In the past decades, engineered nanoparticles have been developed
extensively, and they have gained wide recognition in
variety of commercial and industrial applications. According
to the market survey, the production of metal oxide nanoparticles
was estimated to rise from 2,70,041 tons in 2012 to 16,
63,168 tons by 2020, and among these nanoparticles, aluminum
oxide nanoparticles (Al2O3 NPs) are one of the most
abundantly used engineered nanoparticles (Yang et al. 2012;
Future Markets Inc. 2013). Due to their dielectric and abrasive
properties, Al2O3 NPs have been used as an abrasive agent
and insulators (Prabhakar et al. 2012). Al2O3 NPs are widely
being used in various applications, including alloys, explosives,
rocket fuel, wear-resistant coatings for ships, energetics,
sensors, personal care products, and drug delivery systems
(Darlington et al. 2009; Jiang et al. 2009; Schrand et al.
2010; Sadiq et al. 2011). These nanometer-sized materials also
have enhanced toxicity in comparison to bulk material when
unleashed into the environment (Donaldson et al. 2001).
Al2O3 NPs were found to appreciably induce toxicity by increasing
the frequency of micronucleus occurrence, chromosomal
losses, mutations, and polyploidy (Di Virgilio et al.
2010). The minimal reports on their toxicity and genotoxicity
in spite of their increased use in industries have inspired the
toxicity evaluation of Al2O3 NPs, in particular (Tsaousi et al.
2010).
Nanoparticles can have positive and negative impacts on
higher plants and their consumers in the food chain (Rico et al.
2011). Al2O3 NPs with varying diameters and surface compositions
can be engineered for evading the reticuloendothelial
system (Faraji and Wipf 2009) or can be complexed with
antibodies to attain targeted drug delivery (Arruebo et al.
2009). These ceramic nanoparticles are also known to be beneficial
as they possess antimicrobial properties, which result due to the electrostatic attraction between their positive surface
and the negatively charged bacteria, thus decreasing the
viability ofmicrobes (Mukherjee et al. 2011). In addition, their
high surface reactivity is known to cause increased risk of the
nanoparticles being entrapped in the gill mucus of marine
organisms, and thereby hinder respiratory processes and transportation
of ions (Baker et al. 2014). In mammalian systems,
they can decrease the tight junction protein expression, alter
the properties of the blood-brain barrier, and induce toxicity to
the microvascular endothelium (Chen et al. 2008). Hence, it is
highly mandatory to study the toxicity profile of Al2O3 NPs.
For studies concerning the mutagenesis in higher eukaryotes,
plants have generally been used as a test system in order to
study the effect of nanoparticles (Fiskesjo 1985). Al2O3 NP
exposure are known to cause the rapid depolarization of plasma
membrane, which was more extensive in the distal cellular
portions, and the extent was influenced by the developmental
state of cells (Illéš et al. 2006).Al2O3 NP toxicity can also lead
to the inhibition of basipetal polar transport of auxin in the
outer cortex cells and epidermis (Hasenstein and Evans 1988).
The foremost uncertainty with regard to Al2O3 NP phytotoxicity
is the primary route of action at the subcellular level;
however, the root apex has been cited as the primary injury
site by Ryan et al. (1993).
The easily distinguishable genetic endpoints, including
chromosomal aberrations, alterations in ploidy, and sister
chromatid exchanges, have made the plant system ideal for
studies (Kumari et al. 2011). In addition, the Allium cepa root
chromosomal aberration assay has been authenticated by the
United Nations Environment Programme (UNEP) (Grant
1982) and the International Programme on Chemical Safety
(IPCS) (WHO 1985) as an effective conventional plant bioassay
and as an established standard test for the in situ monitoring
of environmental substances and chemical screening
(Cabrera and Rodriguez 1999). Chromosomal aberrations
have been determined by using A. cepa since 1920s
(Satapathy and Swamy 2013; Kanaya et al. 1994). The clear
mitotic phases, stable chromosome number and karyotype,
diversity of chromosome morphology, rapid response to
genotoxic materials, and the rare occurrences of spontaneous
chromosomal damages are the features that make A. cepa an
excellent plant system for studying the toxicity of nanoparticles
(Firbas and Amon 2013). For genotoxicity screening during
cell division or microspore formation, root tips are shown
to be convenient for microscopic analysis of features like mitotic
or meiotic aberrations, respectively (Kristen 1997).
Kumari et al. (2009) have used the A. cepa as an indicator
to evaluate the genotoxic effect of AgNPs. Cell damages in
A. cepa were observed by Chen et al. (2010) in the presence of
carbon nanoparticles for a range of exposure concentrations
from 10 to 110mg/L. The role of ROS in causing genotoxicity
in A. cepa by ZnO and AgNPs was later on reported by Panda
et al. (2011) and Kumari et al. (2011), respectively. Similarly, Ghodake et al. (2011) have assessed the phytotoxic nature of
cobalt and zinc oxide nanoparticles in A. cepa. Liman (2013)
also used A. cepa as a test system to evaluate the genotoxic
effect of bismuth (III) oxide nanoparticles. Recently, Pakrashi
et al. (2014) have reported the toxic effect of TiO2 NPs on
A. cepa by studying their internalization and ROS generation.
However, there have been just a handful reports that explore
the phytotoxicity of Al2O3 NPs. It has been reported by Kim
et al. (2009) that Al2O3 NPs could affect cultured mammalian
cells by inducing cytotoxicity and primary DNA damage;
however, no mutagens were observed by them.
Thus, the current toxicity study is the first of its nature to
evaluate the cytogenetic effect of Al2O3 NPs in A. cepa. The
various chromosomal aberrations were observed by optical,
fluorescence, and confocal laser microscopy for a wide range
of Al2O3 NP concentrations (0.01, 0.1, 1, 10, 100 μg/mL).
This also happens to be the first study that evaluates the cytogenetic
effect of nanomaterials using A. cepa model at exposure
concentrations ≤1 μg/mL. The cytotoxicity of Al2O3 NPs
on A. cepa root tip cells was evaluated by tracking the changes
in themitotic index and chromosomal aberrations. Further, the
dose-dependent toxicity was substantiated with the help of
surface chemical analysis by FT-IR, internalization/uptake
analyses by ICP-OES, and the antioxidant enzyme (SOD)
assay.