non-enzymatic antioxidants. It is now well-known fact that
plants under severe stress accelerate the production of ROS
that leads to oxidative damage in cell by breaking the
equilibrium between ROS and their scavenger antioxidants.
Increased level of MDA and H 2 O 2 in plant cells are the
indices of oxidative damages caused by free radicals/ROS
produced during stress (Singh et al. 2012). MDA that is a
product of lipid peroxidation resulted in low activities of
antioxidant enzymes. Ghosh et al. (2010) observed an
increase in MDA level at 4 mM of treatment in Allium cepa
suggesting that the lipid peroxidation caused the DNA
damage. Wang et al. (2011) pointed out an increase in ROS
on TiO 2 -NPs exposure caused DNA damage in Arabidopsis
thaliana. Plants have antioxidant systems both enzymatic
(such as superoxide dismutase (SOD), catalase (CAT),
ascorbate peroxidase (APX), guaiacol peroxidase (GPX),
dehydroascorbate reductase (DHAR) and glutathione
reductase (GR) and non-enzymatic (ascorbic acid, proline,
cysteine, non-protein thiols, etc.) to protect themselves from
damaging effects of ROS produced under the adverse situation. Jacob et al. (2013) reported that TiO 2 -NPs modify the
activities of enzymatic antioxidants in Phaseolus vulgaris at
10 and 30 ppm. Moreover, in spinach seedlings treated with
colloidal solution of 0.25% TiO 2 -NPs cause oxidative stress
in chloroplasts (Lei et al. 2008).
TiO 2 -NPs have found to affect antioxidant activities of
SOD, POD and CAT in duckweed (Song et al. 2012),
ascorbate peroxidase (APX) in faba bean (Foltête et al.
2011), MDA in onion and tobacco (Ghosh et al. 2010) and
thiols (GSH) in bean (Castiglione et al. 2014). Song et al.
(2012) observed that in duckweed, TiO 2 -NPs increased the
enzyme activity at concentration lower than 200 mg l
−1 ,
while high concentration (500 mg l
−1 ) caused serious damage to plant cells. Moreover, TiO 2 -NPs exposure also ameliorates plant tolerance to various stresses by alleviating the
toxicity induced by ROS (Lei et al. 2008; Gohari et al.
2020).
3.2.4 Phytotoxicity at Gene Level
In plants, NPs cause cytotoxicity and genotoxicity in terms
of alteration of cell structure, DNA structure, cell division,
micronuclei formation, chromosomal aberrations and DNA
damage (Yan and Chen 2019; Santos Filho et al. 2019).
During the process of entry into a plant cell, TiO 2 -NPs may
use several transporters present on cell wall (Nel et al. 2009;
Rico et al. 2011). The induction of cytotoxic and genotoxic
responses in cell lines of plants and animals on TiO 2 -NPs
exposure evident recently. However, till now the knowledge
in context to molecular mechanisms of TiO 2 -NP-mediated
toxicity in plants is less but still emerging. However, the
progressive technology especially transcriptomics can help
to understand the link between regulation of genes and their
impacts in response to TiO 2 -NPs’ exposure. Ghosh et al.
(2010) evaluated the genotoxic response of Allium cepa and
Nicotiana tabaccum to TiO 2 -NPs using two classical techniques, i.e., comet assay and the DNA laddering. It is concluded that the genotoxic potential of TiO 2 -NPs on onion
and tobacco increased with increasing numbers of
micronuclei formation, chromosomal aberrations and DNA
damage. TiO 2 -NPs exposure to plants or seeds at particular
concentration, resulted in the genotoxicity, mutagenicity and
cytotoxicity.
Castiglione et al. (2011) used TiO 2 -NPs soaked seeds to
know the potential hazards of these NPs on monocots and
dicots, i.e., Zea mays and Vician arbonensis. Results showed
reduction of mitotic index and concentration-dependent
increase in the DNA damage, chromosomal aberrations
and fragmentations. The application of TiO 2 -NPs (seeds
soaking, soil amendments or foliar application) reported to
have genotoxic and cytotoxic impacts in Hordeum vulgare
(Matiellio et al. 2015), Triticum aestivum (Rafique et al.
2018) and Allium cepa (Santos Filho et al. 2019). Zhao et al.
(2016) observed the plasma membrane damage, the presence
of oil bodies and changes in the number of vacuoles in
response to TiO 2 -NPs. Contrary to this, Frazier et al. (2014)
confirmed the activation of the expression profiles of
microRNAs (miRNAs) and gene regulators in response to
TiO 2 -NPs that are known to improve plant development by
increasing plant tolerance to abiotic stresses. Besides,
selected area electron diffraction (SAED) with TEM analysis
done in roots of onion treated with 1000 mg l
−1 of TiO 2 -
NPs (anatase phase; 25 nm) revealed internalization of TiO 2 -
NPs in the vacuole but in the brookite phase that is less
reactive and with comparatively bigger in size (orthorhombic format with 450 nm) and predicted this as a protective
mechanism (Santos Filho et al. 2019).
3.2.5 Impacts on Overall Plant Productivity
and Yield
The overall plant productivity and yield are the resultant of
various parameters of plant, i.e., physiological, biochemical
and molecular. From the above section, it can be observed
that TiO 2 -NPs have a crucial role in regulation of these
processes. Authors who conducted long-term studies on life
cycle of plants are able to conclude results in context to
productivity hence, very few studies are reported. Like,
Raliya et al. (2015b) demonstrated constructive role of TiO 2 -
NPs in plant processes and fecundity of plants. It showed
enhanced fruit yields, biomass and productivity in Solanum
lycopersicum. Another full life cycle experiments on Linum
usitatissimum also reported enhanced seed oil, yield and
protein contents on TiO 2 -NPs exposure (Aghdam et al.
2016). Similarly, favorable results obtained in case of Arachis hypogaea (peanut) planted in soil amended with anatase
TiO 2 -NPs by Rui et al. (2018). Janmohammadi et al. (2016)
observed increased grain yield and biomass for Hordeum
60
R. Singh et al.
plants under severe stress accelerate the production of ROS
that leads to oxidative damage in cell by breaking the
equilibrium between ROS and their scavenger antioxidants.
Increased level of MDA and H 2 O 2 in plant cells are the
indices of oxidative damages caused by free radicals/ROS
produced during stress (Singh et al. 2012). MDA that is a
product of lipid peroxidation resulted in low activities of
antioxidant enzymes. Ghosh et al. (2010) observed an
increase in MDA level at 4 mM of treatment in Allium cepa
suggesting that the lipid peroxidation caused the DNA
damage. Wang et al. (2011) pointed out an increase in ROS
on TiO 2 -NPs exposure caused DNA damage in Arabidopsis
thaliana. Plants have antioxidant systems both enzymatic
(such as superoxide dismutase (SOD), catalase (CAT),
ascorbate peroxidase (APX), guaiacol peroxidase (GPX),
dehydroascorbate reductase (DHAR) and glutathione
reductase (GR) and non-enzymatic (ascorbic acid, proline,
cysteine, non-protein thiols, etc.) to protect themselves from
damaging effects of ROS produced under the adverse situation. Jacob et al. (2013) reported that TiO 2 -NPs modify the
activities of enzymatic antioxidants in Phaseolus vulgaris at
10 and 30 ppm. Moreover, in spinach seedlings treated with
colloidal solution of 0.25% TiO 2 -NPs cause oxidative stress
in chloroplasts (Lei et al. 2008).
TiO 2 -NPs have found to affect antioxidant activities of
SOD, POD and CAT in duckweed (Song et al. 2012),
ascorbate peroxidase (APX) in faba bean (Foltête et al.
2011), MDA in onion and tobacco (Ghosh et al. 2010) and
thiols (GSH) in bean (Castiglione et al. 2014). Song et al.
(2012) observed that in duckweed, TiO 2 -NPs increased the
enzyme activity at concentration lower than 200 mg l
−1 ,
while high concentration (500 mg l
−1 ) caused serious damage to plant cells. Moreover, TiO 2 -NPs exposure also ameliorates plant tolerance to various stresses by alleviating the
toxicity induced by ROS (Lei et al. 2008; Gohari et al.
2020).
3.2.4 Phytotoxicity at Gene Level
In plants, NPs cause cytotoxicity and genotoxicity in terms
of alteration of cell structure, DNA structure, cell division,
micronuclei formation, chromosomal aberrations and DNA
damage (Yan and Chen 2019; Santos Filho et al. 2019).
During the process of entry into a plant cell, TiO 2 -NPs may
use several transporters present on cell wall (Nel et al. 2009;
Rico et al. 2011). The induction of cytotoxic and genotoxic
responses in cell lines of plants and animals on TiO 2 -NPs
exposure evident recently. However, till now the knowledge
in context to molecular mechanisms of TiO 2 -NP-mediated
toxicity in plants is less but still emerging. However, the
progressive technology especially transcriptomics can help
to understand the link between regulation of genes and their
impacts in response to TiO 2 -NPs’ exposure. Ghosh et al.
(2010) evaluated the genotoxic response of Allium cepa and
Nicotiana tabaccum to TiO 2 -NPs using two classical techniques, i.e., comet assay and the DNA laddering. It is concluded that the genotoxic potential of TiO 2 -NPs on onion
and tobacco increased with increasing numbers of
micronuclei formation, chromosomal aberrations and DNA
damage. TiO 2 -NPs exposure to plants or seeds at particular
concentration, resulted in the genotoxicity, mutagenicity and
cytotoxicity.
Castiglione et al. (2011) used TiO 2 -NPs soaked seeds to
know the potential hazards of these NPs on monocots and
dicots, i.e., Zea mays and Vician arbonensis. Results showed
reduction of mitotic index and concentration-dependent
increase in the DNA damage, chromosomal aberrations
and fragmentations. The application of TiO 2 -NPs (seeds
soaking, soil amendments or foliar application) reported to
have genotoxic and cytotoxic impacts in Hordeum vulgare
(Matiellio et al. 2015), Triticum aestivum (Rafique et al.
2018) and Allium cepa (Santos Filho et al. 2019). Zhao et al.
(2016) observed the plasma membrane damage, the presence
of oil bodies and changes in the number of vacuoles in
response to TiO 2 -NPs. Contrary to this, Frazier et al. (2014)
confirmed the activation of the expression profiles of
microRNAs (miRNAs) and gene regulators in response to
TiO 2 -NPs that are known to improve plant development by
increasing plant tolerance to abiotic stresses. Besides,
selected area electron diffraction (SAED) with TEM analysis
done in roots of onion treated with 1000 mg l
−1 of TiO 2 -
NPs (anatase phase; 25 nm) revealed internalization of TiO 2 -
NPs in the vacuole but in the brookite phase that is less
reactive and with comparatively bigger in size (orthorhombic format with 450 nm) and predicted this as a protective
mechanism (Santos Filho et al. 2019).
3.2.5 Impacts on Overall Plant Productivity
and Yield
The overall plant productivity and yield are the resultant of
various parameters of plant, i.e., physiological, biochemical
and molecular. From the above section, it can be observed
that TiO 2 -NPs have a crucial role in regulation of these
processes. Authors who conducted long-term studies on life
cycle of plants are able to conclude results in context to
productivity hence, very few studies are reported. Like,
Raliya et al. (2015b) demonstrated constructive role of TiO 2 -
NPs in plant processes and fecundity of plants. It showed
enhanced fruit yields, biomass and productivity in Solanum
lycopersicum. Another full life cycle experiments on Linum
usitatissimum also reported enhanced seed oil, yield and
protein contents on TiO 2 -NPs exposure (Aghdam et al.
2016). Similarly, favorable results obtained in case of Arachis hypogaea (peanut) planted in soil amended with anatase
TiO 2 -NPs by Rui et al. (2018). Janmohammadi et al. (2016)
observed increased grain yield and biomass for Hordeum
60
R. Singh et al.