly used NPs to enhance abiotic stress tolerance in plants are metal-based (MB) and carbon-based (CB). Among MB, the most widely studied nanomaterials are metal and metal oxides, such as copper, silver, titanium, iron, and zinc; while the most explored CB nanomaterials are carbon nanotubes (CNTs), fullerene (C70), and fullerol (C60(OH)20) [94]. The impact of NPs on plants is determined by a number of variables, including availability, uptake, translocation, and accumulation. The plant cell wall restricts the entry of foreign elements; therefore, effective techniques are needed to introduce advantageous NPs and make them available to plants.

Different factors like size, chemical content, and plant species affect the entry of NPs, which is further influenced by their stability, transport and absorption, toxicity, and accumulation [95]. Particle size, surface charge, and the hydrophobicity of the plant surface all play important roles in their absorption [96]. Additionally, the absorption rate and translocation in plants are directly correlated to the structure of the nanomaterial utilized [97]. All of these elements highlight the requirement for developing and enhancing laboratory techniques to comprehend the NPs physicochemical qualities [98] as they undergo biotransformation in the soil, which has a direct impact on their toxicity and bioavailability. Foliar spraying or incubating isolated cells, roots, pollen, seeds, and protoplast with NPs, direct injection, irrigation of plants with NPs, delivery by biolistic, and hydroponic treatment have all been employed in previous research to make NPs available to the plant cells [99]. Bioaccumulation defines the uptake of NPs by plant roots and travels through apoplastic and symplastic routes to the cortex and pericycle [100].

Nanoparticles entered through the stomata, cuticle, stigma, trichomes, wounds, and lenticels and move through the phloem. They reach the xylem and phloem through the root tip meristem, where the Casparian strips continue to the shoot but have not fully developed. Endocytosis allows NPs to enter cells even when the cell wall, cell membrane, and Casparian strips block their uptake and transport. Additionally, transporters like aquaporins and carrier proteins facilitate their easier entry into cells [101]. The capacity of roots to absorb nutrients can change if NPs accumulate on the surface of the roots. Parenchymatic intercellular gaps in seeds enable NPs to be directly absorbed before being diffused into the cotyledon. Stomata allow for the internalization of NPs larger than 10 nm, which are then delivered to the plant's vascular system *via* apoplastic and symplastic pathways. Once internalized, NPs move through vascular systems carried by phloem alongside sugar flow, move-in both the directions, and eventually build up in organs that could serve as sap-sinks [102]. The apoplastic pathway has been extensively described to enhance the transfer of water nutrients and nonessential metal complexes. Leaf shape and chemical composition of surface waxes limits the entry of NPs through leaf [103]. Hence, to ensure the NPs' effective absorption in plants, it is crucial to consider their size, concentration, and physiological environment.

### **7. Mechanism of translocation and accumulation of NPs in plants**

Mechanism of NPs translocation in different plant cells and organelles has been clarified [96]. Plant cell wall serves as a barrier that manages NPs uptake and establishes solubilization needed to enable their translocation. NPs with a size range of 40–50 nm can easily pass through the cell wall [104]. Composition of NPs affects their mobility through cell membrane or cell wall and also encourages their adsorption to radical exudates. Positively charged NPs have better adherence to cell walls. Their coating and morphology have a big impact on how they behave inside plants and the rhizosphere [105]. After penetration through cells, they go through the shoots [101] and roots are transmitted to various aerial tissues and the seeds [6]. Gold NPs were only collected in the shoots of *Oryza sativa* when used with *Cucurbita pepo*, *Raphanus raphanistrum*, and other plant species. Positively charged gold nanoparticles tend to be quickly absorbed by plant roots [106]. The entry of NPs into the cell is facilitated by capillary action and osmotic pressure [95]. Membrane proteins of NPs, including as receptors and transporters, are altered as a result of their interaction with the outermost layer.

Negatively charged gold nanoparticles are easily translocated from plant roots to shoots. The most stable are SiO2 and TiO2, as they remain present in plant tissues after their uptake. When *Zea mays* is exposed to ZnO NPs hydroponically, most of them are accumulated in its roots and shoots. It is explained by the maximal NPs dissolution in the rhizosphere, which produces the zinc ions and enhances its absorption and translocation in the plant [107]. Soil-grown wheat has also been observed for this perseverance.

Different processes have been identified by the translocation of CeO2 and ZnO NPs into *Glycine max* [108]. CuO NPs have been shown to be capable of moving from *Zea mays* roots to shoots and vice versa. TiO2 NPs with a diameter of 140 nm or larger may translocate in *Triticum aestivum* roots [103]. The data of different reports about accumulation of different NPs in different plant tissues have been summarized in **Table 4**.


#### **Table 4.**

*Accumulation of nanoparticles in different plant species' tissues.*

*Improvement of Abiotic Stress Tolerance in Plants with the Application of Nanoparticles DOI: http://dx.doi.org/10.5772/intechopen.110201*
