charged (Zhou et al. 2011). These accumulated ENPs have
to cross different layers including cuticle, epidermis, cortex,
endodermis, and Casperian strips to translocate into shoots
by xylem vessels. Likewise, in plant leaves, ENPs get
incorporated into cuticles of roots as these are thin. After
incorporation into the cuticle, there are two proposed pathways for the uptake of ENPs through epidermis, i.e.,
apoplastic pathway and symplastic pathway. The apoplastic
pathway is the most studied and according to which intercellular transport of ENPs occurs without crossing the cell
membrane as these NPs diffuse into the intercellular spaces
by penetrating the cell wall (Lv et al. 2019). These ENPs
after passing through epidermis, cortex, and endodermis then
reaches Casperian strip which is made up of lipophilic
compounds and hampers the movement into the vascular
system (Schwab et al. 2016). Nonetheless, sometimes these
ENPs enters the vascular system as Casperian strip has not
yet formed in root tip or through root junctions where
Casperian strip is disjointed (McCully 1995; Dietz and Herth
2011; Lv et al. 2015; Schymura et al. 2017). However, the
symplastic pathway is a cell-to-cell pathway that occurs in
two steps which are the penetration of cell membrane followed by intercellular transfer via plasmodesmata.
According to the literature, the highest feasibility transmembrane pathway for NPs is through endocytosis. After
endocytosis, these ENPs reaches vacuoles and get sequestered as it acts as a sink for solutes and also largest organelle
in the plant cell (Serag et al. 2011; Bao et al. 2016; Huang
et al. 2017). The study on vacuole membrane transport of
NPs suggests that dissolved metal NPs can induce
anti-oxidative responses in plants. These oxidative response
results into the production of thiol-containing glutathione
stimulating hormone (GSH) which binds with metal ions or
with metal transporters resulting in metal detoxification
pathway (Dhankher et al. 2002; Ma et al. 2016). Another
mechanism of ENPs toxicity may be due to the Trojan horse
mechanism in which these NPs are taken up by plants cell
followed by the release of metal ions causing damage to the
cellular structure (Singh and Ramarao 2012). Studies of
ENPs uptake and translocation by plants are still at a very
nascent stage which needs to be studied thoroughly.
4.1 Effects of Engineered Nanoparticles
on Plants
The increasing application of ENPs may have an impact on
ecosystem functioning and food crops. In this section, major
interaction of ENPs are illustrated in terms of some growth
and developmental features (seed germination, biomass,
yield characteristics, shoot/root growth and leaf production),
physiological features (photosynthetic efficiency and effect
on various photosystems), biochemical, and molecular
features (enzymatic and non-enzymatic components). The
response of major crops exposed to different ENPs are presented in Table 1.
4.1.1 Effect of Engineered Nanoparticles
on Growth and Developmental Features
of Plants
Plant morphological parameters such as seed germination,
biomass, leaf area, yield, length, and weight of root and
shoot are indicators of plant’s health. Exposure of carbon
nano onions (CNO) to Cicer arietinum seed showed an
increase in $ 35% in weight, $ 24% in length, $ 17% in
diameter, and $ 16% in height over the control when
exposed to 30 lg mL
−1 of CNO (Tripathi et al. 2017a, b).
Similarly, studies on effects of multiwalled carbon nanotubes
(MWCNTs) (50 lg mL
−1 ) on A. cepa, Arachis hypogaea,
Glycine max, Hordeum vulgare, Triticum aestivum, and Zea
mays showed improvement in root and shoot length, biomass, and seed germination by improving water absorption
potential (Srivastava and Rao 2014; Lahiani et al. 2017).
Yield in terms of crop products is an important parameter for
growth assessment which includes the number of spikes,
grains per spike, and grain weight. Xue et al. (2017) showed
that nanochitin (6 mg kg
−1 )-treated T. aestivum led up to
23% increase in yield and other yield parameters. Nanochitin
showed a prominent increase in the number of spikelets per
spike as apex development and spikelet primordia differentiation was influenced resulting in apex elongation (Xue
et al. 2017). In contrast to previous studies, mesoporous
carbon NPs (150 mg L
−1 ) showed a reduction in root length
(70%), shoot length (57.1%), root fresh weight (34%), and
shoot fresh weight (45%) of Oryza sativa grown hydroponically (Hao et al. 2019).
Metal-based NPs generally show toxic effects on maize
crops due to the release of metal ions (Dimkpa et al. 2012;
Mahmoodzadeh et al. 2013; Nair and Chung 2014a). Ag
NPs exposed to A. hypogaea showed a significant reduction
in plant height, biomass, and yield (Rui et al. 2017). Similar
results were obtained for O. sativa and T. aestivum exposed
to Ag NPs in hydroponics and soil, respectively (Nair and
Chung 2014a; Yang et al. 2018). In contrast, the foliar
application showed no toxicity to Vigna unguiculata at
varied concentrations (Vanti et al. 2019). However, Al 2 O 3
NPs showed improved biomass of Solanum lycopersicum
during foliar applications (Shenashen et al. 2017; Debnath
et al. 2020). CeO 2 NPs are generally used in the automobile
industry, electronics, and fuel additives (Keller et al. 2013).
Different plants have been exposed to different concentrations of CeO 2 NPs to analyze their effect. Zhang et al.
(2017a, b) showed that there is no visible impact of CeO 2
NPs at a concentration of 500 mg kg
−1 , while on increasing
the concentration (1000–2000 mg kg
−1 ), fresh and dry
weight of root and shoot decreased in Lactuca sativa.
Impact of Engineered Nanoparticles on Microbial Communities, Soil …
209
to cross different layers including cuticle, epidermis, cortex,
endodermis, and Casperian strips to translocate into shoots
by xylem vessels. Likewise, in plant leaves, ENPs get
incorporated into cuticles of roots as these are thin. After
incorporation into the cuticle, there are two proposed pathways for the uptake of ENPs through epidermis, i.e.,
apoplastic pathway and symplastic pathway. The apoplastic
pathway is the most studied and according to which intercellular transport of ENPs occurs without crossing the cell
membrane as these NPs diffuse into the intercellular spaces
by penetrating the cell wall (Lv et al. 2019). These ENPs
after passing through epidermis, cortex, and endodermis then
reaches Casperian strip which is made up of lipophilic
compounds and hampers the movement into the vascular
system (Schwab et al. 2016). Nonetheless, sometimes these
ENPs enters the vascular system as Casperian strip has not
yet formed in root tip or through root junctions where
Casperian strip is disjointed (McCully 1995; Dietz and Herth
2011; Lv et al. 2015; Schymura et al. 2017). However, the
symplastic pathway is a cell-to-cell pathway that occurs in
two steps which are the penetration of cell membrane followed by intercellular transfer via plasmodesmata.
According to the literature, the highest feasibility transmembrane pathway for NPs is through endocytosis. After
endocytosis, these ENPs reaches vacuoles and get sequestered as it acts as a sink for solutes and also largest organelle
in the plant cell (Serag et al. 2011; Bao et al. 2016; Huang
et al. 2017). The study on vacuole membrane transport of
NPs suggests that dissolved metal NPs can induce
anti-oxidative responses in plants. These oxidative response
results into the production of thiol-containing glutathione
stimulating hormone (GSH) which binds with metal ions or
with metal transporters resulting in metal detoxification
pathway (Dhankher et al. 2002; Ma et al. 2016). Another
mechanism of ENPs toxicity may be due to the Trojan horse
mechanism in which these NPs are taken up by plants cell
followed by the release of metal ions causing damage to the
cellular structure (Singh and Ramarao 2012). Studies of
ENPs uptake and translocation by plants are still at a very
nascent stage which needs to be studied thoroughly.
4.1 Effects of Engineered Nanoparticles
on Plants
The increasing application of ENPs may have an impact on
ecosystem functioning and food crops. In this section, major
interaction of ENPs are illustrated in terms of some growth
and developmental features (seed germination, biomass,
yield characteristics, shoot/root growth and leaf production),
physiological features (photosynthetic efficiency and effect
on various photosystems), biochemical, and molecular
features (enzymatic and non-enzymatic components). The
response of major crops exposed to different ENPs are presented in Table 1.
4.1.1 Effect of Engineered Nanoparticles
on Growth and Developmental Features
of Plants
Plant morphological parameters such as seed germination,
biomass, leaf area, yield, length, and weight of root and
shoot are indicators of plant’s health. Exposure of carbon
nano onions (CNO) to Cicer arietinum seed showed an
increase in $ 35% in weight, $ 24% in length, $ 17% in
diameter, and $ 16% in height over the control when
exposed to 30 lg mL
−1 of CNO (Tripathi et al. 2017a, b).
Similarly, studies on effects of multiwalled carbon nanotubes
(MWCNTs) (50 lg mL
−1 ) on A. cepa, Arachis hypogaea,
Glycine max, Hordeum vulgare, Triticum aestivum, and Zea
mays showed improvement in root and shoot length, biomass, and seed germination by improving water absorption
potential (Srivastava and Rao 2014; Lahiani et al. 2017).
Yield in terms of crop products is an important parameter for
growth assessment which includes the number of spikes,
grains per spike, and grain weight. Xue et al. (2017) showed
that nanochitin (6 mg kg
−1 )-treated T. aestivum led up to
23% increase in yield and other yield parameters. Nanochitin
showed a prominent increase in the number of spikelets per
spike as apex development and spikelet primordia differentiation was influenced resulting in apex elongation (Xue
et al. 2017). In contrast to previous studies, mesoporous
carbon NPs (150 mg L
−1 ) showed a reduction in root length
(70%), shoot length (57.1%), root fresh weight (34%), and
shoot fresh weight (45%) of Oryza sativa grown hydroponically (Hao et al. 2019).
Metal-based NPs generally show toxic effects on maize
crops due to the release of metal ions (Dimkpa et al. 2012;
Mahmoodzadeh et al. 2013; Nair and Chung 2014a). Ag
NPs exposed to A. hypogaea showed a significant reduction
in plant height, biomass, and yield (Rui et al. 2017). Similar
results were obtained for O. sativa and T. aestivum exposed
to Ag NPs in hydroponics and soil, respectively (Nair and
Chung 2014a; Yang et al. 2018). In contrast, the foliar
application showed no toxicity to Vigna unguiculata at
varied concentrations (Vanti et al. 2019). However, Al 2 O 3
NPs showed improved biomass of Solanum lycopersicum
during foliar applications (Shenashen et al. 2017; Debnath
et al. 2020). CeO 2 NPs are generally used in the automobile
industry, electronics, and fuel additives (Keller et al. 2013).
Different plants have been exposed to different concentrations of CeO 2 NPs to analyze their effect. Zhang et al.
(2017a, b) showed that there is no visible impact of CeO 2
NPs at a concentration of 500 mg kg
−1 , while on increasing
the concentration (1000–2000 mg kg
−1 ), fresh and dry
weight of root and shoot decreased in Lactuca sativa.
Impact of Engineered Nanoparticles on Microbial Communities, Soil …
209