Metallic titanium or TiO2 are regarded both as growth enhancers and further investigation as biofertillizers (Frazier et al. 2014). At 200mg/L concentration TiO2 NP showed increased activity of all antioxidative enzymes (Song et al. 2012). The FTIR data of the cucumber fruits from TiO2 NPs treated plants showed significant amide, lignin and carbohydrates but nanomaterials did not undergo biotransformation confirmed from XRF and XANES upon translocation to edible parts (Servin et al. 2012, 2013).
Cerium dioxide nanoparticles (CeO2 NP) are mainly used in automotive industries, and may interfere with the cell metabolism due to its oxidative properties. Experimental datas showed that in case of wheat and rice plants compromised the quality of grains (Rico et al. 2013, Du et al. 2015). Nickel oxide nanoparticle (NiO NP) was observed to induce ROS accumulation and other antioxidative enzymes and reduces the concentration of plant pigments (Faisal et al. 2013). The mechanism of NiNPs induced phytotoxicity and indication of apoptosis in plants. Ni NP deserves concerns as it causes potential environmental hazards.
Iron oxide nanoparticles (Fe3O4 NP) at lower concentrations were observed to have beneficiary impact on plant and improves germination (Iannone et al., 2016; Li et al., 2016),
Non-metallic nanoparticles:
Nanoparticles have been developed to be used in agriculture as nanopesticides and nanofertilizers preferentially as nanocarrier; (Fraceto et al., 2016; Wang et al., 2016). Non-metallic nanoparticles, such as, single-walled carbon nanotubes (SWCT) and fullerene have been well studied to reveal their nanotoxicity mechanisms (Joner et al., 2008). Chitosan coated nanomaterials were used as herbicide, due to which the efficiency of herbicide was observed to be enhanced (Maruyama et al. 2016). Mesosporous silicon nanoparticles used to deliver DNA and other chemicals in plants (Martin Ortigosa et al. 2014).
Response of certain stress related and developmental genes and further at micro RNA relation level upon interaction with plant and nanoparticles:
The transport of nanomaterial inside the plant depends on size, magnitude, zeta potentials. The size exclusion limit for plant cell wall entry ranges between 5-20nm (Herth et al. 2011). The nanoparticles enters the cell wall through endocytosis and through symplastic transport (Ma et al. 2010) while the large size and high concentration are observed to cause damage to plant cell wall and plasma membrane and interacts with various plant processes (Mirzajani et al., 2013).
Previous studies did not clearly indicated cumulatively the route of entry, translocation of nanoparticles, and its localization along with triggering of signalling cascades with its gene regulation of various stress related genes in plants. But certain reports depicts that AgNP treatment on Arabidopsis was found to trigger ROS molecules that serve as signals to coordinate a wide range of plant cellular events, including hormone perception and transduction (Gechev et al., 2006) and downregulates expression of genes that are associated in response to biotic and hormonal signals.(Kaveh etal.2013). Due to treatment with AgNp could affect auxin and ABA signaling transduction by interacting with the expression of genes such as IAA8 causing its positive induction and NCED3 and RD22 gets downregulated in its presence (Syu et al. 2014). The recent work is done on the exposure of tomato cells to carbon nanotubes (CNTs) that lead to activation of genes for tomato water channel protein (LeAqp2) and causes upregulation of 10 stress related genes. (Khodakovskaya etal.2010). Thus nanoparticles play a vital role in gene regulation of various signaling cascades in plants.
The miRNAs are involved in the RNA silencing and posttranscriptional regulation of gene expression of the plant species. miRNA have been shown to mediate abiotic stress responses such as drought and salinity in plants by altering the gene expression. As the exposure to NPs is a kind of stress conditions imposed upon the plants, one expects that the miRNAs would exhibit actions to mitigate the stress. Using highest concentration 1% Al2O3 NPs upon exposure showed increased expression of miR395,miR397,miR399 (Khodakovskaya et al. 2010)
Studies showed that the gold NPs exposure has improved free radicals and antioxidant activity, thereby alters micro RNAs expression that regulates metabolic processes in plants (Siddiqi & Husen, 2016) mainly expression miR164, miR167, miR395, miR398,miR408 in Arabidopsis sp. Low concentration of TiO2 NPs induced miRNA expression in tobacco seedlings with miR395 and miR399 exhibiting the greatest fold changes of 285 fold and 143 fold respectively (Frazier et al. 2014). The internalised nanoparticles interfere the electron transfort chain of mitochondria and chloroplast which results in oxidative burst with increase in ROS concentration (Dimkpa et al., 2013, Cvjetko et al., 2017). Upon triggering of ROS accumulation after resulting in nanoparticle ���interaction. Upon interaction produces protein modifications, lipid peroxidations and DNA damage.
Concluding Remarks:
From the above review of the literature, it shows an influence of metal and metal oxide nanoparticles on plants but there are necessities to understand the molecular mechanisms of plant nanoparticle interactions in order to get a broad view on the immune responses of a plant with the nanomaterials. As nanomaterials are used intensively in day to day life starting from agriculture to other commercial applications so its effect on the environment must be of greater concern. It is evidenced from the above information that metal and metal oxides nanoparticles in excess are harmful to plants, whereas, when present in traces it can be beneficial for plants.
Few research is there that shows the importance of engineered nanomaterials (ENM) in agriculture where its size, surface charge, and concentration plays an important role but that too at a nascent stage.
But the lacunae that need to be addressed are the impact of metal and metal oxides nanoparticle on plant physiology and molecular biology. The research is also needed to be performed in the area of remediation of nanoparticle from agriculture soil and wastewater.